Dissertations / Theses: 'Crystal Plasticity Finite Element Modelling'

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Zahedi, S. Abolfazl. "Crystal-plasticity modelling of machining." Thesis, Loughborough University, 2014. https://dspace.lboro.ac.uk/2134/14588.

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A machining process is one of the most common techniques used to remove material in order to create a final product. Most studies on mechanisms of cutting are performed under the assumption that the studied material is isotropic, homogeneous and continuous. One important feature of material- its anisotropyis linked to its crystallographic nature, which is usually ignored in machining studies. A crystallographic orientation of a workpiece material exerts a great influence on the chip-formation mechanism. Thus, there is a need for developing fundamental understanding of material's behaviour and material removal processes. While the effect of crystallographic orientation on cutting-force variation is extensively reported in the literature, the development of the single crystal machining models is somewhat limited.

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Xu, Yilun. "On the development of a multi-scale modelling framework to study plasticity and damage through the coupling of finite element crystal plasticity and discrete dislocation plasticity." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/52630.

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The microstructure of polycrystalline materials crucially determines their mechanical performance in engineering applications. A multi-scale modelling approach is capable of representing the microstructure and thus capturing the material performance for various resolution requirement at different scales. Besides, the application of multi-scale modelling effectively reduces expense and improves efficiency of computations without loss of accuracy at sensitive zones. A method of concurrent coupling of planar discrete dislocation plasticity (DDP) and a crystal plasticity finite element (CPFE) method was devised for simulating plastic deformation in large polycrystals with discrete dislocation resolution in a single grain or cluster of grains for computational efficiency; computation time using the coupling method can be reduced by an order of magnitude compared to DDP. The method is based on an iterative scheme initiated by a sub-model calculation, which ensures displacement and traction compatibility at all nodes at the interface between the DDP and CPFE domains. The proposed coupling approach is demonstrated using two plane strain problems: (i) uniaxial tension of a bi-crystal film and (ii) indentation of a thin film on a substrate. The latter demonstrated that the rigid substrate assumption used in earlier discrete dislocation plasticity studies is inadequate for indentation depths that are large compared to the film thickness, i.e. the effect of the polycrystalline plastic substrate modelled using CPFE becomes important. The coupling method can be used to study a wider range of indentation depths than previously possible using DDP alone, without sacrificing the indentation size effect regime captured by DDP. A comprehensive indentation pressure formula has been developed by applying the proposed multi-scale modelling approach on a polycrystalline coating system. Planar nano-sliding and fretting calculations have been performed on thin films modelling by CPFE and DDP at different scales. Results of CPFE simulations provide an understanding of the role of microstructure on the plasticity and crack initiation during a contact problem. Beside, a new DDP computational framework has been proposed for a nano-fretting problem which is able to capture the contact size effect, simulate the dislocation evolution and predict the surface profile variation of thin films. Calculations of DDP simulations potentially provide CPFE simulations with fatigue parameters that is of more physical significance. The method is general and can be applied to any problem where finer resolution of dislocation mediated plasticity is required to study the mechanical response of polycrystalline materials, e.g. to capture size effects locally within a larger elastic/plastic boundary value problem. Also, the model described here will provide further opportunities for directly coupled, three-tiered multi-scale models compromising an overall macroscopic continua having embedded crystal plasticity and discrete dislocation plasticity models, respectively, as the length scale decreases in the area of interest. Finally, the methodology of the proposed coupling method will shed light on archiving a general compatibility of sub-regions and thus benefit other researchers who are working on coupling methods among other scales.

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Dwyer, Liam Paul. "Steps toward a through process microstructural model for the production of aluminium sheet." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/steps-toward-a-through-process-microstructural-model-for-the-production-of-aluminium-sheet(cac0d9a4-0bc5-47e1-ac15-689d02c7c1d4).html.

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Aluminium sheet production is a multi-stage process in which altering processing conditions can drastically alter the size and type of second phase particles found in the final product. The properties of these second phase particles also affects deformation and annealing processes, meaning that any attempt to create a through process model would require the ability to predict both how the particles would develop in the material, and how these particles then affect the alloy moving forward. This project first focuses on gaining insight into how the particles in a model aluminium alloy change during homogenisation heat treatment and hot rolling. This has been accomplished by utilising serial block face scanning electron microscopy (SBF-SEM), a technique which allows the capture of 3D data sets at sub micron resolutions. This has allowed the populations of primary (constituent) and secondary (dispersoid) particles to be analysed at different stages of sheet production, and thus allowing the effects of homogenisation and hot rolling on particle populations to be quantified. To discover how the particles would go on to affect further processing, digital image correlation has been used to examine the localised strain in the alloy near to a selection of particle configurations. This highlighted the heterogeneity in slip behaviour within the alloy and illustrated that plumes of rotation develop near to non deformable regions. Rotation plumes have previously been modelled using a crystal plasticity model, and so further work is also presented expanding upon this model to simulate a variety of particle configurations. This has shown that in the case of single particles, local deformation is dependent on both the aspect ratio of the particle and how it is aligned to the active slip system. With the incorporation of a second particle, the interparticle spacing must also be considered.

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Karamched, Phani Shashanka. "Deformation studies near hard particles in a superalloy." Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:e740592d-8d82-4c12-9bfe-99901d132b60.

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Superalloys have performed well as blade and disc materials in turbine engines due to their exceptional elevated temperature strength, high resistance to creep, oxidation and corrosion as well as good fracture toughness. This study explores the use of a relatively new technique of strain measurement, high resolution electron backscatter diffraction (HR-EBSD) to measure local deformation fields. The heart of the HR-EBSD technique lies in comparing regions in EBSD patterns from a strained region of a sample to those in a pattern from an unstrained region. This method was applied to study the elastic strain fields and geometrically necessary dislocation density (GND density) distribution near hard carbide particles in a nickel-based superalloy MAR-M-002. Significant thermal strains were initially induced by thermal treatment, which included a final cooling from the ageing temperature of 870°C. Elastic strains were consistent with a compressive radial strain and tensile hoop strain that was expected as the matrix contracts around the carbide. The mismatch in thermal expansion coefficient of the carbide particles compared to that of the matrix was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3 x 10¹³ m^–2 in regions around the carbide. These measured elastic strain and GND densities have been used to help develop a crystal plasticity finite element model in another research group and some comparisons under thermal loading have also been examined. Three-point bending was then used to impose strain levels within the range ±12% across the height of a bend bar sample. GND measurements were then made at both carbide-containing and carbide-free regions at different heights across the bar. The average GND density increases with the magnitude of the imposed strain (both in tension and compression), and is markedly higher near the carbide particles. The higher GND densities near the carbides (order of 10¹⁴ per m²) are generated by the large strain gradients produced around the plastically rigid inclusion during monotonic mechanical deformation with some minor contribution from the pre-existing residual deformation from thermal loading. A method was developed of combining the local EBSD measurements with FE modelling to set the average residual strains within the mapped region even when a good strain free reference point was unavailable. Cyclic loading was then performed under four point loading to impose strain levels of about ±8% across the height of bend bar samples. Similar measurements as in the case of monotonic deformation were made at several interruptions to fatigue loading. Observations from the cyclic loading such as slip features, carbide cracking, GND density accumulation have been explored around carbide particles, at regions away from them and near a grain boundary.

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Xie, Mengyin. "X-ray and neutron diffraction analysis and fem modelling of stress and texture evolution in cubic polycrystals." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:c5f8b36c-4728-4c17-8e2e-82b926200019.

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The thesis reports improvements in the characterization techniques for stress and texture in crystalline materials by x-ray and neutron powder diffraction. Furthermore, advances are made in texture evolution modelling and validation against experimental observations. In the beginning, the fundamental assumption of diffraction strain analysis is numerically examined and verified, namely, that the lattice parameter value determined from fitting the diffraction pattern is equal to the average lattice parameter within the gauge volume. Next, the task of shear strain determination from powder diffraction measurements is addressed. A method is developed and implemented for the complete 2D strain tensor determination from the multi-directional energy-dispersive x-ray diffraction patterns. The method not only offers a way to evaluate the shear strain, but also provides a better overall strain averaging approach. Rotation and translation of sample and/or detectors in powder diffraction mode can effectively increase the pole figure coverage and thus the accuracy of texture determination. However, the movements also introduce uncertainties and aberrations into data analysis due to the changes in the diffraction volume and transmitted intensity. In order to overcome these problems, accurate single exposure texture characterization techniques are proposed based on several different powder diffraction setups. Numerical analyses are carried out to prove that any simple texture in cubic polycrystals can be effectively determined using single exposure Debye-Scherrer diffraction pattern analysis. Several experiments are reported on collecting Debye-Scherrer diffraction patterns, multi-directional energy-dispersive x-ray diffraction patterns and multi-directional TOF neutron setup. Efficient data processing procedures of the diffraction patterns for ODF determination are presented. Crystal plasticity finite element models are developed to model the texture evolution in polycrystalline engineering samples during manufacturing. In the present thesis, quantitative measures extracted from orientation distribution function are employed to make precise comparison between the model and experiment. Unlike the simple uni-axial compression and tension considered in the literature, in the present thesis the complex texture evolution during linear friction welding is modelled as a sequence of different shear deformations.

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Furstoss, Jean. "Approche numérique de l'évolution microstructurale des péridotites." Thesis, Université Côte d'Azur, 2020. http://www.theses.fr/2020COAZ4066.

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Cette thèse a pour but de simuler les évolutions microstructurales des roches du manteau supérieur dans des conditions thermomécaniques représentatives de la lithosphère terrestre. En effet, c’est le comportement mécanique de ces roches qui contrôle, au premier ordre la rhéologie de la lithosphère et donc des plaques tectoniques.Les outils utilisés et développés dans ce travail sont basés sur le formalisme level-set (LS) permettant une description implicite de la microstructure et la modélisation de la migration de joint de grains à l’échelle du polycristal. Les évolutions microstructurales sont simulées dans un cadre élément fini (EF) robuste et efficace permettant un couplage avec des calculs de plasticité cristalline décrivant le comportement mécanique de la roche. Une première grande partie de cette thèse est consacrée à la croissance de grain en absence de déformation dans les péridotites. Il est montré, dans un premier temps que la cinétique de croissance d’un agrégat d’olivine (minéral principal des péridotites) n’est pas en accord avec les contraintes naturelles sur la cinétique de croissance de grain des péridotites. Il est ensuite montré que l’introduction de phases secondaires telles que les pyroxènes et le spinelle permet de ralentir la croissance mais ne suffit pas à obtenir des cinétiques cohérentes avec les contraintes naturelles. Finalement il est proposé que les impuretés jouent un rôle important dans la cinétique de croissance des roches mantelliques et que leur prise en compte permet de concilier les contraintes venant des expériences de laboratoire et des observations naturelles.Dans une deuxième grande partie, le modèle constitutif utilisé pour décrire le comportement mécanique de l’olivine dans un cadre de plasticité cristalline est présenté. La manipulation des différents tenseurs dans ce cadre numérique repose sur la construction de bases tensorielles particulières tenant compte des symétries du cristal et permettant l’utilisation d’une élasticité anisotropede manière transparente et naturelle. La formulation mixte EF vitesse-pression est également modifiée pour tenir compte de l’anisotropie élastique. Cette manière de décrire la déformation est ensuite enrichie d’un mécanisme de relaxation sensé représenter les mécanismes, autres que le glissement de dislocation, accommodant la déformation dans les polycristaux d’olivine. Cette description est alors couplée avec le formalisme LS pour simuler les évolutions microstructurales d’un agrégat d’olivine durant la déformation. Ce cadre numérique est enfin utilisé pour étudier la localisation de la déformation, dans les polycristaux d’olivine, par différents types de zone de faiblesse pré-existantes.Finalement, les limites et les perspectives de développement du formalisme numérique pour aboutir à une description fidèle des évolutions microstructurales d’une roche mantellique dans des conditions thermomécaniques de la lithosphère sont discutées
This thesis aims at simulating the microstructural evolutions of upper mantle rocks under thermomechanical conditions representative of the Earth’s lithosphere. Indeed, the mechanical behavior of these rocks controls, at first order, the rheology of the lithosphere and thus of the tectonic plates.The tools used and developed in this work are based on the level-set (LS) formalism allowing an implicit description of the grain boundaries and the modelling of grain boundary migration (GBM) at the polycrystal scale. Thus, the microstructural evolutions are simulated in a robust and efficient finite element (FE) framework allowing a coupling with crystal plasticity (CP) calculations which allows to describe the mechanical behavior of the rock.A first large part of this thesis is devoted to the deformation-free grain growth (GG) in peridotites. Firstly we show that the GG kinetics of olivine (major phase of peridotites), considering only capillarity force, is not in agreement with the natural constraints on the GG kinetics of peridotites. Secondly, it is shown that the introduction of secondary phases such as pyroxenes and spinels can slow GG but is not sufficient to reach kinetics compatible with natural constraints. Finally, it is proposed that impurities play an important role in the GG kinetics of mantel rocks and that taking them into account allows reconciling the constraints coming from laboratory experiments and natural observations.In a second part of the thesis, the constitutive model used to describe the mechanical behavior of olivine in a CP framework is presented. The manipulation of the different tensors in this numerical framework is based on the construction of particular tensor bases considering the symmetries of the crystal and allowing the use of anisotropic elasticity in a straightforward and natural way. The mixed FE velocity-pressure formulation is also modified to take into account the elastic anisotropy. This way of describing the deformation is then enriched with a relaxation mechanism supposed to represent the various processes, other than dislocation glide, accommodating deformation in olivine polycrystals. This description is then coupled with the LS formalism to simulate the microstructural evolutions of an olivine aggregate during deformation. This numerical framework is finally used to study the strain localization in olivine polycrystals along different types of pre-existing shear zones.Finally, the limits and perspectives of the development of numerical formalism to arrive at a faithful description of the microstructural evolutions of a mantle rocks within the lithospheric thermomechanical conditions are discussed

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Al-Harbi, Hamad F. "Crystal plasticity finite element simulations using discrete Fourier transforms." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/51788.

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Crystallographic texture and its evolution are known to be major sources of anisotropy in polycrystalline metals. Highly simplified phenomenological models cannot usually provide reliable predictions of the materials anisotropy under complex deformation paths, and lack the fidelity needed to optimize the microstructure and mechanical properties during the production process. On the other hand, physics-based models such as crystal plasticity theories have demonstrated remarkable success in predicting the anisotropic mechanical response in polycrystalline metals and the evolution of underlying texture in finite plastic deformation. However, the integration of crystal plasticity models with finite element (FE) simulations tools (called CPFEM) is extremely computationally expensive, and has not been adopted broadly by the advanced materials development community. The current dissertation has mainly focused on addressing the challenges associated with integrating the recently developed spectral database approach with a commercial FE tool to permit computationally efficient simulations of heterogeneous deformations using crystal plasticity theories. More specifically, the spectral database approach to crystal plasticity solutions was successfully integrated with the implicit version of the FE package ABAQUS through a user materials subroutine, UMAT, to conduct more efficient CPFEM simulations on both fcc and bcc polycrystalline materials. It is observed that implementing the crystal plasticity spectral database in a FE code produced excellent predictions similar to the classical CPFEM, but at a significantly faster computational speed. Furthermore, an important application of the CPFEM for the extraction of crystal level plasticity parameters in multiphase materials has been demonstrated in this dissertation. More specifically, CPFEM along with a recently developed data analysis approach for spherical nanoindentation and Orientation Imaging Microscopy (OIM) have been used to extract the critical resolved shear stress of the ferrite phase in dual phase steels. This new methodology offers a novel efficient tool for the extraction of crystal level hardening parameters in any single or multiphase materials.

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Kabir, Saiful. "Finite element modelling of photonic crystal fibres." Thesis, City University London, 2007. http://openaccess.city.ac.uk/8592/.

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Photonic crystal fibre (PCF), a new kind of optical fibre, has many air-holes in their cross-section and has potential applications to new optical communication systems. The main objective of this research is the modelling of photonic crystal fibre to identify the fundamental and higher order quasi-TE and TM modes with square, ,rectangular and circular air holes in a square and hexagonal matrix, by using a rigorous full-vectorial H-field based finite element method (FEM). Besides the modal solutions of the effective indices, mode field profiles, spot sizes, modal hybridness, polarization beat length and group velocity dispersion values for equal and unequal air holes; research was carried out to optimize and design highly birefringent PCF. The variation of modal birefringence is shown through the effect of hole diameters, air hole arrangement, structural asymmetry, operating wavelength, and pitch-distance. Birefringence was enhanced by breaking the structural symmetry and this was verified by using unequal air holes. The diameter of two air holes and four air holes in the first ring was changed to break the rotational symmetry and a comparison between the two designs is made in this work. In this work, highly birefringent PCF is designed with higher operating wavelength, larger d2/A value, lower pitch length for a given structural asymmetry. It is identified that birefringence value increases rapidly when d2 is much larger than d. At lower pitch value, one of the highest birefringence values reported so far at wavelertgth of 1.55 J.Jm for an asymmetric PCF using circular air holes. A single polarization guide PCF structure is also achieved. In this study, it has been identified that for fixed d/A and d2/A value, as operating wavelength is increased, birefringence increases significantly. It can also be identified that for higher d/A values, birefringence changes rapidly with A as their corresponding cutoff condition also approaches. One important validation of this work is the existence of modal birefringence for PCF with six-fold rotational symmetry. It is shown that birefringence value of a simple PCF incorporating circular holes but of different diameters is high compared to polarization maintaining Panda or Bow-tie fibres. This research also aims to investigate the modal leakage losses of PCF, by using a semi-vectorial beam propagation method (BPM) based on the versatile FEM. The robust perfectly matched layer (PML) boundary condition has been introduced to the modal solution approach. The effects of d2/A, operating wavelength and number of air holes have been thoroughly detailed and explained. In this study, it has been identified that the confinement loss decreases significantly with the increased number of rings, lower operating wavelength and lower d2/A value. For special case, PCF with large spot-size provides higher leakage loss.

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Hiett, Ben. "Photonic crystal modelling using finite element analysis." Thesis, University of Southampton, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.274031.

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Alankar, Alankar. "Development of a 3D microstructure sensitive crystal plasticity model for aluminum." Pullman, Wash. : Washington State University, 2010. http://www.dissertations.wsu.edu/Dissertations/Spring2010/A_Alankar_020910.pdf.

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Willman, E. J. "Three dimensional finite element modelling of liquid crystal electro-hydrodynamics." Thesis, University College London (University of London), 2009. http://discovery.ucl.ac.uk/16162/.

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Liquid crystals (LC) are used in new applications of increasing complexity and smaller dimensions. This includes complicated electrode patterns and devices incorporating three dimensional geometric shapes, e.g. grating surfaces and colloidal dispersions. In these cases, defects in the liquid crystal director field often play an important part in the operation of the device. Modelling of these devices not only allows for a faster and cheaper means of optimising the design, but sometimes also provides information that would be difficult to obtain experimentally. As device dimensions shrink and complex geometries are introduced, one and two dimensional approximations become increasingly inaccurate. For this reason, a three dimensional finite element computer model for calculating the liquid crystal electro-hydrodynamics is programmed. The program uses the Q-tensor description allowing for variations in the liquid crystal order and is capable of accurately modelling defects in the director field. The aligning effect solid surfaces has on liquid crystals, known as anchoring, is essential to the operation of nearly all LC devices. A simplifying assumption often made in LC modelling is that of strong anchoring (the LC orientation is fixed at the LC- solid surface interface). However, in small scale structures with high electric fields and curved surfaces this assumption is often not accurate. A general expression that can be used to represent various weak anchoring types in the Landau-de Gennes theory is introduced. It is shown how experimentally measurable values can be assigned to the coefficients of the expression. Using the Q-tensor model incorporating the weak anchoring expression, the operation of the Post Aligned Bistable Nematic (PABN) device is modelled. Two stable states, one of higher and the other of lower director tilt angle, are identified. Then, the switching dynamics between these two states is simulated.

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Lloyd, Jeffrey Townsend. "Implications of limited slip in crystal plasticity." Thesis, Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/34808.

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To better understand consequences of classical assumptions regarding deformation mechanisms at the mesoscale, experimental observations of mesoscale deformation are presented. In light of actual micrographics of deformed polycrystals, the Von Mises criterion which states that 5 independent plastic deformation sources are needed at each material point to satisfy compatibility is studied, and the consequences of violating this assumption are presented through comprehensive parametric studies. From these studies, it can be concluded that not only are 5 independent plastic deformation sources not needed or observed at each point, but if less than 5 sources are allowed to be active a new physical understanding of a mechanism for kinematic hardening emerges. Furthermore, for enhanced subgrain rotation and evolution the Von Mises criterion must be violated. The second focus of this work is looking at studies, experiments, and models of mesoscale deformation in order to better understand controlling deformation length scales, so that they can be fed into a combined top-down, bottom-up, non-uniform crystal plasticity model that captures the variability provided by the mesoscale during deformation. This can in turn be used to more accurately model the heterogeneity provided by the response of each grain. The length scale intuited from insight into mesoscale deformation mechanisms through observation of experiments and analytical models is the free slip line length of each slip system, which informs non-uniform material parameters in a crystal plasticity model that control the yielding, hardening, and subsequent softening of each individual slip system. The usefulness of this non-uniform multiscale crystal plasticity model is then explored with respect to its ability to reproduce experimentally measured responses at different strain levels for different size grains. Furthermore, a "Mantle-Core" type model which combines both the non-uniform material parameter model and the limited slip model is created, in which the majority of plastic deformation is accommodated near the grain boundary under multi-slip, and uniform plastic deformation occurs in the bulk dominated by double or triple slip. These models are compared for similar levels of hardening, and the pole figures that result from their deformation are compared to experimental pole figures. While there are other models that can capture the heterogeneity introduced by mesoscale deformation at the grain scale, this combined top-down, bottom-up multiscale crystal plasticity model is by far one of the most computationally efficient as the heterogeneity of the mesoscale is does not emerge by introducing higher order terms, but rather by incorporating the heterogeneity into a simple crystal plasticity formulation. Therefore, as computational power increases, this approach will be among the first that will be able to perform accurate polycrystal level modeling while retaining the heterogeneity introduced by non-local mesoscale deformation mechanisms at the sub-grain scale.

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Lu, Jiawa. "Material characterisation and finite element modelling of the cyclic plasticity behaviour of steels." Thesis, University of Nottingham, 2016. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.716486.

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The aim of the study was to experimentally investigate the cyclic plasticity behaviour of steels, and to simulate low cycle fatigue failure by using two material models on either mesoscale or microscale. UMAT subroutines were developed to allow the cyclic plasticity behaviour to be predicted in the ABAQUS FE software. The fatigue behaviour of a P91 power plant steel at a temperature of 600 °C was studied by performing uniaxial fatigue tests and microstructural analysis using electron microscopy. A continuum damage mechanics approach was coupled to the constitutive equations of the Chaboche elasto-visco-plastic model to describe the low cycle fatigue failure at high temperature. A stress partition method was developed to interpret the cyclic softening behaviour, and used to give an initial estimate of the material constants in the Chaboche model. Low cycle fatigue tests were also carried out for a 304 stainless steel at room temperature. The crystal plasticity finite element method was used to predict the hysteresis loops under cyclic loadings for a single crystal or polycrystals. A series of experimental characterisations, including SEM, TEM, and XRD, were conducted to facilitate the understanding of the mechanisms responsible for the mechanical responses, and to determine part of the material constants required in the multi-scale constitutive equations for FE simulation. This method can be used to predict the crack initiation sites based on the local accumulated plastic deformation and local plastic dissipation energy criterion, but it has limitation in predicting the crack initiation caused by precipitates.

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Tedjaseputra, Erik Nugroho. "Numerical Simulations of Microstructure-based Crystal Plasticity Finite Element Model for Titanium and Nickel Alloys." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1325084673.

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Srivastava, Ankit. "Void Growth and Collapse in a Creeping Single Crystal." Thesis, University of North Texas, 2011. https://digital.library.unt.edu/ark:/67531/metadc84281/.

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Aircraft engine components can be subjected to a large number of thermo-mechanical loading cycles and to long dwell times at high temperatures. In particular, the understanding of creep in single crystal superalloy turbine blades is of importance for designing more reliable and fuel efficient aircraft engines. Creep tests on single crystal superalloy specimens have shown greater creep strain rates for thinner specimens than predicted by current theories. Therefore, it is necessary to develop a more predictive description of creep processes in these materials for them to be used effectively. Experimental observations have shown that the crystals have an initial porosity and that the progressive growth of these voids plays a major role in limiting creep life. In order to understand void growth under creep in single crystals, we have analyzed the creep response of three dimensional unit cells with a single spherical void under different types of isothermal creep loading. The growth behavior of the void is simulated using a three dimensional rate dependent crystal plasticity constitutive relation in a quasi-static finite element analysis. The aim of the present work is to analyze the effect of stress traixiality and Lode parameter on void growth under both constant true stress and constant engineering stress isothermal creep loading.

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Mayeur, Jason R. "Generalized continuum modeling of scale-dependent crystalline plasticity." Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/39635.

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The use of metallic material systems (e.g. pure metals, alloys, metal matrix composites) in a wide range of engineering applications from medical devices to electronic components to automobiles continues to motivate the development of improved constitutive models to meet increased performance demands while minimizing cost. Emerging technologies often incorporate materials in which the dominant microstructural features have characteristic dimensions reaching into the submicron and nanometer regime. Metals comprised of such fine microstructures often exhibit unique and size-dependent mechanical response, and classical approaches to constitutive model development at engineering (continuum) scales, being local in nature, are inadequate for describing such behavior. Therefore, traditional modeling frameworks must be augmented or reformulated to account for such phenomena. Crystal plasticity constitutive models have proven quite capable of capturing first-order microstructural effects such as grain orientation, grain morphology, phase distribution, etc. on the deformation behavior of both single and polycrystals, yet suffer from the same limitations as other local continuum theories with regard to modeling scale-dependent mechanical response. This research is focused on the development, numerical implementation, and application of a novel, physics-based generalized (nonlocal) theory of single crystal plasticity. Two distinct versions of a dislocation-based micropolar single crystal plasticity theory are developed and discussed within the context of more prominent nonlocal crystal plasticity theories. The constitutive models have been implemented in the commercial finite element code Abaqus, and the size-dependent deformation of both single and polycrystalline metals have been studied via direct numerical simulation. A comparison of results obtained from the solution of several equivalent initial-boundary value problems using the developed models and a model of discrete dislocation dynamics has demonstrated the predictive capabilities of the micropolar theory and also highlighted areas for potential model refinement.

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Warner, Matthew David. "Finite element biphasic modelling of articular cartilage : an investigation into crystal induced damage." Thesis, University of Bath, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.341685.

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Some of the most common diseases and disorders that occur in the adult population are those which affect the joints of the musculo-skeletal system. Most joint diseases cause damage to articular cartilage, the soft tissue that acts as a bearing surface within the load-bearing joints. The function of articular cartilage is to provide a wear-resistant joint surface with a very low coefficient of friction and to reduce the compressive stresses experienced at the end of the long bones. Osteoarthritis can be described as the progressive degeneration of articular cartilage. This disorder causes large areas of cartilage in the load-bearing regions of the joint to become split and fragmented, resulting finally in the exposure of underlying bone. Osteoarthritis has been associated with a number of conditions including the deposition of crystals within the tissue. It has been postulated that crystal deposits have the potential to cause mechanical damage to articular cartilage. Two possible damage mechanisms have been identified; localised tissue damage in the vicinity of the crystal aggregate and surface damage induced by the presence of an aggregate. Articular cartilage is a biphasic material as it consists of a fluid phase, composed mainly of water, and a porous-permeable solid phase. The biphasic nature of the tissue was modelled using the ABAQUS/Standard Finite Element software. Failure criteria for the tissue were investigated using this technique and radial stress and radial strain were found to be reliable predictors of damage. Damage threshold values were determined for radial tensile stress (72 kPa) and radial tensile strain (0.144). A finite element model was then developed to investigate the propensity for crystal deposits to cause damage to a cartilage layer under cyclic loading conditions. It was predicted that aggregates embedded deep within the cartilage layer do not have the potential to cause either local or surface damage. Aggregates nearer the articular surface have the potential to cause localised tissue damage, and it was found that this was dependent upon their stiffness.

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Shahi, Mohsen. "Crystal plasticity finite element modeling of slip system activity and post-localization behavior in magnesium alloys." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=32254.

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During recent years, application of light metals has greatly increased in various industries. Magnesium, the lightest of structural metals, and its alloys have gained special attention, and, therefore, the interest in modeling the behavior of these alloys has increased. In many studies, the goal has been finding ways to improve the low formability of Mg alloys. In this thesis, the effect of slip system activity on Mg alloy behavior in both the pre- and post-localization zones is examined. An available crystal plasticity model, that takes into account the initial texture of the material and its evolution with deformation, is modified for the case of HCP materials, and, then, implemented into the commercial finite element software ABAQUS. Employing the crystal plasticity finite element (CPFE) method, the link between micro-deformation on the slip systems in Mg alloys and the macro-scale response of these metals is established. The model is verified for the case of Mg single crystals which are highly anisotropic. The minimum required size of the representative volume element (RVE), i.e. the minimum number of grains and the degree of inhomogeneity in each grain required in the CPFE modeling of Mg alloys is determined. Next, the role of different slip systems present in Mg alloys is studied. The effects of strain-rate sensitivity on the micro- and macro-scale behavior of the material and the link between them are also discussed. Using the observed trends for the slip resistances and strain-rate sensitivity factor, new relations are proposed for the change in these values over the range of warm temperatures (75-250°C). The mechanical response of samples cut from hot-rolled
Au cours des dernières années, l'application de métaux légers a augmenté dans diverses industries. Le magnésium, le plus léger des métaux structuraux, et ses alliages ont acquis une attention particulière et, par conséquent, l'intérêt pour la modélisation du comportement de ces métaux a augmenté. Dans des nombreuses études, l'objectif a été de trouver les moyens d'améliorer la formabilité des alliages de Mg. Dans cette thèse, l'effet de l'activité du système de glissage sur le comportement de l'alliage de Mg dans les zones de pré- et post-localisation est examiné. Un modèle de plasticité cristalline disponible, qui prend en compte la texture initiale du matériau et de son évolution avec la déformation, est modifié pour le cas des matériaux à structure hexagonale compacte, et, ensuite, mis en œuvre dans le programme de calculs d'éléments finis, ABAQUS. Employant la méthode des éléments finis à plasticité cristalline (EFPC), le lien entre la microdéformation sur les systèmes de glissages des alliages du Mg et la réponse à l'échelle macroscopique de ces métaux est établie. Le modèle est vérifié pour le cas des monocristaux de Mg qui sont fortement anisotropes. La taille minimale prescrite de l'élément volumique représentatif (EVR), c'est-à-dire le nombre minimal de grains et le degré d'hétérogénéité dans chaque grain requis dans la modélisation EFPC des alliages du Mg, est déterminée. Ensuite, le rôle des différents systèmes de glissage dans les alliages Mg est étudié. Les effets de la sensibilité à la vitesse de déformation sur le comportement du matériau aux échelles micro- et macroscopique et le$

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19

Han, Songlin. "High temperature deformation modelling and finite element implementation for single crystal turbine blade materials." Thesis, University of Bristol, 2000. http://hdl.handle.net/1983/943aaa75-6406-4a06-9250-9b0ae85a5eae.

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20

Roters, Franz [Verfasser]. "Advanced material models for the crystal plasticity finite element method : development of a general CPFEM framework / Franz Roters." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2011. http://d-nb.info/1018181369/34.

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21

Zeng, Wei. "Advanced Development of Smoothed Finite Element Method (S-FEM) and Its Applications." University of Cincinnati / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439309306.

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22

Johnston, Stephen R. "FINITE ELEMENT SIMULATIONS OF THREE-DIMENSIONAL MICROSTRUCTURALLY SMALL FATIGUE CRACK GROWTH IN 7075 ALUMINUM ALLOY USING CRYSTAL PLASTICITY THEORY." MSSTATE, 2005. http://sun.library.msstate.edu/ETD-db/theses/available/etd-10242005-133331/.

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This thesis discusses plasticity-induced crack closure based finite element simulations of small fatigue cracks in three dimensions utilizing crystal plasticity theory. Previously, modeling has been performed in two dimensions using a double-slip crystal plasticity material model. The goal of this work is to extend that research using a full three-dimensional FCC crystal plasticity material model implementation that accounts for all twelve FCC slip systems. Discussions of Python scripts that were written to perform analyses with the commercial finite element code ABAQUS are given. A detailed description of the modeling methodology is presented along with results for single crystals and bicrystals. The results are compared with finite element and experimental results from the literature. A discussion of preliminary work for the analysis of crack growth around an intermetallic particle is also presented.

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23

Thomas, Joshua Michael. "Simulating the mechanical response of titanium alloys through the crystal plasticity finite element analysis of image-based synthetic microstructures." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1325088641.

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24

O'Meara, Nicholas. "Developing material models for use in finite element predictions of residual stresses in ferritic steel welds." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/developing-material-models-for-use-in-finite-element-predictions-of-residual-stresses-in-ferritic-steel-welds(0f2cfa95-1d35-42be-b224-665252950efc).html.

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Nuclear reactor pressure vessels are constructed by welding low alloy steel forgings together. Welding processes can leave residual stresses which affect the RPV's resistance to fracture. Welding also induces microstructural changes and these changes have a number of associated consequences, including inelastic strains and altering mechanical properties. The extent to which these microstructural changes influence residual stress evolution during welding is not fully understood. The aim of this project is to characterise the microstructural and mechanical response of SA-508 Gr.3 Cl.1 pressure vessel steel to thermal cycles and develop representative models that can be used to determine how these effects influence stress predictions. There is insufficient materials data to inform the models used to predict how phase transformations influence residual stresses. Using the recently developed Gleeble thermo-mechanical simulator, previously unmeasured data characterising the response of the material to weld-like thermal cycles was generated. Variations in the kinetics of austenite formation and decomposition were investigated using dilatometry. It was found that when the steel is subjected to multiple thermal cycles that exceed the austenisation temperature, the behaviour during the first thermal cycle is different to that of subsequent cycles. In the subsequent thermal cycles, two observations were made: 1) the austenite formation rate increases on heating, and 2) for a given cooling rate, the austenite will decompose at lower temperatures into harder phases. It is explained how these changes in behaviour can affect the residual stress distribution in this thesis. Bainitic, austenitic and martensitic samples were generated. The stress-strain behaviour of these phases is presented and has been used to inform mechanical constitutive models. Finite element simulations of autogenous edge welded beams have shown how microstructural changes can affect the residual stress predictions. The extent of the transformed region of the HAZ and the yield stress of the material surrounding this region influences the location and magnitude of the peak tensile residual stress after a weld pass. Changes in mechanical properties induced by tempering bainitic and martensitic samples were quantified experimentally. The reductions in yield stress in bainite and martensite during short tempering heat treatments were found to be significant. A new approach to integrate the observed tempering behaviour into existing models is presented. The data and models presented in this thesis can provide guidance to structural integrity engineers and help produce more accurate and less conservative residual stress predictions for use in structural integrity assessments.

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Shi, Qiwei. "Experimental and numerical studies on the micromechanical crystal plasticity behavior of an RPV steel." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLN009/document.

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Cette thèse vise à étudier le comportement mécanique de l’acier de cuve 16MND5 (ou A508cl3 pour la norme anglaise) à l’échelle de la microstructure en croisant des approches expérimentale et numérique. Plusieurs contributions au développement de l’essai de traction in-situ à l’intérieur de MEB ont été apportées. En premier, les biais de mesure de différentes modalités (BSE, EBSD et SE) d’acquisition d’images sous MEB ont été caractérisés et corrigés. Les images MEB de différentes modalités ont été corrélées de façon précise afin de décrire la topographie de l’éprouvette. Les images d’orientation cristallographique (EBSD) ont été corrélées afin de révéler la rotation cristalline et les champs de déplacement de surface au long de la traction. La déformation élastique de l’éprouvette a été mesurée par corrélation intégrée des images de diffraction électronique à haute-résolution. Les microstructures fines de l’éprouvette à trois dimensions après déformation ont été mesurées par FIB-EBSD. L’essai a également été simulé par calcul de plasticité cristalline sur un maillage 3D, basé sur les microstructures mesurées dans la configuration déformée. Un algorithme a été proposé pour estimer la configuration initiale de l’éprouvette et identifier les paramètres de loi de plasticité en procédant par itérations. Un cas test synthétique 2D a été employé pour valider la faisabilité de l’algorithme. Deux lois de plasticité cristalline ont été testées sur le maillage 3D: dynamique des dislocations des cristaux cubiques centrés, et une version modifiée de la loi Méric-Cailletaud. Pour cette dernière loi, deux jeux de paramètres ont été identifiés pour les ferrites et bainites par recalage des éléments finis
The PhD project is devoted to the study of the mechanical response of the reactor pressure vessel steel A508cl3 (or 16MND5 in French nomenclature) at the microscopic scale by experimental analyses and numerical simulations. Different aspects of in-situ tests inside an SEM chamber have been considered. First, the characterization and corrections of bias and uncertainties of different SEM imaging modalities (SE, BSE, and EBSD) have been performed. Precise registrations of SEM images in different modalities have been developed in order to give a comprehensive description of the sample surface topographies. Crystallographic orientation maps (from EBSD analyses) are registered to measure the crystal rotation and displacement fields along the tensile test. The elastic deformations of the surface are assessed by integrated correlation of high-resolution electron diffraction images. The 3D microstructure of the analyzed sample is revealed a posteriori by combining FIB milling andEBSD images.The experimental test is also simulated by crystal plasticity calculations on a 3D mesh created according to the 3D microstructure observed in the deformed configuration. An algorithm has been proposed to estimate its initial configuration and to identify the plastic parameters iteratively. A synthetic 2D model has been used to prove its feasibility. Two crystal plasticity laws have been validated on the 3D mesh, namely dislocation dynamics for body-centered cubic crystals and a modified version of Méric-Cailletaud model. In thepresent work finite element model updating was used to provide two sets of parameters (for ferrite and bainite) for the latter law

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Fromm, Bradley S. "Linking phase field and finite element modeling for process-structure-property relations of a Ni-base superalloy." Thesis, Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/45789.

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Establishing process-structure-property relationships is an important objective in the paradigm of materials design in order to reduce the time and cost needed to develop new materials. A method to link phase field (process-structure relations) and microstructure-sensitive finite element (structure-property relations) modeling is demonstrated for subsolvus polycrystalline IN100. A three-dimensional (3D) experimental dataset obtained by orientation imaging microscopy performed on serial sections is utilized to calibrate a phase field model and to calculate inputs for a finite element analysis. Simulated annealing of the dataset realized through phase field modeling results in a range of coarsened microstructures with varying grain size distributions that are each input into the finite element model. A rate dependent crystal plasticity constitutive model that captures the first order effects of grain size, precipitate size, and precipitate volume fraction on the mechanical response of IN100 at 650°C is used to simulate stress-strain behavior of the coarsened polycrystals. Model limitations and ideas for future work are discussed.

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Hoang, Ha. "Modélisation numérique de la plasticité des transformations de phase diffusives à l'état solide." Thesis, Rouen, INSA, 2008. http://www.theses.fr/2008ISAM0019.

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Lors d'une transformation de phase à l’état solide d'un acier, l'interaction entre la phase naissante et la phase parente, chacune avec ses propriétés mécaniques propres, génère des contraintes au voisinage de l'interface entre phases. L’accommodation de ces contraintes est réalisée à travers la plastification de la phase parente notamment, celle dont la limite d'élasticité est la plus faible. La transformation se faisant, si une contrainte déviatorique - même faible- est exercée, la plasticité locale sera canalisée dans la direction de la contrainte appliquée et apparaît à l’échelle macroscopique. Cette déformation est appelée plasticité de transformation ou TRIP (TRansformation Induced Plasticity) ; seuls des modèles dédiés peuvent en rendre compte. Cette plasticité peut aussi apparaître sans charge externe durant la transformation, si la phase austénitique a été soumise à un pré-écrouissage juste avant sa transformation. Les modèles de plasticité de transformation actuels ne sont dans ce cas pas toujours à même de reproduire les observations expérimentales. Afin d'identifier les mécanismes responsables de la plasticité issue de transformations diffusives pour différents cas de chargement, une modélisation numérique des conséquences mécaniques de telles transformations est proposée dans ce travail. La résolution, à chaque instant de la transformation, du problème d'interaction mécanique entre phases utilise la méthode des éléments finis. Ceci donne accès à la description locale des champs de contrainte et de déformation dus à cette interaction. Une première approche de la modélisation porte, comme dans la plupart des modèles courants de plasticité de transformation, sur la croissance d'une particule unique de phase naissante interagissant avec la matrice mère. On peut ainsi analyser les hypothèses portées sur les champs mécaniques auxquelles il est fait appel dans les modèles analytiques. Cette approche est ensuite étendue au cas d'un milieu homogène où apparaissent des germes aléatoirement dans le temps et dans l'espace, avec des lois de distribution données. Cette deuxième approche met en évidence l'importance de la densité spatiale de germes et du taux de germination sur les prédictions de TRIP. Elle pose en outre les bases d'une modélisation de transformation diffusive dans un milieu cristallin hétérogène, où les propriétés effectives sont déterminées par moyennation d'ensemble sur des multicristaux. Avec l'une comme l'autre des approches, l'accord qualitatif avec les mesures expérimentales de TRIP est correct, pour le cas classique de chargements constants pendant la transformation comme pour les conséquences d'un pré-écrouissage
During the solid-solid phase transformation of a steel, the interaction between new phase and parent phase, each having its own properties, leads to accommodation stresses in the vicinity of the interface between phases. Dislocations are thus produced in the parent phase, the one which has the lowest yield stress. If an external loading stress -even small- is exerted during the transformation, dislocations result to a permanent strain at the macroscopic scale, in the direction of the load. This strain is called transformation plasticity or TRIP (TRansformation Induced Plasticity); only dedicated models can predict it. This plasticity may also be observed without any external load during the transformation, if the austenitic phase as been pre-hardened just before the transformation. In this latter case, current transformation plasticity models do not always provide correct predictions as compared to experimental observations. A numerical modelling of the mechanical consequences of diffusive transformations is proposed in this work. It is meant to identify the mechanisms which are responsible for the plasticity induced by such transformations for all cases of loading. The finite elements method is used to solve the problem of the mechanical interaction between phases at any instant of the transformation. This gives access to a local description of the stress and strain fields due to this interaction. In a first approach of the modelling inspired from most current transformation plasticity models, a single growing particle interacting with its mother phase is considered. This allows to analyse the hypothesis on mechanical fields according to which analytical formulations of transformation plasticity can be obtained. This approach has then been extended to the case of a homogeneous medium in which nuclei appear randomly in time and space, with prescribed distribution laws. With this improved approach, the importance of the spatial density of nuclei and of the rate of nucleation on TRIP predictions could be evidenced. Besides, this approach provides the basis of a modelling of diffusive transformation in a crystalline heterogeneous material, where the effective properties are determined by ensemble averaging over multicrystals. With both approaches, a correct qualitative agreement with experimental measures could be obtained, in the classical case of constant load during the transformation as well as concerning the consequences of a pre-hardening

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Nguyen, Pham the nhan. "Identification inverse d'un modèle de plasticité cristalline pour un zinc pur à l'aide de tests de nanoindentation et de simulations par éléments finis." Thesis, Reims, 2020. http://www.theses.fr/2020REIMS001.

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L’identification de certaines lois de comportement demeure délicate surtout pour des comportements non linéaires. Plusieurs stratégies d’identification ont été développées en s’appuyant principalement sur le dialogue essai-calcul. La partie expérimentale consiste à mesurer une ou plusieurs quantités physiques qui caractérisent le comportement du matériau testé et la partie théorique permet de calculer ces mêmes quantités. Pour remonter aux propriétés matérielles recherchées, un accord entre les quantités mesurées et calculées doit être visé. Dans cette thèse, on s’intéresse au zinc qui un matériau polycristallin. Celui-ci est modélisé à travers des lois de plasticité cristalline qui permettent de prédire son comportement mécanique sous chargements complexes ainsi que l’évolution de sa microstructure. Les lois de plasticité cristalline implémentées dans le logiciel commercial Abaqus par Huang et Marin que nous avons adapté au cas du zinc ont été utilisées. L’étape d’identification de ces lois non linéaires est basée sur une approche inverse qui tient compte des hétérogénéités des déformations à l’échelle des grains. Pour cela, nous avons utilisé l’essai de nanoindentation qui est un essai hétérogène permettant d’extraire plus d’informations que les essais homogènes. A cet effet, et pour caractériser la plasticité cristalline du zinc, nous avons utilisé les courbes charge - profondeur de pénétration et les profils de déplacements mesurés sur l’empreinte résiduelle sur des grains de différentes orientations cristallographiques mesurées par EBSD. La confrontation des résultats expérimentaux et montrent la bonne adéquation entre les données expérimentales et les modèles identifiés. Les résultats obtenus ont ainsi permis de caractériser les mécanismes de déformation des monocristaux de zinc
The identification of certain behavior laws of remains delicate especially for nonlinear behaviors. Several identification strategies have been developed based mainly on the experiment-calculation dialogue. The experimental part consists in measuring one or more physical quantities which characterize the behavior of the tested material and the theoretical part makes it possible to calculate these same quantities. To go back to the material properties sought, an agreement between the quantities measured and calculated must be aimed at. In this thesis, we are interested in zinc, which is a polycrystalline material. It is modeled through crystal plasticity laws that predict its mechanical behavior under complex loadings and the evolution of its microstructure. The crystal plasticity laws implemented in the commercial software Abaqus by Huang and Marin that we adapted to the case of zinc were used. The step of identifying these non-linear laws is based on an inverse approach that takes into account the heterogeneities of the deformations at the grain scale. For this, we used the nanoindentation test which is a heterogeneous test to extract more information than the homogeneous tests. For this purpose, and to characterize the crystal plasticity of zinc, we used the load - penetration depth curves and the displacement profiles measured on the residual imprint on grains of different crystallographic orientations measured by EBSD. The confrontation of the experimental and numerical results show the good agreement between the experimental data and the identified models. The results obtained made it possible to characterize the deformation mechanisms of zinc single crystals

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Lillemo, Dennis. "Modelling masonry spires : An investigation." Thesis, KTH, Betongbyggnad, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-301245.

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Masonry spires are a typical part of church architecture. Since it is rare that masonry is used as a load-bearing material in the western world today, it is important to maintain and increase the knowledge of modelling masonry structures both from a maintenance point of view and to build new masonry structures. The purpose of this master thesis is to look at and evaluate some different methods to model masonry spires exposed to common loads such as gravity, settlement and wind. The spire of the Salisbury Cathedral is used as a template regarding geometry and mechanical properties for the modelling methods. Two modelling methods are used in the master’s thesis. The first one is the limit analysis method applied to masonry. It is used to calculate a critical thickness for the masonry of the spire for a severe wind load. The second method is the Finite Element Method (FEM). The commercial finite element software Abaqus is used to create the model and the discretization used with the FE modelling is the macro-modelling approach. Concrete Damage Plasticity (CDP) in Abaqus is used as the material model and adapted to masonry. The finite element model consists of the spire itself along with the supporting structure beneath it down to the piers. Four different simulations (jobs) are run with varying wind direction and two of them have settling piers. The results from the finite element simulations indicate that the membrane stresses in the spire faces for the various jobs were not significantly different from one another. One of the jobs with settling piers could not be completed because the tensile stresses in the arches reached the tensile strength capacity of the material. The other simulation with a settlement that did complete did not have any significant difference in stress compared with the simulations without settlements. While the arches and the piers underwent plastic straining the spire itself did not. The stress levels there remained in the linear range for all the completed simulations. The finite element results also agree with the limit analysis. These findings call into question some of the modelling choices. The inclusion of the structure beneath the spire in the finite element model, as a way to study the effect of settlements, did not give more insight into the spire’s behaviour. Furthermore, the method to implement settlements was too inaccurate and another approach should be used to study the effect of settlements on the state of spires. Further work needs to be done on that topic. Improvements can also be made regarding how CDP was adapted for masonry.
Murade tornspiror är en vanlig takkonstruktion inom kyrkoarkitekturen. Eftersom det numera är sällsynt att murverk fungerar som lastbärande material i västvärlden, är det viktigt att upprätthålla och utöka kunskapen om murverkskonstruktioner för både underhåll och nybyggnation. Syftet med denna masteruppsats är att betrakta och utvärdera några olika modelleringsmetoder för murade tornspiror som är utsatta för några typiska laster såsom egentyngd, sättningar och vind. Katedralen i Salisbury används som en modelleringsmall i uppsatsen med avseende på katedralens geometri och materialegenskaper. Två modelleringsmetoder används i uppsatsen. Den första är gränsanalys tillämpad på murverkskonstruktioner. Den används för att beräkna en kritisk tjocklek för tornspiran under en stor vindlast. Den andra metoden är Finita Elementmetoden (FEM). Den kommersiella finita elementprogramvaran Abaqus används för finita elementanalysen och diskretiseringen som används för murverket i finita elementmodellen är makromodellering. Concrete Damage Plasticity (CDP) i Abaqus används som materialmodell och anpassas för murverk. Finita elementmodellen består utav själva tornspiran inklusive de bärande delarna under spiran och ned till pelarna. Fyra olika simuleringar ("jobb") körs med vindlast som angriper från olika riktningar och två av simuleringarna har pelare som sätter sig. Resultaten från simuleringarna visar att membranspänningarna i tornspirans väggar, för de olika jobben, inte skilde sig i någon betydelig grad från varandra. Ett av jobben med pelare som satte sig kunde inte köras klart eftersom dragspänningarna i valvbågarna överskred draghållfastheten på murverket i modellen. Den andra simuleringen med sättningar som kördes klart uppvisade inte några avsevärda skillnader i spänningar i tornspiran jämfört med simuleringarna utan sättningar. Medan plastiska töjningar uppkom i både valvbågarna och pelarna i modellen, uppkom de inte i tornspiran. Spänningsnivåerna i tornspiran var inom det linjära intervallet för alla simuleringar. Resultaten från finita elementanalysen stämde överens med resultaten från gränsanalysen. Analysresultaten ifrågasätter vissa av modelleringsvalen. Att inkludera de bärande delarna under tornspiran i finita elementmodellen, för att undersöka effekten av sättningar, gav inte en större insikt i hur sättningar påverkar tornspiran. Dessutom, var metoden för att tillämpa sättningar för oprecis och en annan metod borde användas. Mer arbete måste utföras vad gäller det ämnet. Sättet att tillämpa CDP för murverk kan också förbättras.

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Pushkareva, Marina. "Study of Void Growth in Commercially Pure Titanium." Thesis, Université d'Ottawa / University of Ottawa, 2017. http://hdl.handle.net/10393/35667.

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The ductile fracture process, which consists of the nucleation, growth and coalescence of microvoids, was extensively studied for materials deforming homogeneously. For materials with a non-homogeneous deformation behavior, such as those having hexagonal closed packed (HCP) crystal structure, experimental and numerical data is lacking. Therefore, the fracture properties of materials with such HCP structure, like titanium (used in aerospace and biomedical applications), zirconium (nuclear industry) and magnesium (manufacturing industry) are not well understood. The main research objective of this Ph.D. thesis is to better understand the mechanisms governing fracture in commercially pure (CP) titanium. In particular, the effect of grain orientation on void growth is investigated. The fracture process of CP titanium was visualized in model materials containing artificial holes. These model materials were fabricated using a femtosecond laser coupled with a diffusion bonding technique to obtain voids in the interior of titanium samples. Diffusion bonding was carried out either above or below the phase transformation temperature resulting in different microstructures. Changes in void dimensions during in-situ straining were recorded in three dimensions using x-ray computed tomography. Void growth obtained experimentally was compared with the Rice and Tracey model which predicted well the average void growth. However, a large scatter in void growth was observed experimentally and was explained in terms of differences in grain orientation which was confirmed by crystal plasticity simulations. It was also shown that grain orientation has a stronger effect on void growth than intervoid spacing and material strength. Intervoid spacing, however, appears to control whether the intervoid ligament failure is ductile or brittle. While this study showed a good agreement between experiments and simulations on average, there is no direct void growth comparison for particular grain orientations. In a follow-up study, an experimental approach was developed to directly relate the growth of a void to its underlying grain orientation. This is achieved by first annealing CP titanium samples below the α-β phase transformation temperature, then performing electron backscatter diffraction iii (EBSD) and finally diffusion bonding the samples together. Samples were then tested in x-ray tomography. This study showed the importance of the local state of strain on void growth. Crystal plasticity simulations that take into account the particular grain orientation and the local state of strain were found to predict well experimental void growth. Crystal plasticity simulations confirmed that the orientation of the voidcontaining grain is more important than the orientation of surrounding grains and more important than the volume fraction of voids, in order to determine void growth. This thesis on the growth and coalescence of voids is important to validate and improve the predictions of ductile fracture models and to design new materials with improved fracture properties.

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Gong, Jicheng. "Microstructural features and mechanical behaviour of lead free solders for microelectronic packaging." Thesis, Loughborough University, 2007. https://dspace.lboro.ac.uk/2134/15102.

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The demands for high density, fine pitch interconnections in electronics systems has seen solder-based approaches for such interconnections miniaturized to the scale of tens of micro meters. At such a small scale, such 'micro joints' may contain only one or a few grains and the resultant mechanical behaviour may not be that for a polycrystalline aggregate, but rather for a single crystal. Since the ~-Sn matrix of SnAgCu solder has a contracted body-centred tetragonal (BCT) structure, such a solder grain is expected to demonstrate a considerably anisotropic behaviour. In such cases the reliability of a Phfree solder is strongly dependent on the local microstructural features, such as the size and orientation of the grains. This thesis presents the investigation of the evolution of microstructure within a joint or at the interface and, the influence of such microstructural features on the meso-scale mechanical behaviour of the Ph-free solder. It includes Evolution of the interface between a molten solder and the Cu substrate To form a joint, the solder alloy is heated and molten, wetting a solid under-bump metallization. After solidification, layers of brittle intermetallic compounds (IMCs) are formed at the interface. In this project, facilities were set up to obtain interfacial reactants at an arbitrary moment of the liquid/solid reaction. Formation and evolution ~ during reflow of SnCu IMCs at the interface between the molten SnAgCu alloy and the Cu UBM was captured and presented for the first time. Formation of phases and IMCs with the body of a liquid SnAgCu solder during solidification The formation behaviour of basic components for a SnAgCu grain (including Sn dendrites, AIDSn and Cu6Sns IMCs) during solidification was investigated. Relationships between the growth behaviour of these components and their internal lattice orientation were studied. The characteristic growth and coupling of AIDSn IMCs and the Sn matrix to form eutectics has been elaborated and presented in this study for - 1- the first time. Based on the results, the forming process of a eutectic SnAgCu grain under the non-equilibrioum solidification condition was illustrated; and major factors that determine the lattice-orientation, size and substructure of the grain were discussed. Meso- and Micro- scale mechanical behaviour of a SnAgCu solder joint To study the size effect on the microstructure, and subsequently, the meso-scale mechanical behaviour, solder joints were manufactured with varying geometries. Shearing tests were performed on these meso-scale joints. The results first demonstrated that the anisotropic characteristics of a SnAgCu grain play an important role in the mechanical behaviour of both a meso-scale solder joint and the adjacent interfacial IMCs. To further investigate the micro-scale deformation and damage mechanisms, micro-mechanical tests were preformed within a SnAgCu grain. Constitutive equations for a SnAgCu grain Based on the experimental results, a crystal model was established to describe the local microstructure-dependent mechanical behaviour. The constitutive equation was implemented by means of the finite element approach, and applied in solder joints of a Flip Chip (FC) package by a multi-scale method. To describe the crystal behaviour at the higher temperature, the model was improved to account for deformations due to vacancy diffusion and thermal expansion. This model was integrated by an implicit approach, and implemented in a full three dimension (3D) finite element (FE) model.

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Fredriksson, Per. "Modelling and simulation of plastic deformation on small scales : interface conditions and size effects of thin films." Doctoral thesis, Stockholm : Hållfasthetslära, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4652.

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33

Castelluccio, Gustavo Marcelo. "A study on the influence of microstructure on small fatigue cracks." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/44719.

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In spite of its signiﬁcance in industrial applications, the prediction of the inﬂuence of microstructure on the early stages of crack formation and growth in engineering alloys remains underdeveloped. The formation and early growth of fatigue cracks in the high cycle fatigue regime lasts for much of the fatigue life, and it is strongly inﬂuenced by microstructural features such as grain size, twins and morphological and crystallographic texture. However, most fatigue models do not predict the inﬂuence of the microstructure on early stages of crack formation, or they employ parameters that should be calibrated with experimental data from specimens with microstructures of interest. These post facto strategies are adequate to characterize materials, but they are not fully appropriate to aid in the design of fatigue-resistant engineering alloys. This thesis considers ﬁnite element computational models that explicitly render the microstructure of selected FCC metallic systems and introduces a fatigue methodology that estimates transgranular and intergranular fatigue growth for microstructurally small cracks. The driving forces for both failure modes are assessed by means of fatigue indicators, which are used along with life correlations to estimate the fatigue life. Furthermore, cracks with meandering paths are modeled by considering crack growth on a grain-by-grain basis with a damage model embedded analytically to account for stress and strain redistribution as the cracks extend. The methodology is implemented using a crystal plasticity constitutive model calibrated for studying the eﬀect of microstructure on early fatigue life of a powder processed Ni-base RR1000 superalloy at elevated temperature under high cycle fatigue conditions. This alloy is employed for aircraft turbine engine disks, which undergo a thermomechanical production process to produce a controlled bimodal grain size distribution. The prediction of the fatigue life for this complex microstructure presents particular challenges that are discussed and addressed. The conclusions of this work describe the mechanistic of microstructural small crack. In particular, the fatigue crack growth driving force has been characterized as it evolves within grains and crosses to other grains. Furthermore, the computational models serve as a tool to assess the eﬀects of microstructural features on early stages of fatigue crack formation and growth, such as distributions of grain size and twins.

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Renner, Emile. "Vers l’identification d’une loi de plasticité monocristalline par analyse topographique d’empreintes de nanoindentation Berkovich." Thesis, Besançon, 2016. http://www.theses.fr/2016BESA2059/document.

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Une méthode d’analyse des topographies résiduelles, obtenues par essais de nanoindentation Berkovich sur trois échantillons de nickel polycristallins cubiques à faces entrées (CFC), a été développée dans cette thèse. L’objectif de la méthode est d’évaluer la richesse de l’information contenue dans les empreintes pour l’identification de tout ou partie des paramètres d’une loi de plasticité monocristalline. La création d’une base de données de topographies résiduelles, mesurées par microscopie à force atomique (AFM), constitue la partie expérimentale du travail. Les distributions et dimensions de bourrelets montrent une grande sensibilité à l’orientation relative indenteur/grain et au taux d’écrouissage. Les topographies obtenues s’avèrent être de véritables « empreintes digitales » du mécanisme de plasticité à l’échelle du grain. L’élaboration sous le code ZéBuLon d’un modèle éléments finis (EF) 3D de l’essai de nanoindentation Berkovich, intégrant la loi de Méric-Cailletaud, permet de retrouver les observations expérimentales. Une étude numérique confirme la sensibilité de la topographie à l’orientation relative indenteur/grain et aux paramètres plastiques, notamment aux coefficients de la matrice d’interaction des dislocations présentes sur les systèmes de glissement. Afin d’évaluer la richesse du contenu informatif des empreintes, un indice d’identifiabilité est proposé. Son calcul est basé sur la multi-colinéarité des vecteurs de sensibilité des topographies résiduelles aux paramètres de la loi. Il permet de quantifier, en fonction des données topographiques prises en compte, le caractère mal posé du problème d’identification paramétrique. Les résultats obtenus montrent que l’identification de quatre à cinq paramètres de la loi de Méric-Cailletaud est envisageable en exploitant seulement deux empreintes. Ces travaux ouvrent la voie à l’identification du comportement à l’échelle du cristal, guidée par l’identifiabilité paramétrique
In this thesis, a method is developed to analyse the residual topographies obtained by Berkovich nanoindentation tests on three face-centered cubic (FCC) polycrystalline nickel samples. The purpose is to measure the information richness of imprints for identifying all or part of parameters of a single crystal plasticity law. The experimental part consists in creating a residual topography database by atomic force microscopy (AFM) measurements on the samples. Pile-up distributions and sizes are largely sensitive to the indenter/grain relative orientation and the hardening rate. The topographies are true “fingerprints” of the plasticity mechanism at the grain scale. A 3D finite element (FE) modelling of nanoindentation test is developed using the code ZeBuLon and making use of the Méric-Cailletaud law. Numerical results show a good agreement with experimental observations and are largely sensitive to the indenter/grain relative orientation and the plastic parameters, including the interaction matrix coefficient specifying the interaction between dislocations on different slip systems. To measure the imprint information content, an identifiability index is proposed. Its calculation is based on the multicollinearity among the sensitivity vectors of topographies to the law parameters. According to the considered topographies, it measures if the numerical model updating problem is ill-posed. The results show that four to five parameters of the Méric-Cailletaud law can be identified by considering two topographies. This work paves the way for identifying the material behaviour at the grain scale using parametric identifiability

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Rukavina, Ivan. "Cyber-physics intrinsic modelling for smart systems." Thesis, Compiègne, 2021. http://bibliotheque.utc.fr/EXPLOITATION/doc/IFD/2021COMP2581.

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Dans le cadre de cette thèse, une approche de calcul de couplage multi-échelle et multi-physique en 2D et en 3D est présentée. La modélisation multi-échelle d’une structure consiste de l’échelle macro qui représente la réponse homogénéisée de la structure entière, tandis que l’échelle micro peut capturer les détails du comportement à la petite échelle du matériau, où des mécanismes inélastiques, tels que la plasticité ou l’endommagement, peuvent être pris en compte. L’intérieur de chaque macro-élément est rempli par le maillage à l’échelle micro qui s’y adapte entièrement. Les deux échelles sont couplées à travers le champ de déplacements imposé à l’interface. Le calcul par éléments finis est effectué, en utilisant une procédure de solution operator-split sur les deux échelles. En 2D, une discontinuité dans le champ de déplacements est introduite à l’échelle macro dans un élément fini Q4, pour pouvoir capturer l’adoucissement comportement d’un matériau piézoélectrique. Un degré de liberté supplémentaire qui représente le voltage est ajouté aux noeuds des macro-éléments de tétraèdre et d’hexaèdre en 3D. La poutre de Timoshenko comportant un modèle de commutation de polarisation est utilisée à l’échelle micro. Également, une formulation multi-échelle de Hellinger-Reissner a été développée et implémentée pour un simple patch test en électrostatique. La procédure proposée est mise en œuvre dans le logiciel de calcul par éléments finis FEAP - Finite Element Analysis Program. Pour simuler le comportement aux deux échelles, FEAP est modifié, et deux versions différentes du code sont obtenues - macroFEAP et microFEAP. Le couplage de ces codes est réalisé avec Component Template Library - CTL qui rend possible l’échange d’informations entre les deux échelles. Les capacités de cette approche multi-échelle en 2D et en 3D sont démontrées dans un environnement purement mécanique, mais aussi multi-physique. La formulation théorique et l’application algorithmique sont présentées, et les avantages de la méthode multi-échelle pour la modélisation des matériaux hétérogènes sont illustrés avec plusieurs exemples numériques
In this thesis, a multi-scale and multi-physics coupling computation procedure for a 2D and 3D setting is presented. When modeling the behavior of a structure by a multi-scale method, the macro-scale is used to describe the homogenized response of the structure, and the micro-scale to describe the details of the behavior on the smaller scale of the material where some inelastic mechanisms, like damage or plasticity, can be taken into account. The micro-scale mesh is defined for each macro-scale element in a way to fit entirely inside it. The two scales are coupled by imposing a constraint on the displacement field over their interface. The computation is performed using the operator split solution procedure on both scales, using the standard finite element method. In a 2D setting, an embedded discontinuity is implemented in the Q4 macroscale element to capture the softening behavior happening on the micro-scale. For the micro-scale element, a constant strain triangle (CST) is used. In a 3D setting, a macro-scale tetrahedral and hexahedral elements are developed, while on the micro-scale Timoshenko beam finite elements are used. This multi-scale methodology is extended with a multi-physics functionality, to simulate the behavior of a piezoelectric material. An additional degree of freedom (voltage) is added on the nodes of the 3D macro-scale tetrahedral and hexahedral elements. For the micro-scale element, a Timoshenko beam element with added polarization switching model is used. Also, a multi-scale Hellinger- Reissner formulation for electrostatics has been developed and implemented for a simple electrostatic patch test. For implementing the proposed procedure, Finite Element Analysis Program (FEAP) is used. To simulate the behavior on both macro and micro-scale, FEAP is modified and two different version of FEAP code are implemented – macroFEAP and microFEAP. For coupling, the two codes are exchanging information between them, and Component Template Library (CTL) is used. The capabilities of the proposed multi-scale approach in a 2D and 3D pure mechanics settings, but also multi-physics environment have been shown. The theoretical formulation and algorithmic implementation are described, and the advantages of the multi-scale approach for modeling heterogeneous materials are shown on several numerical examples

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36

Prasannavenkatesan, Rajesh. "Microstructure-sensitive fatigue modeling of heat treated and shot peened martensitic gear steels." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/31713.

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Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2010.
Committee Chair: David L. McDowell; Committee Member: G. B. Olson; Committee Member: K. A. Gall; Committee Member: Min Zhou; Committee Member: R. W. Neu. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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37

Gmati, Hela. "Modélisation par champ de phase de la rupture des matériaux solides élastiques et élasto-viscoplastiques." Thesis, Paris, HESAM, 2020. http://www.theses.fr/2020HESAE010.

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La méthode des champs de phases, qui a été conçue pour les problèmes d'interface, fournit un formalisme général intéressant pour la modélisation de la rupture. Ce formalisme est donc utilisé dans ce travail afin de construire des lois de comportement qui permettent de modéliser la rupture des matériaux homogènes et hétérogènes (i.e. polycristallins). Plus spécifiquement, deux modèles de comportement, qui utilisent les ingrédients de la mécanique de l'endommagement, sont proposés. Dans le premier cas, typique de la rupture fragile, l'endommagement est gouverné par le stockage d'énergie élastique. Le second modèle se concentre sur le cas où l'endommagement est piloté par le processus de déformation plastique, ce qui est représentatif de l'endommagement ductile ou de fatigue.Le modèle pour la rupture fragile utilise une variable d'endommagement scalaire pour décrire la perte de rigidité progressive. Le gradient de cette variable est traité comme une variable d'état supplémentaire afin de considérer l'augmentation d'énergie de surface due à la fissuration. La prise en compte des effets de fermeture repose sur une décomposition déviatorique/sphérique de l'énergie élastique. L'approche proposée est flexible en cela que des paramètres permettent de contrôler les contributions sphérique et déviatorique à la croissance de l'endommagement. Aussi, la description de la perte de rigidité ne nécessite pas d'hypothèse particulière quant à la classe de symétrie du matériau considéré. L'implémentation numérique du modèle, via la méthode des éléments finis, permet de réaliser des simulations représentatives sous chargement aussi bien statique que dynamique.Le cadre général de la plasticité cristalline est ensuite utilisé pour construire un modèle champs de phases pour les matériaux élasto-viscoplastiques polycristallins. L'approche est semblable à celle utilisée précédemment, à ceci près que le couplage endommagement-écrouissage est introduit. Ce choix de modélisation permet de considérer l'impact des déformations plastiques sur le développement de l'endommagement. Les résultats numériques obtenus avec le modèle proposé permettent de reproduire les aspects essentiels de la rupture ductile et par fatigue des matériaux métalliques
The Phase-Field Method (PFM), which has been designed for interfacial problems, provides an attractive framework for the modelling of fracture. The present work aims at developing some constitutive models within the framework of the PFM to model fracture in homogeneous and polycrystalline materials. For this purpose, two different situations have been examined. For the first situation, which is typical of brittle fracture, the development of damage is driven by the accumulation of elastic strain energy. The second situation is the one where damage is controlled by the development of plastic strains, which is quite common for ductile or fatigue fracture.The phase-field model for brittle fracture uses a scalar damage variable to represent the progressive degradation of mechanical resistance. The spatial gradient of the damage variable, which is treated as an additional external state variable, serves regularization purposes and allows considering the surface energy associated with cracks. The deviatoric/spherical decomposition of elastic strain energy is used to consider closure effects. Some material parameters have been introduced to control the impact of deviatoric and spherical contributions on the development of damage. Also, the proposed strategy is adapted to any class of material symmetry. Numerical implementation is undertaken via the finite element method, where nodal degrees of freedom are the displacement and the damage variable. For illustration purpose, the numerical simulations are carried out under both static and dynamic loading conditions.An extension of the above model to plasticity-driven fracture in polycrystalline materials is also proposed. The framework of crystal plasticity has been used for the construction of constitutive relations. To consider the role of plastic strains on the development of damage, the proposed strategy uses the coupling between damage and hardening. The consequence is that the driving force for damage contains some contributions from isotropic and kinematic hardening variables. According to the numerical results, the important features of ductile and fatigue fracture are correctly reproduced

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Zouaghi, Ahmed. "Modélisation multi-échelle du comportement non linéaire et hétérogène en surface de l'acier AISI H11." Thesis, Ecole nationale des Mines d'Albi-Carmaux, 2015. http://www.theses.fr/2015EMAC0008/document.

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Les outillages de mise en forme en acier martensitique de type AISI H11 sont des pièces critiques dont le comportement en service est étroitement lié à leurs structures internes et à leur évolution. Les conditions des sollicitations lors de la mise en oeuvre du procédé est souvent à l'origine de modifications microstructurales en surface, à savoir la morphologie des lattes de martensite, les orientations cristallographiques, l'état d'écrouissage interne ou encore le profil de surface. Ces aspects peuvent éventuellement altérer les performances mécaniques de l'acier AISI H11. Afin d'appréhender et d'optimiser le comportement mécanique de celui-ci, une approche multi-échelle est mise en oeuvre dans ce travail. Celle-ci s'articule autour d'une investigation expérimentale et d'un traitement numérique. L'étude expérimentale s'attache à reproduire, à l'échelle du laboratoire, des surfaces équivalentes à celles issues lors des procédés de mise en oeuvre des outillages. Des techniques de caractérisation spécifiques, à savoir le MEB, l'EBSD, la nanoindentation ou encore l'altimétrie permettent de mettre en évidence un gradient de la stéréologie du matériau en surface et sous-surface. Les hétérogénéités locales induites concernent la morphologie des lattes de martensite, les orientations cristallographiques, l'état d'écrouissage interne mais également le profil de surface. Des essais mécaniques in-situ associés à la technique de corrélation d'images numériques sont réalisés pour des chargements monotones quasi-statiques et cycliques de type traction-traction. Une investigation des champs mécaniques locaux en surface est ainsi effectuée, elle permet d'analyser les schémas de localisations des déformations non linéaires liés aux artéfacts stéréologiques. Le traitement numérique s'intéresse à une modélisation multi-échelle, et plus particulièrement à des calculs par la méthode des éléments finis sur des microstructures virtuelles générées par tesselations de Voronoï. Celles-ci sont effectuées de manière à reproduire les structures martensitiques et considèrent des relations d'orientations spécifiques (de type Kurdjumov-Sachs) à l'issue du traitement thermique entre les lattes de martensite et le grain austénitique parent. Les équations constitutives du modèle de plasticité cristalline (élasto-viscoplastique) de Méric-Cailletaud sont implantées dans le code de calcul par éléments finis Abaqus dans le cadre de l'hypothèse des petites perturbations (HPP) et de la théorie des transformations finies. La formulation du modèle dans le contexte de la théorie des transformations finies est effectuée dans le cadre d'une description spatiale où la notion de dérivée objective est considérée. Celle-ci consiste en celle d'Oldroyd ou de Truesdell de manière à ce qu'une telle formulation soit équivalente à une description lagrangienne. Le traitement numérique a permis de reproduire de manière qualitative les schémas de localisation en surface mise en évidence lors de l'investigation expérimentale. L'influence des divers paramètres stéréologiques, évoqués ci-dessus, sur les champs mécaniques locaux a été analysée. De par cette approche, il a été possible de mettre en évidence certains mécanismes élémentaires, notamment les effets d'interaction et de surface. Enfin, il a été constaté que la prise en compte des rotations des réseaux cristallins par la théorie des transformations finies permet de relâcher certaines zones de localisation des champs mécaniques autour d'artéfacts stéréologiques
AISI H11 martensitic tool steels are critical mechanical components that behaviour during service is drastically linked to their internal structures and their possible evolution. Their manufacture processes are often at the origin of microstructural changes at the surface, namely the morphology of martensitic laths, the crystallographic orientations, the internal hardening state and the surface profile These aspects can potentially alter the mechanical performance of AISI H11 martensitic steel. In order to get better insight into and optimize its mechanical behaviour, a multi-scale approach involving an experimental investigation and a numerical treatment is taken in this work.The experimental investigation focuses to reproduce, at the laboratory scale, equivalent surfaces to those resulting from tool steels manufacture processes. Specific characterization techniques, namely SEM, EBSD, nanoindentation and altimetry enable to highlight a stereology gradient of the material in surface and sub-surface. The induced local heterogeneities consist in morphology of martensitic laths and crystallographic orientations, internal hardening state and surface profile. In-situ mechanical tests with digital image correlation technique (DIC) are carried out for monotonous quasi-static and tension-tension cyclic loads. An investigation of the local mechanical fields at the surface is thus performed and allows to analyze the localizations schemes of nonlinear strains which are related to stereological artifacts.The numerical treatment is focused on a multi-scale modelling, and more particularly on finite element calculations on virtual microstructures which are generated by Voronoi tesselations. The latters are carried out such that to reproduce martensitic structures and consider a specific orientation relationship between martensitic laths and parent austenitic grains (i.e. Kurdjumov-Sachs) after the heat treatment. The constitutive equations of the (elasto-viscoplastic) crystal plasticity of Méric-Cailletaud are implemented in the finite element code Abaqus in the context of the small strain assumption and the finite strain theory. The formulation of the model in the context of finite strain theory is is given a spatial description where the notion of objective derivative, namely the so called one of Oldroyd or Truesdell, is used in such a way that such formulation is equivalent to a Lagrangian description.The numerical treatment has allowed to qualitatively reproduce the localization patterns at the surface which have been highlighted in the experimental investigation. The influence of the different stereological parameters mentioned above on the local mechanical fields was analyzed. By this approach, it was possible to highlight some elementary mechanisms including interaction and surface effects. Finally, it was found that the inclusion of lattice rotations via the theory of finite strain allows to release certain areas of mechanical fields localization that are related to stereological artifacts

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39

Vu, Hoang Sinh. "Simulations numériques et mesures expérimentales du comportement mécanique de films minces, effets d'echelles." Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENI105.

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40

Guillotin, Alban. "Étude de la rugosité de surface induite par la déformation plastique de tôles minces en alliage d'aluminium AA6016." Phd thesis, Ecole Nationale Supérieure des Mines de Saint-Etienne, 2010. http://tel.archives-ouvertes.fr/tel-00716025.

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Dans le cadre d'un programme de recherche visant à l'allègement de la structure des véhicules, l'origine de lignage dans des tôles en aluminium AA6016 a été étudiée. Ce phénomène, qui peut apparaître à la suite d'une déformation plastique, est apparenté à de la rugosité de surface alignée dans la direction de laminage (DL). Sa présence est néfaste à une bonne finition de surface, et son intensité est appréciée visuellement par les fabricants.Une méthode de quantification rationnelle a été développée. La caractérisation de la distribution morphologique des motifs de rugosité a été rendue possible par l'utilisation de fonctions fréquentielles telle la densité de puissance spectrale. La note globale, construite à partir de la quantification individuelle des composantes de lignage pur et de rugosité globulaire, s'est montrée en bon accord avec les estimations visuelles, et notamment avec le niveau de lignage intermédiaire regroupant plusieurs aspects de surface différents.La microstructure des matériaux à l'état T4 a été expérimentalement mesurée couche de grains par couche de grain à l'aide d'un couplage entre polissage contrôle et acquisition par EBSD. Les 4 à 5 premières couches sous la surface (-120μm) semblent jouer un rôle mécanique prépondérant dans la formation du lignage car elles offrent à la fois une grande taille de grains moyenne, une importante ségrégation d'orientations cristallines, et une forte similitude de longueurs d'onde entre la rugosité de surface et les motifs de la microtexture.Des simulations numériques ont permis de vérifier que les couples de texture identifiés (Cube/Goss, Cube/Aléatoire et Cube/CT18DN) possédaient des différences d'amincissements hors-plans suffisantes pour générer l'ondulation d'une couche d'éléments. En revanche, l'influence mécanique de cette même couche décroit très rapidement avec son enfouissement dans la profondeur et devient négligeable sous plus de 4 couches d'éléments.

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41

Liang, Xiaoyu. "Comportement en fatigue à grand nombre de cycle d’un acier inoxydable 316L obtenu par fabrication additive : effets de la microstructure, de la rugosité et des défauts." Thesis, Paris, HESAM, 2020. http://www.theses.fr/2020HESAE017.

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Cette étude vise à étudier l'influence de la microstructure, de la rugosité et des défauts de surface sur le comportement en fatigue à grand nombre de cycles (FGNC) d'un acier inoxydable 316L obtenu par fabrication additive (FA). Composée d’un volet expérimental et d’un volet numérique, elle est motivée par le fait que les matériaux issus du procédé de FA présentent souvent un état de surface et une microstructure très distincts des couples procédés de fabrication / matériaux conventionnels. Afin de clairement identifier le rôle joué par chacun des facteurs influents sur la réponse en fatigue, différentes techniques de caractérisation (Profilométrie, EBSD, Tomographie RX, dureté …) sont employées et permettent de mettre en évidence un niveau de rugosité important après fabrication ainsi que des textures morphologiques et cristallographiques marquées. Pour ce qui est du comportement sous chargement mécanique, des essais cycliques à déformation totale imposée mettent en évidence un écrouissage cyclique avec durcissement puis adoucissement. Une importante campagne d’essais en fatigue est conduite sous différents modes de chargement (traction, flexion, torsion) et pour différentes configurations d’état de surface (brut de fabrication, poli). L’analyse des faciès de rupture fait apparaître le rôle prépondérant joué par les défauts de type « lack of fusion » sur les mécanismes d’amorçage en surface des fissures de fatigue. Un diagramme de type Kitagawa-Takahashi est construit à partir de l’observation de la taille des défauts à l’amorçage et le rôle des amas de défaut est clairement démontré. L’étude numérique comporte deux parties distinctes avec, d’abord, un travail préliminaire relatif à la construction d’une méthode non locale adaptée à la prise en compte des effets de microstructure en fatigue dans le cas d’un acier 316L corroyé. A partir des données collectées lors de la campagne expérimentale portant sur l’acier SLM 316L, un modèle d'éléments finis tenant compte de la rugosité, des défauts et de la microstructure est construit. Les calculs sont conduits en utilisant un comportement de type élasticité cubique associé ou pas à de la plasticité cristalline. À l'aide d’une approche faisant appel à la statistique des extrêmes, les résultats des simulations EF sont analysés de manière à quantifier les effets respectifs de la rugosité de surface, de la taille et morphologie des grains, de la texture cristallographique et des défauts
This study aims to investigate the influence of both the microstructure and surface defects on the high cycle fatigue (HCF) behavior of a 316L stainless steel obtained by additive manufacturing (AM). Surface defects and microstructure are dominant factors of fatigue behavior, while the AM materials often exhibit distinguished surface state and microstructure compared to conventional materials. The current study begins with an investigation of the material properties that are related to fatigue behavior. Microstructure observations of the powder and fabricated specimens are undertaken. Profilometry and tomography analyses make the inherent defects visible. The hardness, elastic behavior and elastic-plastic behavior are studied via mechanical tests. Then, load-controlled fatigue tests concerning different surface-treated specimens under different loading types are conducted. To reveal the mechanism of fatigue failure in the studied specimens, a comprehensive fractography analysis is carried out. Experimental research reveals the weakening of fatigue strength due to lack-of-fusion defects. Yet, the effect of the microstructural attributes is difficult to evaluate without numerical tools. A preliminary numerical study about the application of the non-local method in an explicit microstructure sensitive model is undertaken to complement the microstructure-sensitive modeling framework. Based on the data collected in the experimental campaign, a finite element model that can take into consideration of the defects and the microstructure of the SLM SS 316L is built up. Finite element analyses are performed with both cubic elasticity and polycrystal plasticity constitutive laws. With the help of the statistical method, the results from the FE model are used to quantitatively assess the influence of surface roughness and microstructural attributes on the fatigue performance of SLM SS 316L

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Chung-ShaoSun and 孫仲紹. "Simulation of Flow Curve and Rolling Texture of Polycrystalline Copper Using Crystal Plasticity Finite Element Method (CP-FEM)." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/766b34.

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"3D Modeling of Incipient Spall Damage in Shocked FCC Multicrystals." Doctoral diss., 2013. http://hdl.handle.net/2286/R.I.16458.

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abstract: Shock loading is a complex phenomenon that can lead to failure mechanisms such as strain localization, void nucleation and growth, and eventually spall fracture. Studying incipient stages of spall damage is of paramount importance to accurately determine initiation sites in the material microstructure where damage will nucleate and grow and to formulate continuum models that account for the variability of the damage process due to microstructural heterogeneity. The length scale of damage with respect to that of the surrounding microstructure has proven to be a key aspect in determining sites of failure initiation. Correlations have been found between the damage sites and the surrounding microstructure to determine the preferred sites of spall damage, since it tends to localize at and around the regions of intrinsic defects such as grain boundaries and triple points. However, considerable amount of work still has to be done in this regard to determine the physics driving the damage at these intrinsic weak sites in the microstructure. The main focus of this research work is to understand the physical mechanisms behind the damage localization at these preferred sites. A crystal plasticity constitutive model is implemented with different damage criteria to study the effects of stress concentration and strain localization at the grain boundaries. A cohesive zone modeling technique is used to include the intrinsic strength of the grain boundaries in the simulations. The constitutive model is verified using single elements tests, calibrated using single crystal impact experiments and validated using bicrystal and multicrystal impact experiments. The results indicate that strain localization is the predominant driving force for damage initiation and evolution. The microstructural effects on theses damage sites are studied to attribute the extent of damage to microstructural features such as grain orientation, misorientation, Taylor factor and the grain boundary planes. The finite element simulations show good correlation with the experimental results and can be used as the preliminary step in developing accurate probabilistic models for damage nucleation.
Dissertation/Thesis
Ph.D. Mechanical Engineering 2013

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Abdolvand, Hamidreza. "MULTI-SCALE MODELING AND EXPERIMENTAL STUDY OF DEFORMATION TWINNING IN HEXAGONAL CLOSE-PACKED MATERIALS." Thesis, 2012. http://hdl.handle.net/1974/7095.

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Zirconium and its alloys have been extensively used in both heavy and light water nuclear reactors. Like other Hexagonal Close-Packed (HCP) materials, e.g. magnesium, zirconium alloys develop different textures during manufacturing process which result in highly anisotropic materials with different responses under different loading conditions. Slip and twinning are two major deformation mechanisms during plastic deformation of zirconium. This dissertation uses various experimental techniques and a crystal plasticity scheme in the finite element framework to study deformation mechanisms in HCP materials with an emphasis on twinning in Zircaloy-2. The current study is presented as a manuscript format dissertation comprised of four manuscript chapters. After a literature review in Chapter 2, Chapter 3 reports steps in developing a crystal plasticity finite element user material subroutine for modeling deformation in Zircaloy-2 at room temperature. It is shown in Chapter 3 that the developed rate dependent equations are capable of capturing evolution of key features, e.g., texture, lattice strains, and twin volume fractions, during deformation by twinning and slip. Chapter 4 reports various assumptions and approaches in modeling twinning where results are compared against neutron diffraction measurements from the literature. It is shown in Chapter 4 that the predominant twin reorientation scheme can explain texture development more precisely than the other schemes discussed. Chapter 5 and 6 are two connected chapters where in the first one the formation of twins is studied statistically and in the second one, local inception and propagation of twins is studied. Numerical results of these two chapters are compared with 2D electron backscattered diffraction measurements, both carried out by the author and from the literature. Results from these two connected chapters emphasize the important role of grain boundary geometry and stress concentration sites on twin nucleation and growth. The four manuscript chapters are followed by summarizing conclusions and suggestions for future work in Chapter 7.
Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2012-04-23 11:50:33.751

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(8464098), Veerappan Prithivirajan. "MODELING FATIGUE BEHAVIOR OF ADDITIVELY MANUFACTURED NI-BASED SUPERALLOYS VIA CRYSTAL PLASTICITY." Thesis, 2020.

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Additive manufacturing (AM) introduces high variability in the microstructure and defect distributions, compared with conventional processing techniques, which introduces greater uncertainty in the resulting fatigue performance of manufactured parts. As a result, qualification of AM parts poses as a problem in continued adoption of these materials in safety-critical components for the aerospace industry. Hence, there is a need to develop precise and accurate, physics-based predictive models to quantify the fatigue performance, as a means to accelerate the qualification of AM parts. The fatigue performance is a critical requirement in the safe-life design philosophy used in the aerospace industry. Fatigue failure is governed by the loading conditions and the attributes of the material microstructure, namely, grain size distribution, texture, and defects. In this work, the crystal plasticity finite element (CPFE) method is employed to model the microstructure-based material response of an additively manufactured Ni-based superalloy, Inconel 718 (IN718). Using CPFE and associated experiments, methodologies were developed to assess multiple aspects of the fatigue behavior of IN718 using four studies. In the first study, a CPFE framework is developed to estimate the critical characteristics of porosity, namely the pore size and proximity that would cause a significant debit in the fatigue life. The second study is performed to evaluate multiple metrics based on plastic strain and local stress in their ability to predict both the modes of failure as seen in fractography experiments and estimate the scatter in fatigue life due to microstructural variability as obtained from fatigue testing. In the third study, a systematic analysis was performed to investigate the role of the simulation volume and the microstructural constraints on the fatigue life predictions to provide informed guidelines for simulation volume selection that is both computationally tractable and results in consistent scatter predictions. In the fourth study, validation of the CPFE results with the experiments were performed to build confidence in the model predictions. To this end, 3D realistic microstructures representative of the test specimen were created based on the multi-modal experimental data obtained from high-energy diffraction experiments and electron backscatter diffraction microscopy. Following this, the location of failure is predicted using the model, which resulted in an unambiguous one to one correlation with the experiment. In summary, the development of microstructure-sensitive predictive methods for fatigue assessment presents a tangible step towards the adoption of model-based approaches that can be used to compliment and reduce the overall number of physical tests necessary to qualify a material for use in application.

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Shantharajanna, H. R. "Elasto-Plastic Modelling Of Fine Grained Soils - A Variable Moduli Approach." Thesis, 1995. http://etd.iisc.ernet.in/handle/2005/2192.

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(7038047), Anudeep Padmanabhan. "Fatigue Analysis of 3D Printed 15-5 PH Stainless Steel - A Combined Numerical and Experimental Study." Thesis, 2019.

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Additive manufacturing (AM) or 3D printing has gained significant advancement in recent years. However the potential of 3D printed metals still has not been fully explored. A main reason is the lack of accurate knowledge of the load capacity of 3D printed metals, such as fatigue behavior under cyclic load conditions, which is still poorly understood as compared with the conventional wrought counterpart.

The goal of the thesis is to advance the knowledge of fatigue behavior of 15-5 PH stainless steel manufactured through laser powder bed fusion process. To achieve the goal, a combined numerical and experimental study is carried out. First, using a rotary fatigue testing experiment, the fatigue life of the 15-5 PH stainless steel is measured. The strain life curve shows that the numbers of the reversals to failure increase from 13,403 to 46,760 as the applied strain magnitudes decrease from 0.214\% from 0.132\%, respectively. The micro-structure analysis shows that predominantly brittle fracture is presented on the fractured surface. Second, a finite element model based on cyclic plasticity including the damage model is developed to predict the fatigue life. The model is calibrated with two cases: one is the fatigue life of 3D printed 17-4 stainless steel under constant amplitude strain load using the direct cyclic method, and the other one is the cyclic behavior of Alloy 617 under multi-amplitude strain loads using the static analysis method. Both validation models show a good correlation with the literature experimental data. Finally, after the validation, the finite element model is applied to the 15-5 PH stainless steel. Using the direct cyclic method, the model predicts the fatigue life of 15-5 PH stainless steel under constant amplitude strain. The extension of the prediction curve matches well with the previously measured experimental results, following the combined Coffin-Manson Basquin Law. Under multi-amplitude strain, the kinematic hardening evolution parameter is incorporated into the model. The model is capable to capture the stresses at varied strain amplitudes. Higher stresses are predicted when strain amplitudes are increased. The model presented in the work can be used to design reliable 3D printed metals under cyclic loading conditions.

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Qureshi, J., and Dennis Lam. "Behaviour of Headed Shear Stud in Composite Beams with Profiled Metal Decking." 2012. http://hdl.handle.net/10454/5917.

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This paper presents a numerical investigation into the behaviour of headed shear stud in composite beams with profiled metal decking. A three-dimensional finite element model was developed using general purpose finite element program ABAQUS to study the behaviour of through-deck welded shear stud in the composite slabs with trapezoidal deck ribs oriented perpendicular to the beam. Both static and dynamic procedures were investigated using Drucker Prager model and Concrete Damaged Plasticity model respectively. In the dynamic procedure using ABAQUS/Explicit, the push test specimens were loaded slowly to eliminate significant inertia effects to obtain a static solution. The capacity of shear connector, load-slip behaviour and failure modes were predicted and validated against experimental results. The delamination of the profiled decking from concrete slab was captured in the numerical analysis which was observed in the experiments. ABAQUS/Explicit was found to be particularly suitable for modelling post-failure behaviour and the contact interaction between profiled decking and concrete slabs. It is concluded that this model represents the true behaviour of the headed shear stud in composite beams with profiled decking in terms of the shear connection capacity, load-slip behaviour and failure modes.

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Azeiteiro, Ricardo José Novo. "Numerical simulation of liquefaction-related phenomena." Doctoral thesis, 2021. http://hdl.handle.net/10316/95382.

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Thesis for the degree of Doctor of Philosophy in Civil Engineering, specialisation in Geotechnics, submitted to the Department of Civil Engineering of the Faculty of Sciences and Technology of the University of Coimbra.
Liquefaction-related phenomena are a major concern for modern societies, at least in seismically active zones, due to their massive destructive potential, capable of causing great economic losses, social disruption and loss of life. Although the great development of numerical tools in recent years has extended the possibilities of performance-based design, it has also revealed the need for more reliable constitutive models, capable of simulating the mechanisms involved in these complex phenomena. In this thesis, a bounding surface plasticity model is implemented into the finite element code FEMEPDYN, developed at the University of Coimbra, and applied to the simulation of liquefaction-related phenomena induced by cyclic loading and observed in element laboratory tests and centrifuge experiments. In the first part of this thesis, the outcome of an extensive laboratory testing programme performed on Hostun sand is presented, including results of bender element tests, as well as of drained and undrained monotonic triaxial compression and extension tests, allowing for the characterisation of the monotonic response of Hostun sand within the very small to large strain range. The effects of the void ratio, consolidation stress state and stress path on the measured response are discussed. Moreover, a state-parameter approach is used in conjunction with critical state soil mechanics concepts to characterise the distinctive states of the response of sand, namely the undrained instability, phase transformation, peak stress ratio, and critical states. The stress-dilatancy characteristics of sand are also investigated. In addition, results of a series of drained and undrained cyclic triaxial tests carried out on Hostun sand are used to assess the key aspects of its cyclic response, including the reduction of the secant shear stiffness and concurrent increase in damping ratio with strain amplitude, the generation of excess pore water pressure with cyclic loading and the undrained cyclic resistance. The second part of this thesis includes a comprehensive description of a bounding surface plasticity model proposed in the literature and adopted in the present study. To increase the overall flexibility and expand the modelling capabilities of the constitutive model, two modifications are introduced to its formulation. Attention is subsequently given to its implementation into the finite element code FEMEPDYN. The operations required in each step of the stress integration scheme are thoroughly described, giving particular emphasis to the operations required by the co-existence of the two yield surfaces of the constitutive model. Validation exercises comprising the simulation of both element laboratory tests and centrifuge experiments and the comparison of the obtained results with those reported in the literature are presented afterwards. The third and final part of this thesis starts with the calibration of the constitutive model against the results of element laboratory tests performed on Hostun sand during the first stage of the research. Subsequently, the ability of the constitutive model to reproduce the response of Hostun sand measured in the laboratory is explored in detail, enabling to characterise the merits and pitfalls of the constitutive model. Its performance is further evaluated by simulating two dynamic centrifuge experiments presented in the literature concerning the performance of shallow foundations resting on saturated deposits of Hostun sand subjected to dynamic loading causing liquefaction. The obtained results suggest that the numerical tool is able to capture accurately important aspects of the sand-structure interaction observed in the centrifuge experiments, such as the generation of large excess pore pressures in sand induced by the applied dynamic loading and consequent alteration of the input motion due to the reduction of sand’s stiffness, as well as the progressive accumulation of large structural settlement with dynamic loading. Furthermore, the impacts of both densification and closely spaced high-capacity vertical drains on the mitigation of liquefaction effects seem also adequately captured in the numerical analysis.
Os fenómenos associados a liquefação são uma preocupação para as sociedades modernas, nomeadamente quando localizadas em zonas sísmicas ativas, devido ao seu enorme potencial de destruição, capaz de provocar elevados prejuízos económicos, alteração ao normal funcionamento da sociedade e perda de vidas humanas. Apesar do grande desenvolvimento de ferramentas numéricas verificado nos últimos anos ter aumentado as possibilidades do uso do dimensionamento baseado no desempenho das estruturas na prática, este progresso também evidenciou a necessidade do desenvolvimento de modelos constitutivos mais robustos, capazes de simular os mecanismos envolvidos neste tipo de fenómenos. Nesta tese, um modelo baseado na teoria bounding surface plasticity é implementado no código de elementos finitos FEMEPDYN, desenvolvido na Universidade de Coimbra, e utilizado para a simulação de fenómenos associados a liquefação induzida por carregamentos cíclicos e observada ensaios elementares de laboratório e ensaios de centrifugadora. Na primeira parte desta tese, são apresentados os resultados obtidos num extenso programa de ensaios elementares realizados sobre a areia de Hostun, incluindo ensaios de bender elements e ensaios triaxiais de compressão e de extensão drenados e não drenados, os quais permitem a caracterização da resposta monotónica da areia de Hostun no domínio das muito pequenas a grandes deformações. Os efeitos do índice de vazios, do estado de tensão na consolidação das amostras e da trajectória de tensão na resposta medida no laboratório são aí discutidos. Para além disso, é utilizada uma metodologia com um parâmetro de estado em conjunto com conceitos de mecânica dos solos de estado crítico para caracterizar estados distintos da resposta de areias, nomeadamente os relativos a instabilidade em condições não drenadas, a transformação de fase, ao valor de pico do rácio de tensão e ao estado crítico. As características da relação entre o estado de tensão e a dilatância de areias são também investigadas. Para complementar, são utilizados resultados obtidos em ensaios triaxiais cíclicos em condições drenadas e não drenadas sobre a areia de Hostun para caraterizar os aspectos principais da sua resposta cíclica, incluindo a redução da rigidez secante ao corte acompanhada do aumento do amortecimento com o nível de extensão, a geração de pressões de água nos poros com o carregamento cíclico e a resistência cíclica não drenada. A segunda parte da tese inclui uma descrição detalhada do modelo baseado na teoria bounding surface plasticity proposto na bibliografia e adotado neste estudo. Para aumentar a versatilidade do modelo, assim como as suas capacidades de modelação, são introduzidas duas modificações na sua formulação. De seguida, apresenta-se a implementação do modelo no código de elementos finitos FEMEPDYN. As operações necessárias em cada passo do método de integração são descritas em detalhe, dando especial atenção às operações requeridas pela coexistência de duas superfícies de cedência no modelo constitutivo. São finalmente apresentados exemplos de validação, incluindo a simulação de ensaios de laboratório elementares e de centrifugadora, comparando-se os resultados obtidos neste estudo com os que estão descritos na bibliografia. A terceira e última parte da tese começa por descrever a calibração do modelo constitutivo utilizando os resultados dos ensaios de laboratório elementares realizados sobre a areia de Hostun durante a primeira fase do trabalho de investigação. De seguida, a capacidade do modelo para reproduzir a resposta medida no laboratório é explorada em detalhe, permitindo a caracterização das capacidades e limitações do modelo constitutivo. O seu desempenho é posteriormente avaliado através da simulação de dois ensaios dinâmicos de centrifugadora, cujos resultados são apresentados na bibliografia, acerca do desempenho de fundações superficiais assentes em depósitos saturados de areia de Hostun quando sujeitas a ações dinâmicas causando liquefação. Os resultados obtidos sugerem que a ferramenta numérica é capaz de prever, de forma adequada, aspectos importantes da interação solo-estrutura observados nos ensaios de centrifugadora, tais como a geração de pressões de água nos poros elevadas no depósito de areia devido ao carregamento dinâmico e a consequente alteração da acção dinâmica aplicada na base do modelo devido à redução da rigidez ao corte da areia, bem como a progressiva acumulação de elevados assentamentos estruturais com o carregamento dinâmico. Para além disso, os impactos da densificação da areia e da introdução de uma malha densa de drenos verticais de alta capacidade na mitigação dos efeitos de liquefação também parecem ser adequadamente simulados na análise numérica.

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(8741097), Ritwik Bandyopadhyay. "ENSURING FATIGUE PERFORMANCE VIA LOCATION-SPECIFIC LIFING IN AEROSPACE COMPONENTS MADE OF TITANIUM ALLOYS AND NICKEL-BASE SUPERALLOYS." Thesis, 2020.

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In this thesis, the role of location-specific microstructural features in the fatigue performance of the safety-critical aerospace components made of Nickel (Ni)-base superalloys and linear friction welded (LFW) Titanium (Ti) alloys has been studied using crystal plasticity finite element (CPFE) simulations, energy dispersive X-ray diffraction (EDD), backscatter electron (BSE) images and digital image correlation (DIC).

In order to develop a microstructure-sensitive fatigue life prediction framework, first, it is essential to build trust in the quantitative prediction from CPFE analysis by quantifying uncertainties in the mechanical response from CPFE simulations. Second, it is necessary to construct a unified fatigue life prediction metric, applicable to multiple material systems; and a calibration strategy of the unified fatigue life model parameter accounting for uncertainties originating from CPFE simulations and inherent in the experimental calibration dataset. To achieve the first task, a genetic algorithm framework is used to obtain the statistical distributions of the crystal plasticity (CP) parameters. Subsequently, these distributions are used in a first-order, second-moment method to compute the mean and the standard deviation for the stress along the loading direction (σ_load), plastic strain accumulation (PSA), and stored plastic strain energy density (SPSED). The results suggest that an ~10% variability in σ_load and 20%-25% variability in the PSA and SPSED values may exist due to the uncertainty in the CP parameter estimation. Further, the contribution of a specific CP parameter to the overall uncertainty is path-dependent and varies based on the load step under consideration. To accomplish the second goal, in this thesis, it is postulated that a critical value of the SPSED is associated with fatigue failure in metals and independent of the applied load. Unlike the classical approach of estimating the (homogenized) SPSED as the cumulative area enclosed within the macroscopic stress-strain hysteresis loops, CPFE simulations are used to compute the (local) SPSED at each material point within polycrystalline aggregates of 718Plus, an additively manufactured Ni-base superalloy. A Bayesian inference method is utilized to calibrate the critical SPSED, which is subsequently used to predict fatigue lives at nine different strain ranges, including strain ratios of 0.05 and -1, using nine statistically equivalent microstructures. For each strain range, the predicted lives from all simulated microstructures follow a log-normal distribution; for a given strain ratio, the predicted scatter is seen to be increasing with decreasing strain amplitude and are indicative of the scatter observed in the fatigue experiments. Further, the log-normal mean lives at each strain range are in good agreement with the experimental evidence. Since the critical SPSED captures the experimental data with reasonable accuracy across various loading regimes, it is hypothesized to be a material property and sufficient to predict the fatigue life.

Inclusions are unavoidable in Ni-base superalloys, which lead to two competing failure modes, namely inclusion- and matrix-driven failures. Each factor related to the inclusion, which may contribute to crack initiation, is isolated and systematically investigated within RR1000, a powder metallurgy produced Ni-base superalloy, using CPFE simulations. Specifically, the role of the inclusion stiffness, loading regime, loading direction, a debonded region in the inclusion-matrix interface, microstructural variability around the inclusion, inclusion size, dissimilar coefficient of thermal expansion (CTE), temperature, residual stress, and distance of the inclusion from the free surface are studied in the emergence of two failure modes. The CPFE analysis indicates that the emergence of a failure mode is an outcome of the complex interaction between the aforementioned factors. However, the possibility of a higher probability of failure due to inclusions is observed with increasing temperature, if the CTE of the inclusion is higher than the matrix, and vice versa. Any overall correlation between the inclusion size and its propensity for damage is not found, based on inclusion that is of the order of the mean grain size. Further, the CPFE simulations indicate that the surface inclusions are more damaging than the interior inclusions for similar surrounding microstructures. These observations are utilized to instantiate twenty realistic statistically equivalent microstructures of RR1000 – ten containing inclusions and remaining ten without inclusions. Using CPFE simulations with these microstructures at four different temperatures and three strain ranges for each temperature, the critical SPSED is calibrated as a function of temperature for RR1000. The results suggest that critical SPSED decreases almost linearly with increasing temperature and is appropriate to predict the realistic emergence of the competing failure modes as a function of applied strain range and temperature.

LFW process leads to the development of significant residual stress in the components, and the role of residual stress in the fatigue performance of materials cannot be overstated. Hence, to ensure fatigue performance of the LFW Ti alloys, residual strains in LFW of similar (Ti-6Al-4V welded to Ti-6Al-4V or Ti64-Ti64) and dissimilar (Ti-6Al-4V welded to Ti-5Al-5V-5Mo-3Cr or Ti64-Ti5553) Ti alloys have been characterized using EDD. For each type of LFW, one sample is chosen in the as-welded (AW) condition and another sample is selected after a post-weld heat treatment (HT). Residual strains have been separately studied in the alpha and beta phases of the material, and five components (three axial and two shear) have been reported in each case. In-plane axial components of the residual strains show a smooth and symmetric behavior about the weld center for the Ti64-Ti64 LFW samples in the AW condition, whereas these components in the Ti64-Ti5553 LFW sample show a symmetric trend with jump discontinuities. Such jump discontinuities, observed in both the AW and HT conditions of the Ti64-Ti5553 samples, suggest different strain-free lattice parameters in the weld region and the parent material. In contrast, the results from the Ti64-Ti64 LFW samples in both AW and HT conditions suggest nearly uniform strain-free lattice parameters throughout the weld region. The observed trends in the in-plane axial residual strain components have been rationalized by the corresponding microstructural changes and variations across the weld region via BSE images.

In the literature, fatigue crack initiation in the LFW Ti-6Al-4V specimens does not usually take place in the seemingly weakest location, i.e., the weld region. From the BSE images, Ti-6Al-4V microstructure, at a distance from the weld-center, which is typically associated with crack initiation in the literature, are identified in both AW and HT samples and found to be identical, specifically, equiaxed alpha grains with beta phases present at the alpha grain boundaries and triple points. Hence, subsequent fatigue performance in LFW Ti-6Al-4V is analyzed considering the equiaxed alpha microstructure.

The LFW components made of Ti-6Al-4V are often designed for high cycle fatigue performance under high mean stress or high R ratios. In engineering practice, mean stress corrections are employed to assess the fatigue performance of a material or structure; albeit this is problematic for Ti-6Al-4V, which experiences anomalous behavior at high R ratios. To address this problem, high cycle fatigue analyses are performed on two Ti-6Al-4V specimens with equiaxed alpha microstructures at a high R ratio. In one specimen, two micro-textured regions (MTRs) having their c-axes near-parallel and perpendicular to the loading direction are identified. High-resolution DIC is performed in the MTRs to study grain-level strain localization. In the other specimen, DIC is performed on a larger area, and crack initiation is observed in a random-textured region. To accompany the experiments, CPFE simulations are performed to investigate the mechanistic aspects of crack initiation, and the relative activity of different families of slip systems as a function of R ratio. A critical soft-hard-soft grain combination is associated with crack initiation indicating possible dwell effect at high R ratios, which could be attributed to the high-applied mean stress and high creep sensitivity of Ti-6Al-4V at room temperature. Further, simulations indicated more heterogeneous deformation, specifically the activation of multiple families of slip systems with fewer grains being plasticized, at higher R ratios. Such behavior is exacerbated within MTRs, especially the MTR composed of grains with their c-axes near parallel to the loading direction. These features of micro-plasticity make the high R ratio regime more vulnerable to fatigue damage accumulation and justify the anomalous mean stress behavior experienced by Ti-6Al-4V at high R ratios.

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