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1
Evans, Christabel. "Micromechanisms and micromechanics of Zircaloy-4." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/14335.
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The micromechanisms of Zircaloy-4 are investigated in relation to texture evolution, hydride formation and fatigue. The Zircaloy-4 plate used throughout this thesis was provided by Rolls- Royce plc, Derby, and was annealed post unidirectional rolling. The effect of strain rate on the texture evolution of Zircaloy-4 was investigated to understand how different processing methods would effect the final texture. Texture evolution during high temperature (550◦C) compression and tension tests were investigated using synchrotron X- ray diffraction in the transverse and rolling directions (TD and RD) at strain rates ranging from 10−4s−1 to 10−1s−1. The post deformation microstructures showed the presence of twins at the higher strain rates (10−1s−1 to 10−2s−1 ), with minimal twinning seen at the slower rates. The pole figures obtained throughout testing showed no texture evolution during tensile testing, regardless of strain rate and the basal poles remained orientated ±30◦between the normal direction (ND) and the transverse direction (TD), which is the original texture for the as received material. During the compression tests specimens tested in the RD showed an evolution in the pole figures as strain rate was increased. At a strain rate of 10−1s−1 a reorientation of the basal poles to lie almost solely in the RD was seen, indicative of twinning. As the strain rate was reduced, this effect diminished and at a strain rate of 10−4s−1only a slight rotation of the basal poles was observed. The Kearns’ factor evolution with strain confirmed this result. These results were then used in an elasto-plastic self-consistent model to simulate the slip and twin levels during deformation. The computational results were consistent with the notion that increasing the strain rate increased twin density, as shown in the post deformation microstructures. To understand the micromechanical effects hydride precipitates have on the alloy, a section of the alloy was charged with hydrogen in a vacuum furnace to 375 ppm ± 50 ppm. Microstructural characterisation of the material indicated that high levels of hydrides forming predominantly at grain boundaries. Nanoindentation tests were carried out at room temperature on individual hydride packets, the surrounding matrix and the as received material to characterise the me- chanical properties. The results obtained from these tests were used in computational modelling scenarios to determine more accurate mechanical properties. The nano-hardness of the matrix was found to be highest (4.64 GPa), followed by the matrix and the as received material (3.62 GPa and 2.74 GPa respectively). As part of the initial scope of this thesis it was the author’s original intention to understand how the presence of hydrides affects dislocation propagation and micro-deformation mechanisms. However, since carrying out the experimental procedures and results analysis, a number of papers have come to the author’s attention which outline the importance of the final processing steps prior to testing. It has been found that mechanical polishing as a method for material preparation induces work hardening into the surface of the material. Although this does not have an affect in macro and indeed micro scale hardness testing, where the tested layer is in the scale of a few microns, this work hardened layer does have a major effect in nano-hardness tests, where the testing layer is in the region of nanometers. As a result of this no dislocation analysis was carried out as it would be impossible to distinguish between dislocations present from mechanical polishing and those induced by the presence of hydrides. In spite of the work hardened layer rendering the absolute hardness values invalid, the relative values in relation to the matrix, hydride and as received material are still of interest. High cycle fatigue tests were carried out on samples taken from the rolling and the transverse direction of the material. Fractographic examination of the samples showed facets in the area immediately surrounding the initiation site. There were only found to be between 10-20 faceted grains, which were confined to this region. These features showed feather-like characteristics, indicative of plastic deformation. Site specific transmission electron microscopy (TEM) was carried out on the initiation facets, showing mostly <c+a> dislocations, although <a> and imperfect dislocations segments were also found to be present. The low dislocation density in these features compared to that of titanium suggests that these features may be quite brittle in nature. Crack propagation was found to occur via striated crack growth. The direction of the striations appear to be affected by grain orientation. TEM analysis of the underlying grain did not show the presence of any dislocations. It is thought that this may be a result of image stresses causing the dislocation to evaporate out the TEM specimen once it is removed from the fracture surface, although further work needs to be done to confirm this.
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Yap, Siaw Fung. "Micromechanics and powder compaction." Thesis, University of Birmingham, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.489036.
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Olsson, Erik. "Micromechanics of Powder Compaction." Doctoral thesis, KTH, Hållfasthetslära (Avd.), 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-159142.
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Compaction of powders followed by sintering is a convenient manufacturing method for products of complex shape and components of materials that are difficult to produce using conventional metallurgy. During the compaction and the handling of the unsintered compact, defects can develop which could remain in the final sintered product. Modeling is an option to predict these issues and in this thesis micromechanical modeling of the compaction and the final components is discussed. Such models provide a more physical description than a macroscopic model, and specifically, the Discrete Element Method (DEM) is utilized. An initial study of the efect of particle size distribution, performed with DEM, was presented in Paper A. The study showed that this effect is small and is thus neglected in the other DEM studies in this thesis. The study also showed that good agreement with experimental data can be obtained if friction effects is correctly accounted for. The most critical issue for accurate results in the DEM simulations is the modeling of normal contact between the powder particles. A unified treatment of this problem for particles of a strain hardening elastic-plastic material is presented in Paper B. Results concerning both the elastic-plastic loading, elastic unloading as well as the adhesive bonding between the particles is included. All results are compared with finite element simulation with good agreement with the proposed model. The modeling of industry relevant powders, namely spray dried granules is presented in Paper C. The mechanical behavior of the granules is determined using two types of micromechanical experiments, granule compression tests and nanoindentation testing. The determined material model is used in an FEM simulation of two granules in contact. The resulting force-displacement relationships are exported to a DEM analysis of the compaction of the granules which shows very good agreement with corresponding experimental data. The modeling of the tangential forces between two contacting powder particles is studied in Paper D by an extensive parametric study using the finite element method. The outcome are correlated using normalized parameters and the resulting equations provide the tangential contact force as function of the tangential displacement for different materials and friction coefficients. Finally, in Paper E, the unloading and fracture of powder compacts, made of the same granules as in Paper C, are studied both experimentally and numerically. A microscopy study showed that fracture of the powder granules might be of importance for the fracture and thus a granule fracture model is presented and implemented in the numerical model. The simulations show that incorporating the fracture of the granules is essential to obtain agreement with the experimental data.<br><p>QC 20150122</p>
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Borodulina, Svetlana. "Micromechanics of Fiber Networks." Doctoral thesis, KTH, Hållfasthetslära (Inst.), 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-188481.
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The current trends in papermaking involve, but are not limited to, maintaining the dry strength of paper material at a reduced cost. Since any small changes in the process affect several factors at once, it is difficult to relate the exact impact of these changes promptly. Hence, the detailed models of the network level of a dry sheet have to be studied extensively in order to attain the infinitesimal changes in the final product. In Paper A, we have investigated a relation between micromechanical processes and the stress–strain curve of a dry fiber network during tensile loading. The impact of “non-traditional” bonding parameters, such as compliance of bonding regions, work of separation and the actual number of effective bonds, is discussed. In Paper B, we studied the impact of the chemical composition of the fiber cell wall, as well as its geometrical properties, on the fiber mechanical properties using the three-dimensional model of a fiber with helical orientation of microfibrils at a range of different microfibril angles (MFA). In order to accurately characterize the fiber and bond properties inside the network, via statistical distributions, microtomography studies on the handsheets have been carried out. This work is divided into two parts: Paper C, which describes the methods of data acquisition and Paper D, where we discuss the extracted data. Here, all measurements were performed at a fiber level, providing data on the fiber width distribution, width-to-height ratio of isotropically oriented fibers and contact density. In the last paper, we utilize data thus obtained in conjunction with fiber morphology data from Papers C and D to update the network generation algorithm in order to produce more realistic fiber networks. We also successfully verified the models with the help of experimental results from dry sheets tested under uniaxial tensile tests. We carry out numerical simulations on these networks to ascertain the influence of fiber and bond parameters on the network strength properties.<br><p>QC 20160613</p>
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Lei, Sheng-Yuan. "Deformation micromechanics in composite structures." Thesis, University of Manchester, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488306.
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Langroudi, Arya Assadi. "Micromechanics of collapse in loess." Thesis, University of Birmingham, 2014. http://etheses.bham.ac.uk//id/eprint/5284/.
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Soil collapse is amongst one of the most significant ground related hazards. A collapsible soil, in particular loess, typically has an open-structure and collapse occurs when as a consequence of the addition of water and/or load the particles rearrange to form a more dense fabric. Collapse leads to a suite of problems for buildings and infrastructures built on or in collapsing soil. Treatment to mitigate collapse often involves in densification. However, such approaches have been reported not always effective enough to combat the problem. This stems from a lack of understanding of soils’ geochemistry and structure, the result of which is an oversimplification of complex geotechnical and geological interactions. An important example of such limited knowledge is the increasing evidence of restoration of the collapsing structure upon wetting-drying cycles, which is widely ignored in the current compaction practice. This research aims to first identifying collapse micro-mechanisms in fine-grained soils, examining the contribution of a handful of soil constituents in collapsibility, and finally developing a practical tool for ground engineers to evaluate the efficiency of the current compaction practice for systematically classified fine-grained soils, and to take modified/novel earthwork approaches where the current practice fails to fully remove the collapse risk.
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Ciomocos, M. T. "Micromechanics of agglomerate damage processes." Thesis, Aston University, 1996. http://publications.aston.ac.uk/14149/.
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This thesis reports a detailed investigation of the micromechanics of agglomerate behaviour under free-fall impact, double (punch) impact and diametrical compression tests using the simulation software TRUBAL. The software is based on the discrete element method (DEM) which incorporates the Newtonian equations of motion and contact mechanics theory to model the interparticle interactions. Four agglomerates have been used: three dense (differing in interface energy and contact density) and one loose. Although the simulated agglomerates are relatively coarse-grained, the results obtained are in good agreement with laboratory test results reported in the literature. The computer simulation results show that, in all three types of test, the loose agglomerate cannot fracture as it is unable to store sufficient elastic energy. Instead, it becomes flattened for low loading-rates and shattered or crushed at higher loading-rates. In impact tests, the dense agglomerates experience only local damage at low impact velocities. Semi-brittle fracture and fragmentation are produced over a range of higher impact velocities and at very high impact velocities shattering occurs. The dense agglomerates fracture in two or three large fragments in the diametrical compression tests. Local damage at the agglomerate-platen interface always occurs prior to fracture and consists of local bond breakage (microcrack formation) and local dislocations (compaction). The fracture process is dynamic and much more complex than that suggested by continuum fracture mechanics theory. Cracks are always initiated from the contact zones and propagate towards the agglomerate centre. Fracture occurs a short time after the start of unloading when a fracture crack "selection" process takes place. The detailed investigation of the agglomerate damage processes includes an examination of the evolution of the fracture surface. Detailed comparisons of the behaviour of the same agglomerate in all three types of test are presented. The particle size distribution curves of the debris are also examined, for both free-fall and double impact tests.
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Gong, Lei. "Deformation micromechanics of graphene nanocomposites." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/deformation-micromechanics-of-graphene-nanocomposites(b4e4780d-738f-4629-9dbb-151b1230bd52).html.
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Graphene nanocomposites have been successfully prepared in this study in the form of a sandwich structure of PMMA/graphene/SU-8. It has been proved that Raman spectroscopy is a powerful technique in the characterisation of the structure and deformation of graphene. The 2D band of the monolayer graphene has been used in the investigation of stress transfer in the graphene reinforced nanocomposites. It has been demonstrated that the 2D band moves towards low frequency linearly under tensile stress, which is shown to be significant method of monitoring the strain in graphene in a deformed specimen. The Raman spectroscopy behaviour under deformation validates that the monolayer graphene acts as a reinforcing role in nanocomposites although it is only one atom thick.A systematic investigation of the deformation of bilayer, trilayer and few-layer graphene has been undertaken with a view to determine the optimum number of layers for the reinforcement of nanocomposites. It has been demonstrated that monolayer graphene is not necessarily the optimum material to use for reinforcement in graphene-based polymer nanocomposites and bilayer graphene will be equally as good as monolayer graphene. There is therefore a balance to be struck in the design of graphene-based nanocomposites between the ability to achieve higher loadings of reinforcement and the reduction in effective Young’s modulus of the reinforcement, as the number of layers in the graphene is increased.Both the G and 2D bands have been found to undergo splitting under high strain levels or asymmetric band broadening in lower strain deformation. The G band polarisation property has been utilized to determine the crystallographic orientation of monolayer graphene by measuring the intensity ratio of G-/G+ bands. Analogously, the 2D band also undergoes strain-induced splitting where the 2D- band has higher Raman shift rate than that of the 2D+ band.
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Pantina, John Peter. "Interactions and micromechanics of colloidal aggregates /." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file 2.77 Mb., 193 p, 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:3221138.
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Russell, Benjamin Peter. "The micromechanics of composite lattice materials." Thesis, University of Cambridge, 2010. https://www.repository.cam.ac.uk/handle/1810/252176.
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Li, Hong. "Experimental micromechanics of composite buckling strength." Thesis, Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/11719.
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Eichhorn, Stephen J. "Deformation micromechanics of regenerated cellulosic fibres." Thesis, University of Manchester, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488200.
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Mohammadi-Aghdam, Mohammad. "Micromechanics of unidirectional metal matrix composites." Thesis, University of Bristol, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.297843.
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Sharma, Rajdeep. "Micromechanics of toughening in polymeric composites." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/38561.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.<br>Includes bibliographical references (leaves 195-206).<br>The macroscopic tensile toughness of glassy homo-polymers is often limited due to either localized crazing followed by failure [e.g., polymethyl-methacrylate (PMMA), polystyrene (PS)], or cavitation-induced brittle failure under high triaxiality [e.g., polycarbonate (PC)]. Judicious choice of polymer composites alleviates the above concerns by spreading the inelastic deformation and damage throughout the material, thereby increasing the macroscopic toughness. This thesis focuses on the micro- and macro-mechanics of two polymer composite systems - ductile/brittle PC/PMMA microlaminates and rubber-toughened PS. Ductile/brittle microlaminates are comprised of alternating layers of ductile polymer (e.g., PC) that inelastically deform by shear yielding, and brittle polymer (e.g., SAN, PMMA) that undergo crazing in tension. The layer thicknesses are typically in the sub-micron to tens of micron range. We have modeled the deformation and axial tensile toughness of PC/PMMA micro-laminates. Experiments indicate that the macroscopic ductility of ductile/brittle polymeric laminates depends on several factors such as the thickness of the brittle and ductile layers, the volume fraction of the ductile component, as well as strain rate.<br>(cont.) The nominally brittle layer can undergo in-elastic deformation by both crazing and shear-yielding, with the relative contribution of these mechanisms being dependent on the laminate morphology and strain rate. In particular, with decreasing brittle layer thickness, the inelastic behavior of the brittle layer is dominated by shear-yielding. We present a micromechanical model for two-phase ductile/brittle laminates that enables us to capture the macroscopic behavior, as well as the underlying micro-mechanisms of deformation and failure, in particular the synergy between crazing and shear yielding. The finite element implementation of our model considers a two-dimensional and three-dimensional representative volume element (RVE), and incorporates continuum-based physics-inspired descriptions of shear yielding and crazing, along with failure criteria for the ductile and brittle layers. The interface, between the ductile and brittle layers, is assumed to be perfectly bonded. The model is used to probe the effect of laminate parameters, such as the absolute and relative layer thicknesses, and material properties on the behavior during tensile loading. In addition, our modeling approach can be generalized to other laminate systems, such as two-phase brittle-1/brittle-2 and three-phase ductile-I/brittle/ductile-2 laminates, as well as to more complex loading conditions.<br>(cont.) Results from our studies reveal that the 2D RVE does not adequately capture the effect of volume fraction of the constituents on the laminate toughness. However, the 3D RVE captures the effect of volume fraction based on the extent of craze tunneling through the width of the specimen; at high volume fractions of PC, crazes emanating from the surface do not tunnel through the specimen width significantly, while at low volume fractions of PC, the crazes tunnel through the entire specimen width. The 3D RVE captures the strain-rate effect on toughness based on the greater rate-sensitivity of shear yielding compared to craze initiation, thereby increasing the craze density in the laminate at higher rates. The length-scale effect is captured by the 3D RVE, based on decrease in the craze opening rate and damage confinement by the PC layers. It is well-known that the incorporation of a small volume fraction (10-25 %) of micron-order size, compliant and well-dispersed rubbery particles in (brittle and crazeable) polystyrene (PS) yields considerable dividends in tensile toughness at the expense of reduction in stiffness and yield strength. In commercial rubber-toughened PS, the rubbery particles often have a composite "salami" morphology, consisting of 70-80 % volume fraction of sub-micron PS occlusions dispersed in a topologically, continuous polybutadiene (PB) phase.<br>(cont.) While it is recognized that these composite particles play the dual role of providing multiple sites for craze initiation in the PS matrix and allow the stabilization of the crazing process through cavitation/fibrillation in the PB-phase within the particle, the precise role of particle morphology, as well as the particle-matrix interface are not well understood or quantified. This work probes the micromechanics and macromechanics of uni-axial tensile deformation and failure in rubber-toughened PS through axi-symmetric finite element representative volume element (RVE) models that can guide the development of blends of optimal toughness. The RVE models reveal the effect on craze morphology and toughness by various factors such as particle compliance, particle morphology, particle fibrillation and particle volume fraction. The principal result of our study is that particle compliance and particle heterogeneity alone cannot account for the macroscopic behavior of HIPS, as well as the experimentally observed craze profile. Fibrillation/cavitation of PB domains within the heterogeneous particle provides the basic key ingredient to account for the micro- and macro-mechanics of HIPS.<br>by Rajdeep Sharma.<br>Ph.D.
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Capolungo, Laurent. "Modeling of the size effect in the plastic behavior of polycrystalline materials." Diss., Available online, Georgia Institute of Technology, 2007, 2007. http://etd.gatech.edu/theses/available/etd-06112007-102904/.
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Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2008.<br>Garmestani, Hamid, Committee Member ; Johnson, Steven, Committee Chair ; McDowell, David, Committee Member ; Qu, Jianmin, Committee Co-Chair ; Cherkaoui, Mohammed, Committee Co-Chair.
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Sertse, Hamsasew M. "Micromechanics Based Failure Analysis of Heterogeneous Materials." Thesis, Purdue University, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10689831.
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<p> In recent decades, heterogeneous materials are extensively used in various industries such as aerospace, defense, automotive and others due to their desirable specific properties and excellent capability of accumulating damage. Despite their wide use, there are numerous challenges associated with the application of these materials. One of the main challenges is lack of accurate tools to predict the initiation, progression and final failure of these materials under various thermomechanical loading conditions. Although failure is usually treated at the macro and meso-scale level, the initiation and growth of failure is a complex phenomena across multiple scales. </p><p> The objective of this work is to enable the mechanics of structure genome (MSG) and its companion code SwiftComp to analyze the initial failure (also called static failure), progressive failure, and fatigue failure of heterogeneous materials using micromechanics approach. The initial failure is evaluated at each numerical integration point using pointwise and nonlocal approach for each constituent of the heterogeneous materials. The effects of imperfect interfaces among constituents of heterogeneous materials are also investigated using a linear traction-displacement model. Moreover, the progressive and fatigue damage analyses are conducted using continuum damage mechanics (CDM) approach. The various failure criteria are also applied at a material point to analyze progressive damage in each constituent. The constitutive equation of a damaged material is formulated based on a consistent irreversible thermodynamics approach. The overall tangent modulus of uncoupled elastoplastic damage for negligible back stress effect is derived. The initiation of plasticity and damage in each constituent is evaluated at each numerical integration point using a nonlocal approach. The accumulated plastic strain and anisotropic damage evolution variables are iteratively solved using an incremental algorithm. The damage analyses are performed for both brittle failure/high cycle fatigue (HCF) for negligible plastic strain and ductile failure/low cycle fatigue (LCF) for large plastic strain. </p><p> The proposed approach is incorporated in SwiftComp and used to predict the initial failure envelope, stress-strain curve for various loading conditions, and fatigue life of heterogeneous materials. The combined effects of strain hardening and progressive fatigue damage on the effective properties of heterogeneous materials are also studied. The capability of the current approach is validated using several representative examples of heterogeneous materials including binary composites, continuous fiber-reinforced composites, particle-reinforced composites, discontinuous fiber-reinforced composites, and woven composites. The predictions of MSG are also compared with the predictions obtained using various micromechanics approaches such as Generalized Methods of Cells (GMC), Mori-Tanaka (MT), and Double Inclusions (DI) and Representative Volume Element (RVE) Analysis (called as 3-dimensional finite element analysis (3D FEA) in this document). </p><p> This study demonstrates that a micromechanics based failure analysis has a great potential to rigorously and more accurately analyze initiation and progression of damage in heterogeneous materials. However, this approach requires material properties specific to damage analysis, which are needed to be independently calibrated for each constituent.</p><p>
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Al-Geroushi, Rajab A. "Probabilistic micromechanics of clay compression and consolidation." Thesis, McGill University, 1988. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=75856.
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In the light of new studies of physico-chemical analysis, fabric, and particle interaction, the consideration of the actual constitution of the soil media is required for actual prediction of the soil media performance under loads. This study has been restricted to the prediction of clay performance under compression loading.<br>A micromechanics approach has been used in this research; a microelement scale has been introduced and a set of formulations (i.e., stress transfer, fluid flow equation, and volumetric constitutive equation) has been developed for this scale. The passage from the microelement to the global scale has been established by using evolution equations. These equations have been solved by using numerical techniques (finite difference and finite element). The prediction of the developed models is discussed.
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Uguz, Agah. "Micromechanics of crack extension in aluminium alloys." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.276988.
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Cheung, Lok Yee Geraldine. "Micromechanics of sand production in oil wells." Thesis, Imperial College London, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.528300.
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O'Donovan, John. "Micromechanics of wave propagation through granular material." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/23921.
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The stiffness of soil is an important parameter that has implications on soil-structure interaction, on the response to earthquake motion and on the response of soils to dynamic loadings. Stiffness reduces and behaves plastically at medium to high strains; however, at small-strain the stiffness has been observed to be a constant value and elastic. Small-strain stiffness governs the soil-structure interaction during construction projects and site response during dynamic loading due to earthquakes and man-made operations. Quantifying stiffness, in particular shear stiffness, at small-strain is difficult due to the effect of sample fabric on the values measured and the resolution of the testing equipment that is available. Wave propagation has been used to measure the stiffness of samples by propagating waves in different directions and in different planes. This thesis aims to examine the propagation of stress waves through a granular medium. Samples were created using the numerical discrete element method (DEM) in two- and three-dimensions. Waves, created by a point source, were propagated through the samples and this propagation was measured using micromechanical data. The speed of the propagating wave was assessed using existing techniques and novel methods developed during the research. The effect of macro-scale parameters, such as sample boundary conditions, and the effect of micro-scale parameters, such as interparticle contact laws, on sample stiffness were examined. Randomly packed samples were created with a quantifiable fabric tensor, measured using the contact force network. Wave propagation in different directions was examined to quantify the effect of inherent anisotropy on the sample stiffness. Samples were confined at anisotropic confining pressures to isolate the effect of induced anisotropy on the sample stiffness. Wave propagation results were compared with the results of small amplitude stress probes for a number of simulations and with experimental work carried out in the University of Bristol.
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Tang, Tian. "Variational Asymptotic Micromechanics Modeling of Composite Materials." DigitalCommons@USU, 2008. https://digitalcommons.usu.edu/etd/72.
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The issue of accurately determining the effective properties of composite materials has received the attention of numerous researchers in the last few decades and continues to be in the forefront of material research. Micromechanics models have been proven to be very useful tools for design and analysis of composite materials. In the present work, a versatile micromechanics modeling framework, namely, the Variational Asymptotic Method for Unit Cell Homogenization (VAMUCH), has been invented and various micromechancis models have been constructed in light of this novel framework. Considering the periodicity as a small parameter, we can formulate the variational statements of the unit cell through an asymptotic expansion of the energy functional. It is shown that the governing differential equations and periodic boundary conditions of mathematical homogenization theories (MHT) can be reproduced from this variational statement. Finally, we employed the finite element method to solve the numerical solution of the constrained minimization problem. If the local fields within the unit cell are of interest, the proposed models can also accurately recover those fields based on the global behavior. In comparison to other existing models, the advantages of VAMUCH are: (1) it invokes only two essential assumptions within the concept of micromechanics for heterogeneous material with identifiable unit cells; (2) it has an inherent variational nature and its numerical implementation is shown to be straightforward; (3) it calculates the different material properties in different directions simultaneously, which is more efficient than those approaches requiring multiple runs under different loading conditions; and (4) it calculates the effective properties and the local fields directly with the same accuracy as the fluctuation functions. No postprocessing calculations such as stress averaging and strain averaging are needed.
The present theory is implemented in the computer program VAMUCH, a versatile engineering code for the homogenization of heterogeneous materials. This new micromechanics modeling approach has been successfully applied to predict the effective properties of composite materials including elastic properties, coefficients of thermal expansion, and specific heat and the effective properties of piezoelectric and electro-magneto-elastic composites. This approach has also been extended to the prediction of the nonlinear response of multiphase composites. Numerous examples have been utilized to clearly demonstrate its application and accuracy as a general-purpose micromechanical analysis tool.
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Olsson, Erik. "Micromechanics of Powder Compaction and Particle Contact." Licentiate thesis, KTH, Hållfasthetslära (Avd.), 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-117608.
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Rashidian, Bizhan. "Micromachined devices based on PVDF-TrFE." Diss., Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/13299.
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Frazier, Albert Bruno. "The use of polyimide for the development of micromachined materials, processes and devices." Diss., Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/15696.
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Milne, Neil Graeme. "Finite element analysis of electrostatic axial-drive and wobble micromotors." Thesis, Heriot-Watt University, 1994. http://hdl.handle.net/10399/1363.
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Osman, Mohammad Shahril. "Measurement of granular contact forces using frequency scanning interferometry." Thesis, Loughborough University, 2002. https://dspace.lboro.ac.uk/2134/7025.
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The propagation of stress within a granular material has been studied for many years, but only recently have models and theories focused on the micromechanical (single grain) level. Experiments at this level are still rather limited in number. For this reason, a system using optical techniques has been developed. The substrate on which the granular bed is assembled is a double layer elastic substrate with high modulus epoxy constituting the top layer and silicone rubber as the bottom layer. In between the two layers, gold is coated which acts as a reflective film. To design the substrate, a Finite Element Analysis package called LUSAS was used. By performing a non-linear contact analysis, the design of the substrate was optimised so as to give a linear response, high stiffness, deflection in the measurable range, and negligible cross-talk between neighbouring grains. Fabrication and inspection techniques were developed to enable samples to be manufactured to this design. The deformation of the gold interface layer is measured using interferometry. The interferometer utilised a frequency tunable laser which acts both as the light source and the phase shifting device. The optical arrangement is based on the Fizeau set-up. This has removed several problems such as multiple reflections and sensitivity to vibration that occurred when using a Mach-Zehnder configuration. A fifteen-frame phase shifting algorithm, was developed based on a Hanning window, which allows the phase difference map to be obtained. This is then unwrapped in order to obtain the indentation profile. The deflection profile is then converted to a single indentation depth value by fitting a Lorentzian curve to the measured data. Calibration of the substrate is carried out by loading at 9 different locations simultaneously. Spatial and temporal variations of the calibration constants are found to be of order 10-15%. Results are presents showing contact force distributions under both piles of sand and under face-centred cubic arrangements of stainless steel balls. Reasonable agreement was obtained in the latter case with both the expected mean force and the probability density function predicted by the so-called 'q' model. The experimental techniques are able to measure small displacements down to a few nanometers. To the best of my knowledge these experiments are the first to employ the interferometer method in attempting to measure the contact force distribution at the base of a granular bed.
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Stapleton, A. M. "Micromechanics and fatigue initiation in Ti-6Al-4V." Thesis, Imperial College London, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.501489.
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Gu, Xiaohong. "Micromechanics of model carbon-fibre/epoxy-resin composites." Thesis, University of Manchester, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488261.
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Hejda, Marek. "Deformation micromechanics of single glass fibre reinforced composites." Thesis, University of Manchester, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491333.
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The current work presents for the first time the application of luminescence spectroscopy in following the micromechanical deformation of glass fibre reinforced composites; this has been achieved using luminescence-active glass fibres prepared from glass doped with small amounts of Sm3+. Glass prepared in this way exhibited several relatively sharp and intense luminescence peaks observed in the range 550 nm to 700 nm. The luminescence band located at 648 nm was used for the calibration of the local strain state of the fibre due to its distinctive linear shift towards lower wavelengths with increasing strain and the factors affecting this shift were studied in detail. The fragmentation of both untreated and silane-treated Sm3+ doped glass fibre has been followed in detail and the behaviour analysed using a classical shear-lag analysis. Silane treatment slightly enhanced adhesion between glass fibre and epoxy resin, which was confirmed by a supplementary fragmentation study, which employed carbon nanotubes dispersed in the silane agent as an additional strain sensor. This work has demonstrated luminescence spectroscopy as a new significant development in the ability to follow local mechanics of the interface between glass fibres and transparent resins.
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Carman, Greg P. "Micromechanics of finite length fibers in composite materials." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/39869.
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Carapella, Elissa E. "Micromechanics of crenulated fibers in carbon/carbon composites." Thesis, This resource online, 1992. http://scholar.lib.vt.edu/theses/available/etd-09192009-040251/.
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Case, Scott Wayne. "Micromechanics of strength-related phenomena in composite materials." Thesis, This resource online, 1993. http://scholar.lib.vt.edu/theses/available/etd-09122009-040447/.
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Guck, Jochen Reinhold. "Optical deformability micromechanics from cell research to biomedicine /." Access restricted to users with UT Austin EID Full text (PDF) from UMI/Dissertation Abstracts International, 2001. http://wwwlib.umi.com/cr/utexas/fullcit?p3037014.
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Smith, Carmen Alexis. "Micromechanics of the through-thickness deformation of paperboard." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/9426.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999.<br>Includes bibliographical references (p. 171-172).<br>An experimental investigation of the behavior of paperboard has been performed, focusing on the identification of the mechanisms of through-thickness deformation. Experiments have been conducted at the microscopic and macroscopic levels, the difference between the two being the length scale. Experiments at the microscopic level were performed in a scanning electron microscope, allowing concurrent viewing of the deformation as it took place with acquisition of load vs. displacement data. The experiments at the macroscopic level confirm the mechanisms observed at the microscopic level and provide more accurate, continuum-level stress-strain data. The motivation for the investigation is the modeling of the creasing process, in which a sheet of paperboard is punched and folded along a narrow line to create a corner for packaging. Creasing experiments indicate that out-of-plane shear damage during punching and normal delamination during folding govern the quality of the resulting crease. Experiments in out-of-plane tension, compression, and simple shear have been performed to investigate the behavior of paperboard under these simple loading conditions. The results show that normal and tangential delamination at the interfaces between layers is of extreme importance in the behavior of paperboard in tension and shear. Damage in the form of micro cracks occurs almost from the onset of strain and culminates in large-scale delamination coincident with a large decrease in the strength of the material. In compression, the behavior is mostly elastic and is governed by densification of the material. The brief initial stages of deformation involve plastic elimination of voids. This is followed by non-linear elastic stiffening of the material via densification.<br>by Carmen Alexis Smith.<br>S.M.
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Le, Menn Flora-Maud. "Micromechanics of cavitation in confined soft polymer layers." Electronic Thesis or Diss., Université Paris sciences et lettres, 2022. http://www.theses.fr/2022UPSLS044.
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Chaque seconde, 29 To de données sont échangées sur Internet et plus de 90% d'entre elles transitent par fibre optique. Les fibres de verre sont protégées par plusieurs couches de résines polymères de propriétés mécaniques différentes, la vitesse de production des câbles conduit à une accumulation de contraintes internes, pouvant mener à un phénomène de cavitation dans la première couche de protection de la fibre de verre. Afin d'étudier ce phénomère, nous avons synthétisé des réseaux élastomères à base d'oligomères fonctionnalisés di-(méth)acrylates (PPG8000) et de monomères acrylates (EHA et PEA) en polymérisation radicalaire par illumination UV flash (1s) de haute intensité. Par leur formulation et leur procédé de polymérisation, nos réseaux sont des systèmes modèles représentatifs des revêtements industriels. Après avoir étudié l'influence du processus de polymérisation sur la structure du réseau, nous avons caractérisé les propriétés mécaniques du matériau (élasticité, énergie de fracture, raidissement) au travers de tests de traction uniaxiale et de gonflement de membrane.Enfin, nous proposons un montage innovant et original pour tester des couches minces confinées de une à plusieurs centaines de micromètres d'épaisseur sous traction principalement hydrostatique dans une géométrie sphère/plan. Le montage, conçu en interne, permet d'observer en temps réel la nucléation et la croissance de cavités pendant le test. Une analyse par éléments finis a permis de relier la force macroscopique mesurée à la pression hydrostatique locale où le premier événement de cavitation se produit. Toutes les formulations et les épaisseurs testées ont montré une résistance à la cavitation bien supérieure à la limite théorique de 5E/6, la diminution de l'épaisseur conduit à une dispersion accrue vers des pressions élevées pour le premier événement de cavitation. La différence de confinement produit un effet reproductible sur le comportement de la cavitation. Les échantillons épais (500 µm) présentent un événement de cavitation critique qui transite vers une fracture de mode I sur l'ensemble de l'échantillon, la diminution de l'épaisseur conduit à des événements de cavitation multiples qui semblent être stabilisés sur une plus grande gamme de pressions appliquées. L'impact de la formulation du réseau sur la transition de ces cavités multiples en fracture de mode I est analysée. Nous suggérons un effet de l'énergie de fracture et du raidissement des différents matériaux sur la morphologie des cavités pendant leur croissance<br>Every second, 29 Tb of data are exchanged on the internet, and more than 90% of it travels through fiber-optic cables. The glass fibres are protected by several layers of polymer resins of different mechanical properties. The rapidity of the coating process leads to a build up of internal stresses that can eventually provoke cavitation in the first coating, decreasing the fibre protection. To shed some light on this cavitation behaviour, we investigated elastomer networks based on di-(meth)acrylate functionalized oligomers (PPG8000) and acrylate monomers (EHA and PEA) polymerised in a flash (less than 1 second at high UV intensities). Both the formulation and the polymerisation protocol are comparable to the industrial coatings, making it a representative model system. After an investigation of the influence of the polymerisation process on the network structure, we characterized the material properties (elasticity, strain hardening, toughness) in uniaxial traction test and bulge inflation.Finally, we designed and built an original set-up to test thin confined layers of one to several hundreds of micrometers under mainly hydrostatic traction in a sphere against flat geometry. With this set-up we could observe the nucleation and growth of the cavities as a function of load in real time. From finite element simulations we also determined the local hydrostatic pressure at the location of the first cavitation event. For all formulations and tested thicknesses, the cavitation resistance was well above the theoretical limit of 5E/6. However, reduced thicknesses also led to an increased dispersion to high values of measured pressures for the first cavitation event. The difference in confinement showed a reproducible effect on the cavitation behaviour. Thick samples (500 µm) presented one cavitation event that transitioned to a mode I fracture over the whole sample: decreasing the confined thickness led to multiple cavitation events that appeared to be stabilized over a longer range of applied load. The transition of these multiple cavitation events into mode I fracture processes was observed and analysed. We saw marked differences between the different model formulations and suggest that both toughness and a high level of strain hardening of the elastomer have an effect on the cavities morphology during their growth
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Brzesowsky, Rolf. "Micromechanics of sand grain failure and sand compaction = Micromechanica van het breukgedrag van zandkorrels en van het compactie gedrag van zand /." [S.l. : s.n.], 1995. http://www.gbv.de/dms/goettingen/19094918X.pdf.
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Fu, Yuhong. "Rapid solution of large-scale three-dimensional micromechanics problems /." Digital version accessible at:, 1998. http://wwwlib.umi.com/cr/utexas/main.
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Xie, Mayue. "Interface problems in micromechanics and effective composite property analysis." Related electronic resource: Current Research at SU : database of SU dissertations, recent titles available full text, 2005. http://wwwlib.umi.com/cr/syr/main.
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Sachtleber, Michael. "Surface roughening and micromechanics of plastically strained aluminum sheets /." Aachen : Shaker, 2005. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=014833476&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.
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Acharya, Sunil. "Micromechanics of asperity interaction in wear a numerical approach /." Connect to OhioLINK ETD Center, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=akron1135267571.
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Dissertation (Ph. D.)--University of Akron, Dept. of Polymer Engineering, 2005.<br>"December, 2005." Title from electronic dissertation title page (viewed 09/17/2006) Advisor, Arkady I. Leonov; Co-Advisor, Joseph P. Padovan; Committee members, Joseph P. Padovan, Gary R. Hamed, Erol Sancaktar, Rudolph J. Scavuzzo, Jr.; Department Chair, Sadhan C. Jana; Dean of the College, Frank N. Kelley; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.
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Raghunathan, Seema L. "Texture and micromechanics of Forged Ti-10V-2Fe-3AI." Thesis, Imperial College London, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485552.
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Ti-lOV-2Fe-3AI is a near-p titanium alloy· primarily used in the landing gear of aircraft. Such components are normally produced by isothermal forging but the sensitivity of the microstructure to temperature makes forging optimisation difficult and therefore, an empirical approach is usually applied. To reduce the resultant high processing costs, it is essential for the industry to accurately l:ffiderstand and predict microstructural evolution and properties with respect to the isothennal forging variables. This thesis considers the evolution of macrotexture and microtexture in Ti-l0V..2Fe-3AI during and after forging. The aim of this research is to improve our understanding of the micromechanics ofthis alloy. t· The effect of aging on the lattice strain behaviour of Ti-l0V-2Fe-3Al was investigated. In-situ synchrotron diffraction was performed while the as-forged condition and the forged and aged were undergoing tensile loading at room temperature. The behaviour ofboth conditions was modelled using a two-phase elastic-plastic self consistent (EPSC) scheme in an attempt to rationalise the material behaviour. It was found that the constrained p phase stiffuess, Ezoo, increased from 45GPa in the asforged condition to 88GPa in the forged and aged material. The model results showed that ell -CIZ increased from 12GPa in the forged condition to 47GPa in the forged and aged material. This can be attributed to an increase in p-stabiliser content and is in agreement with atomistic predictions. The EPSC models, while reasonably successful at reproducing the observed behaviour, do not provide a complete description of the micromechanics of these materials due to the complexity of the microstructures. The macrotexture and microtex-ture were investigated during a.+p forging of Ti-l0V-2Fe-3Al and were characterised using a combination of neutron, in-situ synchrotron X-ray and ex-situ electron backscatter -diffraction (EBSD). The EBSD analysis showed that the measured misorientation distributions at a strain of 0.8 and strain rates of 0.ls-1 and O.Ols-· were similar, with an average misorientation of 2.2°. It was. found that during forging, the moderate cube macrotexture inherited from the parent material evolved into a moderate fibre texture, with the major change occurring between strains of 0.4 and 0.6. The synchrotron diffraction studies allowed the orientation evolution of individual grains to be characterised. At the highest strain rate of 0.15-·, this indicated a change in behaviour from dispersion of the crystal mosaic (peak angular spread) at low strains, to convergence of the crystal mosaic at larger strains. At lower strain rates, only convergence of the crystal mosaic was observed. It is suggested that these results indicate a change in mechanism between deformationcontrolled behaviour during the early stages of deformation and a strain rate of 0.1 s-· and diffusional, recovery-controlled behaviour lower strain rates and higher strains.
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Goutianos, Stergios. "Micromechanics of compressive failure in carbon-fibre polymer composites." Thesis, Queen Mary, University of London, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.411718.
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Yang, Jiecheng. "DEM-CFD analysis of micromechanics for dry powder inhalers." Thesis, University of Birmingham, 2015. http://etheses.bham.ac.uk//id/eprint/6019/.
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Dry powder inhalers (DPIs) are widely used for the therapy of respiratory and pulmonary diseases. In this study, a coupled discrete element method and computational fluid dynamics (DEM-CFD) is employed to investigate the micromechanics of carrier-based DPIs. The effects of van der Waals forces and electrostatic forces on the mixing process, and the influences of air flow and particle-wall impact on the dispersion process are examined. For the mixing of carrier and active pharmaceutical ingredient (API) particles in a vibrating container, it is found that vibration conditions affect the mixing performance. While there is an optimal mixing condition to maximise the number of API particles attaching to the carrier (i.e. contact number) for van der Waals cases, the contact number decreases with increasing vibration velocity amplitude and frequency for electrostatic force cases. It is also revealed that van der Waals forces (short range) and electrostatic forces (long range) result in different mixing behaviours. For the air flow induced and impact induced dispersion, it is found that the dispersion performance improves with increasing air velocity, impact velocity and impact angle, and reduces with increasing work of adhesion. The dispersion performance can be approximated using the cumulative Weibull distribution function governed by the ratio of air drag force to adhesive force or the ratio of impact energy to adhesion energy.
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Chen, Ting-Ting S. M. Massachusetts Institute of Technology. "Microstructure and micromechanics of the sea urchin, Colobocentrotus atratus." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/67360.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>"June 2011." Cataloged from student-submitted PDF version of thesis.<br>Includes bibliographical references (p. 128-132).<br>The purpose of this research is to study the porous microstructure and the micromechanics of the exoskeletal armor of the helmet urchin, Colobocentrotus atratus. Its unusual reduction in spines forms a smooth tiling of millimeter-sized, flattened polygonal protective aboral spines. Each aboral spine articulates with the underlying test via a ball-and-socket joint and the microstructure of each aboral spine is a porous network of single-crystal magnesium-doped calcite with a few percent of intercalated organic. Methods of microstructural characterization and simulation were developed through the investigation of the urchin's microstructure and the finite element model. With the high resolution scans from X-ray microcomputed tomography at the Advanced Photon Source at Argonne National Laboratory, Beamline 2BM, visualization and characterization of a complex porous network in three-dimensions was possible, providing a more quantitative insight into the geometry of the microstructure not characterized before in urchin biology literature. The galleried stereom of the individual spines were found to possess a gradient in volume fraction with distance from the socket ranging from 90% at the ball-and-socket joint to 50% at the outer surface. The axial direction of the galleried structure radiates outwardly from the socket and terminated perpendicular to the outer surface of the aboral spine. Additionally, with the microcomputed tomography results, an efficient and more accurate finite element model of an entire aboral spine along with the microstructural properties was created. The galleried mesh (average pore size ~ 15 microns) was modeled using three-dimensional elastic finite element analysis that consisted of a microstructurally-based parametric representative volume element with periodic boundary conditions. Various loading configurations were simulated to obtain anisotropic stiffness tensors and resulted in an orthotropic effective mechanical behavior with the stiffness in the plane transverse to the long axis of the galleried microstructure (E1, E2) approximately half the stiffness in the axial direction (E3). With parametric simulations, E3 was found to decrease linearly from 0.87 of the solid elastic modulus (Es) to 0.34 of Es as the volume fraction decreases from 0.88 to 0.46. In the transverse direction, E1 and E2 also decrease linearly from 0.49 of Es to 0.18 of Es within the same range of volume fraction. Spatial gradients in density were also modeled, corresponding to the gradient in porosity in the aboral spine. From simulation of blunt indentation by both a conical indenter and flat plate indenter, the graded porosity of the microstructure exhibits an expected lower overall stiffness and lower stress state than the solid material, but also serves to increase the strains near the exterior surface of the aboral spine while reducing the strains near the joint. This open-pore structure and trabeculae alignment results in a directional strengthening due to inhomogeneous deformations in the porous structure and provides resistance against blunt impacts and containment of penetration into the surface of the aboral spine.<br>by Ting-Ting Chen.<br>S.M.
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Venkovic, Nicolas. "Nanoindentation relaxation study and micromechanics of Cement-Based Materials." Master's thesis, Université Laval, 2016. http://hdl.handle.net/20.500.11794/27066.
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Ce travail évalue le comportement mécanique des matériaux cimentaires à différentes échelles de distance. Premièrement, les propriétés mécaniques du béton produit avec un bioplastifiant à base de microorganismes efficaces (EM) sont etudiées par nanoindentation statistique, et comparées aux propriétés mécaniques du béton produit avec un superplastifiant ordinaire (SP). Il est trouvé que l’ajout de bioplastifiant à base de produit EM améliore la résistance des C–S–H en augmentant la cohésion et la friction des nanograins solides. L’analyse statistique des résultats d’indentation suggère que le bioplastifiant à base de produit EM inhibe la précipitation des C–S–H avec une plus grande fraction volumique solide. Deuxièmement, un modèle multi-échelles à base micromécanique est dérivé pour le comportement poroélastique de la pâte de ciment au jeune age. L’approche proposée permet d’obtenir les propriétés poroélastiques requises pour la modélisation du comportoment mécanique partiellement saturé des pâtes de ciment viellissantes. Il est montré que ce modèle prédit le seuil de percolation et le module de Young non drainé de façon conforme aux données expérimentales. Un metamodèle stochastique est construit sur la base du chaos polynomial pour propager l’incertitude des paramètres du modèle à travers plusieurs échelles de distance. Une analyse de sensibilité est conduite par post-traitement du metamodèle pour des pâtes de ciment avec ratios d’eau sur ciment entre 0.35 et 0.70. Il est trouvé que l’incertitude sous-jacente des propriétés poroélastiques équivalentes est principalement due à l’énergie d’activation des aluminates de calcium au jeune age et, plus tard, au module élastique des silicates de calcium hydratés de basse densité.<br>This work assesses the mechanical behavior of cement-based materials through different length scales. First, the mechanical properties of concrete produced with effective microorganisms (EM)-based bioplasticizer are investigated by means of statistical nanoindentation, and compared to the nanomechanical properties of concrete produced with ordinary superplasticizer (SP). It is found that the addition of EM-based bioplasticizer improves the strength of C–S–H by enhancing the cohesion and friction of solid nanograins. The statistical analysis of indentation results also suggests that EM-based bioplasticizer inhibits the precipitation of C–S–H of higher density. Second, a multiscale micromechanics-based model is derived for the poroelastic behavior of cement paste at early age. The proposed approach provides poroelastic properties required to model the behavior of partially saturated aging cement pastes. It is shown that the model predicts the percolation threshold and undrained elastic modulus in good agreement with experimental data. A stochastic metamodel is constructed using polynomial chaos expansions to propagate the uncertainty of the model parameters through different length scales. A sensitivity analysis is conducted by post-treatment of the meta-model for water-to-cement ratios between 0.35 and 0.70. It is found that the underlying uncertainty of the effective poroelastic proporties is mostly due to the apparent activation energy of calcium aluminate at early age and, later on, to the elastic modulus of low density calcium-silicate-hydrate.
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Bandorawalla, Tozer Jamshed. "Micromechanics-Based Strength and Lifetime Prediction of Polymer Composites." Diss., Virginia Tech, 2002. http://hdl.handle.net/10919/26445.
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With the increasing use of composite materials for diverse applications ranging from civil infrastructure to offshore oil exploration, the durability of these materials is an important issue. Practical and accurate models for lifetime will enable engineers to push the boundaries of design and make the most efficient use of composite materials, while at the same time maintaining the utmost standards of safety. The work described in this dissertation is an effort to predict the strength and rupture lifetime of a unidirectional carbon fiber/polymer matrix composite using micromechanical techniques. Sources of material variability are incorporated into these models to predict probabilistic distributions for strength and lifetime. This approach is best suited to calculate material reliability for a desired lifetime under a given set of external conditions.
A systematic procedure, with experimental verification at each important step, is followed to develop the predictive models in this dissertation. The work begins with an experimental and theoretical understanding of micromechanical stress redistribution due to fiber fractures in unidirectional composite materials. In-situ measurements of fiber stress redistribution are made in macromodel composites where the fibers are large enough that strain gages can be mounted directly onto the fibers. The measurements are used to justify and develop a new form of load sharing where the load of the broken fiber is redistributed only onto the nearest adjacent neighbors. The experimentally verified quasi-static load sharing is incorporated into a Monte Carlo simulation for tensile strength modeling. Very good agreement is shown between the predicted and experimental strength distribution of a unidirectional composite.
For the stress-rupture models a time and temperature dependent load-sharing analysis is developed to compute stresses due an arbitrary sequence of fiber fractures. The load sharing is incorporated into a simulation for stress rupture lifetime. The model can be used to help understand and predict the role of temperature in accelerated measurement of stress-rupture lifetimes. It is suggested that damage in the gripped section of purely unidirectional specimens often leads to inaccurate measurements of rupture lifetime. Hence, rupture lifetimes are measured for [90/0_3]_s carbon fiber/polymer matrix specimens where surface 90 deg plies protect the 0 deg plies from damage. Encouraging comparisons are made between the experimental and predicted lifetimes of the [90/0_3]_s laminate. Finally, it is shown that the strength-life equal rank assumption is erroneous because of fundamental differences between quasi-static and stress-rupture failure behaviors in unidirectional polymer composites.<br>Ph. D.
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Pullawan, Tanittha. "Interfacial micromechanics of all-cellulose nanocomposites using Raman spectroscopy." Thesis, University of Manchester, 2012. https://www.research.manchester.ac.uk/portal/en/theses/interfacial-micromechanics-of-allcellulose-nanocomposites-using-raman-spectroscopy(e1d41897-368a-4b80-8683-7ee554b5731f).html.
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All-cellulose nanocomposites, in which both the matrix and the reinforcing fillers are cellulose, were successfully prepared using different solvents (LiCl/DMAc and NaOH/urea) and different sources of cellulose whiskers (cotton and tunicate whiskers) as control parameters. The volume fraction of cellulose whiskers was varied from 5 to 20%. The mechanical properties and crystallinity of the nanocomposites were characterised using tensile testing and X-ray diffraction. The tensile properties and crystallinity of nanocomposite samples are associated with the stiffness and crystallinity of the cellulose whiskers. Using Raman spectroscopic technique, the stiffnesses of single cotton and tunicate whiskers are found to be approximately 56-105 and 118-221 GPa respectively, assuming two dimensional (2D) and three dimensional (3D) arrangements. A comparison between two different solvent systems showed that the stress transfer of nanocomposites prepared using NaOH/urea is lower than samples prepared using LiCl/DMAc due to the thermal sensitivity of NaOH/urea solvent system. In this study, the N,N-dimethyl acetamide (DMAc) containing LiCl was used as the main solvent system due to the dissolution process is well controllable. Raman band located at ~1095 cm-1 and ~895 cm-1 related to the reinforcing phase and matrix material have been monitored during deformation. It is shown that Raman spectroscopy can be used to discriminate the matrix-fibre interactions in all-cellulose nanocomposites. By wetting the nanocomposite samples, it was shown that three interactions can be “switched off”; namely whisker-matrix interfaces, interactions between whiskers and interactions within the cellulose matrix. Disabling these interactions leads to the lack of stress-transfer within the composite. By drawing in wet conditions and subsequent drying, preferred orientation was introduced into the all-cellulose nanocomposite films. The effect of a 1.2 T magnetic field on the orientation of cellulose whiskers was also investigated. The presence of a magnetic field during the curing process of the nanocomposites has been shown to induce partial orientation of the whiskers.
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Zhang, Pan. "A virtual grain structure representation system for micromechanics simulations." Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/9656.
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Representing a grain structure within a combined finite element computer aided engineering environment is essential for micromechanics simulations. Methods are required to effectively generate high-fidelity virtual grain structures for accurate studies. A high-fidelity virtual grain structure means a statistically equivalent structure in conjunction with desired grain size distribution features, and must be represented with realistic grain morphology. A family of controlled Poisson Voronoi tessellation (CPVT) models have been developed in this work for systematically generating virtual grain structures with the aforementioned properties. Three tasks have been accomplished in the development of the CPVT models: (i) defining the grain structure’s regularity that specifies the uniformity of a tessellation as well as deriving a control parameter based on the regularity; (ii) modelling the mapping from a grain structure’s regularity to its grain size distribution; and (iii) establishing the relation between a set of physical parameters and a distribution function. A one-gamma distribution function is used to describe a grain size distribution characteristic and a group of four physical parameters are employed to represent the metallographic measurements of a grain size distribution property. Mathematical proofs of the uniqueness of the determination of the distribution parameter from the proposed set of physical parameters have been studied, and an efficient numerical procedure is provided for computing the distribution parameter. Based on the general scheme, two- and three-dimensional CPVT models have been formulated, which respectively define the quantities of regularity and control parameters, and model the mapping between regularity and grain size distribution. For the 2D-CPVT model, statistical tests have been carried out to validate the accuracy and robustness of regularity and grain size distribution control. In addition, micrographs with different grain size distribution features are employed to examine the capability of the 2D-CPVT model to generate virtual grain structures that meet physical measurements. A crystal plasticity finite element (CPFE) simulation of plane strain uniaxial tension has been performed to show the effect of grain size distribution on local strain distribution. For the 3D-CPVT model, a set of CPFE analyses of micro-pillar compression have been run and the effects of both regularity and grain size on deformation responses investigated. Further to this, a multi-zone scheme is proposed for the CPVT models to generate virtual gradient grain structures. In conjunction with the CPVT model that controls the seed generating process within individual zones, the multi-zone CPVT model has been developed by incorporating a novel mechanism of controlling the seed generation for grains spanning different zones. This model has the flexibility of generating various gradient grain structures and the natural morphology for interfacial grains between adjacent zones. Both of the 2D- and 3D-CPVT models are capable of generating a virtual grain structure with a mean grain size gradient for the grain structure domain and grain size distribution control for individual zones. A true gradient grain structure, two simulated gradient grain structure, and a true gradient grain structure with an elongated zone have been used to examine the capability of the multi-zone CPVT model. To facilitate the CPFE analyses of inter-granular crack initiation and evolution using the cohesive zone models, a Voronoi tessellation model with non-zero thickness cohesive zone representation was developed. A grain boundary offsetting algorithm is proposed to efficiently produce the cohesive boundaries for a Voronoi tessellation. The most challenging issue of automatically meshing multiple junctions with quadrilateral elements has been resolved and a rule-based method is presented to perform the automatically partitioning of cohesive zone junctions, including data representation, edge event processing and cut-trim operations. In order to demonstrate the novelty of the proposed cohesive zone modelling and junction partitioning schemes, the CPFE simulations of plane strain uniaxial tension and three point bending have been studied. A software system, VGRAIN, was developed to implement the proposed virtual grain structure modelling methods. Via user-friendly interfaces and the well-organised functional modules a virtual grain structure can be automatically generated to a very large-scale with the desired grain morphology and grain size properties. As a pre-processing grain structure representation system, VGRAIN is also capable of defining crystallographic orientations and mechanical constants for a generated grain structure. A set of additional functions has also been developed for users to study a generated grain structure and verify the feasibility of the generated case for their simulation requirements. A well-built grain structure model in VGRAIN can be easily exported into the commercial FE/CAE platform, e.g. ABAQUS and DEFORM, via script input, whereby the VGRAIN system is seamlessly integrated into CPFE modelling and simulation processing.
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Shabrov, Maxim N. "Micromechanical modeling of void nucleation in two phase materials /." View online version; access limited to Brown University users, 2005. http://wwwlib.umi.com/dissertations/fullcit/3174672.
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