To see the other types of publications on this topic, follow the link: Crack modeling.
Dissertations / Theses on the topic 'Crack modeling'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Crack modeling.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Christy, Clifford T. "Numerical modeling of a propagating crack." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1993. http://handle.dtic.mil/100.2/ADA272604.
Full text
2
Verma, Dhirendra. "Stochastic modeling of fatigue crack growth." Case Western Reserve University School of Graduate Studies / OhioLINK, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=case1054565393.
Full text
3
Hafezi, M. H., R. Alebrahim, and T. Kundu. "Crack propagation modeling using Peridynamic theory." SPIE-INT SOC OPTICAL ENGINEERING, 2016. http://hdl.handle.net/10150/622515.
Full textAbstract:
Crack propagation and branching are modeled using nonlocal peridynamic theory. One major advantage of this nonlocal theory based analysis tool is the unifying approach towards material behavior modeling- irrespective of whether the crack is formed in the material or not. No separate damage law is needed for crack initiation and propagation. This theory overcomes the weaknesses of existing continuum mechanics based numerical tools (e.g. FEM, XFEM etc.) for identifying fracture modes and does not require any simplifying assumptions. Cracks grow autonomously and not necessarily along a prescribed path. However, in some special situations such as in case of ductile fracture, the damage evolution and failure depend on parameters characterizing the local stress state instead of peridynamic damage modeling technique developed for brittle fracture. For brittle fracture modeling the bond is simply broken when the failure criterion is satisfied. This simulation helps us to design more reliable modeling tool for crack propagation and branching in both brittle and ductile materials. Peridynamic analysis has been found to be very demanding computationally, particularly for real-world structures (e.g. vehicles, aircrafts, etc.). It also requires a very expensive visualization process. The goal of this paper is to bring awareness to researchers the impact of this cutting-edge simulation tool for a better understanding of the cracked material response. A computer code has been developed to implement the peridynamic theory based modeling tool for two-dimensional analysis. A good agreement between our predictions and previously published results is observed. Some interesting new results that have not been reported earlier by others are also obtained and presented in this paper. The final objective of this investigation is to increase the mechanics knowledge of self-similar and self-affine cracks.
4
Kim, Yun-Jae. "Modeling fully plastic, plane strain crack growth." Thesis, Massachusetts Institute of Technology, 1993. http://hdl.handle.net/1721.1/12223.
Full text
5
Palettas, Panickos Neophytos. "Stochastic modeling and predictions for fatigue crack propagation /." The Ohio State University, 1988. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487597424135962.
Full text
6
Gualandi, Gabriele. "Crack modeling and crack propagation in structures using damage model and extended finite element techniques." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2012. http://amslaurea.unibo.it/3931/.
Full text
7
Hennessey, Conor Daniel. "Modeling microstructurally small crack growth in Al 7075-T6." Thesis, Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53947.
Full textAbstract:
Fatigue of metals is a problem that affects almost all sectors of industry, from energy to transportation, and failures to account for fatigue or incorrect estimations of service life have cost many lives. To mitigate such fatigue failures, engineers must be able to reliably predict the fatigue life of components under service conditions. Great progress has been made in this regard in the past 40 years; however one aspect of fatigue that is still being actively researched is the behavior of microstructurally small cracks (MSCs), which can diverge significantly from that of long cracks. The portion of life spent nucleating and growing a MSC over the first few grains/phases can consume over 90% of the total fatigue life under High Cycle Fatigue (HCF) conditions and is the primary source of the scatter in fatigue lives. Therefore, the development of robust fatigue design methodologies requires that the MSC regime of crack growth can be adequately modeled. The growth of microstructurally small cracks is dominated by influence of the local heterogeneity of the microstructure and is a highly complex process. In order to successfully model the growth of these microstructurally small cracks (MSCs), two computational frameworks are necessary. First, the local behavior of the material must be modeled, necessitating a constitutive relation with resolution on the scale of grain size. Second, a physically based model for the nucleation and growth of microstructurally small fatigue cracks is needed. The overall objective of this thesis is best summarized as the introduction these two computational frameworks, a crystal plasticity constitutive model and fatigue model, specifically for aluminum alloy 7075-T6, a high-strength, low density, precipitation hardened alloy used extensively in aerospace applications. Results are presented from simulations conducted to study the predicted crack growth under a variety of loading conditions and applied strain ratios, including uniaxial tension-compression and simple shear at a range of applied strain amplitudes. Results from the model are compared to experimental results obtained by other researchers under similar loading conditions. A modified fatigue crack growth algorithm that captures the early transition to Stage II growth in this alloy will also be presented.
8
Hartmaier, Alexander. "Modeling of crack-tip plasticity in Tungsten single crystals." [S.l. : s.n.], 2000. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB9444852.
Full text
9
Palmos, Epameinondas. "Modeling of Lamb waves and application to crack identification." Thesis, Monterey, California : Naval Postgraduate School, 2009. http://edocs.nps.edu/npspubs/scholarly/theses/2009/Sep/09Sep%5FPalmos.pdf.
Full textAbstract:
Thesis (M.S. in Mechanical Engineer and M.S. in Mechanical Engineering)--Naval Postgraduate School, September 2009.Thesis Advisor(s): Kwon, Young W. ; Pollak, Randall D. "September 2009." Description based on title screen as viewed on November 5, 2009. Author(s) subject terms: Lamb Waves, modeling, finite element, longitudinal, and shear mode, ANSYS, debonding, sensor, piezoelectric. Includes bibliographical references (p. 93-96). Also available in print.
10
Gonçalves, da Silva Bruno Miguel. "Modeling of crack initiation, propagation and coalescence in rocks." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/55156.
Full textAbstract:
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2009.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 254-260).
Natural or artificial fracturing of rock plays a very important role in geologic processes and for engineered structures in and on rock. Fracturing is associated with crack initiation, propagation and coalescence, which have been studied experimentally and analytically by many researchers. The analytical models developed to describe the initiation and propagation of cracks in brittle materials can be incorporated in Finite Element (FE) and Displacement Discontinuity (DD) codes. Corresponding research has been going on at MIT and has led to the development of a DDM code - FROCK - which currently uses a stress-based criterion proposed by Bobet (1997) to model crack initiation and propagation. Even though the predictions obtained with this criterion generally correspond to the experimental results, there are cases, in which the results obtained with FROCK are not satisfactory. This thesis proposes and implements new crack initiation and propagation criteria in the DDM code FROCK, namely a strain-based criterion and two stress-dependent criteria. It also studies the crack initiation and propagation processes numerically, using the FEM code ABAQUS. Existing crack initiation and propagation criteria (stress, strain and energy based) are also investigated with ABAQUS. The crack development processes are studied by modeling pairs of pre-existing flaws (double-flaw geometries) embedded in specimens subjected to vertical compressive loads in both ABAQUS and FROCK. For the different flaw arrangements studied, the difference between the stress and strain fields around the flaw tip gradually increases as the horizontal distance between the inner flaw tips increases. In terms of crack initiation, the results obtained with the stress and strain-based criteria studied were more consistent with the experimental observations than the results obtained with the energy-based criterion. The proposed strain-based criterion implemented in FROCK yielded better results than Bobet's stress-based criterion currently used in FROCK, for the five flaw arrangements studied. The results obtained with the two proposed stress-dependent criteria indicate that the critical shear stress at which a crack propagates in rock does not depend upon the normal stress applied, since the best crack propagation results were obtained for very low or zero friction angles.
by Bruno Miguel Gonçalves da Silva.
S.M.
11
Chiluveru, Sudhir. "Computational modeling of crack initiation in cross-role piercing." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/39325.
Full textAbstract:
Thesis (S.M.)--Massachusetts Institute of Technology, Computation for Design and Optimization Program, 2007.Includes bibliographical references (p. 81-89).
Thle Mannesmann process is the preferred method in the oil industry for fabrication of hollow pipes. The critical phenomenon in this process is the formation of a small round hole at the center of the cylindrical billet ahead of the piercing plug. In this work the crack initiation that leads to the creation of tile small hole has been modeled. The Gurson-Tvergaard-Needlemnan model of porous plasticity is used to simulate the Mannesmann effect. The appearance of a crack at the center of the cylindrical bar is demonstrated and the stress profiles, plastic equivalent strain profiles and porosity distribution during the deformation process are analyzed. The influence of various parameters in the model on the evolution of porosity in tile specimen is studied. Other simple ductile fracture criteria that are proposed in literature are also implemented. An interface model for fracture using the discontinuous Galerkin framework combined with a cohesive fracture law is implemented. This approach and its advantages are illustrated in the application of tensile loading of a simple beam specimen.
by Sudhir Chiluveru.
S.M.
12
Shastry, Vijay. "Modeling of dislocation emission and cleavage at crack tips /." The Ohio State University, 1994. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487858106116655.
Full text
13
Gordon, Ali Page. "Crack Initiation Modeling of a Directionally-Solidified Ni-base Superalloy." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/10468.
Full textAbstract:
Combustion gas turbine components designed for application in power generation equipment are subject to periodic replacement as a result of cracking, damage, and mechanical property degeneration that render them unsafe for continued operation. In view of the significant costs associated with inspecting, servicing, and replacing damaged components, there has been much interest in developing models that not only predict service life, but also estimate the evolved state of the material. This thesis explains manifestations of microstructural damage mechanisms that facilitate fatigue crack nucleation in directionally-solidified (DS) Ni-base superalloy components exposed to elevated temperatures and high stresses. In this study, models were developed and validated for damage and life prediction using DS GTD-111 as the subject material. This material has a chemical composition and grain structure designed to withstand creep damage occurring in blades of gas-powered turbines. The service conditions in these components, which generally exceed 600C, facilitate the onset of one or more damage mechanisms related to fatigue, creep, or environment.
The study was divided into an empirical phase, which consisted of experimentally simulating service conditions in fatigue specimens, and a modeling phase, which entailed numerically simulating the stress-strain response of the material. Tests have been carried out to simulate a variety of thermal, mechanical, and environmental operating conditions endured by longitudinally (L) and transversely (T) oriented DS GTD-111. In some cases, tests in extreme environments/temperatures were needed to isolate one or at most two of the mechanisms causing damage. Microstructural examinations were carried out via microscopy. A continuum crystal plasticity model was used to simulate the material behavior in the L and T orientations. The constitutive model was implemented in ABAQUS and a parameter estimation scheme was developed to obtain the material constants. A physically-based model was developed for correlating crack initiation life based on the experimental life data. Assuming a unique relationship between the damage fraction and cycle fraction with respect to cycles to crack initiation for each damage mode, total crack initiation life has been represented in terms of the individual damage components observed at the end state of crack initiation.
14
Potirniche, Gabriel Petru. "FINITE ELEMENT MODELING OF CRACK TIP PLASTIC ANISOTROPY WITH APPLICATION TO SMALL FATIGUE CRACKS AND TEXTURED ALUMINUM ALLOYS." MSSTATE, 2003. http://sun.library.msstate.edu/ETD-db/theses/available/etd-06242003-220551/.
Full textAbstract:
For the characterization of crack advance in mechanical components and specimens under monotonic and fatigue loading, many engineering approaches use the assumption that the plastic deformation at the crack tip is isotropic. There are situations when this assumption is not correct, and the modeling efforts require additional correction factors that account for this simplification. The goal of this work is to study two cases where the plastic anisotropy at the crack tip is predominant and influences the magnitude crack-tip parameters, which in turn determine the amount of crack advance under applied loading. At the microstructural level, the small crack issue it is a long-standing problem in the fatigue community. Most of the small crack models consider that the plastic deformation at the crack tip is isotropic. The proposed approached for analyzing small crack growth is to perform finite element simulation of small cracks growing in a material that is assigned single crystal plastic properties. The nature of the plastic deformation of the material at the crack tip in the intra-granular regions could be accurately described and used for modeling small crack growth. By employing finite element analyses for stationary and growing cracks, the main characteristics of the plastic deformation at the crack tip, such as plastic zone sizes and shapes, crack-tip opening displacements, crack-tip opening stresses, are quantified and crack growth rates are determined. Ultimately, by using this crystal plasticity model calibrated for different microstructures, important time and financial resources for real experiments for the study of small cracks can be spared by employing finite element simulations. At macroscale, it is widely known that the manufacturing processes for aluminum alloys results in highly anisotropic microstructures, known as textures. The plastic behavior of these types of materials is far from isotropic and even the use of classical anisotropic yield criteria, such as that on Hill (Hill, 1950), is far from producing accurate results for describing the plastic deformation. Two of these anisotropic yield functions are implemented into finite element code ANSYS and stationary cracks are studied in a wide variety of textures. Significant variations of the plastic deformation at the crack due to the anisotropy are revealed.
15
Karedla-Ravi, Shankar. "Modeling of crack tip high inertia zone in dynamic brittle fracture." Texas A&M University, 2003. http://hdl.handle.net/1969.1/5783.
Full textAbstract:
A phenomenological cohesive term is proposed and added to an existing
cohesive constitutive law (by Roy and Dodds) to model the crack tip high inertia region
proposed by Gao. The new term is attributed to fracture mechanisms that result in high
energy dissipation around the crack tip and is assumed to be a function of external
energy per volume input into the system. Finite element analysis is performed on
PMMA with constant velocity boundary conditions and mesh discretization based on the
work of Xu and Needleman. The cohesive model with the proposed dissipative term is
only applied in the high inertia zone i.e., to cohesive elements very close to the crack tip
and the traditional Roy and Dodds model is applied on cohesive elements in the rest of
the domain. It was observed that crack propagated in three phases with a speed of 0.35cR
before branching, which are in good agreement with experimental observations. Thus,
modeling of high inertia zone is one of the key aspects to understanding brittle fracture.
16
Barker, Vincent Mark. "Thermo-mechanical fatigue crack growth modeling of a nickel-based superalloy." Thesis, Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/44714.
Full textAbstract:
A model was created to predict the thermo-mechanical fatigue crack growth rates under typical engine spectrum loading conditions. This model serves as both a crack growth analysis tool to determine residual lifetime of ageing turbine components and as a design tool to assess the effects of temperature and loading variables on crack propagation. The material used in the development of this model was a polycrystalline superalloy, Inconel 100 (IN-100).
The first step in creating a reliable model was to define the first order effects that influence TMF crack growth in a typical engine spectrum. Load interaction effects were determined to be major contributors to lifetime estimates by influencing crack growth rates based upon previous load histories. A yield zone model was modified to include temperature dependent properties that controlled the effects of crack growth retardation and acceleration based upon overloads and underloads, respectively. Multiple overload effects were included in the model to create enhanced retardation compared to single overload tests. Temperature interaction effects were also considered very important due to the wide temperature ranges of turbine engine components. Oxidation and changing temperature effects were accounted for by accelerating crack growth in regions that had been affected by higher temperatures. Constant amplitude crack growth rates were used as a baseline, upon which load and temperature interaction effects were applied. Experimental data of isolated first order effects was used to calibrate and verify the model.
Experimental data provided the means to verify that the model was a good fit to experimental results. The load interaction effects were described by a yield zone model, which included temperature dependent properties. These properties were determined experimentally and were essential in the model's development to include load and temperature contributions. Other interesting factors became apparent through testing. It was seen that specific combinations of strain rate and temperature would lead to serrated yielding, discovered to be the Portevin-Le Chatelier effect. This effect manifested itself as enhanced hardening, leading to unstable strain bursts in specimens that cyclically yielded while changing temperature.
17
Psaila-Dombrowski, Maureen J. (Maureen Justine). "Modeling of crack and crevice chemistry in light water reactor environments." Thesis, Massachusetts Institute of Technology, 1990. http://hdl.handle.net/1721.1/13997.
Full text
18
Khattab, Hussein A. "Finite element response modeling of crack geometries induced by hydraulic fracturing /." The Ohio State University, 1985. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487261919112032.
Full text
19
Hafezi, Mohammad Hadi, and Mohammad Hadi Hafezi. "Peridynamic Modeling and Extending the Concept to Peri-Ultrasound Modeling." Diss., The University of Arizona, 2017. http://hdl.handle.net/10150/625456.
Full textAbstract:
In this dissertation, a novel fast modeling technique called peri-ultrasound that can model both linear and nonlinear ultrasonic behavior of materials is developed and implemented. Nonlinear ultrasonic response can detect even very small material non- linearity. Quantification of the material nonlinearity at the early stages of damage is important to avoid catastrophic failure and reduce repair costs. The developed model uses the nonlocal continuum-based peridynamic theory which was found to be a good simulation tool for handling crack propagation modeling, in particular when multiple cracks grow simultaneously. The developed peri-ultrasound modeling tool has been used to model the ultrasonic response at the interface of two materials in presence of an interface crack. Also, the stress wave propagation in a half-space (or half-plane for a 2-dimensional problem) with boundary loading is investigated using peri-ultrasound modeling. In another simulation, well-established two-dimensional Lamb's problem is investigated where the results are verified against available analytical solution. Also, the interaction between the surface wave and a surface breaking crack is studied.
20
Graffeo, Jeffrey K. "Mathematical modeling of adhesive layer cracks utilizing integral equations." Thesis, This resource online, 1995. http://scholar.lib.vt.edu/theses/available/etd-05022009-040448/.
Full text
21
Wang, Mei. "Electromagnetic modeling of distributed coaxial cable crack sensors in reinforced concrete members." Diss., Rolla, Mo. : Missouri University of Science and Technology, 2008. http://scholarsmine.mst.edu/thesis/pdf/Wang_09007dcc804a050d.pdf.
Full textAbstract:
Thesis (M.S.)--Missouri University of Science and Technology, 2008.Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed March 31, 2008) Includes bibliographical references (p. 48-49).
22
Kumar, Geetha Ramkumar. "Finite element modeling of crack detection in aircraft structures using electrical current." Morgantown, W. Va. : [West Virginia University Libraries], 2004. https://etd.wvu.edu/etd/controller.jsp?moduleName=documentdata&jsp%5FetdId=3620.
Full textAbstract:
Thesis (M.S.)--West Virginia University, 2004.Title from document title page. Document formatted into pages; contains viii, 82 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 80-81).
23
Aslan, Ozgur. "Numerical modeling of fatigue crack growth in single crystal nickel based superalloys." Paris, ENMP, 2010. http://pastel.archives-ouvertes.fr/pastel-00540893.
Full textAbstract:
Les composants monocristallins fonctionnant à des températures élevées sont soumis à des conditions de chargement thermo-mécanique sévères. La géométrie et le comportement de ces composants sont très complexes. Un défi majeur est de développer des modèles mathématiques afin de prévoir l'initiation et la propagation de fissures en présence de contraintes importantes et de forts gradients de température. Dans ce cas, le comportement élastoviscoplastique fortement anisotrope du matériau étudié (superalliage à base Ni) doit être pris en compte. Le modèle correspondant doit être en mesure de rendre compte de la croissance anisotrope des fissures et de leur bifurcation dans des champs de contrainte complexes. De plus, le modèle doit être capable de prédire non seulement le taux de croissance des fissures mais aussi les chemins de fissuration. La mécanique de l'endommagement anisotrope est un cadre théorique bien adapté au développement de modèles de croissance de fissures dans les monocristaux. Au cours d'études précédentes, une loi de comportement couplant plasticité cristalline et endommagement cyclique a été développée, démontrant l'intérêt de cette approche, mais aussi ses limites, notamment du fait de la dépendance au maillage des résultats. Le développement récent de modèles non-locaux dans le cadre de la mécanique des milieux continus pourrait ainsi aider à surmonter ces difficultés. Une grande base expérimentale existe concernant l'initiation et la propagation de fissures dans les superalliages monocristallins à base de nickel. Les simulations thermomécaniques par éléments finis des aubes de turbine fournissent des informations détaillées sur la distribution des contraintes et des déformations plastiques, en particulier près de singularités géométriques comme les trous et les fentes de refroidissement. Tout d'abord, sur la base de la théorie de la plasticité cristalline qui établit un lien solide entre les contraintes et les déformations plastiques, un modèle découplé en mécanique de l'endommagement basé sur l'historique des calculs par éléments finis sera présenté. Ensuite, un modèle d'endommagement incrémental basé sur les milieux généralisés sera proposé et enfin, les prédictions du modèle pour l'initiation et la croissance de micro-fissures en résolvant le problème de dépendance au maillage seront discutésSingle crystal components operating at elevated temperatures are subjected to severe thermomechanical loading conditions. The geometry and behaviour of these components are now very complex. A major issue is to develop models to predict crack initiation and crack growth in the presence of strong stress and temperature gradients. The strongly anisotropic elastoviscoplastic behaviour of the material which is a single crystal nickel base superalloy, must be taken into account. The corresponding model should be able to account for anisotropic crack growth and crack bifurcation in complex stress elds. Moreoever the model must be capable of predicting not only the crack growth rate but also the non-straight crack paths. Anisotropic damage mechanics is a well-suited theoretical framework for the development of crack growth models in single crystals. A model coupling crystal plasticity and cyclic damage has been developed in a previous project, that shows the interest of the approach, but also its current limits, in particular the strong mesh dependence of the results. Recent development of nonlocal models within the framework of the mechanics of generalized continua could help overcoming these difficulties. A large experimental basis exists concerning initiation and crack growth in single crystal nickel base superalloys. Finite element simulations of the thermomechanics of turbine blades provide detailed information about stress and plastic strain distribution, in particular near geometrical singularities like cooling holes and slits. First of all, on the basis of crystal plasticity theory which provides a solid link between stress and plastic strains, an uncoupled damage mechanics model based on the history of FE calculations will be presented. Afterwards, an incremental damage model based on generalized continua will be proposed and model predictions for the initiation and growth of microcracks by solving the mesh dependency, will be discussed
24
Efthymiadis, Panos. "Multiscale experimentation & modeling of fatigue crack development in aluminium alloy 2024." Thesis, University of Sheffield, 2015. http://etheses.whiterose.ac.uk/7735/.
Full textAbstract:
The objective of this research project is to be able to understand the role of various microstructural features on Fatigue Crack Initiation (FCI) of metallic alloys. By employing a novel experimental set-up, mechanical testing was performed in situ within an SEM chamber, and the deformation of the individual grains was observed real time. A physically-based Crystal Plasticity (CP) model was then developed that accurately predicts the macro and micro mechanical behaviour for Al2024 T3. An experimentally informed FCI criterion was developed that accounts for the effect of local slip bands and the applied local strains. While ‘precious’ insights were given on the small crack growth regime observing the occurring microscale phenomena. FCI is a multiscale process and thus evaluating the microscale does not cover fully the understanding of local deformation and damage. Thus a multiscale DIC process was employed to better understand the macro and mesoscale as well. 3D Digital Image Correlation (DIC) was employed and the strain distributions (at the sample scale) were obtained for various loading conditions. High magnification camera based 2D DIC was then used and the strain measurements were also extracted at clusters of grains. Useful observations were given for the different strain components (εxx, εyy, εxy). Finally the total fatigue lifetime of the component was compared to the modeled FCI for various loading conditions.
25
Pluma, Reyes Jorge A. "Fracture Control Modeling With The Finite Element Method." DigitalCommons@CalPoly, 2019. https://digitalcommons.calpoly.edu/theses/2096.
Full textAbstract:
This thesis investigates the feasibility and usability of the finite element method approach in the design of crack arresting devices. Current design and manufacturing practices are improving structures' susceptibility to fracture, in particular brittle fracture; however, cracks in structures are still observed within their lifespans due to severe unexpected service conditions, poor designs, or faulty manufacturing. Crack arrester systems can be added during service to prolong the longevity of structures with sub-critical or critical flaws. Fracture properties of different specific structures under specific services can be obtained experimentally, however, experiments are expensive and of high complexity. Alternatively, the finite element method can reduce these factors and provide reliable solutions. Finite element analysis conducted provides insight into the modeling process and the effectiveness of the simulation of fracture problems. Fracture mechanics technology in conjunction with the finite element method allows for the evaluation of the effectiveness of introducing crack arresters to a flawed structure. Additionally, the simulation of recorded crack arrester experiments alongside analytic methods are used to verify the finite element analysis results. The work in this thesis verifies the validity of using the finite element approach in designing crack arrester systems for flawed structures and suggests further investigation be done with variation in crack arrester types.
26
Abdeljawad, Fadi Faeq. "Microstructural Modeling of CSL Grain-Boundary Effects and Crack Growth in F.C.C. Polycrystals." NCSU, 2006. http://www.lib.ncsu.edu/theses/available/etd-01222006-115908/.
Full textAbstract:
A new multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolutionary equations of mobile and immobile dislocation densities is developed. Dislocation densities were modeled as internal state variables that provide a more detailed microstructural description of the material?s inelastic deformation and interrelated physical mechanisms that control different failure modes. Specialized microstructurally-based finite-element schemes have been used to investigate the effects of crystallographic orientations of the grains and grain-boundaries (GBs), grain shape and size, (GB) misorientation and the dependency of GB strength and mechanical properties on specific CSL misorientations on the inelastic finite deformation and failure mode mechanisms in f.c.c. polycrystalline aggregates. A Voronoi algorithm was used to generate grains and GBs with random shapes and sizes. The combined effects of GB misorientation, structure and geometry, strain hardening, localized plastic shear slip, intensive regions of crystal lattice rotation and the evolution, interaction and accumulation of dislocation densities on the nucleation and growth of intergranular and transgranular failure modes in f.c.c. polycrystalline aggregates were studied. Results from this study are consistent with experimental observations that microstructures with desired material properties, such as resistivity to crack nucleation, can be achieved by the control of the crystallographic orientation distribution of the grain aggregate and CSL GB orientations. Results from this study show that transgranular failure modes are dominant in aggregates with a high frequency of Sigma-3 GBs, and intergranular fracture modes dominate the aggregate with a high frequency of Sigma-17b GBs.
27
Yang, Jidong. "Road crack condition performance modeling using recurrent Markov chains and artificial neural networks." [Tampa, Fla.] : University of South Florida, 2004. http://purl.fcla.edu/fcla/etd/SFE0000567.
Full text
28
VARGAS, ELVIS YURI MAMANI. "CRACK MODELING USING GENERALIZED WESTERGAARD STRESS FUNCTIONS IN THE HYBRID BOUNDARY ELEMENT METHOD." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2015. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=26857@1.
Full textAbstract:
PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIROCOORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
PROGRAMA DE EXCELENCIA ACADEMICA
Apresenta-se uma formulação do método híbrido dos elementos de contorno para a análise de problemas planos de potencial e de elasticidade que, apesar de completamente geral para domínios finitos, é mais apropriada a aplicações de mecânica da fratura. A formulação exige integrações apenas ao longo do contorno e usa como soluções fundamentais, para interpolar campos no domínio, funções generalizadas do tipo Westergaard, inspiradas numa proposta feita por Tada et al. em 1993. Os conceitos de elementos de contorno são semelhantes aos conceitos apresentados por Crouch e Starfield em 1983, mas em um contexto variacional que permite interpretações mecânicas das equações matriciais resultantes. Problemas de topologia geral podem ser modelados, como ilustrado para domínios infinitos ou multiplamente conexos. A formulação é diretamente aplicável à solução de problemas de placas com entalhes ou trincas curvas internas ou de bordo, pois permite a descrição adequada de altos gradientes de tensão, sendo uma ferramenta simples para a avaliação de fatores de intensidade de tensão. Além disso, é possível determinar, num processo iterativo, a zona plástica ao redor da ponta de uma trinca. Esta tese tem foco no desenvolvimento matemático da formulação para problemas de potencial e de elasticidade. Vários exemplos numéricos de validação são apresentados.
A particular implementation of the hybrid boundary element method is presented for the two dimensional analysis of potential and elasticity problems, which, although general in concept, is suited for fracture mechanics applications. The formulation requires integrations only along the boundary and uses fundamental solutions to interpolate fields in the domain. Generalized Westergaard stress functions, as proposed by Tada et al in 1993, are used as the problem s fundamental solutions. The proposed formulation leads to displacement-based concepts that resemble those presented by Crouch and Starfield, although in a variational framework that leads to matrix equations with sound mechanical meanings. Problems of general topology, such as in the case of unbounded and multiply-connected domains, may be modeled. The formulation, which is directly applicable to notches and generally curved, internal or external cracks, is especially suited for the description of the stress field in the vicinity of crack tips and is an easy means of evaluating stress intensity factors. The plastic phenomenon is taken into account around the crack tip through an iterative process. This thesis focuses on the mathematical fundamentals of the formulation of potential and elasticity problems. Several validating numerical examples are presented.
29
Adair, Benjamin Scott. "Characterization and modeling of thermo-mechanical fatigue crack growth in a single crystal superalloy." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/52191.
Full textAbstract:
Turbine engine blades are subjected to extreme conditions characterized by significant and simultaneous excursions in both stress and temperature. These conditions promote thermo-mechanical fatigue (TMF) crack growth which can significantly reduce component design life beyond that which would be predicted from isothermal/constant load amplitude results. A thorough understanding of the thermo-mechanical fatigue crack behavior in single crystal superalloys is crucial to accurately evaluate component life to ensure reliable operations without blade fracture through the use of "retirement for cause" (RFC). This research was conducted on PWA1484, a single crystal superalloy used by Pratt & Whitney for turbine blades. Initially, an isothermal constant amplitude fatigue crack growth rate database was developed, filling a void that currently exists in published literature. Through additional experimental testing, fractography, and modeling, the effects of temperature interactions, load interactions, oxidation and secondary crystallographic orientation on the fatigue crack growth rate and the underlying mechanisms responsible were determined. As is typical in published literature, an R Ratio of 0.7 displays faster crack growth when compared to R = 0.1. The effect of temperature on crack growth rate becomes more pronounced as the crack driving force increases. In addition secondary orientation and R Ratio effects on crack growth rate were shown to increase with increasing temperature. Temperature interaction testing between 649°C and 982°C showed that for both R = 0.1 and 0.7, retardation is present at larger alternating cycle blocks and acceleration is present at smaller alternating cycle blocks. This transition from acceleration to retardation occurs between 10 and 20 alternating cycles for R = 0.1 and around 20 alternating cycles for R = 0.7. Load interaction testing showed that when the crack driving force is near KIC the overload size greatly influences whether acceleration or retardation will occur at 982°C. Semi-realistic spectrum testing demonstrated the extreme sensitivity that relative loading levels play on fatigue crack growth life while also calling into question the importance of dwell times. A crack trajectory modeling approach using blade primary and secondary orientations was used to determine whether crack propagation will occur on crystallographic planes or normal to the applied load. Crack plane determination using a scanning electron microscope enabled verification of the crack trajectory modeling approach. The isothermal constant amplitude fatigue crack growth results fills a much needed void in currently available data. While the temperature and load interaction fatigue crack growth results reveal the acceleration and retardation that is present in cracks growing in single crystal turbine blade materials under TMF conditions. This research also provides a deeper understanding of the failure and deformation mechanisms responsible for crack growth during thermo-mechanical fatigue. The crack path trajectory modeling will help enable "Retirement for Cause" to be used for critical turbine engine components, a drastic improvement over the standard "safe-life" calculations while also reducing the risk of catastrophic failure due to "chunk liberation" as a function of time. Leveraging off this work there exists the possibility of developing a "local approach" to define a crack growth forcing function in single crystal superalloys.
30
Gigliotti, Luigi. "Assessment of the applicability of XFEM in Abaqus for modeling crack growth in rubber." Thesis, KTH, Hållfasthetslära (Inst.), 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-103919.
Full textAbstract:
The eXtended Finite Element Method is a partition of unity based method, particularly suitable for modelling crack propagation phenomena, without knowing a priori the crack path. Its numerical implementation is mostly achieved with stand-alone codes. The implementation of the eXtended Finite Element Method in commercial FEA softwares is still limited, and the most famous one including such capabilities is Abaqus TM. However, due to its relatively recent intro-duction, XFEM technique in Abaqus has been proved to provide trustable results only in few simple benchmark problems involving linear elastic material models.In this work, we present an assessment of the applicability of the eXtendend Finite Element Method in Abaqus, to deal with fracture mechanics problems of rubber-like materials. Results are provided for both Neo-Hookean and Arruda-Boyce material models, under plane strain conditions. In the rst part of this work, a static analysis for the pure Mode-I and for a 45o mixed-Mode load condition, whose objective has been to evaluate the ability of the XFEM technique in Abaqus, to correctly model the stress and displacement elds around a crack tip, has been performed. Outcomes from XFEM analysis with coarse meshes have been compared with the analogous ones obtained with highly re ned standard FEM discretizations. Noteworthy, despite the remarkable level of accuracy in analyzing the displacement eld at the crack tip, concerning the stress eld, the adoption of the XFEM provides no bene ts, if compared to the standard FEM formulation. The only remarkable advantage is the possibility to discretize the model without the mesh con-forming the crack geometry. Furthermore, the dynamic process of crack propagation has been analyzed by means of the XFEM. A 45o mixed-Mode and a 30o mixed-Mode load condition are analyzed. In particular, three fundamental aspects of the crack propagation phenomenon have been investigated, i.e. the instant at which a pre-existing crack starts to propagate within the body under the applied boundary conditions, the crack propagation direction and the predicted crack propagation speeds. According to the obtained results, the most inuent parameters are thought to be the elements size at the crack tip hand the applied displacement ratev. Severe diculties have been faced to attain convergence. Some reasonable motivations of the unsatisfactory convergence behaviour are proposed.
31
Lee, James H. "Modeling of crack initiation and growth in solid rocket propellants using macromechanics and micromechanics theories." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1996. http://handle.dtic.mil/100.2/ADA319521.
Full textAbstract:
Thesis (M.S. in Mechanical Engineering) Naval Postgraduate School, September 1996.Thesis advisor(s): Young W. Kwon. "September 1996." Includes bibliographical references (p. 85-86). Also available online.
32
Li, Shi. "Modeling of active crack damage control and the active fatique damage control of adhesive joint." Thesis, This resource online, 1992. http://scholar.lib.vt.edu/theses/available/etd-08222009-040330/.
Full text
33
Amaro, Robert L. "Thermomechanical fatigue crack formation in a single crystal Ni-base superalloy." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/37172.
Full textAbstract:
This research establishes a physics-based life determination model for the second generation single crystal superalloy PWA 1484 experiencing out-of-phase thermomechanical fatigue (TMF). The life model was developed as a result of a combination of critical mechanical tests, dominant damage characterization and utilization of well-established literature. The resulting life model improves life prediction over currently employed methods and provides for extrapolation into yet unutilized operating regimes. Particularly, the proposed deformation model accounts for the materials' coupled fatigue-environment-microstructure response to TMF loading. Because the proposed model is be based upon the underlying deformation physics, the model is robust enough to be easily modified for other single crystal superalloys having similar microstructure. Future use of this model for turbine life estimation calculations would be based upon the actual deformation experienced by the turbine blade, thereby enabling turbine maintenance scheduling based upon on a "retirement for a cause" life management scheme rather than the currently employed "safe-life" calculations. This advancement has the ability to greatly reduce maintenance costs to the turbine end-user since turbine blades would be removed from service for practical and justifiable reasons. Additionally this work will enable a rethinking of the warranty period, thereby decreasing warranty related replacements. Finally, this research provides a more thorough understanding of the deformation mechanisms present in loading situations that combine fatigue-environment-microstructure effects.
34
Lee, Ji Soo. "Time-Dependent Crack Growth in Brittle Rocks and Field Applications to Geologic Hazards." Diss., The University of Arizona, 2007. http://hdl.handle.net/10150/193784.
Full textAbstract:
The primary focus of this research is to evaluate the time-dependent crack growth in rocks using lab tests and numerical modeling and its application to geologic hazard problems. This research utilized Coconino sandstone and Columbia granite as the study materials and produced the subcritical crack growth parameters in both mode I and II loadings using the rock materials. The mode I loading test employs three different types of fracture mechanics tests: the Double Torsion (DT), the Wedge Splitting (WS), and the Double Cantilever Beam (DCB) test. Each test measured the mode I crack velocity. The DT test indirectly measured the crack velocity using the load relaxation method. The WS and DCB tests directly measured the crack velocity by monitoring using a video recording. The different mode I subcritical crack growth parameters obtained from the three tests are discussed. For the mode II loading test, this study developed a new shear fracture toughness test called the modified Punch-Through Shear (MPTS). The MPTS test conducted at different loading rates produced the mode II subcritical crack growth parameters. These fracture mechanics tests were calibrated and simulated using the distinct element method (DEM) and the finite element method (FEM). DEM analysis employed the particle flow code (PFC) to simulate the mixed mode crack growth and to match with the failure strength envelop of the triaxial compressive tests. FEM analysis employed the Phase2 program to analyze the crack tip stress distribution and the FRANC2D program to calculate the modes I and II stress intensity factors. The fracture mechanics tests and numerical modeling showed well the dependency of the mode II subcritical crack growth parameters according to confining pressure, loading rate, and the mode II fracture toughness. Finally, the UDEC modeling based on DEM is utilized in this study to forecast the long-term stability of the Coconino rock slope, as one of geologic hazards. The fracture mechanics approach is implemented in the program using the modes I and II subcritical crack growth parameters obtained from the lab tests and numerical modeling. Considering the progressive failure of rock bridges due to subcritical crack growth, the UDEC results predicted the stable condition of the Coconino rock cliff over 10,000 years. This result was validated by comparing it with the previous planar failure case.
35
Cai, Qingbo. "Finite element modelling of cracking in concrete gravity dams." Thesis, Pretoria : [s.n.], 2007. http://upetd.up.ac.za/thesis/available/etd-01302008-160623.
Full text
36
Hösthagen, Anders. "Thermal Crack Risk Estimation and Material Properties of Young Concrete." Licentiate thesis, Luleå tekniska universitet, Byggkonstruktion och brand, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-65495.
Full textAbstract:
This thesis presents how to establish a theoretical model to predict risk of thermal cracking in young concrete when cast on ground or an arbitrary construction. The crack risk in young concrete is determined in two steps: 1) calculation of temperature distribution within newly cast concrete and adjacent structure; 2) calculation of stresses caused by thermal and moisture (due to self-desiccation, if drying shrinkage not included) changes in the analyzed structure. If the stress reaches the tensile strength of the young concrete, one or several cracks will occur. The main focus of this work is how to establish a theoretical model denoted Equivalent Restraint Method model, ERM, and the correlation between ERM models and empirical experiences. A key factor in these kind of calculations is how to model the restraint from any adjacent construction part or adjoining restraining block of any type. The building of a road tunnel and a railway tunnel has been studied to collect temperature measurements and crack patterns from the first object, and temperature and thermal dilation measurements from the second object, respectively. These measurements and observed cracks were compared to the theoretical calculations to determine the level of agreement between empirical and theoretical results. Furthermore, this work describes how to obtain a set of fully tested material parameters at CompLAB (test laboratory at Luleå University of Technology, LTU) suitable to be incorporated into the calculation software used. It is of great importance that the obtained material parameters describe the thermal and mechanical properties of the young concrete accurately, in order to perform reliable crack risk calculations. Therefore, analysis was performed that show how a variation in the evaluated laboratory tests will affect the obtained parameters and what effects it has on calculated thermal stresses.
37
Mokashi, Prasad Shrikant. "Numerical modeling of homogeneous and bimaterial crack tip and interfacial cohesive zones with various traction-displacement laws." Columbus, Ohio : Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1180621217.
Full text
38
Wu, Tao. "Theoretical modeling and experimental characterization of stress and crack development in parts manufactured through large area maskless photopolymerization." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/54274.
Full textAbstract:
Large Area Maskless Photopolymerization (LAMP) is a disruptive additive manufacturing technology developed in the Direct Digital Manufacturing Laboratory at Georgia Tech. Due to polymerization shrinkage during the layer-by-layer curing process, stresses are accumulated that can give rise to cracks and delaminations along the interfaces between adjacent layers. The objective of this doctoral dissertation is to investigate the mechanisms of stress evolution and cracking/delamination during the LAMP manufacturing process through theoretical modeling and experimental characterization methods. The evolving conversion degree in a layer was characterized through Fourier Transform Infrared Spectroscopy and this leads to a so-called print-through curve. The polymerization shrinkage strain in each exposed layer was calculated on the basis of the theoretical relationship between the volumetric shrinkage and the degree of conversion. Furthermore, the material’s elastic modulus, which also evolves with the degree of conversion, was characterized by three-point bending tests. With the degree of conversion, cure-dependent modulus and shrinkage strain as the three primary inputs, finite element modeling was conducted to dynamically simulate the layer-by-layer manufacturing process and to predict the process-induced stresses. To investigate the fracture process, Mode I and Mode II interlaminar fracture toughness of the LAMP-built laminates was characterized, using the double cantilever beam (DCB) test and the end notched flexure (ENF) test, respectively. In order to predict the crack initiation and propagation occurring in a LAMP-built part, a mixed-mode cohesive element model was developed. The Mode I and Mode II cohesive parameters, which are used to describe the bilinear constitutive behavior of the cohesive elements, were determined by matching the numerical load-deflection curves to the experimental ones obtained from the DCB tests and the ENF tests, respectively. Using this model, the fracture of a hollow-cylinder part was analyzed and the simulation results were compared with experiments. Finally, several possible strategies for mitigating the shrinkage related defects were investigated. Reducing the overall polymerization shrinkage, optimizing the print-through curve and delaying the gel point of resin composite were demonstrated to be effective in reducing stresses and cracks.
39
Zhou, Jun. "Numerical Modeling of Ductile Fracture." University of Akron / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=akron1384774266.
Full text
40
Hartmaier, Alexander [Verfasser]. "Modeling of crack-tip plasticity in tungsten single crystals / Max-Planck-Institut für Metallforschung, Stuttgart. Vorgelegt von Alexander Hartmaier." Stuttgart : Max-Planck-Inst. für Metallforschung, 2000. http://d-nb.info/962399469/34.
Full text
41
Hammood, Mohammed Naji. "A Meso-Macro Numerical Approach for Chloride Diffusivity Modeling Taking into Account Chloride Binding and Crack Evolution in Concrete." Thesis, Nantes, 2017. http://www.theses.fr/2017NANT4066/document.
Full textAbstract:
La pénétration des ions chlorure est la principale cause de la dégradation des structures en béton, par corrosion des armatures, entraînant un impact sévère sur leur durabilité et leur durée de vie. La pénétration de ces agents agressifs pourrait être favorisée davantage par la présence de fissures. Dans cet thèse, nous avons utilisé la méthode des éléments finis (EF) pour résoudre l'équation de la loi de Fick couplée à la capacité de fixation d’ions chlorure afin de modéliser la diffusion des ions chlorure à l’échelle mésoscopique. Dans un premier temps, nous avons considéré une représentation 3D d’un matériau, sain, hétérogène biphasé (comme le béton) ou les inclusions (granulats) sont noyées dans une matrice de mortier. Le problème des interfaces (inclusion/matrice) a été résolu en utilisant la méthode E-FEM (Embedded Finit Element Method). Au niveau de ces interfaces, nous avons introduit une discontinuité faible du champ de concentration de chlorures. Une approche d’homogénéisation par moyennes spatiales se basant sur les travaux de Pouya est également utilisée pour prédire les tenseurs de diffusivité macroscopiques des matériaux biphasiques. La comparaison avec l'équation de Maxwell et des résultats expérimentaux a été réalisée pour montrer la précision de l’approche numérique proposée. Dans un second temps, l’approche méso-macro est représentée pour introduire un modèle numérique capable de fournir des informations macroscopiques (tenseur de diffusion moyen) intégrant le niveau d’ouverture de fissure, le chemin de fissuration et l’hétérogénéité des matériaux quasi fragiles tels que les matériaux cimentaires (béton, mortier, ….). Dans ce cas, des points clés du processus de fissuration comme l’évolution d’une fissuration répartie vers une fissuration localisée (macro-fissure(s)), la tortuosité de la fissure et son anisotropie sont intégrées naturellement dans la diffusivité macroscopique. En fin, le tenseur défini est ensuite utilisé afin d'estimer la durée de vie des structures en béton, y compris l'effet de l'endommagement et de la mésostructure interneThe penetration of chloride ions has an essential responsibility in the degradation of concrete structures caused by reinforcement corrosion leading to a severe impact on the durability and service life of concrete structures. The problem becomes more critical with the existence of cracking which accelerate the penetration of chloride ions into concrete cover. In this work, the FE formulation for the numerical modelling of chloride ions diffusion accounting for chloride binding capacity in mesoscale concrete is introduced. The mesostructure is based on a twophase 3D representation of heterogeneous materials, such as concrete, where stiff aggregates are embedded into a mortar matrix. For this purpose, we turn to the Embedded Finite Element Method (E-FEM). This is performed by introducing a weak discontinuity in the chloride concentration field for finite elements where the physical interface is present. Numerical spatial homogenization experiments based on Pouya’s works are also performed on 3D mesostructures to compute macroscopic diffusivity tensors accounting for two-phase material. Comparison with Maxwell's equation and experimental results are carried out to show the accuracy of the proposed numerical approach. Finally, the meso-macro approach is presented to introduce a numerical model capable of providing macroscopic information (mean diffusivity tensor) integrating the level of crack opening, crack path and heterogeneity of materials in quasi-brittle concrete. The mesoscale coupling with the mass transport part is based on Fick’s Law with a modified diffusion coefficient taking into account crack opening and aggregates. The macroscopic diffusivity tensor integrates more complex features such as the cracking evolution process, tortuosity of the crack’s path, inducedanisotropy and presence of aggregates. The defined tensor is used afterwards in order to estimate the service-life of concrete structures, including the effect of the cracking and the internal mesostructure
42
McKellar, Dougan Kelk. "A dislocation model of plasticity with particular application to fatigue crack closure." Thesis, University of Oxford, 2001. http://ora.ox.ac.uk/objects/uuid:45183b90-017f-4ac1-9550-94772a0ca88b.
Full textAbstract:
The ability to predict fatigue crack growth rates is essential in safety critical systems. The discovery of fatigue crack closure in 1970 caused a flourish of research in attempts to simulate this behaviour, which crucially affects crack growth rates. Historically, crack tip plasticity models have been based on one-dimensional rays of plasticity emanating from the crack tip, either co-linear with the crack (for the case of plane stress), or at a chosen angle in the plane of analysis (for plane strain). In this thesis, one such model for plane stress, developed to predict fatigue crack closure, has been refined. It is applied to a study of the relationship between the apparent stress intensity range (easily calculated using linear elastic fracture mechanics), and the true stress intensity range, which includes the effects of plasticity induced fatigue crack closure. Results are presented for all load cases for a finite crack in an infinite plane, and a method is demonstrated which allows the calculation of the true stress intensity range for a growing crack, based only on the apparent stress intensity range for a static crack. Although the yield criterion is satisfied along the plastic ray, these one-dimensional plasticity models violate the yield criterion in the area immediately surrounding the plasticity ray. An area plasticity model is therefore required in order to model the plasticity more accurately. This thesis develops such a model by distributing dislocations over an area. Use of the model reveals that current methods for incremental plasticity algorithms using distributed dislocations produce an over-constrained system, due to misleading assumptions concerning the normality condition. A method is presented which allows the system an extra degree of freedom; this requires the introduction of a parameter, derived using the Prandtl-Reuss flow rule, which relates the magnitude of slip on complementary shear planes. The method is applied to two problems, confirming its validity.
43
Zhang, Wen [Verfasser]. "Dynamic modeling of crack propagation in ceramic laminate-toughened composites with weak interfaces by using discrete element method / Wen Zhang." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2014. http://d-nb.info/1049556941/34.
Full text
44
Lau, Kingsley. "Corrosion of Epoxy-Coated Reinforcement in Marine Bridges with Locally Deficient Concrete." Scholar Commons, 2010. https://scholarcommons.usf.edu/etd/1696.
Full textAbstract:
Epoxy-coated rebar (ECR) has been used in approximately 300 Florida bridges, in an attempt to control corrosion of the substructure in the splash-evaporation zone. Early severe ECR corrosion was observed in the substructure of several Florida ECR bridges (Group 1) where the substructure was built with permeable concrete of high apparent chloride diffusivity DApp. Other ECR bridges built during the same period and having similar DApp were projected to show corrosion damage starting on the following decade. Examination of several of those bridges (Group 2) confirmed that projection. Other recently examined Florida ECR bridges (Groups 3 and 4) were built with very low to moderate permeability concrete having correspondingly low to moderate DApp values at normally sound concrete locations. Those bridges were projected not to show early corrosion at normal locations and that projection has also been confirmed. However, some incidence of thin structural cracks exists affecting a small fraction of the substructure. Chloride transport there is much faster than through the matrix in otherwise low permeability concrete and work has confirmed that early corrosion can develop there.
A predictive ECR corrosion model was applied that replicated most of the damage function features observed in the field. The model divides the substructure in separate elements with individual chloride exposure, concrete permeability, concrete rebar cover, and extent of ECR coating imperfections. Additionally, a model for projecting impact of preexisting cracking on corrosion damage was developed. The projections indicate that relatively isolated cracking should only create topical concrete damage with reduced maintenance requirements. However, model projections indicated that even though assuming that the incidence of damage is limited to a small region around the crack, if the crack orientation with respect to the rebar were adverse and chloride transport were greatly enhanced (as it could be expected in relatively wide cracks), corrosion damage from localized concrete deficiencies could significantly increase maintenance costs.
Electrochemical Impedance Spectroscopy (EIS) measurements of ECR in extracted cores showed good potential for non destructive characterization of the extent of coating damage. A possible method accounting for frequency dispersion effects in the high frequency response (of importance to assess extent of defects) was introduced.
45
Anand, Nagarajan. "A Conforming to Interface Structured Adaptive Mesh Refinement for Modeling Complex Morphologies." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1574436067401755.
Full text
46
Du, Wei 1962. "Numerical modeling of mixed mode multiple crack propagation and its application to the simulation of nonlinear rock deformation and borehole breakout." Diss., The University of Arizona, 1997. http://hdl.handle.net/10150/282298.
Full textAbstract:
Rock is a very heterogeneous material, containing structural weakness at all scales. These weaknesses include grain boundaries, pores, and cracks on the small scale, and joints, faults, and bedding planes on the large scale. Nonlinear rock deformation in the low-temperature, low-confinement regime is due primarily to the growth of cracks from these weaknesses and the coalescence of cracks to form macroscopic structural features. Another important aspect of rock deformation and failure is the statistical distribution of weaknesses in the initial microstructure. Borehole breakout is the process by which portions of a borehole wall fracture or spall when subjected to compressive stresses. Studies of borehole breakout in the past twenty years include experiments, field studies, and numerical modeling. With regards to the numerical modeling of borehole breakout, the rock surrounding the borehole is considered as a nonlinear continuum material in most of the previous approaches. Experiments and field studies, however, have shown that the heterogeneous and discontinuous nature of rock has a strong impact on the mechanics of borehole breakout. This dissertation describes a numerical model that has been developed to simulate the damage of rock and the corresponding non-linear stress-strain behavior, and also the progression of borehole breakout in heterogeneous and discontinuous rock by mixed mode crack growth, interaction, and coalescence. The rock is simulated as an elastic material containing a random distribution of cracks. As compressive load is applied, the initial cracks grow, interact, and coalesce to form macroscopic fractures. The numerical model was developed by making a series of modifications to the displacement discontinuity code of Crouch and Starfield (Crouch & Starfield, 1983). The most important modifications include modifying the boundary element for the calculation of stress intensity factors, adding Coulomb friction for closed portions of cracks, adding a crack generator, and adding an algorithm for crack coalescence. The numerical model is used to simulate the non-linear deformation and the progression of breakout in Westerly granite, and the results are realistic.
47
Emami, Anahita. "Investigation on Physics-based Multi-scale Modeling of Contact, Friction, and Wear in Viscoelastic Materials with Application in Rubber Compounds." Diss., Virginia Tech, 2018. http://hdl.handle.net/10919/97008.
Full textAbstract:
This dissertation aims to contribute towards the understanding and modeling of tribological phenomena of contact, friction, and wear in viscoelastic materials with application in rubber compounds. Tribiological properties of rubber compounds are important for many applications such as tires, shoe heels and soles, wiper blades, artificial joints, O-ring seals, and so on. In all these applications, the objective is to maximize the friction coefficient to avoid slipping and reduce the wear rate to improve the life expectancy and performance of the products.
The first topic in this study focuses on a novel multiscale contact theory proposed by Persson and explains the advantages of this theory over other classical contact theories. The shortcomings of this theory are also investigated, and three methods are proposed to improve Persson's original contact model by correcting the approximation of deformation in the contact area. The first method is based on the original Greenwood and Williamson (GW) contact theory, which neglects the effect of elastic coupling between asperities. The second method is based on an improved version of GW theory, which considers the elastic coupling effect of asperities in an approximate way. The third method is based on the distribution of local peaks of asperities, which is particularly suitable to determine the fraction of a skewed height profile involved in tribological processes. This method can be implemented within the framework of other proposed methods. Since the height profiles of rough surfaces studied in this dissertation are approximately normally distributed, the second correction method is applied to the original contact model to calculate the real contact area and friction coefficient.
The second topic addresses the theoretical model of hysteresis friction in viscoelastic materials. The multiscale temperature rise of the rubber surface due to hysteresis friction is also modeled and the effect of flash temperature on the real contact area and friction coefficient is studied. Since the hysteresis friction is not the only mechanism involved in the rubber friction, a semi-empirical model is added to the hysteresis model to include the contribution of adhesion and other processes on the real contact area. Based on the improved multiscale contact theory, a pressure-dependent friction model is also developed for viscoelastic materials, which is in good agreement with experimental results.
The third topic deals with the theory of stationary crack propagation in viscoelastic materials and the effect of crack tip flash temperature on the instability of crack propagation observed in some experimental results in the literature. Initially, a theoretical model is developed to calculate the tearing energy vs crack tip velocity in a Kelvin-Voigt rubber model. Besides, two coupled iterative algorithms are developed to calculate the temperature field around the crack tip in addition to the tearing energy as a function of crack tip velocity. In this model, the effect of crack tip flash temperature on the tearing energy is considered to update the relation between tearing energy vs crack tip velocity, which also affects the flash temperature. A theoretical model is also developed to calculate the contribution of the hysteresis effect to the tearing energy vs crack tip velocity using the dynamic modulus master curve of a rubber compound. Then, the low-frequency fatigue test results are compared with the theoretical predictions and used in the framework of powdery rubber wear theory to calculate the stationary rubber wear rate due to fatigue crack propagation.
Moreover, a sliding friction and wear test set-up, with both indoor and outdoor testing capability, is developed to validate the theoretical models. The experimental results confirm that the theoretical model can successfully predict the friction coefficient when there is no trace of thermochemical degradation on the rubber surface. Investigating the wear mechanism of rubber samples on three different surfaces reveals that the contribution of fatigue wear rate is less important than other wear mechanisms such as abrasive wear due to sharp asperities or thermochemical degradation due to a significant rise of temperature on the contact area. Finally, the correlation between friction coefficient and wear rate on different surfaces is studied, and it is found that the relation between friction and wear rate strongly depends on the dominant wear mechanism, which is determined by the surface characteristics, sliding velocity, normal load, and contact flash temperature.PHD
48
El-Hajjar, Rani Fayez. "Experimental study and analytical modeling of translayer fracture in pultruded FRP composites." Diss., Available online, Georgia Institute of Technology, 2004:, 2004. http://etd.gatech.edu/theses/available/etd-03152004-230942/unrestricted/elhajjar%5Frani%5Ff%5F200405%5Fphd.pdf.
Full textAbstract:
Thesis (Ph. D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2004.Dr. Zureick, Abdul-Hamid, Committee Member; Dr. White, Donald, Committee Member; Dr. Saxena, Ashok, Committee Member; Dr. Jacobs, Laurence, Committee Member; Dr. Haj-Ali, Rami, Committee Chair; Dr. Armanios, Erian, Committee Member. Vita. Includes bibliographical references (leaves 164-172).
49
Afshari, Mana. "Vibration- and Impedance-based Structural Health Monitoring Applications and Thermal Effects." Diss., Virginia Tech, 2012. http://hdl.handle.net/10919/27954.
Full textAbstract:
Structural Health Monitoring (SHM) is the implementation of damage detection and characterization algorithms using in vitro sensing and actuation for rapidly determining faults in structural systems before the damage leads to catastrophic failure. SHM systems provide near real time information on the state of the integrity of civil, mechanical and aerospace structures. A roadblock in implementing SHM systems in practice is the possibility of false positives introduced by environmental changes. In particular, temperature changes can cause many SHM algorithms to indicate damage when no damage exists. While several experimentally based efforts have been attempted to alleviate temperature effects on SHM algorithms, fundamental research on the effects of temperature on SHM has not been investigated.
The work presented in this dissertation composes of two main parts: the first part focuses on the experimental studies of different mechanical structures of aluminum beams, lug samples and railroad switch bolts. The experimental study of the aluminum lug samples and beams is done to propose and examine methods and models for in situ interrogation and detection of damage (in the form of a fatigue crack) in these specimen and to quantify the smallest detectable crack size in aluminum structures. This is done by applying the electrical impedance-based SHM method and using piezoceramic sensors and actuators. Moreover, in order to better extract the damage features from the measured electrical impedance, the ARX non-linear feature extraction is employed. This non-linear feature extraction, compared to the linear one, results in detection of damages in the micro-level size and improves the early detection of fatigue cracks in structures. Experimental results also show that the temperature variation is an important factor in the structural health monitoring applications and its effect on the impedance-based monitoring of the initiation and growth of fatigue cracks in the lug samples is experimentally investigated. The electrical impedance-based SHM technique is also applied in monitoring the loosening of bolted joints in a full-scale railroad switch and the sensitivity of this technique to different levels of loosening of the bolts is investigated.
The second part of the work presented here focuses on the analytical study and better understanding of the effect of temperature on the vibration-based SHM. This is done by analytical modeling of the vibratory response of an Euler-Bernoulli beam with two different support conditions of simply supported and clamped-clamped and with a single, non-breathing fatigue crack at different locations along the length of the beam. The effect of temperature variations on the vibratory response of the beam structure is modeled by considering the two effects of temperature-dependent material properties and thermal stress formations inside the structure. The inclusion of thermal effects from both of these points of view (i.e. material properties variations and generation of thermal stresses) as independent factors is investigated and justified by studying the formulations of Helmholtz free energy and stresses inside a body. The effect of temperature variations on the vibratory response of the cracked beam are then studied by integrating these two temperature-related effects into the analytical modeling. The effect of a growing fatigue crack as well as temperature variations and thermal loadings is then numerically studied on the deflection of the beam and the output voltage of a surface-bonded piezoceramic sensor.Ph. D.
50
Zhang, Gongwang. "THE FORMATION MECHANISM OF α-PHASE DISPERSOIDS AND QUANTIFICATION OF FATIGUE CRACK INITIATION BY EXPERIMENTS AND THEORETICAL MODELING IN MODIFIED AA6061 (AL-MG-SI-CU) ALLOYS." UKnowledge, 2018. https://uknowledge.uky.edu/cme_etds/90.
Full textAbstract:
AA6061 Al alloys modified with addition of Mn, Cr and Cu were homogenized at temperatures between 350 ºC and 550 ºC after casting. STEM experiments revealed that the formation of α-Al(MnFeCr)Si dispersoids during homogenization were strongly affected by various factors such as heating rate, concentration of Mn, low temperature pre-nucleation treatment and homogenization temperature. Through analysis of the STEM results using an image software Image-Pro, the size distributions and number densities of the dispersoids formed during different annealing treatments were quantitatively measured. It was revealed that increasing the heating rate or homogenization temperature led to a reduction of the number density and an increase in size of the dispersoids. The number density of dispersoids could be markedly increased through a low temperature pre-nucleation treatment. A higher Mn level resulted in the larger number density, equivalent size and length/width ratio of the dispersoids in the alloy.
Upsetting tests on two of these Mn and Cr-containing AA6061 (Al-Mg-Si-Cu) Al alloys with distinctive Mn contents were carried out at a speed of 15 mm s-1 under upsetting temperature of 450 ºC after casting and subsequent homogenization heat treatment using a 300-Tone hydraulic press. STEM experiments revealed that the finely distributed α-Al(MnFeCr)Si dispersoids formed during homogenization showed a strong pinning effect on dislocations and grain boundaries, which could effectively inhibit recovery and recrystallization during hot deformation in the two alloys. The fractions of recrystallization after hot deformation and following solution heat treatment were measured in the two alloys with EBSD. It was found that the recrystallization fractions of the two alloys were less than 30%. This implied that the finely distributed α-dispersoids were rather stable against coarsening and they stabilized the microstructure by inhibiting recovery and recrystallization by pinning dislocations during deformation and annealing at elevated temperatures. By increasing the content of Mn, the effect of retardation on recrystallization were further enhanced due to the formation of higher number density of the dispersoids.
STEM and 3-D atom probe tomography experiments revealed that α-Al(MnFeCr)Si dispersoids were formed upon dissolution of lathe-shaped Q-AlMgSiCu phase during homogenization of the modified AA6061 Al alloy. It was, for the first time, observed that Mn segregated at the Q-phase/matrix interfaces in Mn-rich regions in the early stage of homogenization, triggering the transformation of Q-phase into strings of Mn-rich dispersoids afterwards. Meanwhile, in Mn-depleted regions the Q-phase remained unchanged without segregation of Mn at the Q-phase/matrix interfaces. Upon completion of α-phase transformation, the atomic ratio of Mn and Si was found to be 1:1 in the α-phase. The strengthening mechanisms in the alloy were also quantitatively interpreted, based on the measurements of chemical compositions, dispersoids density and size, alloy hardness and resistivity as a function of the annealing temperature. This study clarified the previous confusion about the formation mechanism of α-dispersoids in 6xxx series Al alloys.
Four-point bend fatigue tests on two modified AA6061 Al alloys with different Si contents (0.80 and 1.24 wt%, respectively) were carried out at room temperature, f = 20 Hz, R = 0.1, and in ambient air. The stress-number of cycles to failure (S-N) curves of the two alloys were characterized. The alloys were solution heat treated, quenched in water, and peak aged. Optical microscopy and scanning electron microscopy were employed to capture a detailed view of the fatigue crack initiation behaviors of the alloys. Fatigue limits of the two alloys with the Si contents of 0.80 and 1.24 wt% were measured to be approximately 224 and 283.5 MPa, respectively. The number of cracks found on surface was very small (1~3) and barely increased with the applied stress, when the applied stress was below the yield strength. However, it was increased sharply with increase of the applied stress to approximately the ultimate tensile strength. Fatigue crack initiation was predominantly associated with the micro-pores in the alloys. SEM examination of the fracture surfaces of the fatigued samples showed that the crack initiation pores were always aspheric in shape with the larger dimension in depth from the sample surface. These tunnel-shaped pores might be formed along grain boundaries during solidification or due to overheating of the Si-containing particles during homogenization.
A quantitative model, which took into account the 3-D effects of pores on the local stress/strain fields in surface, was applied to quantification of the fatigue crack population in a modified AA6061 Al alloy under cyclic loading. The pores used in the model were spherical in shape, for simplicity, with the same size of 7 μm in diameter. The total volume fraction of the pores in the model were same as the area fraction of the pores measured experimentally in the alloy. The stress and strain fields around each pore near the randomly selected surface in a reconstructed digital pore structure of the alloy were quantified as a function of pore position in depth from the surface using a 3-D finite element model under different stress levels. A micro-scale Manson-Coffin equation was used to estimate the fatigue crack incubation life at each of the pores in the surface and subsurface. The population of fatigue cracks initiated at an applied cyclic loading could be subsequently quantified. The simulated results were consistent with those experimentally measured, when the applied maximum cyclic stress was below the yield strength, but the model could not capture the sudden increase in crack population at UTS, as observed in the alloy. This discrepancy in crack population was likely to be due to the use of the spherical pores in the model, as these simplified pores could not show the effects of pore shape and their orientations on crack initiation at the pores near surface. Although it is presently very time-consuming to calculate the crack population as a function of pore size and shape in the alloy with the current model, it would still be desirable to incorporate the effects of shape and orientation of the tunnel-shaped pores into the model, in the future, in order to simulate the fatigue crack initiation more accurately in the alloy.