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1
McFadyen, Neil B. (Neil Barry) Carleton University Dissertation Engineering Mechanical. "Fatigue crack growth in semi-elliptical surface cracks." Ottawa, 1987.
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2
CORBANI, SILVIA. "CRACK GROWTH WITH PARTIAL BENDING-INDUCED CRACK CLOSURE." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2012. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=23847@1.
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PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIROCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
Neste trabalho são investigadas experimentalmente e numericamente as mudanças de geometria em trincas inicialmente passantes submetidas a carregamento remoto de flexão pura induzindo fechamento parcial das faces da trinca. Esse crescimento de trinca pode ocorrer numa variedade de estruturas com defeitos pré-existentes, tais como fuselagens de aviões, cascos de navios, vasos de pressões e pontes metálicas. O carregamento de flexão pura ocasiona regiões de tração e compressão na frente da trinca. É inquestionável que parte das faces da trinca sob compressão fecha independentemente de qualquer mecanismo de fechamento; e outra parte das faces da trinca, por outro lado, sob tração cresce mudando gradualmente de geometria. Após realizar ensaios em corpos-de-prova de aço ASTM A-36, foi observado que tais carregamentos geram uma quina na frente da trinca, que é a transição de uma geometria parcialmente passante e um trecho remanescente da geometria inicial. Para entender a distribuição do fator de intensidade de tensão em tais frentes de trinca, suas geometrias foram reproduzidas em um modelador tridimensional de mecânica da fratura linear elástica, o FRANC3D, acoplado a um programa de análise de elementos finitos (ABAQUS). Com este sistema acoplado, foram executadas análises considerando efeitos não lineares causados pelo contato das faces da trinca sob compressão. Verificou-se a necessidade de propor metodologias para tratamento dos resultados numéricos na quina, obtendo-se predições eficientes das mudanças na geometria da trinca. Contudo, a estimativa de vida, quando se compara taxas de crescimento da trinca obtidas em um corpo-de-prova sob tração cíclica e as taxas em um corpo-de-prova sob flexão com fechamento parcial da trinca, foi melhor reproduzida usando um fator de correção de fechamento da trinca. Adicionalmente, uma série de expressões empíricas normalizadas para geometrias da trinca e fatores de intensidade de tensão são propostas.
This work investigates experimentally and numerically how the front of initially through edge cracks in plate changes after they pass to be remotely fatigue loaded under pure bending to induce partial closure of the crack faces. This type of crack growth problem can occur in a variety of structures with preexisting defects, such as aircraft fuselages, ship hulls, pressure vessels components, and steel bridges. The bending loads induce tension and compression regions along the crack front, with the part of the crack faces that work under compression undoubtedly closed by the load, independently of any other closure mechanism. The part of the crack faces that work under tension; on the other hand, crack grows by fatigue gradually changing its shape. After performing tests on ASTM A36 steel specimens, it was observed that the bending load induces a kink on the crack front, in the transition between the part through crack created on the tension side and initial crack geometry. To understand the distribution of the stress intensity factor along such crack fronts, the measured crack shapes were reproduced in a three-dimensional fracture mechanics modeler (FRANC3D) coupled to a finite element analysis program (ABAQUS). With this coupled system, linear elastic stress analysis simulations were performed considering the nonlinear effects caused by the crack face contact in the compressed region. In particular, methods had to be proposed to treat numerical noise around the kink. The proposed methodology efficiently predicts the observed crack front shape changes; although the observed fatigue lives were better reproduced using a crackclosure correction factor when compared to crack growth data obtained from standard compact tension specimens. In addition, a series of normalized empiric expressions for both crack front shapes and stress intensity factors are proposed.
3
Hejman, Ulf. "On initiation of chemically assisted crack growth and crack propagation paths of branching cracks in polycarbonate." Licentiate thesis, Malmö högskola, Teknik och samhälle, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:mau:diva-7790.
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Stress corrosion, SC, in some cases gives rise to stress corrosion cracking, SCC, which differs from purely stress intensity driven cracks in many aspects. They initiate and grow under the influence of an aggressive environment in a stressed substrate. They grow at low load and may branch. The phenomenon of SCC is very complex, both the initiation phase and crack extension itself of SCC is seemingly associated with arbitrariness due to the many unknown factors controlling the process. Such factors could be concentration of species in the environment, stress, stress concentration, electrical conditions, mass transport, and so on.In the present thesis, chemically assisted crack initiation and growth is studied with special focus on the initiation and branching of cracks. Polycarbonate plates are used as substrates subjected to an acetone environment. Experimental procedures for examining initiation and branching in polycarbonate are presented. An optical microscope is employed to study the substrate.The attack at initiation is quantified from pits found on the surface, and pits that act as origin for cracks is identified and the distribution is analysed. A growth criterion for surface cracks is formulated from the observations, and it is used to numerically simulate crack growth. The cracks are seen to coalesce, and this phenomenon is studied in detail. Branching sites of cracks growing in the bulk of polycarbonate are inspected at the sample surface. It is found that the total width of the crack branches are approximately the same as the width of the original crack. Also, angles of the branches are studied. Further, for comparison the crack growth in the bulk is simulated using a moving boundary problem based algorithm and similar behaviour of crack branching is found.
4
Ahmad, Haider Yousif. "Fatigue crack growth at notches." Thesis, University of Sheffield, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360410.
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5
Shin, C. S. "Crack growth at stress concentrations." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.355680.
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6
Sheu, Yih-Chyun. "Dynamic elastic-viscoplastic crack growth /." The Ohio State University, 1988. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487587604133479.
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7
Patel, Surendra Kumar. "Experimental And Numerical Studies On Fatigue Crack Growth Of Single And Interacting Multiple Surface Cracks." Thesis, Indian Institute of Science, 2000. https://etd.iisc.ac.in/handle/2005/276.
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Design based on damage tolerance concepts has become mandatory in high technology structures. These concepts are also essential for evaluating life extension of aged structures which are in service beyond originally stipulated life. Fracture analysis of such structures in the presence of single or multiple three-dimensional flaws is essential for this approach. Surface cracks are the most commonly occurring flaws and development of accurate methods of analysis for such cracks is essential for structural integrity evaluation of newly designed or aged structures. The crack fronts of these surface flaws are usually approximated mathematically to be of either part-elliptical or part-circular in geometry. In this thesis, some of the issues related to fatigue crack growth of single and multiple surface cracks are studied in detail. Here emphasis is given to the development of simple and accurate post-processing techniques to estimate stress intensity factors for surface cracks, development and/or implementation of simple numerical methods to simulate three-dimensional single and multiple cracks in fatigue and their experimental verification.
Modified virtual crack closure integral (MVCCI) technique for estimation of strain energy release rates has been improved (chapter II) to deal with curved crack front and unequal elements across the crack front. The accuracy of this method is evaluated and presented in this chapter for certain benchmark surface flaw problems. The improved MVCCI is used in the investigation of interaction between multiple surface cracks in three-dimensional solids. The interaction effects are studied for both interacting and coalescing phases as observed to occur in the growth of multiple surface cracks. Extensive numerical work is performed to study the effects of various parameters such as aspect ratio, thickness ratio, interspacing on the interaction factors. These solutions are used in formulating empirical equations to estimate interaction factors. This facilitated the development of a simple semi-analytical method to study fatigue crack growth of multiple cracks.
The growth of surface cracks under fatigue loading in the finite width specimens of an aero-engine superalloy has been studied experimentally (presented in chapter III). Four configurations for single semi-elliptical cracks are considered. Fatigue crack growth is simulated by two models viz. two degrees of freedom and "multi degrees of freedom with ellipse fit'. These models are sometimes referred to as semi-analytical models as the crack growth is predicted by numerical integration combining Paris equation with an empirical form of stress intensity factor solution. In order to use two degrees of freedom model for fatigue crack growth prediction of semi-elliptical cracks, empirical solution for the Ml range of geometric parameters for stress intensity factor is required for the considered
configurations. The available Newman-Raju solution is useful for this purpose within a limited range of surface crack length to width (c/W) of the specimen. Based on the present finite element results, the empirical equations are developed for extended values of c/W. It is well understood that the fatigue prediction for two-dimensional crack can be improved by inclusion of crack closure effects. Usually, in semi-analytical models for growth of surface cracks under fatigue loading, the crack closure is included as a ratio of crack closure factor at surface and depth locations of semi-elliptical crack. In the present work, this ratio for the considered material of specimens is obtained by an experimental study. The difference in characteristics of preferred propagation path between semi-elliptical crack in a finite width plate and a wide plate is clearly brought out.
Current crack growth predictions for most of the structures are based on the presence of only a single crack. However, in structures several cracks may initiate simultaneously within a stress critical zone and may interact depending upon their geometry, spatial location, structure geometry and mode of loading. In this work various configurations of twin semi-elliptical cracks have been studied by experiments. The beachmarks created on the specimens during experiments are used in the investigation of crack shape progression during fatigue. A three degrees of freedom crack growth model for interacting and coalescing cracks has been proposed. The experimentally determined crack shape and lives have been compared with the corresponding values from numerical simulation.
The correlation of experimental results with numerical predictions was carried out through improved MVCCI for eight-noded brick elements. This has worked well in the configurations analysed. However, it is known in literature that there are benefits of using 20-noded singular elements. There could be special situations where the regular elements could fail, and singular elements could be essential. For this purpose, further development of MVCCI were carried out using 20-noded quarter-point elements (presented in chapter IV). Also a novel technique of decomposed crack closure integral (DCCI) was developed (presented in chapter V) for both regular and singular elements to represent the variation of MVCCI more accurately along the crack front.
It is well known that quarter-point elements at crack front produce the required singularity at the crack tip and give accurate stress distribution with fewer degrees of freedom than conventional elements. Thus to develop more efficient post-processing tools, the MVCCI expressions are formulated for 20-noded singular quarter-point element for various assumptions regarding stress and displacement distributions in the elements across the crack front. A comprehensive study is presented (chapter IV) on MVCCI for 20-noded singular brick element including various simplified expressions for three-dimensional part-through cracks in pure and mixed-mode state of deformation of fracture. The developed MVCCI expressions are also valid for 15-noded quarter-point Penta elements. The reduction in model size can further be obtained if 12-noded three-dimensional singular element is employed at the crack front and eight-noded elements are used away from the crack front. The MVCCI expressions are also developed for 12-noded singular element and their accuracy is evaluated by numerical solutions.
Presently, MVCCI, estimates the average stress intensity factor at the center of each element along the crack front. In this thesis, a Decomposed Crack Closure Integral (DCCI) is formulated to represent an assumed variation of stress intensity factor along the crack front in each element. The DCCI is formulated for 8-noded brick, 20-noded conventional brick and 20-noded singular brick elements. The numerical examples presented here deal with three-dimensional problems of patch repair technology and part-through cracks. The technique showed a major advantage for the patch repair problems where SIF variations along the crack front are of significance and large mesh sizes are computationally expensive. This along with MVCCI for 12-noded and 20-noded singular elements formed a part of the work on development of accurate and effective post-processing tools.
It is expected that the present work will be helpful in damage tolerance design and assessment of aerospace structures and the experimental work performed as a part of this thesis will enhance confidence in the damage tolerance analysis.
The thesis is concluded in chapter VI presenting the contributions of this thesis and projecting future lines of work possible in this area.
8
Patel, Surendra Kumar. "Experimental And Numerical Studies On Fatigue Crack Growth Of Single And Interacting Multiple Surface Cracks." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/276.
Full textAbstract:
Design based on damage tolerance concepts has become mandatory in high technology structures. These concepts are also essential for evaluating life extension of aged structures which are in service beyond originally stipulated life. Fracture analysis of such structures in the presence of single or multiple three-dimensional flaws is essential for this approach. Surface cracks are the most commonly occurring flaws and development of accurate methods of analysis for such cracks is essential for structural integrity evaluation of newly designed or aged structures. The crack fronts of these surface flaws are usually approximated mathematically to be of either part-elliptical or part-circular in geometry. In this thesis, some of the issues related to fatigue crack growth of single and multiple surface cracks are studied in detail. Here emphasis is given to the development of simple and accurate post-processing techniques to estimate stress intensity factors for surface cracks, development and/or implementation of simple numerical methods to simulate three-dimensional single and multiple cracks in fatigue and their experimental verification.
Modified virtual crack closure integral (MVCCI) technique for estimation of strain energy release rates has been improved (chapter II) to deal with curved crack front and unequal elements across the crack front. The accuracy of this method is evaluated and presented in this chapter for certain benchmark surface flaw problems. The improved MVCCI is used in the investigation of interaction between multiple surface cracks in three-dimensional solids. The interaction effects are studied for both interacting and coalescing phases as observed to occur in the growth of multiple surface cracks. Extensive numerical work is performed to study the effects of various parameters such as aspect ratio, thickness ratio, interspacing on the interaction factors. These solutions are used in formulating empirical equations to estimate interaction factors. This facilitated the development of a simple semi-analytical method to study fatigue crack growth of multiple cracks.
The growth of surface cracks under fatigue loading in the finite width specimens of an aero-engine superalloy has been studied experimentally (presented in chapter III). Four configurations for single semi-elliptical cracks are considered. Fatigue crack growth is simulated by two models viz. two degrees of freedom and "multi degrees of freedom with ellipse fit'. These models are sometimes referred to as semi-analytical models as the crack growth is predicted by numerical integration combining Paris equation with an empirical form of stress intensity factor solution. In order to use two degrees of freedom model for fatigue crack growth prediction of semi-elliptical cracks, empirical solution for the Ml range of geometric parameters for stress intensity factor is required for the considered
configurations. The available Newman-Raju solution is useful for this purpose within a limited range of surface crack length to width (c/W) of the specimen. Based on the present finite element results, the empirical equations are developed for extended values of c/W. It is well understood that the fatigue prediction for two-dimensional crack can be improved by inclusion of crack closure effects. Usually, in semi-analytical models for growth of surface cracks under fatigue loading, the crack closure is included as a ratio of crack closure factor at surface and depth locations of semi-elliptical crack. In the present work, this ratio for the considered material of specimens is obtained by an experimental study. The difference in characteristics of preferred propagation path between semi-elliptical crack in a finite width plate and a wide plate is clearly brought out.
Current crack growth predictions for most of the structures are based on the presence of only a single crack. However, in structures several cracks may initiate simultaneously within a stress critical zone and may interact depending upon their geometry, spatial location, structure geometry and mode of loading. In this work various configurations of twin semi-elliptical cracks have been studied by experiments. The beachmarks created on the specimens during experiments are used in the investigation of crack shape progression during fatigue. A three degrees of freedom crack growth model for interacting and coalescing cracks has been proposed. The experimentally determined crack shape and lives have been compared with the corresponding values from numerical simulation.
The correlation of experimental results with numerical predictions was carried out through improved MVCCI for eight-noded brick elements. This has worked well in the configurations analysed. However, it is known in literature that there are benefits of using 20-noded singular elements. There could be special situations where the regular elements could fail, and singular elements could be essential. For this purpose, further development of MVCCI were carried out using 20-noded quarter-point elements (presented in chapter IV). Also a novel technique of decomposed crack closure integral (DCCI) was developed (presented in chapter V) for both regular and singular elements to represent the variation of MVCCI more accurately along the crack front.
It is well known that quarter-point elements at crack front produce the required singularity at the crack tip and give accurate stress distribution with fewer degrees of freedom than conventional elements. Thus to develop more efficient post-processing tools, the MVCCI expressions are formulated for 20-noded singular quarter-point element for various assumptions regarding stress and displacement distributions in the elements across the crack front. A comprehensive study is presented (chapter IV) on MVCCI for 20-noded singular brick element including various simplified expressions for three-dimensional part-through cracks in pure and mixed-mode state of deformation of fracture. The developed MVCCI expressions are also valid for 15-noded quarter-point Penta elements. The reduction in model size can further be obtained if 12-noded three-dimensional singular element is employed at the crack front and eight-noded elements are used away from the crack front. The MVCCI expressions are also developed for 12-noded singular element and their accuracy is evaluated by numerical solutions.
Presently, MVCCI, estimates the average stress intensity factor at the center of each element along the crack front. In this thesis, a Decomposed Crack Closure Integral (DCCI) is formulated to represent an assumed variation of stress intensity factor along the crack front in each element. The DCCI is formulated for 8-noded brick, 20-noded conventional brick and 20-noded singular brick elements. The numerical examples presented here deal with three-dimensional problems of patch repair technology and part-through cracks. The technique showed a major advantage for the patch repair problems where SIF variations along the crack front are of significance and large mesh sizes are computationally expensive. This along with MVCCI for 12-noded and 20-noded singular elements formed a part of the work on development of accurate and effective post-processing tools.
It is expected that the present work will be helpful in damage tolerance design and assessment of aerospace structures and the experimental work performed as a part of this thesis will enhance confidence in the damage tolerance analysis.
The thesis is concluded in chapter VI presenting the contributions of this thesis and projecting future lines of work possible in this area.
9
Vethe, Stine. "NUMERICAL SIMULATION OF FATIGUE CRACK GROWTH." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for produktutvikling og materialer, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18721.
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The purpose of this study was to explore the posibilities and challenges with simulating fatigue crack growth (FCG) by the extended finite element method (XFEM). Another aim was to develope a procedure for XFEM FCG simulations in Abaqus by means of scripting. Finally was the procedure used to simulate FCG in an API standard, cone shaped threaded connection. Different FCG models were reviewed by a limited litterature search and a procedure 2D FCG simulations was carried out by a python script. The procedure succeeded with the simulation of FCG when applied to a model with refined mesh around the crack tip. In the suggested partial tasks of the thesis description were a procedure 3D FCG simulation also suggested, but as this required more computer capacity than available in the study this was not carried out.
10
Baldie, Keith David. "Crack growth in hardened cement paste." Thesis, Imperial College London, 1986. http://hdl.handle.net/10044/1/37934.
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Isaksson, Per. "On crack growth under compressive stresses /." Luleå, 2001. http://epubl.luth.se/1402-1544/2001/06/index.html.
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Medina, Velarde Jose Luis. "Fast ductile crack growth in panels." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.343563.
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Chambers, A. C. "Mixed mode creep/fatigue crack growth." Thesis, University of Nottingham, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.328464.
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14
Loo-Morrey, Marianne. "Crack growth transition in Udimet 720." Thesis, University of Southampton, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.245339.
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Ratsimba, Christian H. H. "Fatigue crack growth of filled elastomers." Thesis, Queen Mary, University of London, 2000. http://qmro.qmul.ac.uk/xmlui/handle/123456789/26243.
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In the past, the use of a fracture mechanics approach to describe crack growth in elastomers has been shown to work well for specimens of simple test geometry, simply loaded. This has been the case because elastic strain energy density (e.s.e.d.) functions could reliably be used to calculate both the magnitude of elastic stored energy available to drive a crack and the magnitude of the rate of release of such energy as the crack grows. The aim of this thesis was to investigate the applicability of such a methodology to situations of more complex loading. To this end two novel test-piece geometries were developed. The first consisted of a pure shear geometry with the sample having been pre-strained in the longitudinal direction to varying extents, hence introducing a type of bi-axial deformation. The second consisted of a pure shear geometry test-piece inclined at 30° to the horizontal and loaded in the vertical direction, hence inducing simultaneously pure shear and simple shear loading. Both types of test-piece were used to study the validity of the particular e.s.e.d. functions, the energetics and mechanics of crack growth and crack growth geometries on a macro and micro scale. The constants in particular e.s.e.d. functions were determined by uniaxially deforming in pure shear each of the carbon black reinforced materials used in this study. The resulting functions became progressively less good at predicting the elastic strain energy in the novel geometry test-pieces as the deformation modes became more complex. Anisotropy induced by deforming specimens in one direction was not easily removed even by an imposed large deformation in another direction. Nevertheless, the functions were successfully used to predict crack growth directions in the 30° inclined test-piece. However in the pre-strain pure shear test-pieces the functions significantly underestimated the elastic strain energy. Hence the real energies had to be determined from the forces and extensions measured during cyclic crack growth tests. In these tests crack growth rates for a given tearing energy (elastic energy release rate) increased as the magnitude of the pre-strain increased. This significant weakening was associated with the development of a strain induced molecular and carbon black anisotropy.
16
Crouch, Barry A. "High speed crack growth in polymers." Thesis, Imperial College London, 1986. http://hdl.handle.net/10044/1/37984.
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Djavanroodi, Faramarz. "Creep and creep-fracture crack growth." Thesis, Imperial College London, 1989. http://hdl.handle.net/10044/1/47412.
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Van, Arsdell William W. (William Walker). "Subcritical crack growth in polysilicon MEMS." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/10439.
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Lin, Xiaobin. "Numerical simulation of fatigue crack growth." Thesis, University of Sheffield, 1995. http://etheses.whiterose.ac.uk/14437/.
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This thesis describes the advances made by the author on the direct modelling of the fatigue growth of planar cracks A versatile step-by-step 3D finite element technique has been developed, which calculates the stress intensity factors at a set of points on the crack front according to linear elastic fracture mechanics principles and then applies a fatigue crack growth la\v to this set of points to obtain a new crack profile The software DUCK developed has a good capability of automatic remeshing so that the fatigue propagation of arbitrary shaped cracks can be conveniently followed The stress intensity factor calculation using the 3D finite element method has been improved and widely verified to be of good accuracy generally, which provides an important guarantee for the fatigue growth prediction. Several major problems associated with the stress intensity factor calculation, such as crack front definition, free surface layer and mesh abutting the crack front, as well as J-integral path independence, have been discussed A variety of cracked geometries of either theoretical or practical significance have been modelled by the numerical technique, including internal defects in infinite solids, surface cracks in finite thickness plates, round bars and pressure vessels, and initially multiple surface small cracks. Many results useful for the fatigue assessment of each kind of crack, such as stress intensity factor variations, fatigue shape changes and fatigue lives, have been obtained. By comparing with existing numerical methods and some limited experimental data found in the literature, it is shown that the numerical simulation technique is reliable and can predict the fatigue propagation of complex crack configurations, which is seldom possible for methods with assuming crack front configurations Meanwhile, due to the highly automated procedure or the technique, it has become straightforward to model the fatigue growth of practical cracked components, as demonstrated by the computations in this thesis. It is believed that such a numerical simulation technique has a great potential and will play an useful role in the area of fatigue study and assessment.
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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.
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Zhang, Hong Carleton University Dissertation Engineering Mechanical. "Fatigue crack growth and coalescence study." Ottawa, 1993.
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22
Fazluddin, Shahed. "Crack growth resistance in nuclear graphite." Thesis, University of Leeds, 2002. http://etheses.whiterose.ac.uk/879/.
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R-curve behaviour is often used for evaluating crack growth resistance in quasibrittle materials but few studies have focused on polycrystalline graphite. In this study, R-curve behaviour in three commercial grade nuclear graphites, of varying structure and properties, is compared using an optical method, a theoretical compliance method, and, a potential drop (PD) technique to measure crack length. Two graphites are coarse-grained and the third a fine grained graphite. Both 3-point bend and compact tension specimens are used. The fine-grain graphite shows lowest resistance, with the coarse-grained materials displaying similar R-curves. The compliance method is simplest but assumes the material is linear elastic, producing similar R-curves to the optical method. The PID method seriously underestimates the crack length due to crack face bridging, causing the R-curves to show a falling behaviour. The shortfall in the PID measurements presents a novel way of physically measuring the bridging zone length. The graphites display similar variation in the apparent bridging zone length despite the difference in grain size. Higher resistance in the coarser material results from increased crack path deflection, coupled with stronger grain bridging traction within the bridging zone. The bridging zone length is longer in compact tension specimens than in 3-point bend specimens, explaining partly why higher fracture energy is recorded in compact tension. In oxidised graphite, the crack growth resistance reduces but the coarser materials still show useful resistance. The fine-grain graphite shows a tendency toward flat R-curve behaviour with increasing oxidation. The mechanisms responsible for toughening in non-oxidised graphite prevail in oxidised material but diminish with increasing porosity and loss of binder phase. A preliminary study of the nano-indentation behaviour of the nuclear graphites reveals a similar response in all the materials. Anelastic hysteresis in the loadunload response is found with little residual deformation. There is also evidence of a creep effect during the dwell period at the maximum load. Comparison of the average indentation modulus with the bulk flexural and sonic modulus indicates that the nano-indentation technique is unable to sense bulk modulus changes such as occur with oxidation or the forming process. Instead, the method is susceptible to localised structural inhomogeneities.
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Lakkundi, Avinash Basavraj Soschinske Kurt A. "Prediction of crack growth in aircraft engine bleed air ducts using finite element and crack growth analysis." Diss., Click here for available full-text of this thesis, 2006. http://library.wichita.edu/digitallibrary/etd/2006/t011.pdf.
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Thesis (M.S.)--Wichita State University, Dept. of Mechanical Engineering."May 2006." Title from PDF title page (viewed on October 19, 2006). Thesis adviser: Kurt Soschinske. Includes bibliographic references (leaves 71-75).
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Hannes, Dave. "Growth of cracks at rolling contact fatigue." Thesis, KTH, Hållfasthetslära (Avd.), 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-33659.
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Rolling contact fatigue is a problem encountered with many machine elements.In the current report a numerical study has been performed in order to predictthe crack path and crack propagation cycles of a surface initiated rolling contactfatigue crack. The implementation of the contact problem is based on theasperity point load mechanism for rolling contact fatigue. The practical studiedproblem is gear contact. Different loading types and models are studied andcompared to an experimental spall profile. Good agreement has been observedconsidering short crack lengths with a distributed loading model using normalloads on the asperity and for the cylindrical contact and a tangential load on theasperity. Several different crack propagation criteria have been implemented inorder to verify the validity of the dominant mode I crack propagation assumption.Some general characteristics of rolling contact fatigue cracks have beenhighlighted. A quantitative parameter study of the implemented model hasbeen performed.Utmattning med rullande kontakter är ett ofta förekommande problem för många maskinelement. I den aktuella rapporten utfördes en numerisk studieför att förutsäga sprickvägen hos utmattningssprickor som initierats i ytan vidrullande kontakter. Implementeringen av kontaktproblemet bygger på asperitpunktlastmekanismen för rullande kontakter. Studien av kontaktproblemetär tillämpad till kugghjul. Olika belastningstyper och modeller studeradesoch jämfördes med profilen hos en experimentell spall. Bra överensstämmelseobserverades för korta spricklängder när en modell med fördelad belastninganvänds för en belastningstyp där en normalbelastning agerar på asperiten ochvid cylindriska kontakten och en tangentialbelastning införs på asperiten. Olikakriterier för spricktillväxt implementerades för att verifiera giltigheten av antagandetatt mode I spricktillväxt är dominant. Några generella kännetecken avutmattningssprickor med rullande kontakter framhävdes. En kvantitativ parameterstudie för den implementerade modellen utfördes.
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Bian, Lichun, and 卞立春. "Fatigue crack growth analysis of metallic plates with an inclinedsemi-elliptical crack." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2001. http://hub.hku.hk/bib/B31242844.
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Fuleki, Daniel M. Carleton University Dissertation Engineering Mechanical and Aerospace. "Crack initiation and short crack growth characteristics of notched merl 76 specimens." Ottawa, 1995.
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Williams, Joshua Marc. "EVALUATION OF FATIGUE CRACK GROWTH SOFTWARE FOR USE ON CRACKS IN COMPLEX GEOMETRIES." MSSTATE, 2009. http://sun.library.msstate.edu/ETD-db/theses/available/etd-04032009-104840/.
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Fatigue-crack growth data for the lower arm of the Apache helicopters scissor assembly is presented from an Army alternate source qualification test. The lower arm model is imported to finite element analysis software to obtain the stress state at a crack location. The stress state and geometry are used in seven fatigue-crack growth cases in NASGRO and AFGROW, with an additional four cases discussed briefly. The results from the fatigue-crack growth routines are compared to the fatigue-crack growth data from the Armys test. One case reproduces the crack growth data prior to breakthrough. Some cases are shown to be more applicable to this configuration than others are. The process of performing fatigue life estimates is discussed. Suggestions are made on the viability of this approach and possible future avenues for development.
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Nosair, S. I. M. "Fatigue crack growth in aluminium alloy structures." Thesis, University of Salford, 1986. http://usir.salford.ac.uk/26840/.
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Fatigue cracks have been reported in the super structure of the surface ships in the navy. Repair techniques have been proposed and applied on compact tension specimens. Such techniques were; either by creating compressive residual stresses using welding anisotropy, local plastic deformation and spot heating. Or, constraining the crack tip using tightened bolt or steel/carbon fibre patches. Complete F.C.G. data were obtained for the metal in the as-rolled and welded conditions. A metallurgical study has also been made. Fractography, the influence of residual stresses and second phases have been studied. The study showed that superior crack resistance was obtained from spot heated specimens, which were very simple to prepare. Considerable retardation was also obtained using welding anisotropy, local plastic deformation and tightened bolts. Patching seems to provide limited improvement. The influence of compressive residual stress was found to improve the crack resistance. The influence of the inclusions was found to depend on their morphology. Medium sized and homogeneously distributed inclusions provided a retarding effect on the crack speed. More inclusion interaction was reported in the presence of residual stresses.
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Recalde, Juan Jose. "Estimating Crack Growth in Temperature Damaged Concrete." NCSU, 2009. http://www.lib.ncsu.edu/theses/available/etd-11252009-111845/.
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Evaluation of the structural condition of deteriorated concrete infrastructure and evaluation of new sustainable cementitious materials require an understanding of how the material will respond to applied loads and environmental exposures. A fundamental understanding of how microstructural changes in these materials relate to changes in mechanical properties and changes in fluid penetrability is needed. The ability to provide rapid, inexpensive assessment of material characteristics and relevant engineering properties is valuable for decision making and asset management purposes. In this investigation, the effects of changes in dynamic elastic properties with water content and fluid penetrability properties before and after a 300 °C exposure were investigated as they relate to estimates of the crack density parameter, developed from the work by OâConnell and Budiansky (1974; 1977) on dry and saturated crack media. The experimental and analytical techniques described in this dissertation allow calculation of a value for the crack density parameter nondestructively from differences in wet and dry dynamic shear modulus of relatively thin disks. The techniques were used to compare a conventional concrete mixture and several mixtures with enhanced sustainability characteristics. The analysis provided quantitative assessment of changes with high temperature damage and autogenous healing, and provided estimates of increases in mean crack trace lengths. The three enhanced sustainable materials investigated were a very high fly ash mixture (mixture F), a magnesium phosphate cement based mortar (mixture M), and a magnesium phosphate cement based concrete (mixture G), and were compared to a conventional concrete mixture (mixture C). The results showed that water interaction, deterioration due to damage, and autogenous healing recovery were different for mixtures G and M than the concrete mixtures C and F based on portland cement. A strong correlation was found between log(API), Gd and crack density parameter. The findings imply that the test method and related analysis can be used to evaluate the validity of current standard test methods to new âgreenâ construction materials and therefore be a useful screening tool as well as providing important insight into microstructural changes in concrete under various exposures.
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Ljustell, Pär. "On predictions of fatigue crack growth rates /." Stockholm : Royal Institute of Technology, Department of Solid Mechanics, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-281.
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Chahardehi, Amir Ebrahim. "Fatigue Crack Growth in Complex Stress Fields." Thesis, Cranfield University, 2008. http://hdl.handle.net/1826/3481.
Full textAbstract:
Fatigue crack growth has been traditionally modelled using LEFM through the use of
the Paris law. This requires an accurate method for stress intensity factor (K)
calculation. Weight functions have been developed for one-dimensional cracks (e.g.
edge and through cracks); these are functions that enable separation of the loading and
geometry and considering the effect of each one of these two factors on the stress
intensity factor (SIF) separately. They have been proven to be useful for arbitrary stress
distributions where an accurate empirical formula for the stress intensity factor does not
exist. Such cases include residual stress fields due to surface treatments or welds.
However, in the case of surface cracks, or part-through cracks, the problem of
modelling the growth of these cracks poses two main questions, namely, how should the
Paris law be generalised to suit the two-dimensional scenario, and under arbitrary
loadings, how can the SIFs be calculated for these cracks. Current solutions involve
tedious mathematical calculations and are complicated functions.
In this thesis, the concept of root mean square (RMS) SIF is examined and by drawing
mathematical analogy with the one-dimensional case, a novel weight function is derived
which enables calculation of RMS SIF values for a range of semi-elliptical surface
cracks under arbitrary loadings. The accuracy of the weight function is verified through
comparisons with finite elements results for a variety of loadings/geometries. The
simplicity of the weight function construction method makes it a useful tool for fatigue
life predictions where incremental recalculations of SIF is required as the crack grows.
Surface treatments such as shot peening and laser peening are used for crack growth
retardation. It is generally believed that it is through the introduction of what is termed
‘beneficiary compressive residual stresses’ that crack retardation occurs. The
compressive residual stresses are superimposed on the ‘detrimental tensile stresses’ due
to loading and hence lead to a lower SIF level. By having such a strong tool as weight
functions, this general belief can be put to test. To this end, a set of experiments were
carried out to study the behaviour of cracks in residual stress fields arising from laser
peening. Edge cracks were grown in partially-peened specimens. Neutron diffraction
stress measurements were taken and stress profiles were obtained for these specimens.
Measurements of strain fields near the crack show the interaction between the crack and
the stress field induced by the peening process. The effect of laser peening on crack
growth is discussed and recommendations for future work are proposed.
Overall the thesis proposes a weight function for surface cracks the uniqueness of which
is in its simplicity, and develops an understanding of the nature of induced and transient
stresses in laser-peened components. The concept of ‘effective fatigue stress’ is
introduced and its calculation is described, and conclusions are drawn from the nature of
this stress distribution.
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Henderson, M. B. "Fatigue crack growth in single crystal superalloys." Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.314993.
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Ojala, Ira O. "Stress corrosion crack growth in porous sandstones." Thesis, University of Edinburgh, 2004. http://hdl.handle.net/1842/501.
Full textAbstract:
Stress corrosion crack growth occurs when the chemical weakening of strained crack tip bonds facilitates crack propagation. I have examined the effect of chemical processes on the growth of a creack population by carrying out triaxial compression tests on Clashach and Locharbriggs sandstones at temperatures of 25-80 degrees C and at strain rates of 10-5 to 10-8/s. The axial strain, permiability, acoustic emission (AE) activity and the pore fluid chemistry were monitored continuously during these tests. Rock strength is reduced in the presence of water and on the application of a slower strain rate. Elastic modulus also decreases with decreasing strain rate. Microstructural observations indicate that microfracturing is more pervasive in the slow strain rate tests in comparison to the high strain rate tests. Damage parameters derived from the AE data predict the stress-strain curves adequately. The accumulation of damage is more rapid in the slow strain rate tests than in the high strain rate tests. The exit pore fluid silica (Si) concentrations correlate with the main microfracturing domains of the stress-strain curve. In the strain hardening phase of the Locharbriggs tests the Si concentrations and AE damage increase exponentially. The small reactive surface area and the temperature dependance of the Si concentration in the Locharbriggs tests suggest that silica is dissolving actively from the growing crack tips and that reaction rates contribute towards this signal. the Locharbriggs Si signal and damage parameters are strongly correlated by a power law relationship. the obseved strain rate and environment dependance of mechanical properties of Locharbriggs sandstone can be uniquely attributed to crack growth by the stress corrosion mechanism. In the Clashach tests the damage accumulation is best described by a powe-law. The AE activity of both sandstones exhibits clear fore- and aftershock sequences that are well modeled by the Omori law with a power law exponent that is close to unity. The Clashach Omori decay parameter correlates with test temperature, indicating a faster decay of aftershock activity at a higher temperature. The permeability evolution also displays a distinct strain rate dependence. At high strain rates permeability correlates with microcrack damage. At slow strain rate the fluid flow properties correlate with mean effective stress or pore fluid ion concentrations. These observations suggest that brittle fracturing, chemical reaction and hydraulic properties of porous sandstone are strongly coupled processes in the crust.
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Persson, Johan. "On dynamic crack growth in discontinuous materials." Doctoral thesis, Mittuniversitetet, Avdelningen för naturvetenskap, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:miun:diva-24960.
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In this thesis work numerical procedures are developed for modeling dynamic fracture of discontinuous materials, primarily materials composed of a load-bearing network. The models are based on the Newtonian equations of motion, and does not require neither stiffness matrices nor remeshing as cracks form and grow. They are applied to a variety of cases and some general conclusions are drawn. The work also includes an experimental study of dynamic crack growth in solid foam. The aims are to deepen the understanding of dynamic fracture by answering some relevant questions, e.g. What are the major sources of dissipation of potential energy in dynamic fracture? What are the major differences between the dynamic fracture in discontinuous network materials as compared to continuous materials? Is there any situation when it would be possible to utilize a homogenization scheme to model network materials as continuous? The numerical models are compared with experimental results to validate their ability to capture the relevant behavior, with good results. The only two plausible dissipation mechanisms are energy spent creating new surfaces, and stress waves, where the first dominates the behavior of slow cracks and the later dominates fast cracks. In the numerical experiments highly connected random fiber networks, i.e. structures with short distance between connections, behaves phenomenologically like a continuous material whilst with fewer connections the behavior deviates from it. This leads to the conclusion that random fiber networks with a high connectivity may be treated as a continuum, with appropriately scaled material parameters. Another type of network structures is the ordered networks, such as honeycombs and semi-ordered such as foams which can be viewed as e.g. perturbed honeycomb grids. The numerical results indicate that the fracture behavior is different for regular honeycombs versus perturbed honeycombs, and the behavior of the perturbed honeycomb corresponds well with experimental results of PVC foam.
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Sulaiman, Salim J. "Fracture and crack growth in recycled mixes." Thesis, University of Dundee, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320522.
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Monahan, Craig C. "Early fatigue crack growth in offshore structures." Thesis, University College London (University of London), 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.271908.
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Li, Xu-Dong. "On kinetics of small fatigue crack growth." Thesis, Open University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.296607.
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Kim, Yun-Jae. "Modeling fully plastic, plane strain crack growth." Thesis, Massachusetts Institute of Technology, 1993. http://hdl.handle.net/1721.1/12223.
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Isogai, Takeshi. "Creep-fatigue crack growth in engineering materials." Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627408.
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Jivkov, Andrey P. "On crack growth in functionally graded materials." Licentiate thesis, Luleå tekniska universitet, 1999. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-25814.
Full textAbstract:
Stress intensity factors' behaviour is studied for long plane cracks interacting with a region of functionally graded elastic material. The region is assumed embedded into a large body treated as a homogeneous elastic continuum. The analysis is limited to small deviations of the graded region's elastic modulus from that of the surrounding body (Poisson's ratio is kept constant) and analytical solutions are sought using a perturbation technique. Emphasis is laid on the case of an infinite strip, which admits a closed form solution. A cosine change of the modulus of elasticity is treated, furnishing the solution for arbitrary variation in the form of a Fourier's expansion. Finite element analysis is subsequently performed for investigating the scope of validity of the analytical solution. The results for a set of finite changes of the elastic modulus are compared with the analytical predictions, and a remarkably wide range of validity is demonstrated. New functions, suitable for non-homogeneous material description, are introduced to approach the case of non-constant Poisson's ratio. The properties and possible applications of these functions are examined.Godkänd; 1999; 20070320 (ysko)
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Bold, P. E. "Multiaxial fatigue crack growth in rail steel." Thesis, University of Sheffield, 1990. http://etheses.whiterose.ac.uk/14807/.
Full textAbstract:
In the introduction to a recent symposium on rolling contact fatigue, R.A.Smith stated that it was difficult to apply our greatly increased understanding of metal fatigue, to rolling contact fatigue, because of "the apparent lack of alternating tensile stresses to drive the cracks." He went on to say "alternating shear stresses are easily found, but the reproduction of continuous crack growth controlled by shear (Mode II in fracture mechanics terms), has proved to be near impossible." This project has demonstrated that under specific conditions this mode of growth does occur. The project began by studying rolling contact fatigue defects, in particular the 'squat' defect in railway lines, and the stress analyses that have been performed on them. It was concluded that the largest stress cycle experienced by the cracks must be a shear stress. It. series of tests were then performed that loaded a crack in pure shear, or a mixture of tension and shear, looking at the effects of using fully reversed shear loading, and the effects of applying tensile mean stresses to reduce the friction on the crack flanks. However these tests all produced less than one millimetre of mode II growth, before the cracks arrested or branched. The final series of tests however applied a tensile load cycle before each shear load cycle. This time coplanar growth was produced, that is the crack grew in the direction of the maximum shear stress. This type of load cycle is a simplification of the load cycle calculated by Bower and Johnson of Cambridge University, where the tensile load is produced by fluid trapped in the crack. Two crack growth rate formulae were produced that fitted the data, indicating that the growth rate was dependent on both the tensile and the shear parts of the cycle.
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Geary, W. "Fatigue crack growth in iron silicon alloys." Thesis, Sheffield Hallam University, 1985. http://shura.shu.ac.uk/20609/.
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A technique for accurately monitoring fatigue crack growth at near threshold growth rates has been established. The characteristics of near threshold fatigue crack growth of a number of iron-silicon alloys has been quantitatively and qualitatively investigated. Relationships have been established relating the stress intensity factor, AK, and the fatigue crack growth rate da/dN. At fatigue crack growth rates approaching threshold the material has shown some microstructural sensitivity and this has been related to the stress intensity factor and the yield stress. A relationship has been shown to exist between the value of the threshold stress intensity factor and the inverse root of the grain size, d~2, for each of the alloys investigated. A model for near threshold fatigue crack growth has been proposed and includes the contributions made by grain size and crack tip plasticity. This work has also shown that fatigue crack closure plays an important role in the micromechanisms of fatigue crack growth near the threshold at low R ratio s. A number of mechanisms have been identified: crack closure due tothe presence of oxidation products on fracture surfaces in tests conducted in air, and closure due to the presence of fatigue fretting, facet contact and a contribution of mixed mode opening.
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Shademan, Sassan Steven. "Mechanism-based models of fatigue crack growth /." The Ohio State University, 2000. http://rave.ohiolink.edu/etdc/view?acc_num=osu1488203857249745.
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Lindblom, David. "Crack growth based FEM with embedded discontinuities." Thesis, KTH, Hållfasthetslära, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-298347.
Full textAbstract:
In recent years there have been a major developments in the computational mechanics community when it comes to modelling of fracture mechanics. There are now several well established numerical methods that are implemented in commercial programs such as: Phase Field Modelling, Enhanced Assumed Strain (EAS), Smeared Crack Methods, Extended Finite Element Method (XFEM) and Partition of Unity Finite Element Method (PUFEM). This work has been based on PUFEM and it has been of interest to investigate if this numerical method can be combined with plastic deformation. The reason for this is that it has been known that complex structural and material phenomenon such as embrittlement of steels or composites show a variation of fracture toughness. Thus more advanced approaches are required to capture the response of such structures and materials. The analysis was split in to three parts. First, a benchmark analysis was done with linear tetrahedral elements and, which has been implemented in previous applications. Thereafter the same analysis was done for quadratic tetrahedral elements. Finally, the interface between plasticity and PUFEM was implemented and was analysed with the same geometries as in the benchmark case. The analysis show that it is possible to combine plasticity in PUFEM setting and that it has a possibility to be used in future applications.Under dem senaste åren har det skett en massiv utveckling inom beräkningsmekaniken när det kommer till att modellera brottmekaniska fenomen. Det finns nu ett flertal väletablerade numeriska metoder som är implementerade i kommersiella program såsom: Phase Field Modelling, Enhanced Assumed Strain (EAS), Smeared Crack Methods, Extended Finite Element Method (XFEM) and Partition of Unity Finite Element Method (PUFEM). Detta arbete har fokuserat på (PUFEM) och det har varit av intresse att se om denna metod kan kombineras tillsammans med plastisk deformation. Anledningen till att detta har varit av intresse är på grund av att det finns ett flertal komplexa struktur - och materialfenomen såsom försprödning av stål och kompositer som uppvisar en variation i sin duktilitet. Detta medför att mer avancerade metoder behöver tillämpas för att fånga responsen av dessa strukturer och material. Analysen som har genomförts var uppdelad i tre delar. Först analyserades ett problem med linjär tetrahedriska element som ett riktmärke, detta har implementerats i tidigare applikationer. Därefter analyserades samma problem fast med tethraderiska element med kvadratisk interpolation. Slutligen så integrerades plasticitet med PUFEM, där samma geometrier analyserades som riktmärke. Den analysen som har genomfört visar att det går att kombinera plastiska deformation tillsammans med PUFEM och att denna metod har potentialen att användas i framtida applikationer.
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Doré, Matthew James. "An investigation of fatigue crack growth acceleration." Thesis, Open University, 2016. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.700486.
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The problem of fatigue crack growth acceleration was investigated in crack propagation studies and endurance testing. The study was driven by the needs of designers and researchers alike, to provide a better understanding of the mechanisms associated with accelerated growth, and recommendations on the use of Miner's rule to calculate fatigue life under variable amplitude loading. The study was conducted on S355 structural steel and 6082 T6 aluminium alloy using centre-crack tension (CCT) specimens, with and without additional welding, and longitudinal fillet welded specimens. Crack growth rates under simple sequence loading and more complex variable amplitude CV A) loading, all cycling down from fixed tensile stress levels, were determined using optical or direct current potential' drop methods and scanning electron microscope examination of fracture surface striations. Under simple loading sequences, comprising two magnitudes of stress range, the presence of tensile underloads resulted in accelerated growth rates compared with those based on constant amplitude (CA) loading. Various possible mechanisms to explain crack growth acceleration and factors that might influence it, notably crack closure and welding residual stress, were evaluated. The most promising outcome came from finite element analysis (FEA) of the crack tip stress and strain. This showed that whereas under CA loading the material near the crack tip cycled about zero mean stress, the mean stress was tensile after the application of a tensile underload, thus resulting in a higher crack growth rate. Fatigue endurance testing of welded joints performed under the same types of loading confirmed that Miner's rule overestimated the actual lives, consistent with the occurrence of acceleration. Thus, it was concluded that modification of the principle and application of Miner's rule is required to allow for stress interactions that cause crack growth acceleration. Preliminary design recommendations were made.
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Santos, e. Lucato Sergio Luis dos. "Constraint induced crack initiation and crack growth at electrode edges in piezoelectric ceramics." Phd thesis, [S.l.] : [s.n.], 2002. http://elib.tu-darmstadt.de/diss/000191.
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Rehman, Atiq-Ur. "An investigation of methods of reducing fatigue crack growth by artificial crack-closure effects." Thesis, University of Salford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315379.
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Wan, Victor. "Fundamental modelling studies of fatigue crack nucleation & microstructurally short crack growth in superalloy." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/61023.
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Current lifing techniques for metal superalloys have largely remained empirical, particularly those addressing the nucleation or initiation of fatigue cracks. However, there is growing interest within both the academic literature and industrial users to move away from the approaches which are dependent on empirical models. Instead, there has been greater focus on the advancement of developing fundamental mechanistically-based models to predict alloy behaviour in order to help better understand and predict defect nucleation processes. Early lifing methods in face centred cubic (fcc) and body centred cubic (bcc) polycrystalline alloys are of interest due to the relevance of cubic alloys in industrial applications, where microstructural fatigue behaviour to crack nucleation and short crack growth are studied. A stored energy criterion for fatigue crack nucleation is introduced which is validated with ferritic steel polycrystal specimens to reveal new evidence and address the scattered cycles to crack nucleation. The criterion is extended into a 3D crystal plasticity finite element model (CPFEM) representation of RS5 Nickel superalloy where the method provided a new perspective to quantify microstructurally sensitive cycles to fatigue crack nucleation in RS5 alloys. Comparisons of experimental and CPFEM investigation of microstructural stress distributions is presented across a polycrystalline copper where fatigue hotspots identified provided new insight of common nucleation hotspots, and is typically associated on/near grain boundaries. The work is extended using FEM to address Microstructural Sensitive Short Crack Growth (MSCG) in microstructurally different ferritic notched specimens. Our assessments and methodology introduced based on Extended Finite Element Method (XFEM) revealed information on the role of anisotropy to better capture the MSCG paths. In addition, user-defined materials and grain boundary properties were introduced to address the difficulties in capturing intergranular cracks where grain boundary properties were introduced to promote the grain boundary cracks as witnessed in experiments.
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Meyer, Arnd. "A New Efficient Preconditioner for Crack Growth Problems." Universitätsbibliothek Chemnitz, 2006. http://nbn-resolving.de/urn:nbn:de:swb:ch1-200601600.
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Nagaralu, Ramesh. "Fatigue Crack Growth Under Residual Stresses Around Holes." MSSTATE, 2005. http://sun.library.msstate.edu/ETD-db/theses/available/etd-11112005-143754/.
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Compressive residual stresses induced by tensile overloads, compressive under loads, or by a cold-expansion process in specimens containing a circular hole and their influence on subsequent fatigue crack growth in aluminum alloys are studied. The finite element method is used to calculate residual stresses. The superposition method, which uses crack-tip stress intensity factors for cases involving remote loading and residual stresses, is used to calculate crack growth life for three kinds of tests from the literature: (1) fatigue of a circular hole specimen after an overload or under load, (2) single crack growing from a circular hole after a severe tensile overload, and (3) single crack growing from a circular hole after cold-working, reaming and notching. All specimens were subjected to subsequent constant amplitude loading. The superposition method worked fairly well for most cases, but tended to over predict fatigue life for small cracks and for cracks growing under residual stresses, which produce compressive (maximum and minimum) stress intensity factors.