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Tesi sul tema "Fracture mechanics"
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1
Srivastava, Ankit. "Mechanics and Mechanisms of Creep and Ductile Fracture". Thesis, University of North Texas, 2013. https://digital.library.unt.edu/ark:/67531/metadc283799/.
Testo completoAbstract (sommario):
The main aim of this dissertation is to relate measurable and hopefully controllable features of a material's microstructure to its observed failure modes to provide a basis for designing better materials. The understanding of creep in materials used at high temperatures is of prime engineering importance. Single crystal Ni-based superalloys used in turbine aerofoils of jet engines are exposed to long dwell times at very high temperatures. In contrast to current theories, creep tests on Ni-based superalloy specimens have shown size dependent creep response termed as the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal creep tests were performed on uncoated Ni-based single crystal superalloy sheet specimens with two thicknesses and under two test conditions: a low temperature high stress condition and a high temperature low stress condition. At the high temperature, surface oxidation induced microstructural changes near the free surface forming a layered microstructure. Finite element calculations showed that this layered microstructure gave rise to local changes in the stress state. The specimens also contained nonuniform distribution of initial voids formed during the solidification and homogenization processes. The experiments showed that porosity evolution could play a significant role in the thickness debit effect. This motivated a basic mechanics study of porosity evolution in single crystals subjected to creep for a range of stress states. The study was performed using three-dimensional finite deformation finite element analysis of unit cells containing a single initially spherical void in a single crystal matrix. The materials are characterized by a rate-dependent crystal plasticity constitutive relation accounting for both primary and secondary creep. The effect of initial void spacing and creep exponent was also explored. Based on the experimental observations and results of finite element calculations a quantitative mechanistic model is proposed that can account for both bulk and surface damage effects and assess their relative roles in the observed thickness debit effect. Another set of calculations aim at relating the crack growth resistance and fracture surface morphology to material microstructure for ductile structural metals. The process that governs the ductile fracture of structural materials at room temperature is one of nucleation, growth and coalescence of micron scale voids, and involves large plastic deformations. Experimental studies have shown that fracture surfaces in a wide variety of materials and under a wide variety of loading conditions have remarkable scaling properties. For thirty years, the hope to relate the statistical characterization of fracture surfaces to a measure of a material's crack growth resistance has remained unfulfilled. Only recently has the capability been developed to calculate sufficient amounts of three dimensional ductile crack growth in heterogeneous microstructures to obtain a statistical characterization of the predicted fracture surfaces. This development has enabled the exploration of the relation of both fracture toughness and fracture surface statistics to material properties and microstructure when the fracture mechanism is one of void nucleation, growth and coalescence. The relation of both toughness and the statistical properties of fracture surfaces in calculations of heterogeneous microstructures to various microstructural features is discussed and a remarkable correlation between fracture surface roughness and fracture toughness is shown for the first time.
2
Mansfield-Williams, H. D. "Mode 11 fracture mechanics in solid wood and fracture mechanics in laminated veneer lumber". Thesis, Brunel University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.390815.
Testo completo
3
Tiernan, Declan Martin. "Collocation studies in fracture mechanics and quantum mechanics". Thesis, Queen's University Belfast, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318739.
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4
Denning, Shawn Patrick. "Fracture mechanics of sandwich structures". Thesis, Wichita State University, 2013. http://hdl.handle.net/10057/6809.
Testo completoAbstract (sommario):
Over the past few decades both the demand for knowledge as well an increase in the application of composite materials has boomed. This has led to an intensive focus by the research community to gather information and validate the use of such materials. Sandwich composites have been a particular interest and an intense demand for further understanding was sparked by the catastrophic rudder failure of Air Transat 961. This thesis focuses on understanding fracture mechanics and damage tolerance within sandwich composites. Facesheet disbond has severe impacts on the material systems strength and stiffness, and large disbonds can often lead to catastrophic component failure. Understanding how these disbonds grow is paramount to recognizing the limitations of sandwich composites.
This thesis has several objectives. First, determine the fracture toughness of various sandwich composite material systems under going quasi-static loading. Second, determine how variations within the material systems such as facesheet thickness, core type, cell size and core density effect the results. Third, determine how the failure modes such as adhesive, pullout and core alter the results. Fourth, determine how fluid ingression effects fracture toughness. Fifth, provide baseline data for further testing and modeling.Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering
5
Matthews, C. "Fracture mechanics of volcanic eruptions". Thesis, University College London (University of London), 2009. http://discovery.ucl.ac.uk/16280/.
Testo completoAbstract (sommario):
Seismology is a key tool in the forecasting of volcanic eruptions. The onset of an eruption is often preceded and accompanied by an increase in local seismic activity, driven by fracturing within the edifice. For closed systems, with a repose interval of the order of a century or more, this fracturing must occur in order to create a pathway for the magma to reach the surface. Time-to-failure forecasting models have been shown to be consistent with seismic acceleration patterns prior to eruptions at volcanoes in subduction zone settings. The aim of this research is to investigate the patterns in seismic activity produced by a failure model based on fundamental fracture mechanics, applied to a volcanic setting. In addition to the time series of earthquake activity, statistical measures such as seismic b-value are also analysed and compared with corresponding data from the field and laboratory studies. A greater understanding of the physical factors controlling fracture development and volcano-tectonic activity is required to enhance our forecasting capability. The one dimensional, fracture mechanics grid model developed in this work is consistent with the theory of growth and coalescence of multi-scale fractures as a controlling factor on magma ascent. The multi-scale fracture model predicts an initial exponential increase in the rate of seismicity, progressing to a hyperbolic increase that leads to eruption. The proposed model is run with variations in material and load properties, and produces exponential accelerations in activity with further development to a hyperbolic increase in some instances. In particular, the model reproduces patterns of acceleration in seismicity observed prior to eruptions at Mt. Pinatubo (1991) and Soufriere Hills (1995). The emergence of hyperbolic activity is associated with a mechanism of crack growth dominated by interaction and coalescence of neighbouring cracks, again consistent with the multi-scale fracture model. The model can also produce increasing sequences of activity that do not culminate in an eruption; an occurrence often observed in the field. Scaling properties of propagating fractures are also considered. The seismic bvalue reaches a minimum at the time of failure, similar to observations from the field and measurements of acoustic emissions in the laboratory. Similarly, the fractal dimension describing the fracture magnitude distribution follows trends consistent with other observations for failing materials. The spatial distribution of activity in the model emerges as a fractal distribution, even with an initially random location of fractures along the grid. Significant shifts in the temporal or spatial scaling parameters have been proposed as an indication of change in controlling factors on a volcanic system, and therefore represent a relatively unexplored approach in the art of eruption forecasting.
6
Richardson, James Bruce. "The mechanics of fracture healing". Thesis, University of Aberdeen, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.290866.
Testo completoAbstract (sommario):
The mechanics applied to healing fractures vary widely. At one extreme rigid internal fixation is advocated, while at the other early mobilisation is recommended using external splints. Kuhn's method of paradigm orientated research was used to define the historical context of current assumptions regarding fracture healing. Conflict between the various schools of thought is the main evidence for failure of these assumptions and the need to evolve a new perspective on fracture healing. A paradigm is presented which proposes healing by external callus as an early stage and 'primary healing' as the later stage as of one continuous but changing process. A fundamental hypothesis was tested: that mechanics is the major control of fracture healing in man. A multicentre study of 102 patients with serious fractures were treated with external skeletal fixation. In 60 patients rigid external fixation was applied. In the remaining 42 the same fixation device was used, but adapted to apply 1 to 2mm of cyclic axial micromovement across the fracture. A piston applied 500 cycles of movement over a 30 minute period each day until this could be achieved by the patient on weight-bearing. Objective assessment required development of new techniques of measuring fracture stiffness and defining the point of healing. This objective measure, and clinically defined healing, were significantly faster in the group treated with micromovement (two-way analysis of variance, p = 0.005 and 0.03, respectively). Repeated injury by plastic deformation is proposed to maintain callus growth in the first phase of healing. Evidence for the required parameters of movement was gathered from the trial of micromovement, from measurements in 4 cases of epiphyseolysis and also 8 patients undergoing arthrodesis. It would appear appropriate to apply cyclic axial displacement of 2mm within the first two weeks from injury and of consistent direction until sufficient bulk of callus is formed. Thereafter axial compaction is appropriate in a second phase where callus matures. The mechanics that govern remodelling were considered to apply to the final phase. Failure of a cell culture model to display obvious results from cyclic loading may indicate that the response to mechanical loading is indirect. Intermediate and mechanically dependent biochemical and bioelectrical factors are discussed.
7
MacLennan, Iain James. "Two parameter engineering fracture mechanics". Thesis, University of Glasgow, 1996. http://theses.gla.ac.uk/6756/.
Testo completoAbstract (sommario):
The object of this work was to investigate and expand on previously carried out research into elastic-plastic crack tip fields using the first two terms of the Williams expansion to characterise the degree of crack tip constraint. As a precursor to this research a history of fracture mechanics is also presented. In the present work crack tip fields in small scale yielding have been detennined using modified boundary layer formulations in an attempt to model the influence of the second order term of the Williams expansion, the T -stress. The prime object of this thesis was to investigate and expand on previously carried out research into a two parameter characterisation of elastic-plastic crack tip fields using the second parameter of the Williams expansion(T), which attempts to characterise the degree of crack tip constraint. Modified Boundary Layer formulations in conditions of plane strain were implemented to derive a suitable reference solutions, against which the effects of out of plane strains can be compared and the validity of presently established reference fields can be gauged. The effect of out of plane non-singular stress, S, on the crack tip stress field were also considered, where constraint was largely determined by T. A wide range of analyses have been carried out, from the microstructural scale to complete engineering components in an attempt to characterise crack tip stress fields. The ability to apply two parameter fracture concepts to real engineering structures requires methods for calculating T for complex components with realistic semi-elliptical defects. A simple engineering method for achieving this was developed making use of linespring elements in the finite element package ABAQUS. This approach was validated by the calculation of T for semi-elliptical cracks at the chord-brace intersection of a tubular welded joint, modelled using the mesh generation program PATRAN. The micromechanics of cleavage, using the Ritchie-Knott-Rice model have also been constructed. This work relates the ratio of J for unconstrained and constrained geometries to critical microstructural distance, critical cleavage stress and the toughness ratio on the strainhardening effect. The elastic-plastic behaviour of short and deeply cracked bend bars has previously been described by Betegon and Hancock based on the first two terms of the Williams expansion. A local cleavage criterion has been applied to these fields to indicate the effect of loss of constraint on lower shelf toughness of shallow cracked bend bars. The work models the maximum temperature at which cleavage can occur in these geometries to show the effect of constraint and aJW ratio of cracked bend bars on the ductile-brittle transition temperature. This has also been backed by a significant experimental research program. Finally constraint dependent toughness has been considered in relation to failure assessment methodologies. A simple engineering method for modifying these Failure Assesssment Diagrams has been presented, this consists of considering the constraint matched toughness of the strucutre. This procedure recovers the original Failure Assessment Line and unifies the constraint dependent fracture toughness within defect assessment schemes which utilise Failure Assessment Diagrams.
8
Jonsson, Anders. "Integral equation methods for fracture mechanics and micro-mechanical problems". Doctoral thesis, KTH, Solid Mechanics, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3336.
Testo completo
9
Stehn, Lars. "Tensile fracture of ice : test methods and fracture mechanics analysis". Doctoral thesis, Luleå tekniska universitet, Byggkonstruktion och -produktion, 1993. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-18394.
Testo completoAbstract (sommario):
This thesis is concerned with several aspects of fracture of both brackish (low salinity) sea ice and freshwater ice. The tests and analyses are confined to tensile, or in fracture mechanics language, Mode I, fracture. A large part of this thesis is dedicated to demonstrate that Linear Elastic Fracture Mechanics (LEFM) can be applicable on ice by laboratory and in-situ tests of defined specimens. All interpretations are made using the dicipline of LEFM.First, the development of a field test equipment called FIFT ( a Field Instrument for Fracture toughness Tests on ice) is described. The FIFT is used in both field and laboratory fracture toughness tests on brackish sea ice from the Gulf of Bothnia to describe porosity effects on the apparent fracture toughness, KQ, and estimate crack velocities. An appropriate speciment size, in terms of notch sensitivity, is then provided valid for grain sizes ranging from 1.6 to nearly 100 mm.An augmented use of the FIFT is then described where fracture toughness tests are performed on S1 type freshwater ice to investigate if similarities exist in the local KI fields for three different fracture geometries. The results indicate that, under comparable conditions, KQ is similar for all of the geometries. However, the type of specimen, has a marked influence on the character of the fracture surface.Then, the influence of structural anisotropy on the fracture toughness of S1 ice is investigated by fabricating and testing three different fracture geometries from a single ice core. This approach is suitable for both field and, as in this work, laboratory studies. There is a wide scatter in the KQ values. Possible explanations to the results are discussed in terms of the microstructural influences and specimen size effects.Finally, crack growth resistance measurements on large grained S1 ice is conducted. A new fracture geometry is used which is found to be extremely favorable of promoting stable, stick-slip, crack growth over a large portion of the uncracked ligament. Now a complete characterization of the fracture resistance curve is therefore possible, A negative fracture resistance KR-curve is evaluated for the S1 ice at -16°C.Godkänd; 1993; 20070426 (ysko)
10
Liu, Guoning. "Application of fracture mechanics in electrical/mechanical failures of dielectrics /". View abstract or full-text, 2006. http://library.ust.hk/cgi/db/thesis.pl?MECH%202006%20LIU.
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11
Vasic, Svetlana. "Applications of fracture mechanics to wood". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape2/PQDD_0030/NQ65471.pdf.
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12
Ohlsson, Ulf. "Fracture mechanics analysis of concrete structures /". Luleå, 1995. http://epubl.luth.se/avslutade/0348-8373/179/index.html.
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13
Pollitz, Ernest Theodore. "Studies on elastic-plastic fracture mechanics". Thesis, Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/18363.
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14
Carling, Michael John. "Fracture mechanics of short fibre composites". Thesis, Imperial College London, 1988. http://hdl.handle.net/10044/1/46986.
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15
Quintana, Alonso Ignacio. "Fracture mechanics of brittle lattice materials". Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611458.
Testo completo
16
Karstensen, Annette Degn. "Constraint estimation schemes in fracture mechanics". Thesis, University of Glasgow, 1996. http://theses.gla.ac.uk/7228/.
Testo completoAbstract (sommario):
Elastic-plastic crack tip constraint has been estimated for the common planar crack geometries. These include single edge cracked bars in tension and bending, centre cracked panels, and double edge cracked bars. The description of the stress field has been modified from a one parameter characterisation, based on K or J, to include a second term. The second parameter is a non-singular term, described either in terms of an elastic T-stress or a Q field. The limits of one and two parameter characterisation are discussed for single edge bars in tension and bending and for centre cracked panels. For the single edge geometries the two parameter characterisation was found to extend the characterisation well beyond the one parameter approach. For the centre cracked panels the two parameter characterisation in terms of a J-Q approach was found to be more accurate than a J-T approach at high levels of deformation. For the single edge cracked bending and tension geometries the second parameter Q is divided into two components named QT and QP. QT is an elastic term which depends on the elastic T stress and is independent of the distance from the crack tip. The second component, QP, arises from global bending on the uncracked ligament. This is a distance dependent term which depends on the level of deformation and can either be expressed as a function of the load normalised by the limit load or as a function of the plastic component of the J-integral. For single edge bend bars the constraint estimation provides a basis for a method of predicting fracture toughness using local failure approach, where the failure criterion is expressed in terms of the stress level ahead of the crack tip. Finally the constraint of mixed mode problem has been analysed and compared with that of pure Mode I problems.
17
Zhang, Guihua. "Development of numerical approaches to predict ductile and cleavage fracture of structural materials". Akron, OH : University of Akron, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=akron1196174104.
Testo completoAbstract (sommario):
Dissertation (Ph. D.)--University of Akron, Dept. of Mechanical Engineering, 2007."December, 2007." Title from electronic dissertation title page (viewed 03/27/2008) Advisor, Xiaosheng Gao; Committee members, T.S. Srivatsan, Fred Choy, Wieslaw Binienda, Kevin Kreider; Department Chair, Celal Batur; Dean of the College, George K. Haritos; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.
18
Kasturi, Krishna Kumar. "Microcracking Fracture Toughness for Graphite Epoxy Composite Laminates Using Finite Fracture Mechanics". ScholarWorks@UNO, 2006. http://scholarworks.uno.edu/td/427.
Testo completoAbstract (sommario):
Graphite/epoxy composite laminates are widely used in the aerospace industry. However the industry cannot take full advantage of the superior strength and stiffness of composite materials until their failure mechanisms can be thoroughly understood by engineers. Recognizing the importance of such understanding, the current study was undertaken to determine the microcracking fracture toughness in composite laminates using the energy release rate criteria. Three materials of specific interest in the aerospace industry – IM7/977-2, IM7/5555 and IM7/5276-1 – were analyzed. To evaluate the microcracking fracture toughness, displacement controlled static tensile tests were performed. Microcrack density (the number of microcracks per unit length) was measured as a function of applied stress. The data were analyzed to obtain the microcracking fracture toughness (Gm) for each material system. The value of Gm can be used to predict the microcracking behavior of composite laminates made from the corresponding material.
19
Averett, Rodney Dewayne. "Fracture Mechanics of High Performance Nylon Fibers". Thesis, Georgia Institute of Technology, 2004. http://hdl.handle.net/1853/5238.
Testo completoAbstract (sommario):
A fracture mechanics protocol appropriate for small fibers (35 micron diameter) is presented, which allows for the determination of the strength limitations of high performance nylon 6,6 fibers. Specifically, linear elastic fracture mechanics (LEFM) techniques are employed in addition to elastic-plastic fracture mechanics (EPFM) theories to achieve this.
We assume that a minute semi-elliptical flaw of an unknown size exists in the specimen, as a result of the detrimental effects of the manufacturing process (melt spinning). Next, we seek to propagate this flaw in a stable manner through an ancillary process such as high cycle or low cycle fatigue (load-unload). After propagation, uniaxial tensile experiments are performed on the fatigued samples, by which the crack growth eventually becomes catastrophic during the process. After performing scanning electron microscopy (SEM) techniques and reviewing fractography, we are able to determine the critical flaw size and ligament length that leads to unstable crack propagation. These results are substituted into the appropriate LEFM equations and are in close agreement with material properties for nylon 6,6. A discussion is provided that draws parallel to the topics discussed in the literature investigation and the experimental results of this study.
20
Walker, Peter David. "“CAD-on” interfaces - a fracture mechanics characterization". Thesis, University of British Columbia, 2017. http://hdl.handle.net/2429/62663.
Testo completoAbstract (sommario):
Abstract
Objective: “CAD-on” crowns, consisting of CAD/CAM milled lithium disilicate (LS2) veneers
glass-fused to CAD/CAM milled yttrium oxide stabilized tetragonal zirconia polycrystal (Y-TZP)
framework, have shown promise in increasing veneer fracture resistance. The glass fusion
technique is purported to result in stronger bonding between veneer and framework when
compared to conventional veneering. The objective of this study was to apply fracture mechanics
methodology to characterize the interfaces present in “CAD-on” crowns.
Methods: The notchless triangular prism (NTP) specimen fracture toughness (KIC) test was used
to determine interfacial KIC. Four groups, each consisting of 6X6X6X12mm NTP specimens
(n=22), were produced from IPS Emax CAD (LS2), IPS Emax ZirCAD (Y-TZP), and IPS Emax
ZirPress and crystal connect™(CC) fusing glass. Groups I (Emax/CC/Emax), II (Zir/CC/Zir), and
III (Zir/CC/Emax) utilized half-size (6X6X6X6mm) NTP specimens approximated under
vibration with the connecting glass and sintered according to manufacturers’ guidelines. Group IV
specimens were coated with ZirLiner and pressed with IPS Emax ingots. The specimens were
tested using a computer controlled (Bluehill) Instron 4301. Results were analyzed with one-way
ANOVA, Scheffé multiple means comparisons (α=0.05) and Weibull statistics. All fractured
surfaces were characterized with a light microscope. Selected fractured interfaces were
characterized under a scanning electron microscope.
Results: Groups I-III demonstrated a cohesive mode of failure. Number and size of defects
appeared to correlate with the variability of K1C values. There were no significant differences
between the KIC values of the “CAD-on” interfaces. Interfacial KIC values were limited by KIC of
CC. The “CAD-on” KIC value was significantly greater than that of the ZirPress control.
Conclusion: Based on the results obtained, KIC of interfaces produced during the “CAD-on”
technique appear to be limited by the interfacial KIC of the connecting glass and the defects
produced during processing. In this study, “CAD-on” produced veneers had stronger interfacial
KIC than a conventionally veneered control group.Dentistry, Faculty of
Graduate
21
Abdussalam, Saleh R. "Damage and fracture mechanics of composite materials". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape8/PQDD_0005/NQ41601.pdf.
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22
Du, Jiaji. "A fracture mechanics study of concrete failure /". Thesis, Connect to this title online; UW restricted, 1988. http://hdl.handle.net/1773/7060.
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23
Sticko, Simon. "Smooth Particle Hydrodynamics Applied to Fracture Mechanics". Thesis, Uppsala universitet, Tillämpad mekanik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-202881.
Testo completoAbstract (sommario):
A numerical method commonly referred to as smooth particle hydrodynamics (SPH) is implemented in two dimensions for solid mechanics in general and fracture mechanics in particular. The implementation is tested against a few analytical cases: a vibrating plate, a bending plate, a modus I crack and a modus II crack. A conclusion of these tests is that a better way of treating a shortcoming of SPH called tensile instability is needed. A study is made on the best choice of a vital parameter called the smoothing radius, and it is found that a good choice of the smoothing radius is roughly 1.5 times the initial particle spacing.
24
Sun, Gexin. "Application of fracture mechanics to mine design". Thesis, University of Nottingham, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.480734.
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Banerjee, Anuradha. "The fracture mechanics of bi-material systems". Thesis, University of Glasgow, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.398770.
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26
Anandarajah, Arumugam. "Fracture mechanics of adhesive joints in shear". Thesis, University of Dundee, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.357160.
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27
Portillo, Oscar. "Fracture mechanics of bitumen and asphalt mixes". Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608512.
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28
Abdul, Manan Muhamad Saifuildin. "Fracture mechanics analysis of multiple edge cracks". Thesis, University College London (University of London), 2008. http://discovery.ucl.ac.uk/1444086/.
Testo completoAbstract (sommario):
This thesis is concerned with the evaluation of crack propagation in the presence of other cracks within the same two-dimensional body. The parameter known as the Stress Intensity Factor (SIF) is used within Linear Elastic Fracture Mechanics (LEFM) to predict crack propagation rate knowing the appropriate material properties. Unfortunately the number of SIF solutions for multiple cracks is very small especially for real engineering cases. This thesis investigates the use of the SIF weight function for the efficient evaluation of SIFs for two one-dimensional cracks in close proximity to each other. The SIF weight function has proved to be a powerful tool for the evaluation of single cracks however, has not before been used to solve multiple crack problems. The main objective of the thesis is to investigate the mechanics of multiple cracks through an experimental and numerical analysis programme and to develop engineering solutions for prediction of multiple crack behaviour in steels and materials that exhibit LEFM characteristics. A Finite Element (FE) approach was employed to model multiple crack interaction. In addition, the FE model was used to study the non-uniform stress distribution caused by the interaction effect between cracks. Over one hundred FE models were analysed for this study. A major experimental programme was conducted to study the interaction effect between two edge cracks. A total of seven specimens with different crack geometries were completed under fatigue loading in tension. The purpose of this experimental work was to better understand the mechanisms of crack interaction and to provide information for validation of the numerical anlayses. The experimental results show that cracks in close proximity to each other interact to varying degrees depending on relative crack lengths, crack separation and plate width. A novel weight function method was developed in order to predict SIFs of two edge cracks under uniform tension. The crack interaction effect was established using the idea of non-uniform stress distributions along the potential crack plane due to the presence of an additional edge crack. Generally the novel weight function approach shows good results compared to finite element analysis. Finally, further work to explore the wide range of SIFs for multiple cracks using the weight function method is identified and proposed.
29
Huang, Xiyong. "Probabilistic fracture mechanics by boundary element method". Thesis, Imperial College London, 2010. http://hdl.handle.net/10044/1/6192.
Testo completoAbstract (sommario):
In this work, a new boundary element method is presented for the Probabilistic Fracture Mechanics analysis. The method developed allows the probabilistic analysis of cracked structure accomplished by the dual boundary element method (DBEM), in which the traction integral equation is used on one of the crack faces as opposed to the usual displacement integral equation. The stress intensity factors and their first order derivatives are evaluated for mode-I and mixed-mode fracture problems. A new boundary element formulation is derived and implemented to evaluate the design variables sensitivities. This method involves the solution of matrix systems formed by the direct differentiation of the discretised dual boundary element equations with respect to the each random parameter. The derivatives of fracture parameters with respect to design variables are calculated using implicit differentiation method (IDM) in DBEM for mode-I and mixed-mode fracture problems. The gradient of performance function is determined analytically and the total derivative method (TDM) is used in probabilistic fatigue crack growth problems. The randomness in the geometry, material property and the applied stress are considered in 2-D fracture problems; while initial crack size, final crack size, material property and applied stress are considered in fatigue crack growth. Uncertainties in other aspects of the problem can be included. First-Order Reliability Method (FORM) is used for predicting the reliability of cracked structures. The Hasofer Lind Rackwitz Fiessler algorithm is used to find the most probable point, referred as reliability index. Finally, the validation and applications of the stochastic boundary element coupled with FORM are presented. Numerical calculations are shown to be in good agreement either with the analytical solution or Monte Carlo Simulation.
30
Alessi, Roberto. "Variational approach to fracture mechanics with plasticity". Palaiseau, Ecole polytechnique, 2013. http://pastel.archives-ouvertes.fr/pastel-00847970.
Testo completoAbstract (sommario):
Dans le cadre des systèmes rate-independent, un modèle de plasticité avec endommagement, visant à la description des processus de rupture ductile, est proposé et étudié par une formulation variationnelle. Une fissure cohésive, ou ductile, se produit lorsque le champ de déplacement subit une discontinuité, tout en étant encore associé à une contrainte de traction non nulle. Prévoir et décrire efficacement les phénomènes de rupture ductile est une tâche cruciale pour de nombreux matériaux d'ingénierie (métaux, polymères,. . . ), comme en témoigne le grand intérêt de la communauté scientifique sur le sujet. Modèles d'endommagement à gradient ont été fructueusement utilisé pour la description des ruptures fragiles: dans ce cas, une fois que le niveau d'endommagement atteint sa valeur maximale, une fissure est créée lorsque la traction entre les deux faces opposées tombe immédiatement à zéro. D'autre part, le modèle de plasticité parfaite pourrait décrire la formation de la glisse plastique au niveau constant de contrainte. Par conséquent, afin de décrire les effets typiques d'une rupture cohésive, l'idée principale consiste à coupler, par une approche variationnelle, le modèle de plasticité parfaite et un modèle d'endommagement à gradient. L'utilisation d'une approche variationnelle se traduit par une formulation faible et sans dérivées, fournit des moyens efficaces pour traiter les notions de bifurcation et de stabilité, est intrinsèquement discret et indique une manière naturelle et rationnelle pour définir des algorithmes numériques efficaces. L'incorporation des effets d'endommagement dans un modèle de plasticité n'est pas une idée nouvelle. Néanmoins, le modèle proposé présente de nombreux aspects originaux comme le couplage entre la plasticité et l'endommagement et la façon avec laquelle l'évolutions des variables se trouvent. L'approche variationnelle s'appuie simplement sur trois concepts: une condition d'irréversibilité, une condition de stabilité globale, locale ou différentielle et le bilan énergétique. Le modèle résultant possède une grande flexibilité dans les réponses possibles couplées, en fonction des paramètres constitutifs. Ces diverses réponses sont d'abord examinées avec un test d'une barre unidimensionnelle en traction quasi-statique en assumant une évolution homogène qui met en évidence les principales caractéristiques du modèle. La discussion sur la stabilité des solutions homogènes conduit à l'existence d'une longueur de la barre critique qui à son tour dépend de la longueur interne caractéristique du matériel. En considérant des barres plus longues par rapport à cette valeur critique, on démontre que la réponse homogène devient instable. Par conséquence une localisation doit apparaitre dans la barre. Une construction de localisation est ensuite proposée, qui prend explicitement en compte la condition d'irréversibilité sur le champ d'endommagement. Ceci permet d'étudier l'évolution non homogène et la réponse globale. Il s'avère que, en général, une fissure cohésive apparaît au centre de la zone d'endommagement avant la rupture. A ce stade, la déformation plastique se localise comme une mesure de Dirac qui devient responsable de cette fissure cohésive. On obtient la loi cohésive associée en termes de paramètres du modèle et retrouve la loi de fracture cohésive postulée par Barenblatt. Enfin, un schéma de résolution numérique est proposé, qui est basé sur un algorithme de minimisation alternée, et mis en œuvre par une librairie d'éléments finis uniquement pour le test de barre en traction. Même si l'espace d'éléments finis adoptés ne peut pas incorporer les discontinuités, les résultats numériques s'accordent parfaitement avec les solutions analytiques. Néanmoins, les développements futurs visent à étendre les simulations dans un cadre à deux / trois dimensions et de tester une méthode d'éléments finis généraliséeIn the framework of rate-independent systems, an elastic-plastic-damage model, aimed at the description of ductile fracture processes, is proposed and investigated through a variational formulation. A cohesive, or ductile, crack occurs when the displacement field suffers a discontinuity whilst still being associated to a non-vanishing tensile stress. To predict and effectively describe ductile fracture phenomena is a crucial task for many engineering materials (metals, polymers,. . . ), as testified by the great interest of the scientific community on the subject. Gradient damage models have been fruitfully used for the description of brittle fractures: in such cases, once the damage level reaches its maximum value, a crack is created where the traction between the two opposite lips immediately drops to zero. On other other hand, the perfect plasticity model could describe the formation of plastic slips at constant stress level. Hence, in order to describe the typical effects of a cohesive fracture, the main idea is to couple, through a variational approach, the perfect plasticity model and a softening gradient damage model. The use of a variational approach results in a weak and derivative-free formulation, gives effective means to deal with the concepts of bifurcation and stability, is intrinsically discrete and indicates a natural and rational way to define efficient numerical algorithms. Embedding damage effects in a plasticity model is not a new idea. Nevertheless the proposed model presents many original aspects as the coupling between plasticity and damage and the way the governing equations of the variables are found. The variational approach relies simply on three concepts: the irreversibility condition, a global, local or differential stability condition and the energy balance. The resulting model possesses a great flexibility in the possible coupled responses, depending on the constitutive parameters. These various responses are first considered by investigating in a one-dimensional quasi-static traction bar test a homogeneous evolution which highlights the main features of the model. The discussion about the stability of the homogeneous solutions leads to the existence of a critical bar length which in turn depends on the characteristic internal material length. For bars that are longer than this critical value the homogeneous response is proven to become unstable and a localization must appear. A construction of localization is then proposed which explicitly takes into account the irreversibility condition on the damage field. This allows the non-homogeneous evolution and the global response to be investigated. It turns out that in general a cohesive crack appears at the center of the damage zone before the complete rupture. At this point the plastic strain localises as a Dirac measure which becomes responsible for this cohesive crack. The associated cohesive law is obtained in a closed form in terms of the parameters of the model and it recovers the cohesive fracture law postulated by Barenblatt. Finally, a numeric scheme is proposed, which is based on an alternate minimization algorithm, and implemented through a finite element library only for the one-dimensional traction bar test. Although the adopted finite element spaces do not embed discontinuities, the numeric results agree perfectly with the analytic solutions. This is due to a kind of numeric regularisation. Nevertheless, future developments aim to extend the simulations in a two/three-dimensional setting and test a generalized finite element method
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Pfeil, Michael Channing. "A fracture mechanics analysis of electronic packages /". The Ohio State University, 1995. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487862399450324.
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Liu, Shubin Carleton University Dissertation Engineering Aerospace. "Boundary element analysis in contact fracture mechanics". Ottawa, 1994.
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Yan, Jia-Hau. "Elastic-plastic fracture mechanics of compact bone". [Gainesville, Fla.] : University of Florida, 2005. http://purl.fcla.edu/fcla/etd/UFE0011641.
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Li, Yong <1985>. "Fracture Mechanics Investigation of Structures with Defects". Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amsdottorato.unibo.it/5946/1/li_yong_tesi.pdf.
Testo completoAbstract (sommario):
Fracture mechanics plays an important role in the material science, structure design and industrial production due to the failure of materials and structures are paid high attention in human activities. This dissertation, concentrates on some of the fractural aspects of shaft and composite which have being increasingly used in modern structures, consists four chapters within two parts.
Chapters 1 to 4 are included in part 1. In the first chapter, the basic knowledge about the stress and displacement fields in the vicinity of a crack tip is introduced. A review involves the general methods of calculating stress intensity factors are presented. In Chapter 2, two simple engineering methods for a fast and close approximation of stress intensity factors of cracked or notched beams under tension, bending moment, shear force, as well as torque are presented. New formulae for calculating the stress intensity factors are proposed. One of the methods named Section Method is improved and applied to the three dimensional analysis of cracked circular section for calculating stress intensity factors. The comparisons between the present results and the solutions calculated by ABAQUS for single mode and mixed mode are studied. In chapter 3, fracture criteria for a crack subjected to mixed mode loading of two-dimension and three-dimension are reviewed. The crack extension angle for single mode and mixed mode, and the critical loading domain obtained by SEDF and MTS are compared. The effects of the crack depth and the applied force ratio on the crack propagation angle and the critical loading are investigated. Three different methods calculating the crack initiation angle for three-dimension analysis of various crack depth and crack position are compared. It should be noted that the stress intensity factors used in the criteria are calculated in section 2.1.
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Li, Yong <1985>. "Fracture Mechanics Investigation of Structures with Defects". Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amsdottorato.unibo.it/5946/.
Testo completoAbstract (sommario):
Fracture mechanics plays an important role in the material science, structure design and industrial production due to the failure of materials and structures are paid high attention in human activities. This dissertation, concentrates on some of the fractural aspects of shaft and composite which have being increasingly used in modern structures, consists four chapters within two parts.
Chapters 1 to 4 are included in part 1. In the first chapter, the basic knowledge about the stress and displacement fields in the vicinity of a crack tip is introduced. A review involves the general methods of calculating stress intensity factors are presented. In Chapter 2, two simple engineering methods for a fast and close approximation of stress intensity factors of cracked or notched beams under tension, bending moment, shear force, as well as torque are presented. New formulae for calculating the stress intensity factors are proposed. One of the methods named Section Method is improved and applied to the three dimensional analysis of cracked circular section for calculating stress intensity factors. The comparisons between the present results and the solutions calculated by ABAQUS for single mode and mixed mode are studied. In chapter 3, fracture criteria for a crack subjected to mixed mode loading of two-dimension and three-dimension are reviewed. The crack extension angle for single mode and mixed mode, and the critical loading domain obtained by SEDF and MTS are compared. The effects of the crack depth and the applied force ratio on the crack propagation angle and the critical loading are investigated. Three different methods calculating the crack initiation angle for three-dimension analysis of various crack depth and crack position are compared. It should be noted that the stress intensity factors used in the criteria are calculated in section 2.1.
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Oates, William Sumner. "Fracture of Ferroelectric Materials". Diss., Georgia Institute of Technology, 2004. http://hdl.handle.net/1853/4769.
Testo completoAbstract (sommario):
Ferroelectric materials continue to find increasing use in actuator, sensor and transducer design. Questions regarding lifetime and reliability remain a concern due to the inherent low fracture toughness and complex material behavior. The poling procedure required for use in actuator and sensing devices introduces anisotropy in elastic and dielectric coefficients as well as piezoelectric coupling between the mechanical and electrical fields. This introduces complex fracture behavior which necessitates advanced analytical techniques and fracture characterization.
In this dissertation, fracture mechanics of ferroelectric materials is evaluated by employing different analytical techniques and experimental methodology. The theoretical work has focused on linear piezoelectric coupling that accounts for the influence of anisotropy and heterogeneity on fracture. A new orthotropic rescaling technique is presented that explicitly solves the anisotropic linear elastic piezoelectric crack problem in terms of material coefficients. The effects of heterogeneities on electric field induced microfracture are analyzed by implementing a crack at the edge of a heterogeneous piezoelectric inclusion. A positive, flaw-localized driving force is realized when permeable crack face boundary conditions are considered.
The experimental portion of the work evaluates fracture behavior in the ferroelectric ceramic, lead zirconate titanate (PZT), and the ferroelectric relaxor single crystal PZN-4.5%PT. Relative humidity and electric boundary conditions are shown to have significant effects on crack kinetics in PZT. Fracture anisotropy in single crystal PZN-4.5%PT is characterized using the Single-Edge V-notch Beam (SEVNB) method and Vickers indentations. Scanning electron micrographs are used to determine the crack profile which leads to a prediction of crack tip toughness and local energy release rate. A weak cleavage plane is identified in the single crystal relaxor which contains a significantly lower toughness in comparison to the ferroelectric ceramic PZT.
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Gerde, Eric Stendahl. "Fracture and friction /". Full text (PDF) from UMI/Dissertation Abstracts International, 2001. http://wwwlib.umi.com/cr/utexas/fullcit?p3008334.
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Arteaga-Gomez, Joaquin M. "Development of crack generation and propagation algorithms for computational structural mechanics". Fairfax, VA : George Mason University, 2009. http://hdl.handle.net/1920/3430.
Testo completoAbstract (sommario):
Thesis (M.S.)--George Mason University, 2009.Vita: p. 64. Thesis director: Rainald Löhner. Submitted in partial fulfillment of the requirements for the degree of Master of Science in Computational Sciences. Title from PDF t.p. (viewed June 10, 2009). Includes bibliographical references (p. 60-63). Also issued in print.
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Hosoglu, Selcuk. "Cellular automata an approach to wave propagation and fracture mechanics problems". Thesis, Monterey, Calif. : Naval Postgraduate School, 2006. http://bosun.nps.edu/uhtbin/hyperion.exe/06Dec%5FHosoglu.pdf.
Testo completoAbstract (sommario):
Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, December 2006.Thesis Advisor(s): Young W. Kwon. "December 2006." Includes bibliographical references (p. 63-64). Also available in print.
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Reakes, Clayton E. IV. "Nonlinear Fracture Mechanics Analysis of Threaded Fastener Geometry". University of Akron / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=akron1447425945.
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Hu, Kai Xiong. "Stress and fracture analysis for systems with inhomogeneities". Diss., The University of Arizona, 1993. http://hdl.handle.net/10150/186588.
Testo completoAbstract (sommario):
The inhomogeneities situated in materials render the diversification of composite families, and can provide synergistic effects for tailoring materials to a specified and often hostile environment. The work presented here focuses on the fracture and stress analysis of systems with various inhomogeneities. In Chapter 1, interactions among cracks and rigid-line inclusions are investigated. Rigid-line inclusions are represented by a distribution of forces while cracks are modeled by the standard dislocation approach. Chapter 2 presents an analysis of composite systems with interacting cracks and a dilute distribution of inclusions. A damage analysis procedure is developed to evaluate the effective properties of such composites. Chapter 3 examines multiple void-crack interactions. The formulation is based on a mixture of dislocations and tractions. Chapter 4 presents an approach to modeling bridged crack systems. A fully regular integral equation formulation is developed and the approach is ideally suited for the analysis of systems with large number of closely spaced inhomogeneities. The integral equations of different forms, developed throughout the dissertation can also be utilized to evaluate and verify various micromechanical models. The possible future extensions and the major limitations of the present work are briefly discussed in Chapter 5.
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Dhansay, Nur Mohamed. "Fracture mechanics based fatigue and fracture toughness evaluation of SLM Ti-6Al-4V". Master's thesis, University of Cape Town, 2015. http://hdl.handle.net/11427/24326.
Testo completoAbstract (sommario):
The focus of this research project was to determine experimentally the fatigue and fracture toughness characteristic, from a fracture mechanics perspective, of Ti-6Al-4V titanium alloy manufactured by Selective Laser Melting (SLM). Three build orientations are considered where a fatigue crack is grown parallel and two are grown perpendicular to the build orientation. The project then endeavours to generate a fracture mechanics based Paris equation from the fatigue crack growth rate results and together with the fracture toughness, fatigue life predictions may be determined based on crack propagation lifetimes. SLM is an Additive Manufacturing (AM) technique whereby an object is fabricated in a layerwise manner via the use of lasers, directly from a 3D CAD model. This process allows for the manufacture of complex designs in its net or near net shape form, which is not possible with conventional manufacturing techniques. There are minimal amounts of material wastage and it potentially eliminates post manufacture machining and processing costs. Ti- 6Al-4V is used in many applications where high strength at low density is required at moderate temperatures. Corrosion resistance qualities of the alloy are also considered for many applications. Some of the applications where this alloy is used include turbine engine components, aircraft structural components, aerospace fasteners, high-performance automotive parts, marine applications, medical implant devices and sports equipment. Due to the large use of the alloy in industry and with the potential benefits of manufacturing by SLM, there is a great need for investigating SLM Ti-6Al-4V as a viable alternative to conventional casting, forging and machining. There is limited literature covering the fatigue crack growth rate and fracture toughness of SLM Ti-6Al-4V and the effect of build orientation on these characteristics. However, it is clear, from the limited available literature that fatigue crack growth rate behaviour is affected by build orientation, and so this project investigates the effect of these orientations, and aims to contribute to understanding why these orientation effects occur. Since there is even less literature available on the fracture toughness of SLM Ti-6Al-4V with respect to build orientation, this project also endeavours to characterise orientation effects on fracture toughness, if any, and compares these with those of conventionally manufacture Ti-6Al-4V.
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Gieseke, Brian G. "Mechanics and mechanisms of creep-fatigue crack growth in Cu-1 wt% Sb". Diss., Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/19982.
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Yin, Xiaochen. "Concrete fracture process zone characteristics /". Thesis, Connect to this title online; UW restricted, 1997. http://hdl.handle.net/1773/10183.
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Hauch, Jens Andreas. "Dynamic fracture in brittle materials /". Digital version accessible at:, 1998. http://wwwlib.umi.com/cr/utexas/main.
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Bahr, Douglas. "Fracture mechanics characterization of a single crystal nickel alloy". Thesis, Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/19604.
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Zhang, Wenbing. "A method and program for quantitative description of fracture data and fracture data extrapolation from scanline or wellbore data /". May be available electronically:, 2001. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
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Abbas, Safdar [Verfasser]. "High gradient XFEM for fracture mechanics / Safdar Abbas". Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2012. http://d-nb.info/1025514246/34.
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Terfas, Osama Abdulhamid. "Quantification of constraint in three-dimensional fracture mechanics". Thesis, University of Glasgow, 2010. http://theses.gla.ac.uk/2119/.
Testo completoAbstract (sommario):
Abstract The role of crack tip constraint in three dimensional fracture mechanics has been investigated under elastic-plastic conditions using finite element techniques. Out-of-plane constraint loss has been identified by comparing the mean stress of the three dimensional cracked body with a reference plane strain configuration. This has allowed the quantification of constraint loss due to thickness. This is important for fitness-for-service procedures where the use of standard thick deeply cracked samples inherently leads to conservative assessments. The proximity to plane–strain conditions was investigated, as well as the J-integral along the crack fronts of typical fracture mechanics specimens. It was shown that deep cracks (a/w=0.5) were significantly affected by out-of-plane constraint loss, while the effect was smaller for shallow cracks (a/w=0.1) when in-plane effects were dominant, where a is the crack length and w is the width of the specimen. The out-of-plane effect was confirmed experimentally with a series of fracture mechanics tests on thin and thick deeply cracked fracture mechanics samples. Computational and experimental studies showed that geometries with B/w=0.2 maintained high constraint conditions at the centre plane and exhibited a low fracture toughness, where B is the thickness of the specimen. As such they can be used to measure the plane strain fracture toughness (JIc) as long as the thickness and the ligament exceed 20J/σ0. The increased slope of the resistance JR curve and enhanced fracture toughness were correlated to the loss of out-of-plane constraint that developed in thinner samples (B/w=0.1). A procedure to incorporate the effects of out-of-plane constraint in the R6 failure assessment diagram was proposed. A procedure was developed to determine ductile crack growth of semi-elliptical surface cracks in flat plates. The procedure used the J-a resistance curve developed from standard high and low constraint geometries in conjunction with an analysis of the crack tip stress field using finite element modelling. This allowed the evolution of crack shape under ductile tearing to be modelled. The majority of the work was devoted to the study of surface breaking semi-elliptical cracks subject to bending, uniaxial tension or biaxial loading. Both the mean stress and J-integral were geometry and load dependent, and were non-uniformly distributed around the crack front. Crack growth was dependent on the level of crack tip constraint, and the original crack shape was generally not retained after ductile tearing. In bending the crack growth was suppressed in the thickness direction and the crack extended significantly sub-surface in a stable manner so that the crack adopted a boat shape. In tension the crack extended through the thickness and this was accompanied with extensive growth in the angular range 45ْ-70ْ. In biaxial loading higher constraint levels were observed, however the overall trend of crack growth was similar to uniaxial tension. Finally, the results from the finite element modelling and the crack growth procedure were verified with experimental data. Excellent agreement in the crack shape patterns was observed between the test data and the crack growth models.
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Moore, Andrew J. "Strain analysis using ESPI applied to fracture mechanics". Thesis, Loughborough University, 1993. https://dspace.lboro.ac.uk/2134/11889.
Testo completoAbstract (sommario):
Electronic speckle pattern interferometry (ESPI) has become an established technique for surface deformation studies. However, difficulties remain in the practical use of ESPI, primarily because results require skilled and time-consuming interpretation. The work reported here has enabled automated acquisition and processing of ESPI displacement data. Further processing has enabled the components of surface strain and stress to be, determined automatically. Such measurements are of great importance for experimental fracture mechanics studies, a quantitative approach to measuring the severity of defects in a loaded structure. Having established the importance of experimental optical methods in fracture mechanics, and the particular advantages of ESPI, a study has been undertaken to determine whether ESPI can be used for quantitative fracture mechanics measurements. Automated analysis for in-plane displacement measurements with ESPI was achieved by the phase-stepping technique. Numerical differentiation of the displacement data allowed surface strain to be evaluated. The accuracy of such measurements was investigated, particularly with regard to speckle noise inherent in the data. Speckle noise limits the accuracy of all measurements, and a practical threshold for displacement and strain of ±0.03~m and ±6~strain was found. From these considerations, two new phase-stepping algorithms have been proposed for ESPI. The first offers improved accuracy by the way it eliminates speckle noise; the second has allowed phase-stepping of ESPI addition fringes for the first time. · In the past ESPI has been restricted to uniaxial measurements. A new interferometer design enabled displacement to be measured along two axes simultaneously: extension to three-dimensional sensitivity is discussed. Automated displacement and strain measurements, recorded with the dual-sensitivity interferometer, are presented for a cantilever loaded at its free end.