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Showing papers on "Fracture mechanics published in 1993"


Book
24 Sep 1993
TL;DR: In this article, the authors present a unified continuum, microstructural and atomistic treatment of modern day fracture mechanics from a materials perspective, focusing on the basic elements of bonding and microstructure that govern the intrinsic toughness of ceramics.
Abstract: This is an advanced text for higher degree materials science students and researchers concerned with the strength of highly brittle covalent–ionic solids, principally ceramics. It is a reconstructed and greatly expanded edition of a book first published in 1975. The book presents a unified continuum, microstructural and atomistic treatment of modern day fracture mechanics from a materials perspective. Particular attention is directed to the basic elements of bonding and microstructure that govern the intrinsic toughness of ceramics. These elements hold the key to the future of ceramics as high-technology materials - to make brittle solids strong, we must first understand what makes them weak. The underlying theme of the book is the fundamental Griffith energy-balance concept of crack propagation. The early chapters develop fracture mechanics from the traditional continuum perspective, with attention to linear and nonlinear crack-tip fields, equilibrium and non-equilibrium crack states. It then describes the atomic structure of sharp cracks, the topical subject of crack-microstructure interactions in ceramics, with special focus on the concepts of crack-tip shielding and crack-resistance curves, and finally deals with indentation fracture, flaws, and structural reliability.

3,550 citations


Journal ArticleDOI
TL;DR: In this article, a potential function of the components of the crack face displacements is used to generate the tractions along the interface where the fracture processes causing separation occur, and the two main parameters characterizing this potential are the work of separation per unit area and a peak normal stress.
Abstract: Calculations are reported for the mixed mode toughness of an interface joining an elastic-plastic solid to a solid which does not yield plastically. A potential function of the components of the crack face displacements is used to generate the tractions along the interface where the fracture processes causing separation occur. The two main parameters characterizing this potential are the work of separation per unit area and a peak normal stress. This description of the interface separation process is embedded within the continuum description as a boundary condition on the interface linking the adjoining solids. Small-scale yielding in plane strain is considered with the remote field specified by the magnitude and phase of the mixed mode stress intensity factors. Crack growth resistance curves are computed for a range of the most important nondimensional material parameters and for various combinations of remote mixed mode loading. Particular emphasis is placed on the ratio of the steady-state interface toughness to the “intrinsic” work of separation as it depends on plastic yielding and on the combination of modes 1 and 2. Plasticity enhances the interface toughness for all modes of loading, but substantially more so in the presence of a significant mode 2 component of loading than in near-mode 1 conditions.

675 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that fault growth is a self-similar process in which fault displacement d scales linearly with fault length L, which implies that fracture energy increases linearly as the fault length grows.
Abstract: We report progress made in the last few years on the general problem of the mechanism of fault growth and the scaling laws that result Results are now conclusive that fault growth is a self-similar process in which fault displacement d scales linearly with fault length L Both this result and the overall nature of along-strike fault displacement profiles are consistent with the Dugdale-Barenblatt elastic-plastic fracture mechanics model In this model there is a region of inelastic deformation near the crack tip in which there is a breakdown from the yield strength of the unfractured rock to the residual frictional strength of the fault over a breakdown length S and displacement d0 Limited data also indicate that S and d0 also scale linearly with L, which implies that fracture energy G increases linearly with L The scaling parameters in these relationships depend on rock properties and are therefore not universal In our prime field locality, the Volcanic Tableland of eastern California, we have collected data over 2 orders of magnitude in scale range that show that faults obey a power law size distribution in which the exponent C in the cumulative distribution is ∼13 If the fault is growing within the brittle field, the zone of inelastic deformation consists of a brittle process zone which leaves a wake of fractured rock adjacent to the fault Preliminary results of modeling the process zone are consistent with observations now in hand both in predicting the preferred orientation of cracks in the process zone wake and the rate of falloff of crack density as a function of distance from the fault The preferred orientation of these cracks may be used to infer the mode and direction of propagation of the fault tip past the point in question According to the model, the width of the process zone wake may be used to infer the length of the fault at the time its tip passed the measurement point, but data have not yet been collected to verify this prediction If the fault displacement has been accumulated by repeated seismic slips, each of these will sweep the fault with a crack tip stress field of a smaller spatial extent than that of the fault tip stress field, producing an inner, more intensely fractured, process zone wake This may be the mechanism that creates the cataclasite zone, rather than simple frictional wear, as has been previously supposed

418 citations


Journal ArticleDOI
TL;DR: In this article, closed form solutions for all three modes of fracture for an infinite piezoelectric medium containing a center crack subjected to a combined mechanical and electrical loading were obtained.
Abstract: Closed form solutions for all three modes of fracture for an infinite piezoelectric medium containing a center crack subjected to a combined mechanical and electrical loading were obtained. The explicit mechanical and electrical fields near the crack tip were derived, from which the strain energy release rate and the total potential energy release rate were obtained by using the crack closure integral. The suitability in using the stress intensity factor, the total energy release rate, or the mechanical strain energy release rate as the fracture criterion was discussed.

377 citations


01 Jan 1993
TL;DR: In this paper, a combined experimental and numerical approach is adopted to investigate fracture processes in concrete, where the failure of concrete subjected to mixed mode I and II loading is studied by means of four-point shear tests on single and double edge notched beams.
Abstract: A combined experimental and numerical approach is adopted to investigate fracture processes in concrete. The experimental programme focuses on the failure of concrete subjected to mixed mode I and II loading. The influence of shear load on the nucleation and propagation of cracks in concrete is studied by means of four-point-shear tests on single and double edge notched beams. A numerical model for simulating fracture is developed in which the heterogeneous microstructure of concrete is implemented. The model is used to carry out simulations of different fracture experiments. In Chapter 1 of this report the subject of the investigation is clarified. Chapter 2 deals with a summary of research regarding tensile fracture and combined tensile and shear fracture of concrete. An overview of different types of experiments is given. Furthermore numerical models and simulations of fracture tests are discussed. The experiments conducted in the present investigation are described in Chapter 3. Beam specimens with one or two notches, made of different concrete mixes are loaded in fourpoint- shear. All experiments are carried out under displacement control using a closed loop hydraulic system. In the developed test set-up experiments can be carried out either with freely rotating or fixed supports. The results of the various experiments are presented in Chapter 4. The experimental outcome is presented by means of crack patterns, different load-deformation curves and details of cracks obtained with an optical microscope. In Chapter 5 the numerical model is explained. Different ways of implementing heterogeneity are presented. The determination of the various input parameters is discussed. Simulations of different types of experiments are presented in Chapter 6, i.e. uniaxial tensile tests, four-point-shear tests, pull-out of anchor bolts and mixed mode tests on plate specimens. The last chapter includes a discussion of the results and a summary of the conclusions. The main conclusion derived from the experimental part of this investigation is that fracture in concrete is a mode I mechanism, even if the external loading on a specimen is a combination of tensile and shear. The numerical model developed has proved able to predict fracture in concrete quite accurately. Simulations with the model increase insight into the fracture mechanism.

362 citations


Book
31 Mar 1993
TL;DR: In this paper, the authors proposed a method for estimating the crack-tip plastic zone by estimating the elastic-plastic stress field in a 3D model of a crack, and showed that the model can be used to detect cracks in composite materials.
Abstract: Conversion table Preface to the Second Edition Preface 1: Introduction 1.1. Conventional failure criteria 1.2. Characteristic brittle failures 1.3. Griffith's work 1.4. Fracture mechanics References 2: Linear Elastic Stress Field in Cracked Bodies 2.1. Introduction 2.2. Crack deformation modes and basic concepts 2.3. Westergaard method 2.4. Singular stress and displacement fields 2.5. Stress intensity factor solutions 2.6. Three-dimensional cracks Examples Problems Appendix 2.1 References 3: Elastic-Plastic Stress Field in Cracked Bodies 3.1. Introduction 3.2. Approximate determination of the crack-tip plastic zone 3.3. Irwin's model 3.4. Dugdale's model Examples Problems References 4: Crack Growth Based on Energy Balance 4.1. Introduction 4.2. Energy balance during crack growth 4.3. Griffith theory 4.4. Graphical representation of the energy balance equation 4.5. Equivalence between strain energy release rate and stress intensity factor 4.6. Compliance 4.7. Crack stability Examples Problems References 5: Critical Stress Intensity Factor Fracture Criterion 5.1 . Introduction 5.2. Fracture criterion 5.3. Variation of Kc with thickness 5.4. Experimental determination of K1c 5.5. Crack growth resistance curve (R-curve) method 5.6. Fracture mechanics design methodology Examples Problems Appendix 5.1 References 6: J-Integral and Crack Opening Displacement Fracture Criteria 6.1. Introduction 6.2. Path-independent integrals 6.3. J-integral 6.4. Relationship between the J-integral and potential energy 6.5. J-integral fracture criterion 6.6. Experimental determination of the J-integral 6.7. Stable crack growth studied by the J-integral 6.8. Crack opening displacement (COD)fracture criterion Examples Problems References 7. Strain Energy Density Failure Criterion: Mixed-Mode Crack Growth 7.1. Introduction 7.2. Volume strain energy density 7.3. Basic hypotheses 7.4. Two-dimensional linear elastic crack problems 7.5. Uniaxial extension of an inclined crack 7.6. Ductile fracture 7.7. The stress criterion Examples Problems References 8: Dynamic Fracture 8.1. Introduction 8.2. Mott's model 8.3. Stress field around a rapidly propagating crack 8.4. Strain energy release rate 8.5. Crack branching 8.6. Crack arrest 8.7. Experimental determination of crack velocity and dynamic stress intensity factor Examples Problems References 9: Fatigue and Environment-Assisted Fracture 9.1. Introduction 9.2. Fatigue crack propagation laws 9.3. Fatigue life calculations 9.4. Variable amplitude loading 9.5. Environment-assisted fracture Examples Problems References 10: Micromechanics of Fracture 10.1. Introduction 10.2. Cohesive strength of solids 10.3. Cleavage fracture 10.4. Intergranular fracture 10.5. Ductile fracture 10.6. Crack detection methods References 11: Composite Materials 11.1. Introduction 11.2. Through4hickness cracks 11.3. Interlaminar fracture References 12: Thin Films 12.1. Introduction 12.2. Interfacial failure of a bimaterial system 12.3. Steady-state solutions for cracks in bilayers 12.4. Thin films under tension 12.5. Measurement of interfacial fracture toughness References 13: Nanoindentation 13.1. Introduction 13.2. Nanoindentation for measuring Young's modulus and hardness 13.3. Nanoindentation for measuring fracture toughness 13.4. Nanoindentation for measuring interfacial fracture toughness

356 citations


Book ChapterDOI
01 Jun 1993
TL;DR: In this article, a lattice-plane range parameter is proposed as a critical scaling dimension in the brittle crack description, and the critical crack-opening dimensions using this same model confirm that the intrinsic separation process indeed operates at the atomic level.
Abstract: Until now we have approached crack propagation from the continuum viewpoint. Nonetheless, repeated allusions have been made in chapters 3 and 5 to the fundamental limitations of any such approach that disregards the atomic structure of solids. There we argued for the incorporation of a lattice-plane range parameter as a critical scaling dimension in the brittle crack description. We noted that the Barenblatt cohesion-zone model avoids reference to the atomic structure by resorting to the Irwin slit description of cracks; yet estimates of the critical crack-opening dimensions using this same model confirm that the intrinsic separation process indeed operates at the atomic level. The Elliot lattice half-space model of sect. 3.3.2 represents one attempt to incorporate an essential element of discreteness. The phenomenological kinetic models of sect. 5.5, with their presumption of energy barriers, represent another. However, those models are at the very least quasi-continuous. In brittle fracture, as in any thermodynamic process, the final answers must be sought at the atomic or molecular level. On the other hand, while an atomistic approach provides greater physical insight into the crack problem, it inevitably involves greater mathematical complexity. Classically, solids may be represented as manybody assemblages of point masses (atoms) linked by springs (bonds). We will see that the mass–spring representation can lead us to a deeper understanding of brittle cracks. But even this representation is oversimplistic. In some cases, particularly when the crack interacts with environmental species, it is necessary to consider atoms as elastic spheres rather than point masses, to allow properly for molecular size effects.

325 citations


Journal ArticleDOI
Allan M. Rubin1
TL;DR: In this paper, a Barenblatt model that treats fracture resistance as an internal cohesive stress acting at the crack tip is used to investigate the effect of confining pressure on rock tensile failure.
Abstract: Field observations indicate that zones of inelastic deformation produced at the tips of propagating dikes can be much larger than those produced at the tips of tensile cracks in laboratory experiments. This is in direct conflict with the concept that fracture toughness and fracture energy are rock properties, independent of crack size and loading configuration. A Barenblatt model that treats fracture resistance as an internal cohesive stress acting at the crack tip is used to investigate the effect of confining pressure on rock tensile failure. When the confining pressure exceeds the cohesive strength of the rock, as it does at depths greater than several hundred meters, Linear Elastic Fracture Mechanics is inapplicable and the near-tip stress field of a propagating crack is determined by the crack size and loading configuration as well as by rock properties. As inelastic deformation depends upon the near-tip stress field, it follows that fracture energy may also depend upon crack size and loading configuration. For a propagating dike, the near-tip stress field is dominated by the large suction acting within a small (∼several meter) cavity at the tip generated by viscous flow of magma within the dike. Perturbations to the ambient stress are on the order of the cavity suction and act over regions on the order of the cavity length. The tip cavity pressure may be maintained by exsolution of magmatic volatiles or by influx of host rock pore fluids; inelastic deformation is enhanced by the latter. For a tip cavity pressure maintained by influx of pore fluids, the pore pressure exceeds the least compressive stress off the dike plane, even while it equals the least compressive stress at the dike tip. This can lead to tensile failure off the dike plane and the formation of observed dike-parallel joints. Shear stresses scale with the cavity suction and may produce shear failure off the dike plane; such deformation is generally enhanced if the dike is intruded perpendicular to the least compressive stress. For sills intruded parallel to bedding, shear failure in the form of bedding plane slip can lead to the observed blunting and fingering of the intrusion front. Because the tip cavity grows with dike size, the energy consumed by rock fracture also increases with dike size and is potentially as significant for large dikes as for small dikes, a view not adopted by existing fluid mechanical models of dike propagation.

292 citations


Journal ArticleDOI
TL;DR: In this article, an interdisciplinary view of metal fatigue in polycrystalline metals is presented, where fatigue resistance is defined in terms of the difficulty of crack growth in one of two possible directions, the first being related to the texture of a material, and the second to the orientation of the applied loading system.
Abstract: An interdisciplinary view of metal fatigue in polycrystalline metals is presented. Fatigue resistance is defined in terms of the difficulty of crack growth in one of two possible directions, the first being related to the texture of a material, and the second to the orientation of the applied loading system. The fatigue initiation phase is considered to be negligible for polycrystalline metals, and fatigue limits are equated to one of two threshold conditions, one quantified in terms of microstructural fracture mechanics, and the other determined by continuum mechanics. The importance of the intensity and distribution of microstructural barriers to fatigue crack growth is underlined, especially in relation to mechanical conditions such as stress–strain state and to material conditions such as grain size and the shape and orientation of inclusions and their size relative to microstructural barriers.MST/1883

278 citations


Journal ArticleDOI
TL;DR: In this article, two limiting thresholds to fatigue crack propagation are discussed, one is related to the microstructural texture and this threshold may therefore be deemed a material-based threshold, while the second threshold is related only to the stress state at the tip of a substantial defect.
Abstract: Two limiting thresholds to fatigue crack propagation are discussed. The first threshold is related to the microstructural texture and this threshold may therefore be deemed a material-based threshold. The second threshold is mechanically-based, and is related only to the stress state at the tip of a substantial defect. The material-based threshold is characterized in terms of Microstructural Fracture Mechanics (MFM) and the mechanically-based threshold is characterized in terms of Linear Elastic Fracture Mechanics (LEFM). The former condition is important when considering the fatigue limit of materials and components, while the latter is more applicable to the fatigue limit of structures. The different factors which affect the two threshold conditions are briefly presented. Finally, this paper discusses aspects of MFM relevant to the fatigue resistance of metals and components.

216 citations


Journal ArticleDOI
TL;DR: In this paper, a crack propagation in multilayer piezoelectric actuators made of Pb((Ni1/3Nb2/3), Ti, Zr)O3 ceramics with an interdigital electrode configuration was observed dynamically under an applied cyclic electric field using charge coupled device microscopy.
Abstract: Crack propagation in multilayer piezoelectric actuators made of Pb((Ni1/3Nb2/3), Ti, Zr)O3 ceramics with an interdigital electrode configuration was observed dynamically under an applied cyclic electric field using charge coupled device microscopy. The crack was observed only under a high electric field, and it healed under zero field. The crack was initiated at the internal electrode edge and propagated from the electrode edge in three directions. The electric-field-induced displacement was measured simultaneously with the propagation. The displacement of the multilayer actuator became gradually smaller and asymmetric with respect to the sign of the field with increased driving cycle.

Journal ArticleDOI
TL;DR: In this paper, a subcritical fracture growth model is used to generate equilibrium crack geometries, where fracture length distributions and spacing are modeled as proportional to the n-th power of the mode I stress intensity.
Abstract: Fracture networks are examined in the light of subcritical crack growth theory. Examples of equilibrium crack geometries are generated using a fracture mechanics model that explicitly tracks the propagation of multiple fractures. It is determined that propagation velocity as modeled using a subcritical fracture growth law exerts a controlling influence on fracture length distributions and spacing. Velocity is modeled as proportional to the n-th power of the mode I stress intensity. Numerous, closely spaced, similar length fractures result for n=1, with many en echelon arrays forming due to fracture interaction. Increasing the value of n results in the growth of fewer fractures that are more widely spaced. Fractures tend to cluster in narrow zones, with limited fracture growth in the intervening areas. The spacing between zones is controlled by the stress shielding effects of longer fractures on shorter ones. The amount of time required for fracture pattern development is also influenced by the subcritical velocity exponent, n. At low n, patterns take seconds to minutes to develop, while patterns generated at higher n can require hundreds of years or more.

Journal ArticleDOI
TL;DR: In this paper, the mixed-mode bending (MMB) delamination test has been studied in detail and the results from this test method compared to those obtained from the fixed-ratio mixedmode (FRMM) test method.

Journal ArticleDOI
TL;DR: A review of the application of fracture mechanics to ceramics can be found in this article, where the authors present a state-of-the-art review of fracture-based analysis of brittle materials.
Abstract: Research within the past two decades has achieved a dramatic upsurge of improvements in the mechanical properties of engineering ceramics. These improvements have often been made through increased toughness by novel toughening mechanisms such as the stress induced phase transformation, microcracking, fibre/whisker crack bridging, etc. These may occur not only in the frontal process zone ahead of a sharp crack, but also in the following crack wake region. The consequences of these microfracture processes and mechanisms in the wake and the crack bridging regions are significant, for they result in very complex fracture processes and they create many critical issues and difficulties in the experimental determination of the fracture resistance of brittle materials. The lack of a physical basis for a fracture criterion in the present fracture mechanics framework adds further confusion to fracture mechanics studies. This paper is a state of the art review of the application of fracture mechanics to brit...

Journal ArticleDOI
TL;DR: In this paper, an application of the dual boundary element method to the analysis of mixed-mode crack growth in linear elastic fracture mechanics is described, where crack-growth processes are simulated with an incremental crack-extension analysis based on the maximum principal stress criterion which is expressed in terms of the stress intensity factors.

Journal ArticleDOI
A. Yuse1, Masaki Sano1
25 Mar 1993-Nature
TL;DR: Study of crack propagation in glass plates caused by sudden but carefully controlled cooling observes a transition from straight to regular, wavy cracks as the tip speed increases, and suggests that this transition is a Hopf bifurcation, like those seen in a variety of other nonlinear systems.
Abstract: THE study of fracture is an old topic1, but only recently has an understanding begun to emerge of crack formation, propagation and morphology (which is often fractal) 2–8. When a brittle material such as glass is broken under tensile stress9, the cracks have a complicated morphology10. Fineberg et al.11 showed that this process may be caused by a dynamic instability, whereby the speed of crack propagation increases until it approaches the speed of sound: at this point, complex structures appear. But crack morphology in quasistatic fracture, where the speed of the crack tip is much smaller than the speed of sound, can also exhibit marked changes12. Here we present studies of crack propagation in glass plates caused by sudden but carefully controlled cooling. We observe a transition from straight to regular, wavy cracks as the tip speed increases. The scaling behaviour of an appropriately defined relaxation time suggests that this transition is a Hopf bifurcation13, like those seen in a variety of other nonlinear systems. At still higher speeds, the oscillatory cracks split into first two and then four or more branches.

Journal ArticleDOI
TL;DR: In this paper, a simple conventional Brazilian test, which is normally used for the determination of rock tensile strength, was applied to measure rock fracture toughness, and the results compared favourably with those determined by the Chevron bending specimen method, one of the recently proposed international standard methods by the ISRM.

Journal ArticleDOI
TL;DR: In this paper, an interface model for delamination using spring elements is introduced, which can handle singular and non-singular problems in a unified way and requires no assumption of the existence of initial defects and no knowledge of the direction of crack growth.

Journal ArticleDOI
TL;DR: In this paper, an elliptical-arc surface crack in a round bar under cyclic axial loading with constant amplitude is considered, and the stress-intensity factor along the crack front is calculated through a finite-element analysis by employing isoparametric solid elements.

ReportDOI
TL;DR: In this article, the authors explore the fundamental concepts of the J-Q description of crack-tip fields, the fracture toughness locus and micromechanics approaches to predict the variability of macroscopic fracture toughness with constraint under elastic-plastic conditions.
Abstract: Two complementary methodologies are described to quantify the effects of crack-tip stress triaxiality (constraint) on the macroscopic measures of elastic-plastic fracture toughness J and Crack-Tip Opening Displacement (CTOD). In the continuum mechanics methodology, two parameters J and Q suffice to characterize the full range of near-tip environments at the onset of fracture. J sets the size scale of the zone of high stresses and large deformations while Q scales the near-tip stress level relative to a high triaxiality reference stress state. The material's fracture resistance is characterized by a toughness locus Jc(Q) which defines the sequence of J-Q values at fracture determined by experiment from high constraint conditions (Q∼0) to low constraint conditions (Q<0). A micromechanics methodology is described which predicts the toughness locus using crack-tip stress fields and critical J-values from a few fracture toughness tests. A robust micromechanics model for cleavage fracture has evolved from the observations of a strong, spatial self-similarity of crack-tip principal stresses under increased loading and across different fracture specimens. We explore the fundamental concepts of the J-Q description of crack-tip fields, the fracture toughness locus and micromechanics approaches to predict the variability of macroscopic fracture toughness with constraint under elastic-plastic conditions. Computational results are presented for a surface cracked plate containing a 6:1 semielliptical, a=t/4 flaw subjected to remote uniaxial and biaxial tension. Crack-tip stress fields consistent with the J-Q theory are demonstrated to exist at each location along the crack front. The micromechanics model employs the J-Q description of crack-front stresses to interpret fracture toughness values measured on laboratory specimens for fracture assessment of the surface cracked plate.

Journal ArticleDOI
TL;DR: In this article, a new fracture toughness test, the edge crack torsion method, has been developed for characterizing the mode III delamination behavior of composites, based on a laminate specimen subjected to torsions to propagate an edge delamination crack in its midplane.
Abstract: A new fracture toughness test, the edge crack torsion method, has been developed for characterizing the mode III delamination behavior of composites. The test is based on a laminate specimen subjected to torsion to propagate an edge delamination crack in its midplane. The crack growth mode of the specimen has been deduced to be mode III from fracture mechanics principles. The torsional behavior and the corresponding fracture parameter GIIIC have been analyzed on the basis of plate torsion and laminate theory. Edge crack torsion tests were performed to measure GIIIC of several carbon fiber/epoxy composite systems. Laminate layups were optimized to yield linear elastic fracture behavior of the specimens. The specimens were also sufficiently compliant to allow GIIIC to be readily obtained by using the compliance calibration method. The deformation characteristics of the specimens were found to follow the laminate torsion description. SEM observations showed fracture surface morphology varying with resin microstructures and, for the case of an untoughened matrix, were consistent with what was reported in the literature for other proposed mode III fracture tests.

Journal ArticleDOI
TL;DR: In this article, a commercial polybutylene terephthalate/polycarbonate/impact modifier (PBT/PC/IM) blend was used to study the fracture mechanisms involved at different temperatures under both impact and static loading.
Abstract: Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were employed in the morphology and fracture mechanisms studies on a commercial polybutylene terephthalate/polycarbonate/impact modifier (PBT/PC/IM) blend. The fracture mechanisms involved at different temperatures under both impact and static loading were revealed. It was found that massive plastic deformation of the matrix material occurred after rubber particle cavitation; and it was this plastic deformation that was responsible for the drastic enhancement in fracture toughness although the widespread cavitation did absorb a considerable amount of energy as well. The major source of toughness was the same for both impact and static fracture tests, but the toughening processes became effective at a much lower temperature under static than impact conditions. The sequence of toughening events was also observed using TEM.

Journal ArticleDOI
TL;DR: In this article, the mechanical properties of icosahedral AlCuFe quasicrystals were compared with those of related crystalline intermetallic compounds by means of indentation and three-point bending techniques.
Abstract: Mechanical properties of icosahedral AlCuFe quasicrystals were investigated and compared with those of related crystalline intermetallic compounds by means of indentation and three-point bending techniques. Quasicrystalline phases possess high hardness (HV ≈ 1000) and high ability for elastic recovery (H/E > 0.08) but exhibit low toughness against unstable fracture (KIC ≈ 1 MPa m12). Icosahedral particles embedded in Al2Cu or in AlFe crystalline matrices lead to increasing resistance against subcritical crack propagation. In contrast to cleavage#1 fracture in crystalline Al13Fe4, the icosahedral quasicrystals exhibit a rough, quasicleavage#1 fracture surface. Evidences for plastic deformation in the quasicrystals were found and its mechanism is discussed.

Journal ArticleDOI
TL;DR: In this paper, a study of transverse cracking mechanism in composite laminates using a singular hybrid finite element model is presented, which provides the global structural response as well as the precise local crack-tip stress fields.
Abstract: A study of transverse cracking mechanism in composite laminates is presented using a singular hybrid finite element model. The model provides the global structural response as well as the precise local crack-tip stress fields. An elasticity basis for the problem is established by employing Lekhnitskii's complex variable potentials and method of eigenfunction expansion. Stress singularities associated with the transverse crack are obtained by decomposing the deformation into the symmetric and antisymmetric modes and proper boundary conditions. A singular hybrid element is thereby formulated based on the variational principle of a modified hybrid functional to incorporate local crack singularities. Axial stiffness reduction due to transverse cracking is studied. The results are shown to be in very good agreement with the existing experimental data. Comparison with simple shear lag analysis is also given. The effects of stress intensity factors and strain energy density on the increase of crack density are analyzed. The results reveal that the parameters approach definite limits when crack densities are saturated, an evidence of the existence of characteristic damage state.

Journal ArticleDOI
TL;DR: In this paper, a connected-grain model developed earlier to study the modulus of elasticity as a power-law of density was extended to consider the dependence of the flexural strength of polycrystalline ceramics on porosity.
Abstract: A connected-grain model developed earlier to study the modulus of elasticity as a power-law of density was extended to study the dependence of the flexural strength of polycrystalline ceramics on porosity. Relations were derived for specific surface fracture energy, fracture toughness and flexural strength as power laws of (1 −p), wherep is porosity. Model validity was confirmed with data on α-alumina, UO2, Si3N4, and the YBa2Cu3O7−δ superconductor.

Journal ArticleDOI
TL;DR: In this paper, a theory for cleavage cracking surrounded by pre-existing dislocations is proposed, where the dislocations are assumed not to emit from the crack front, and an elastic cell, of size comparable to dislocation spacing or dislocation cell size, is postulated to surround the crack tip.
Abstract: A theory is proposed for cleavage cracking surrounded by pre-existing dislocations. Dislocations are assumed not to emit from the crack front. It is argued that the pre-existing dislocations, except for occasional interceptions with the crack front, are unlikely to blunt the major portion of the crack front, so that the crack front remains nanoscopically sharp, advancing by atomic decohesion. The fracture process therefore consists of two elements: atomic decohesion and background dislocation motion. An elastic cell, of size comparable to dislocation spacing or dislocation cell size, is postulated to surround the crack tip. This near-tip elasticity accomodates a large stress gradient, matching the nanoscopic, high cohesive strength to the macroscopic, low yield strength. Consequences of this theory are explored in the context of slow cleavage cracking, stress-assisted corrosion, fast running crack, fatigue crack growth, constraint effects, and mixed mode fracture along metal/ceramic interfaces. Computational models and experiments to ascertain the range of validity of this theory are proposed.

Journal ArticleDOI
TL;DR: Three factors were suggested to explain why the microcracks seem to prefer to grow in the interbead matrix: the presence of BaSO4, shrinkage during the curing process, and the different polymerization processes of the bead and the interBead polymers.

Journal ArticleDOI
TL;DR: The steady states of a crack moving in a triangular lattice are calculated and shown to become unstable at a certain velocity.
Abstract: The steady states of a crack moving in a triangular lattice are calculated and shown to become unstable at a certain velocity.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated cracking in multilayered ceramic/metal composites and found that the dominant cracking behavior depends on the volume fraction and yield strength of the metal.
Abstract: Investigations of cracking in multilayered ceramic/metal composites are presented. Two aspects are considered: crack renucleation across intact single metal layers and subsequent crack extension. Crack renucleation criteria are determined and compared with predictions. High-resolution strain-mapping techniques are employed to determine the surface strain fields surrounding cracks. Good agreement is found between these experimental measurements and the predictions of a small-scale yielding model. Subsequent crack progression occurs either by the extension of a dominant, nearly planar crack or by the formation of a zone of periodically spaced cracks. Both patterns are analyzed. The dominant cracking behavior is found to depend on the volume fraction and yield strength of the metal.

Journal ArticleDOI
TL;DR: The results of an experimental study of fatigue fracture of geometrically similar high-strength concrete specimens of very different sizes are reported and analyzed in this article, where an adjustment for the size effect needs to be introduced, of a similar type as previously introduced for normal concrete.
Abstract: The results of an experimental study of fatigue fracture of geometrically similar high-strength concrete specimens of very different sizes are reported and analyzed. Three-point bend notched beams were subjected to cyclic loading. The number of cycles to failure ranged from 200 to 41,000. It was found that Paris law for the crack length increment per cycle as a function of the stress intensity factor, which was previously verified for normal concrete, is also applicable to high-strength concrete. However, for specimens of different sizes, an adjustment for the size effect needs to be introduced, of a similar type as previously introduced for normal concrete. A linear regression plot estimating the size-adjustment parameters is derived. An LEFM (linear elastic fracture mechanics)-type calculation of the deflections under cyclic loading on the basis of the size-adjusted Paris law yields correct values for the terminal phase but grossly underpredicts the initial deflections. Overall, the results underscore the importance of considering fatigue fracture growth in the case of high-strength concrete structures subjected to large, repeated loads, and taking into account the very high brittleness under fatigue loading.