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Showing papers on "Fracture toughness published in 1987"


Journal ArticleDOI
TL;DR: In this paper, the influence of the nucleation and growth of micro-voids in the material near the tip of a crack is investigated, and numerical analyses of the stress and strain fields are based on finite strain theory, so that crack tip blunting is fully accounted for.
Abstract: An elastic-Viscoplastic model of a ductile, porous solid is used to study the influence of the nucleation and growth of micro-voids in the material near the tip of a crack. Conditions of small scale yielding are assumed, and the numerical analyses of the stress and strain fields are based on finite strain theory, so that crack tip blunting is fully accounted for. An array of large inclusions or inclusion colonies, with a relatively low strength, results in large voids near the crack tip at a rather early stage, whereas small second phase particles in the matrix material between the inclusions require large strains before cavities nucleate. Various distributions of the large inclusions, and various critical strains for nucleation of the small scale voids between the inclusions, are considered. Localization of plastic flow plays an important role in determining the failure path between the crack tip and the nearest larger void, and the path is strongly sensitive to the distribution of the large inclusions. Values of the J-integral and the crack opening displacement at fracture initiation are estimated, together with values of the tearing modulus during crack growth, and these values are related to experimental results.

376 citations


Journal ArticleDOI
TL;DR: In this article, a fracture mechanics model is developed for nontransforming ceramics that show an increasing toughness with crack extension (R-curve, or T -curve) behavior, treating the increased crack resistance as the cumulative effect of grain bridging restraints operating behind the advancing tip.
Abstract: A fracture mechanics model is developed for nontransforming ceramics that show an increasing toughness with crack extension (R-curve, or T-curve, behavior). The model derives from the observations in Part I, treating the increased crack resistance as the cumulative effect of grain bridging restraints operating behind the advancing tip. An element of discreteness is incorporated into the formal distribution function for the crack-plane restraining stresses, to account for the primary discontinuities in the observed crack growth. A trial forceseparation function for the local bridge microrupture process is adopted, such that an expression for the microstructureassociated crack driving (or rather, crack closing) force may be obtained in analytical form. The description can be made to fit the main trends in the measured toughness curve for a coarse-grained alumina. Parametric evaluations from such fits conveniently quantify the degree and spatial extent of the toughening due to the bridging. These parameters could be useful in materials characterization and design. It is suggested that the mechanics formulation should be especially applicable to configurations with short cracks or flaws, as required in strength analysis.

357 citations


Journal ArticleDOI
TL;DR: In this article, a further-miniaturized specimen punch (SP) test was developed to extract fracture strain and strength information from TEM disc specimens as small as 3 mm diameter and 0.25 mm in thickness.

325 citations


Journal ArticleDOI
TL;DR: In this article, the effects of such parameters as the filler volume fraction, particle size, aspect ratio, modulus and strength of filler, resin-filler adhesion and toughness of the matrix on the stiffness, strength and resilience of particulate filled epoxide resins have been evaluated.
Abstract: The effects of such parameters as the filler volume fraction, particle size, aspect ratio, modulus and strength of filler, resin-filler adhesion and toughness of the matrix on the stiffness, strength and toughness of particulate filled epoxide resins have been evaluated. The mechanisms of deformation and rupture in these multiphase materials are discussed, illustrated byin situ mechanical tests in the scanning electron microscope.

245 citations


Journal ArticleDOI
TL;DR: In this paper, the fracture mechanics stress intensity, K, measured for the cleavage strength of carbon steel by Professor Yokobori and colleagues, at Tohoku University and elsewhere, is shown to follow a Hall-Petch dependence on average grain diameter, l, in accordance with the model-based relationship K = c's 1 2 [σ 0 +kl −1 2 ] for which c' is a numerical factor, 5 is the effective length of the local plastic zone associated with unstable crack growth, σ 0 is a friction stress for appropriate dislocation movement within the poly

238 citations


Journal ArticleDOI
TL;DR: In this paper, a stress intensity approach is used to analyze tensile failure of brittle matrix composites that contain unidirectionally aligned fibers held in place by friction, and explicit relations are derived for the matrix cracking stress (noncatastrophic failure mode), the condition for transition to a catastrophic failure mode, and the fracture toughness in a region of catastrophic failure.

217 citations


Journal ArticleDOI
TL;DR: In this paper, Tungsten-carbide cobalt alloys in the range of technically relevant compositions were fractured under controlled conditions, and stable and unstable crack propagation were shown to be equivalent with respect to crack geometry and energy release rates.
Abstract: Tungsten-carbide cobalt alloys in the range of technically relevant compositions an microstructures were fractured under controlled conditions. Stable and unstable crack propagation are shown to be equivalent with respect to crack geometry and energy release rates. Based on microscopic observations and on measurements using SEM micrographs, the nature of the process zone, the maximum extension of plastic deformation, the types of crack paths, and the area fractions of these crack paths are determined. The results, which are in partial disagreement with earlier work, provide the basis for the qualitative understanding and the quantitative description of fracture processes in composites combining a ductile phase embedded in a brittle matrix.

209 citations


Journal ArticleDOI
TL;DR: In this paper, the Double Cantilever Beam (DCB) test geometry was utilized to investigate the rate ef fects on Mode I interlaminar fracture toughness in graphite/PEEK and graphite-epoxy composites.
Abstract: The Double Cantilever Beam (DCB) test geometry was utilized to investigate the rate ef fects on Mode I interlaminar fracture toughness in graphite/PEEK and graphite/epoxy composites. The tests were...

197 citations



Journal ArticleDOI
L. Ewart1, Subra Suresh1
TL;DR: In this article, it was shown that the phenomenon of crack growth under cyclic compressive stresses exhibits a macroscopically similar behaviour in a wide range of materials spanning the very ductile metals to extremely brittle solids, although the micromechanics of this effect are very different among the various classes of materials.
Abstract: Stable crack growth is observed in notched plates of polycrystalline alumina subject to fully compressive far-field cyclic loads at room temperature in a moist air environment andin vacuo. The fatigue cracks propagate at a progressively decreasing velocity along the plane of the notch and in a direction macroscopically normal to the compression axis. The principal failure events leading to this effect are analysed in terms of notch-tip damage under the far-field compressive stress, microcracking, frictional sliding and opening of microcracks, and crack closure. An important contribution to such Mode I crack growth arises from the residualtensile stresses induced locally at the notch-tip when the deformation within the notch-tip process zone leaves permanent strains upon unloading from the maximum nominal compressive stress. It is shown that the phenomenon of crack growth under cyclic compressive stresses exhibits a macroscopically similar behaviour in a wide range of materials spanning the very ductile metals to extremely brittle solids, although the micromechanics of this effect are very different among the various classes of materials. The mechanisms of fatigue in ceramics are compared and contrasted with the more familiar examples of crack propagation under far-field cyclic compression in metallic systems and the implications for fracture in ceramic-metal composites and transformation toughened ceramic composites are highlighted. Strategies for some important applications of this phenomenon are recommended for the study of fracture mechanisms and for the measurement of fracture toughness in brittle solids.

189 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of the stress ratio on near-threshold growth of delamination fatigue cracks was investigated with unidirectional laminates made from Ciba Geigy 914C prepegs (T300/914) and from Toray P305 pre-pegs.

Journal ArticleDOI
TL;DR: F fracture toughness was highest in the more heavily filled resins, independent of degree of conversion in the matrix, and producing a pre-crack in certain composites reduced the value of their fracture toughness compared with notched-only specimens.
Abstract: The fracture toughness of a variety of dental composites was evaluated using notched bending specimens, with and without pre-cracks. The pre-crack simulates more accurately a sharp, natural flaw in a material than does a notch, and is standard procedure in fracture-toughness evaluation. The fracture toughness was related to the filler composition and degree of conversion in the composite resins. In general, fracture toughness was highest in the more heavily filled resins, independent of degree of conversion in the matrix. The results also show that producing a pre-crack in certain composites reduced the value of their fracture toughness compared with notched-only specimens.

Journal ArticleDOI
TL;DR: In this article, the fracture process zone and its effect on fracture toughness were examined for Charcoal and Rockville granite, average grain sizes of 1 and 10mm, and the experimental evidence indicated an effective crack to be composed of a traction free length and a ligament process zone, which was observed to form a multiconnected region within the macrocrack.

Journal ArticleDOI
TL;DR: In this article, specific essential works of plane stress ductile tearing of polyethylenes were obtained from deeply edge-notched tension specimens, with either single or double notches, by extrapolating the straight line relationship between the total specific fracture work and ligament length to zero ligament.
Abstract: The specific essential works of plane stress ductile tearing of several high- and ultrahigh-molecular-weight polyethylenes were obtained from deeply edge-notched tension specimens, with either single or double notches, by extrapolating the straight line relationship between the total specific fracture work and ligament length to zero ligament. Provided the fracture morphologies of the torn ligament are not widely different, the specific essential work (we) is a material property dependent on thickness but independent of specimen geometry. The specific essential fracture work also can be identified with Jc the critical value of the J-integral along a contour immediately bordering the fracture process zone at the crack tip. There is good agreement between the experimental we values and theoretical Jc estimates for these polyethylene materials.

Journal ArticleDOI
TL;DR: In this article, the effects of alloy composition and aging treatment on plane strain fracture initiation toughness (Klc, Jlc) and crack growth resistance (characterized by the tearing modulus, TR) were investigated in high purity alloys belonging to the Al-Li-Cu-Zr system.

Journal ArticleDOI
TL;DR: In this paper, the effect of water on the deformation, fracture and strength of Lac du Bonnet granite has been investigated through a variety of experimental techniques: standard short-term tests for compressive strength and fracture toughness, and longer-term, time dependent experiments that measure creep, static fatigue and slow crack velocity.

Journal ArticleDOI
TL;DR: In this paper, the static fracture toughness of oil shale, which is a typical layered rock containing fossil fuel, was measured using static tests, and the fracture toughness was determined using a stress intensity factor method, a compliance method and a J- integral based method.

Journal ArticleDOI
TL;DR: In this article, double cantilever beam (DCB) specimens were made using unidirectional lay-ups of T6C/Hx205 composite material in which the delaminating halves were placed at angles of 0, 1.5, and 3 degrees to each other.
Abstract: The possibility to eliminate fiber bridging or at least to reduce it, and to evaluate an alternative approach for determination of in situ mode 7 fracture toughness values of composite matrix materials were investigated. Double cantilever beam (DCB) specimens were made using unidirectional lay-ups of T6C/Hx205 composite material in which the delaminating halves were placed at angles of 0, 1.5, and 3 degrees to each other. The small angles between the delaminating plies were used to avoid fiber nesting without significantly affecting mode I teflon insert. The DCB specimens were fabricated and it was found that: (1) the extent which fiber bridging and interlaminar toughness increase with crack length can be reduced by slight cross ply at the delamination plane to reduce fiber nesting; (2) some fiber bridging may occur even in the absence of fiber nesting; (3) the first values of toughness measured ahead of the thin teflon insert are very close to the toughness of the matrix material with no fiber bridging; (4) thin adhesive bondline of matrix material appears to give toughness values equal to the interlaminar toughness of the composite matrix without fiber bridging.

Journal ArticleDOI
TL;DR: In this paper, a new model for determining elastic/plastic indentation is presented, which generalizes Johnson's incompressible core model to a compressible material and allows the indentation pressure to be transmitted via a misfitted inclusion core beneath the indenter which is surrounded by a hemispherical plastic zone.
Abstract: A new model for determining elastic/plastic indentation is presented This model generalizes Johnson's incompressible core model to a compressible material and allows the indentation pressure to be transmitted via a misfitted inclusion core beneath the indenter which is surrounded by a hemispherical plastic zone The internal stress field inside the core is obtained by applying Eshelby's spherical inclusion problem together with Hill's spherical-cavity expansion analysis The plastic deformation considered here exactly ensures compatibility between the volume of a material displaced by the indenter and that accommodated by expansion The analysis explains the essential relationships between the dimensions of the indentation and plastic zone and the dominant material properties; yield stress, hardness and elastic modulus The solution is extended to evaluate the indentation fracture toughness by taking into account the reduced half-space constraint by the image force

Journal ArticleDOI
TL;DR: In this article, the deformation process of the small punch test and its use to characterize the fracture properties of materials were investigated. But the results of biaxial equivalent fracture strain can be related almost linearly to fracture toughness (JIc) for elastic-plastic behavior.
Abstract: The purpose of this investigation is to understand the deformation process of the small punch test and to use it to characterize the fracture properties of materials. By comparisons between recrystallization-etch results and perfect-rigid plastic analysis, the deformation process of the small punch test has been clearly divided into three regions. Equivalent fracture strain under biaxial stress state has been obtained from 10 by 10 by 0.5 mm thin sheet samples using both a recrystallization-etch technique and a semi-analytical method to characterize the fracture property of the material. The results of biaxial equivalent fracture strain can be related almost linearly to fracture toughness (JIc) for elastic-plastic behavior.

Journal ArticleDOI
TL;DR: In this paper, fracture toughness measurements on single crystals of YBa2Cu3Ox, the phase responsible for superconductivity above liquid-nitrogen temperatures, were performed and showed that these crystals are susceptible to moisture-enhanced nonequilibrium crack propagation.
Abstract: We report fracture toughness measurements on single crystals of YBa2Cu3Ox, the phase responsible for superconductivity above liquid‐nitrogen temperatures. Indentation crack length measurements on the (010) orthorhombic crystal growth faces revealed the (100) and (001) planes as preferred fracture planes. The toughness of these planes is Kc=1.1±0.3 MPa m1/2, and the hardness H=8.7±2.4 GPa. The observed growth of both radial and lateral cracks in ambient air suggests that these crystals are susceptible to moisture‐enhanced nonequilibrium crack propagation.

Journal ArticleDOI
TL;DR: In this article, the fracture toughness of epoxy and polyethersulfone was measured using three point bend tests (ASTM: E-399-81), and the influence of the morphology of the epoxy/polyether sulfone blend on its fracture toughness and toughening mechanism has been studied.
Abstract: Compatibility or miscibility of a polyethersulfone (ICI: Victrex 100P) and a tetrafunctional epoxy (Ciba-Geigy: MY-720), cured with an aromatic anhydride, has been studied using scanning electron microscopy, x-ray microanalysis, and dynamic mechanical spectroscopy. Fracture toughness of epoxy and a blend of epoxy and polyethersulfone has been measured using three point bend tests (ASTM: E-399-81). Fracture surfaces were examined by x-ray microanalysis for detecting concentration of sulfur, present in polyethersulfone, in the matrix and precipitated phase. The influence of the morphology of the epoxy/polyethersulfone blend on its fracture toughness and toughening mechanism has been studied. Fracture toughness values of unmodified and modified cured epoxy resin were evaluated as a function of test temperature.

Journal ArticleDOI
TL;DR: In this paper, the authors extended the maximum circumferential tensile stress theory to anisotropic solids, and formulated the criteria in terms of the maximization of the ratio of the Maximum Circular Tensile Stress (MCTS) over the material critical tensile strength.

Journal ArticleDOI
TL;DR: In this article, the static and cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature, and it was shown that the minimum time to failure was almost the same, in spite of differences in loading mode or frequency.
Abstract: The static and cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation-induced flaw at the center, were tested under a static or cyclic load applied by four-point bending. Sintered silicon nitride was shown to be susceptible to static and cyclic fatigue failure. Comparing the static and cyclic fatigue lifetimes at frequencies from 0.01 to 10 Hz, it was shown that minimum time to failure was almost the same, in spite of differences in loading mode or frequency. However, cyclic stress decreased the scatter in lifetime by reducing the upper limit. Moreover, the cyclic fatigue limit was significantly lower than the static fatigue limit. High-magnification fractography revealed a fatigue failure dominated by intergranular cracking with partial transgranular failure at perpendicularly elongated crystals. This suggests that the intergranular fatigue crack can be arrested at grain-boundary triplets, and also can be reactivated by subsequent cyclic loading. The crack growth rate, calculated from the fatigue lifetime, showed three characteristic regions having a plateau at 70% to 90% of the fracture toughness, which suggests a possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.

Journal ArticleDOI
TL;DR: Values have been established for the critical stress intensity factor (Kc) and the critical strain energy release rate (Gc), which depend on the orientation of the cortical bone, as well as on bone density, the velocity of crack propagation and specimen geometry.

01 Jan 1987
TL;DR: In this article, the authors have demonstrated that the solid particle erosion rate can be taken as a simple and highly reproducible statistical measure of the susceptibility of Si and GaAs to contact damage in the micron range.
Abstract: In recent years, the growing field of semiconductor micromechanics has created an increasing demand for strength data on semiconductors and for adequate tests and evaluations of their mechanical properties. In a recently published paper, the authors have demonstrated that the solid particle erosion rate can be taken as a simple and highly reproducible statistical measure of the susceptibility of Si and GaAs to contact damage in the micron range. In the present work the scope is broadened to include several new crystal orientations (and one new doping level), as well as three new materials: Ge, InP and InAs, for which hardness and fracture toughness (KIc) values are determined by means of the indentation technique. KIc values are also derived from erosion data by means of a brittle fracture model based on a recently reported mechanism of non-lateral spalling in single-crystal semiconductors. These values are compared with literature results obtained by the indentation technique and four-point bend tests. The materials tested are ranked with respect to elastic properties, microhardnesses, fracture toughnesses, and sensitivities to contact damages in general. The influence of crystallographic orientation on room temperature hardness and micro-fracture properties is clearly established, but the influence of doping is found to be either negligible or small.

BookDOI
01 Oct 1987
TL;DR: The first four days were devoted to introductory and overview lectures on theory of fracture, solid state chemistry and physics of fracture and solution chemistry, and structure and properties of interfaces as discussed by the authors.
Abstract: : The first four days were devoted to introductory and overview lectures on theory of fracture, solid state chemistry and physics of fracture, solution chemistry, and structure and properties of interfaces After a one-day break, three days of workshop sessions and a summary lecture were held Workshop topics included novel aspects of fracture, intergranular embrittlement, hydrogen embrittlement, and stress corrosion and corrosion fatigue Content: Chemistry and Physics of Fracture: Mechanics of Brittle Cracking of Crystal Lattices and Interfaces; Theory of Fracture; Plastic Processes at Crack Tips; Plastic Flow Instabilities at Crack Tips; Distributed Damage Processes in Fracture; A comparison of Void Growth and Ductile Failure in Plane and Axisymmetric States of Strain; Solid State Chemistry and Physics of Fracture; Theoretical Approaches to Materials Design: Intergranular Embrittlement; Interatomic Forces and the Simulation of Cracks; Application of the Embedded Atom Method to Hydrogen Embrittlement; Solution Chemistry; Surface Chemistry in Aqueous Solutions; Crack Tip Electrochemistry: Electrochemical Thermodynamics and Kinetics and Their Application to the Study of Stress Corrosion Cracking; Structure and Properties of Interfaces; Universal Properties of Bonding at Metal Interfaces; Structure of Grain Boundaries and Interfaces; In Situ TEM Studies of Crack Tip Deformation in Molybdenum; Hydrogen Embrittlement; The Role of Hydrogen Transport in Hydrogen Embrittlement

Journal ArticleDOI
TL;DR: In this article, it was shown that the crack velocity-strain rate relationship in the presence of multiple cracked specimens can be computed by considering the number of cracks, in which case it is possible to calculate (involving electrochemical data) the crack velocities.
Abstract: One of the problems in relation to predicting the rates of defect growth in structures displaying stress corrosion cracking (SCC) is that knowledge relating to realistic stress corrosion crack velocities is very limited. In monotonic slow strain rate tests (SSRTs) taken to total failure at usual strain rates and in tests on precracked specimens at stress intensity factors in excess of KISCC, stress corrosion crack velocities may be appreciably higher than are likely to occur in service, except in the very late stages of crack growth before a service failure. These differences probably develop partly because of differences in the effective crack tip strain rate, and consideration is given to various expressions for calculating the latter from the applied strain rate. For multiple cracked specimens, as in SSRTs, the importance of considering the number of cracks is demonstrated, in which case it is possible to calculate (involving electrochemical data) the crack velocity-strain rate relationships i...

Journal Article
TL;DR: In this paper, the strength, fracture toughness, and oxidation behavior of a hot-pressed SiC-TiB/sub 2/ composite (with B and C additions) were investigated.
Abstract: The strength, fracture toughness, and oxidation behavior of a hot-pressed SiC-TiB/sub 2/ composite (with B and C additions) were investigated. Adding 15 vol% TiB/sub 2/ to an SiC matrix increased the mean strength and mean fracture toughness of the composite by 28% and 45%, respectively. Oxidation of the SiC-15-vol%-TiB/sub 2/ composite occurred slowly at 1000C and 1200C but was catastrophic at 1400C.

Journal ArticleDOI
TL;DR: In this paper, a large deformation finite element analysis was made to study the deformation, stress and strain, and void ratio near the crack tip under mixed mode plane strain loading conditions, employing Gurson's constitutive equation which has taken into account the effects of void nucleation and growth.
Abstract: In ductile fracture, voids near a crack tip play an important role. From this point of view, a large deformation finite element analysis has been made to study the deformation, stress and strain, and void ratio near the crack tip under mixed mode plane strain loading conditions, employing Gurson's constitutive equation which has taken into account the effects of void nucleation and growth. The results show that: (i) one corner of the crack tip sharpens while the other corner blunts, (ii) the stress and strain distributions except for the near crack tip region, can be superimposed by normalizing distance from the crack tip by a crack tip deformation length, i.e., a steady-state solution under a mixed mode condition has been obtained, (iii) the field near a crack tip can be divided into four characteristic fields ( K field, HRR field, blunted crack tip field, and damaged region), and (iv) the strain and void volume fraction become concentrated in the sharpened part of a crack tip with increasing Mode II component.