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Showing papers on "Fracture toughness published in 1991"


Journal ArticleDOI
TL;DR: In this article, a two-parameter fracture mechanics approach for tensile mode crack tip states in which the fracture toughness and the resistance curve depend on Q, i.e., JC(Q) and JR(Δa, Q), is proposed.
Abstract: Central to the J-based fracture mechanics approach is the existence of a HRR near-tip field which dominates the actual field over size scales comparable to those over which the micro-separation processes are active. There is now general agreement that the applicability of the J-approach is limited to so-called high-constraint crack geometries. We review the J-annulus concept and then develop the idea of a J-Q annulus. Within the J-Q annulus, the full range of high- and low-triaxiality fields are shown to be members of a family of solutions parameterized by Q when distances are measured in terms of J σ 0 , where σ0 is the yield stress. The stress distribution and the maximum stress depend on Q alone while J sets the size scale over which large stresses and strains develop. Full-field solutions show that the Q-family of fields exists near the crack tip of different crack geometries at large-scale yielding. The Q-family provides a framework for quantifying the evolution of constraint as plastic flow progresses from small-scale yielding to fully yielded conditions, and the limiting (steady-state) constraint when it exist. The Q value of a crack geometry can be used to rank its constraint, thus giving a precise meaning to the term crack-tip constraints, a term which is widely used in the fracture literature but has heretofore been unquantified. A two-parameter fracture mechanics approach for tensile mode crack tip states in which the fracture toughness and the resistance curve depend on Q, i. JC(Q) and JR(Δa, Q), is proposed.

1,023 citations


Journal ArticleDOI
TL;DR: In this paper, the principal toughening mechanism of a substantially toughened, rubber-modified epoxy has again been shown to involve internal cavitation of the rubber particles and the subsequent formation of shear bands.
Abstract: The principal toughening mechanism of a substantially toughened, rubber-modified epoxy has again been shown to involve internal cavitation of the rubber particles and the subsequent formation of shear bands. Additional evidence supporting this sequence of events which provides a significant amount of toughness enhancement, is presented. However, in addition to this well-known mechanism, more subtle toughening mechanisms have been found in this work. Evidence for such mechanisms as crack deflection and particle bridging is shown under certain circumstances in rubber-modified epoxies. The occurrence of these toughening mechanisms appears to have a particle size dependence. Relatively large particles provide only a modest increase in fracture toughness by a particle bridging/crack deflection mechanism. In contrast, smaller particles provide a significant increase in toughness by cavitation-induced shear banding. A critical, minimum diameter for particles which act as bridging particles exists and this critical diameter appears to scale with the properties of the neat epoxy. Bimodal mixtures of epoxies containing small and large particles are also examined and no synergistic effects are observed.

484 citations


Journal ArticleDOI
TL;DR: A review of fracture toughness of fiber composites can be found in this article, where various methods for improving the fracture toughness by means of interface control are discussed. But, the focus is on the failure mechanisms leading to eventual fracture.

478 citations


Journal ArticleDOI
D.J. Lloyd1
TL;DR: In this article, the tensile deformation and fracture behavior of the aluminium alloy 6061 reinforced with SiC has been investigated, and it is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing.
Abstract: The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.

427 citations


Book ChapterDOI
TL;DR: In this article, the applicability of the J-integral test procedure to test short crack specimens in the temperature region below the initiation of ductile tearing where J 1 c cannot be measured was investigated.
Abstract: This study investigates the applicability of the J-integral test procedure to test short crack specimens in the temperature region below the initiation of ductile tearing where J 1 c cannot be measured. The current J-integral test procedure is restricted to determining the initiation of ductile tearing and requires that no specimen demonstrates brittle cleavage fracture. The J 1 c test specimen is also limited to crack-depth to specimen-width ratios (a/W) between 0.50 and 0.75. In contrast, the crack tip opening displacement (CTOD) test procedure can be used for testing throughout the entire temperature-toughness transition region from brittle to fully ductile behavior. Also, extensive research is being conducted to extend the CTOD test procedure to the testing of short crack specimens (alW ratios of approximately 0.15). The CTOD and J-integral fracture parameters are compared both analytically and experimentally using square (cross-section) three-point bend specimens of A36 steel with a/W ratios of 0.50 (deep crack) and 0.15 (short crack). Three-dimensional elastic-plastic finite element analyses are conducted on both the deep crack and the short crack specimens. The measured J-integral and CTOD results are compared at various levels of linear-elastic and elastic-plastic behavior. Experimental testing is conducted throughout the lower shelf and lower transition regions where stable crack growth does not occur. Very good agreement exists between the analytical and experimental results for both the short crack and deep crack specimens. Results of this study show that both the J-integral and the CTOD fracture parameters work well for testing in the lower shelf and lower transition regions where stable crack growth does not occur. A linear relationship is shown to exist between J-integral and CTOD throughout these regions for both the short and the deep crack specimens. These observations support the consideration to extend the J-integral test procedure into the temperature region of brittle fracture rather than limiting it to J 1 c at the initiation of ductile tearing. Also, analyzing short crack three-point bend specimen (a/W < 0.15) records using the load versus load-line displacement (LLD) record has great potential as an experimental technique. The problems of accurately measuring the CMOD of short crack specimens in the laboratory without affecting the crack tip behavior may be eliminated using the J-integral test procedure.

427 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of heat treatment on the mechanical properties of Nb-Nb5-Si3 two-phase alloys having compositions Nb 10 and 16 pct Si (compositions quoted in atomic percent) was investigated.
Abstract: The effect of heat treatment on the mechanical properties of Nb-Nb5-Si3 two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb5Si3 matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.

268 citations


Journal ArticleDOI
TL;DR: In this paper, a model is proposed that describes the failure of glassy polymers in the crazing regime, based on the realization that the cross-tie fibrils, which are known to exist between primary polymers, can have a profound effect on the failure mechanics of a craze as they can transfer stress between the broken and unbroken polymers.
Abstract: A model is proposed that describes the failure of glassy polymers in the crazing regime. This model is based on the realization that the cross-tie fibrils, which are known to exist between primary fibrils in all crazes, can have a profound effect on the failure mechanics of a craze as they can transfer stress between the broken and unbroken fibrils. A very simple model of crack tip stress amplification caused by the cross-ties is shown to work well in the prediction of the fracture toughness of a bulk, high molecular weight, glassy polymer

240 citations


Journal ArticleDOI
TL;DR: A series of networks of diverse crosslink density were prepared using bifunctional epoxide prepolymers of different molecular weight, crosslinked with diamine diphenyl sulphone, and their fracture behaviour investigated as discussed by the authors.
Abstract: A series of networks of diverse crosslink density were prepared using bifunctional epoxide prepolymers of different molecular weight, crosslinked with diamine diphenyl sulphone, and their fracture behaviour investigated. The same set of resins was also modified with a reactive rubber. The fracture toughness regularly decreased on increasing the crosslink density for all formulations. The addition of the rubber gave rise to a marked increase in toughness, though it magnified the influence of the molecular weight of the prepolymer. Tests performed with blunt notches showed that the fracture toughness was maximum at medium crosslink densities. A dispersion of rubber particles caused a toughness increase through the formation of microcavities ahead of the crack tip. Failure was preceded by a rapid volume increase caused by void coalescence.

228 citations


Book
01 Jan 1991
TL;DR: In this article, the authors proposed a strengthening mechanism for fibrous composite materials based on the plasticity theory of metal matrix composites and showed that the strengthening mechanism can improve tensile and compressive properties of in situ composites.
Abstract: Part 1 Strengthening mechanisms: plasticity theories for fibrous composite materials, G.J.Dvorak strengthening of metal matrix composites due to dislocation generation through CTE mismatch, R.J.Arsenault strengthening behaviour of in situ composites, T.H.Courtney. Part 2 Mechanical properties: tensile and compressive properties of metal matrix composites, R.J.Arsenault mechanical behaviour of metal matrix composites under high strain rates and impact loadings, M.Taya creep behaviour of metal matrix composites, M.Taya. Part 3 Fracture and fatigue: fracture toughness of particulate metal matrix composites, D.L.Davidson fatigue of metal matrix composites, K.K.Chawla fatigue of discontinuously reinforced metal matrix composites, J.K.Shang and R.O.Ritchie. Part 4 Physical and chemical properties: dynamic mechanical properties, A.Wolfenden and J.M.Wolla thermal expansion of metal matrix composites, T.A.Hahn electrical conductivity in continuous fibre composites, J.E.Schoutens corrosion, P.P.Trzaskoma.

218 citations


Journal ArticleDOI
TL;DR: In this article, the martensitic transformation zones surrounding cracks and indentations have been observed for cracks growing parallel to the layers as well as for those that were oriented normal to the layer.
Abstract: Laminar composites, containing layers of Ce-ZrO2 and either Al2O3 or a mixture of Al2O3 and Ce-ZrO2, have been fabricated using a colloidal method that allowed formation of layers with thicknesses as small as 10 μm. Strong interactions between these layers and the martensitic transformation zones surrounding cracks and indentations have been observed. In both cases, the transformation zones spread along the region adjacent to the layer, resulting in an increased fracture toughness. The enhanced fracture toughness was observed for cracks growing parallel to the layers as well as for those that were oriented normal to the layers.

210 citations


Journal ArticleDOI
TL;DR: In this paper, a plane-strain finite-element analysis and a local stress-based criterion for cleavage fracture are combined to establish specimen size requirements (deformation limits) for testing in the transition region which assure a single parameter characterization of the crack-tip stress field.
Abstract: Single edge-notched bend (SENB) specimens containing shallow cracks (a/W < 0.2) are commonly employed for fracture testing of ferritic materials in the lower-transition region where extensive plasticity (but no significant ductile crack growth) precedes unstable fracture. Critical J-values Jc) for shallow crack specimens are significantly larger (factor of 2–3) than the Jc)-values for corresponding deep crack specimens at identical temperatures. The increase of fracture toughness arises from the loss of constraint that occurs when the gross plastic zones of bending impinge on the otherwise autonomous crack-tip plastic zones. Consequently, SENB specimens with small and large a/W ratios loaded to the same J-value have markedly different crack-tip stresses under large-scale plasticity. Detailed, plane-strain finite-element analyses and a local stress-based criterion for cleavage fracture are combined to establish specimen size requirements (deformation limits) for testing in the transition region which assure a single parameter characterization of the crack-tip stress field. Moreover, these analyses provide a framework to correlate Jc)-values with a/W ratio once the deformation limits are exceeded. The correlation procedure is shown to remove the geometry dependence of fracture toughness values for an A36 steel in the transition region across a/W ratios and to reduce the scatter of toughness values for nominally identical specimens.

Journal ArticleDOI
TL;DR: In this article, the size-adjusted Paris law is combined with size-effect law for fracture under monotonic loading, which leads to a size adjusted Paris law, which gives the crack length increment per cycle as a power function of the amplitude of size adjusted stress intensity factor.
Abstract: Crack growth caused by load repetitions in geometrically similar notched concrete specimens of various sizes is measured by means of the compliance method. It is found that the Paris law, which states that the crack length increment per cycle is a power function of the stress intensity factor amplitude, is valid only for one specimen size (the law parameters being adjusted for that size) or asymptotically, for very large specimens. To obtain a general law, the Paris law is combined with size-effect law for fracture under monotonic loading, proposed previously by Bazant. This leads to a size-adjusted Paris law, which gives the crack length increment per cycle as a power function of the amplitude of size-adjusted stress intensity factor. The crack growth is also characterized in terms of the nominal stress amplitude.

Journal ArticleDOI
TL;DR: In this article, the authors extended the cohesive crack model to mixed mode propagation and an experimental confirmation is provided by testing four-point shear specimens of concrete, where a constant crack mouth sliding displacement rate is imposed, so that is is possible to control and detect the snapback load vs deflection branches.

Journal ArticleDOI
TL;DR: In this paper, the dependence of the energy release rate ratio on the in-plane stress was analyzed and the non-dimensional stress parameter, σ 0(a/E* Ti)1/2, where a is the initial length of the kink into the substrate, E* is a modulus quantity, and Ti is the fracture energy of the interface.
Abstract: A crack lying in the interface between two brittle elastic solids can advance either by continued growth in the interface or by kinking out of the interface into one of the adjoining materials. This competition can be assessed by comparing the ratio of the energy release rates for interface cracking and for kinking out of the interface to the ratio of interface toughness to substrate toughness. The stress parallel to the interface, σ0, influences the energy release rate of the kinked crack and can significantly alter the conditions for interface cracking over substrate cracking if sufficiently large. This paper provides the dependence of the energy release rate ratio on the in-plane stress. The nondimensional stress parameter which emerges is, σ0(a/E* Ti)1/2, where a is the initial length of the kink into the substrate, E* is a modulus quantity, and Ti is the fracture energy of the interface. An experimental observation of the cracking of reaction product layers in bonds between Ti(Ta) and Al2O3 is rationalized by the theory.

Journal ArticleDOI
TL;DR: In this paper, the authors considered a semi-infinite crack propagating through a brittle matrix material, which contains a regular distribution of tough particles, and the effect of the crack bowing between obstacles is included by means of an incremental perturbation method.
Abstract: The toughness of a brittle material may be substantially improved by adding small quantities of tough particles to the solid. Three mechanisms may be responsible. Firstly, the front of a crack propagating through the solid can be trapped by the particles, causing it to bow out between them. Secondly, the particles may remain intact in the wake of the crack, thereby pinning its faces and reducing the crack tip stress intensity factors. Finally, the toughness may be enhanced by frictional energy dissipation as particles are pulled out in the wake of the crack. This paper estimates the improvement in toughness that might be expected due to these mechanisms, by means of a three-dimensional model. The analysis considers a semi-infinite crack propagating through a brittle matrix material, which contains a regular distribution of tough particles. Particles in the wake of the crack are modelled by finding an appropriate distribution of point forces that pin the crack faces; and the effect of the crack bowing between obstacles is included by means of an incremental perturbation method based on work byRice [J. Appl. Mech.56, 619 (1985)]. The calculation predicts the shape of the crack as it propagates through the solid; the resulting R-curve behaviour; and the length of the bridged zone in the wake of the crack.

Journal ArticleDOI
TL;DR: In this article, the distribution of temperature at the tips of dynamic propagating cracks in two heat treatments of AISI 4340 carbon steel was investigated experimentally using an array of eight high speed indium antimonide, infrared detectors.
Abstract: T he heat generated due to plastic deformation at the tip of a dynamically propagating crack in a metal causes a large local temperature increase at the crack tip which is expected to affect the selection of failure modes during dynamic fracture and to thus influence the fracture toughness of the material. The distribution of temperature at the tips of dynamically propagating cracks in two heat treatments of AISI 4340 carbon steel was investigated experimentally using an array of eight high speed indium antimonide, infrared detectors. Experiments were performed on wedge loaded, compact tension specimens with initially blunted cracks, producing crack speeds ranging from 1900 to 730 m/s. The measurements provide the spatial distribution of temperature increase near the crack tip on the specimen surface. Temperature increases were as high as 465° C over ambient and the region of intense heating (greater than 100° C temperature rise) covered approximately one third of the active plastic zone on the specimen surface. The observed temperature increase profiles clearly show the three-dimensional nature of the fracture process near the specimen surface and provide valuable information regarding the dynamic formation of shear lips and their role in the dissipation of energy during dynamic crack growth. Preliminary temperature measurements performed on side-grooved specimens are also reported.

Journal ArticleDOI
TL;DR: In this article, the authors used plane strain elastic-plastic finite element analysis and a local criterion for cleavage fracture to establish specimen size requirements for the ductile-brittle transition region.
Abstract: The article utilizes plane strain elastic-plastic finite element analysis and a local criterion for cleavage fracture to establish specimen size requirements for the ductile-brittle transition region. Critical J and CTOD values, relative to the small-scale yielding value, were predicted as a function of specimen size, strain hardening exponent, and a/W. These analyses predict an increase in the apparent toughness with decreasing specimen size due to a loss in crack tip constraint; this effect is particularly pronounced in shallow notched specimens and low hardening materials. For deeply notched bend and compact specimens, the following size requirement must be met for critical J values for cleavage to be size independent: b , B , a > 200 J c σ Y where b is ligament length, B is thickness, a is crack length, and σγ is flow stress. This criterion is eight times more severe than the size requirements in ASTM E 813-87, but it is less stringent than the requirements of ASTM E 399-83. In order for a CTOD value to be nearly size independent, it must be less than 1/300 times the relevant specimen dimensions. The constraint loss in shallow notched specimens is usually far too rapid to obtain J-controlled cleavage fracture, but the analyses presented in this article provide a means for correcting fracture toughness data for constraint loss. Predictions of the effect of a/W on toughness in the transition region agree favorably with experimental data. Future work will consider the effects of specimen thickness and ductile tearing on transition region toughness.

Journal ArticleDOI
TL;DR: In this article, C addition (2 wt%) to MoSi2 acted as deoxidant, removing the otherwise ubiquitous siliceous grain boundary phase in hot-pressed samples, and causing formation of SiC and Mo5Si3C1 (a variable-composition Nowotny phase).
Abstract: C addition (2 wt%) to MoSi2 acted as a deoxidant, removing the otherwise ubiquitous siliceous grain boundary phase in hot-pressed samples, and causing formation of SiC and Mo5Si3C1 (a variable-composition Nowotny phase). Both hardness and fracture toughness of the C-containing alloy were higher than those of the C-free (and oxygen-rich) material; more significantly, the fracture toughness of the MoSi2+ 2% C alloy increased from 5.5 MPa·m1/2 at 800°C to ∼11.5 MPa·m1/2 at 1400°C.

Journal ArticleDOI
TL;DR: In this paper, the authors present the blister test for the measurement of interface fracture toughness for a thin film bonded to an elastic substrate, where the load applied in order to debond the film is either hydrostatic pressure or a point load.

Journal ArticleDOI
TL;DR: In this article, a strip crack closure model based on the original Dugdale-Barenblatt model was investigated for various aspects of fatigue crack growth behavior, and a variable constraint factor was introduced into the model to account for the 3D effect at the crack tip.

Journal ArticleDOI
TL;DR: In this paper, the authors measured fracture toughness of SiC(whisker)-reinforced alumina and a sintered silicon nitride by direct measurements of lengths of cracks associated with Vickers indentation flaws.
Abstract: R-curves for a sinter/HIPed SiC(whisker)-reinforced alumina and a sintered silicon nitride were assessed by direct measurements of lengths of cracks associated with Vickers indentation flaws. The fracture toughness measurements based on (a) initial (as-indented) crack lengths, (b) equilibrium growth of cracks during increasing far-field loading, and (c) crack lengths corresponding to unstable fracture showed definitive trends of R-curves for both materials. The fracture mechanics analyses employed an indenter-material constant that was independently estimated using a physical model for the residual driving force and a free surface correction factor that accounted for the effects of size and shape of the cracks on stress intensity. It is shown that R-curve estimations based on crack length measurements have the intrinsic advantage that crack length dependence of fracture toughness is not assumed a priori as is done in conventional analysis based on strength. The measured fracture toughness of SiC(whisker)-reinforced alumina was in agreement with the prediction of a toughening model based on crack bridging by partially debonded whiskers.

Book
01 Jan 1991
TL;DR: In this article, the role of matrix properties on the toughness of thermoplastic composites has been discussed, and the relevance of fatigue and the design of fatigue studies is discussed.
Abstract: 1. Introduction (F.N. Cogswell). The keys to thermoplastic composites. Envoi. Bibliography. 2. Physical and mechanical properties of high-performance thermoplastic polymers and their composites (N.J. Johnston, T.W. Towell, P.M. Hergenrother). General features of thermoplastic matrices. Neat resins and properties. Composite properties and performance. Thermoset matrices versus thermoplastic matrices. Challenges. References. 3. Semicrystalline polymer matrix materials (J.M. Schultz). Introduction: typical matrix polymers. Microstructure. Microstructure and properties. Kinetics. Methods of controlling spherulite size. Aging. Modeling of crystallization during molding. References. 4. Short fiber reinforced thermoplastics (D.G. Brady, J.L. Kardas). Introduction. Short fibers for reinforcement. Mechanical property performance and limitations in short-fiber systems. Processing short-fiber composites. Applications for short-fiber composites. Appendices. References. 5. Long fiber molding materials (J.M. Crosby). Introduction. Production methods of LFRTP molding materials. Fiber microstructures in LFRTPs. Mechanical properties. Fabrication/processing. References. 6. Continuous-fiber thermoplastic composites (A.Y. Lou, T.P. Murtha, J.E. O'Connor, D.G. Brady). Introduction. Thermoplastic matrix resins. Reinforcing fibers. Product forms. Mechanical properties. Processing. Applications. References. 7. Creep behaviour of thermoplastic composites (A.A. Ogale). Definitions. Physical aging of thermoplastics. Viscoelastic response of thermoplastics. Viscoelastic response of thermoplastic composites. References. 8. Fracture of thermoplastic composites (K. Friedrich, L.A. Carlsson, J.W. Gillespie, J. Karger-Kocsis). Fracture behaviour of unfilled thermoplastic. Fracture of injection-molded short fiber reinforced PEEK. Interlaminar fracture of PEEK composites. 9. The role of matrix properties on the toughness of thermoplastic composites (W.L. Bradley). Micromechanisms of interlaminar fracture. Strain-field mapping around crack tips during interlaminar fracture. Mode-I and mode-II critical energy release rates for various composite material systems. Models for predicting delamination fracture toughness. Summary. References. 10. Fatigue of thermoplastic composites (D.R. Moore). The relevance of fatigue and the design of fatigue studies. Factors affecting fatigue of thermoplastics. Fatigue of discontinuous thermoplastic composites. Fatigue of continuous-fibre thermoplastic composites.

Journal ArticleDOI
TL;DR: In this paper, the authors established the range of in-plane fracture mode mixtures and contact zone sizes that can be obtained from an edge-cracked bimaterial strip under biaxial applied displacements.
Abstract: The paper establishes the range of in-plane fracture mode mixtures and contact zone sizes that can be obtained from an edge-cracked bimaterial strip under biaxial applied displacements. The development of a suitable loading device for and the application of crack opening interferometry to interfacial crack initiation experiments is described. The crack initiation process under bond-normal loading is examined in detail for a glass/epoxy interface in order to establish a hybrid optical interference/finite element analysis technique for extracting mixed-mode fracture parameters.

Journal ArticleDOI
TL;DR: In this paper, the size effect method, previously developed for concrete and mortar, is demonstrated for rock and the measured maximum load values are used to obtain fracture energy, fracture toughness and effective size of the fracture process zone.

Proceedings ArticleDOI
TL;DR: In this paper, the effect of varying amounts of Laves phase on the mechanical properties of wrought and cast + HIP Inconel718 is discussed, and methods for controlling Laves phases in cast+HIP+INconel 718 are discussed.
Abstract: The effect of varying amounts of Laves phase on the mechanical properties of wrought and cast + HIP Inconel718 is discussed. When present as a continuous or semicontinuous grain boundary network in wrought Inconel718, Laves phase dramatically reduces room temperature tensile ductility and ultimate tensile strength, with room temperature impact and fracture toughness properties and elevated temperature ductility also reduced. Laves may also act as a preferred crack initiation and propagation site, resulting in reduced low cycle fatigue (LCF) p blty d ca a i i an accelerated fatigue crack growth rates. Laves present as large globular aggregates in cast+HIP Inconel 718 significantly reduces room temperature tensile and elevated temperature stress rupture properties. In addition, the phase acts as a preferred crack initiation and propagation site, resulting in significant reductions in smooth and notch LCF capability and an accelerated fatigue crack growth rate. Methods for controlling Laves phase in wrought and cast +HIP Inconel 718 are discussed. Superalloys 718,625 and Various Derivatives Edited by Edward A. JJxia The Minerals, Metals & Materials Society, 1991

Journal ArticleDOI
TL;DR: In this article, the authors investigated the fatigue behavior of 3-mol%yttria-stabilized tetragonal zirconia polycrystals and found that microcracking is the dominant mechanism of fatigue damage, and that nucleation of fatigue crack is usually not necessary.
Abstract: ~Uniaxial tension-compression fatigue behavior of 3-mol%yttria-stabilized tetragonal zirconia polycrystals was investigated. Hysteresis in the stress-plastic strain curve featured cumulative plastic strain and weakened elastic stiffness. Fracture statistics in terms of cycle-to-failure depends strongly on the maximum stress and less on the stress amplitude. Preexisting processing flaws were identified as the fracture origins in all cases. We suggest that microcracking is the dominant mechanism of fatigue damage, that nucleation of fatigue crack is usually not necessary, and that fatigue lifetime is primarily controlled by crack propagation, which is most sensitive to the maximum stress. [Key words: fatigue, fracture, zirconia, cracks, stress.]

Journal ArticleDOI
TL;DR: In this article, the Al2O3/Au interface has been measured for the first time subject to conditions that exclude stress corrosion, and it has been shown that crack growth occurs with a rising resistance governed by intact metal ligaments in the crack wake.
Abstract: Crack propagation has been measured for the Al2O3/Au interface subject to conditions that exclude stress corrosion. Crack growth has been shown to occur with a rising resistance, governed by intact metal ligaments in the crack wake. The level of resistance also increases as the metal layer thickness increases. Crack extension occurs by a combination of plastic void growth and interface debonding. The fracture energies are much larger than the work of adhesion, but appreciably smaller than those expected for ductile interface fracture. The fracture energy is nevertheless dominated by plastic dissipation, which increases at larger metal layer thicknesses.

Journal ArticleDOI
TL;DR: In this paper, the fracture toughness increases up to 11.3MPa·m1/2 with the increase in the grain size, and a prominent crack bridging along the fracture surface behind the propagating crack tip is found to be the possible toughening mechanism.
Abstract: Improved KIC values of monolithic Si3N4 ceramics will be expected by growing-in a matrix of microcrystals of β-phase Si3N4-an appreciable amount of extensively large, rod-shape crystals of the same phase. It is anticipated that the large grains should play as crack impediments like whiskers mixed in composite ceramics. The test materials prepared by changing firing conditions had such well-grown crystals of different sizes, ranging from 2 to 10 microns in diameter. The fracture toughness (measured by the SEPB method) increases up to 11.3MPa·m1/2 with the increase in the grain size. Prominent crack bridging along the fracture surface behind the propagating crack tip is found to be the possible toughening mechanism.

Journal ArticleDOI
TL;DR: In this paper, the aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination, and it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening.
Abstract: The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M23C6 carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.

Journal ArticleDOI
TL;DR: In this article, the elastic-plastic fracture toughness, characterized by the crack tip opening displacement (CTOD), was analyzed for short crack test specimens with a/W ratios of 0.15 and 0.50 throughout lower-shelf and lower-transition regions.
Abstract: Short crack test specimens (a/W ≪ 0.50) are frequently employed when conventional deep crack specimens are either inappropriate or impossible to obtain, for example, in testing of particular microstructures in weldments and in-service structures containing shallow surface flaws. Values of elastic-plastic fracture toughness, here characterized by the crack tip opening displacement (CTOD), are presented for square (cross-section) three-point bend specimens with a/W ratios of 0.15 and 0.50 throughout the lower-shelf and lower-transition regions. Three dimensional, finite-element analyses are employed to correlate the measured load and crack mouth opening displacement (CMOD) values to the corresponding CTOD values, thus eliminating a major source of experimental difficulty in previous studies of shallow crack specimens. In the lower-transition region, where extensive plasticity (but no ductile crack growth) precedes brittle fracture, critical CTOD values for short crack specimens are significantly larger (factor of 2–3) than the CTOD values for deep crack specimens at identical temperatures. Short crack specimens are shown to exhibit increased toughness at the initiation of ductile tearing and decreased brittle-to-ductile transition temperatures. Numerical analyses for the two a/W ratios reveal large differences in stress fields ahead of the crack tip at identical CTOD levels which verify the experimentally observed differences in critical CTOD values. Correlations of the predicted stresses with measured critical CTOD values demonstrate the limitations of single-parameter fracture mechanics (as currently developed) to characterize the response.