Showing papers on "Fracture toughness published in 1992"

The relation between crack growth resistance and fracture process parameters in elastic-plastic solids

Abstract: CKA~K growth initiation and subsequent resistance is computed for an elastic-plastic solid with an idealized traction separation law specified on the crack plane to characterize the fracture process. The solid is specified by its Young’s modulus, E, Poisson’s ratio, v, initial tensile yield stress, (or, and strain hardening exponent, N. The primary parameters specifying the traction-separation law of the fracture process are the work of separation per unit area, To. and the peak traction, 6. Highly refined calculations have been carried out for resistance curves. K,(Arr), for plane strain, mode I growth in small-scale yielding as dependent on the parameters characterizing the elastic-plastic properties of the solid and its fracture process. With K,, = [El-,/( I ~ v’)] ’ 2 as the intensity needed to advance the crack in the absence ofplasticity, K,J& is presented in terms of its dependence on the two most important parameters, d/nr and N, with special emphasis on initiation toughness and steady-state toughness, Three applications of the results are made : to predict toughnesss when the fracture process is void growth and coalescence, to predict the role of plasticity on interface toughness for similar materials bonded together, and to illuminate the role of plasticity in enhancing toughness in dual-phase solids. The regime of applicability of the present model to ductile fracture due to void growth and coalescence, wherein multiple voids interact within the fracture process zone, is complementary to the regime of applicability of models describing the interaction between a single void and the crack tip. The two mechanism regimes are delineated and the consequence of a transition between them is discussed.

Family of crack-tip fields characterized by a triaxiality parameter—II. Fracture applications

Abstract: C entral to the J-based fracture mechanics approach is the concept of J-dominance whereby J alone sets the stress level as well as the size scale of the zone of high stresses and strains. In Part I the idea of a J Q annulus was developed. Within the annulus, the plane strain plastic near-tip fields are members of a family of solutions parameterized by Q when distances are normalized by J σ 0 , where σ0is the yield stress, J and Q have distinct roles: J sets the size scale over which large stresses and strains develop while Q scales the near-tip stress distribution and the stress triaxiality achieved ahead of the crack. Specifically, negative (positive) Q values mean that the hydrostatic stress is reduced (increased) by Qσ0 from the Q = 0 plane strain reference state. Therefore Q provides a quantitative measure of crack-tip constraint, a term widely used in the literature concerning geometry and size effects on a material's resistance to fracture. These developments are discussed further in this paper. It is shown that the J Q approach considerably extends the range of applicability of fracture mechanics for shallow-crack geometries loaded in tension and bending, and deep-crack geometries loaded in tension. The J Q theory provides a framework to organize toughness data as a function of constraint and to utilize such data in engineering applications. Two methods for estimating Q at fully yielded conditions and an interpolation scheme are discussed. The effects of crack size and specimen type on fracture toughness are addressed.

Failure mechanisms of polymer interfaces reinforced with block copolymers

Abstract: The fracture toughness (characterized by the critical energy release rate G c ) of interfaces between polystyren (PS) and poly(2-vinylpyridine) (PVP) reinforced with poly(styrene-b-2-vinylpyridine) was measured with a double cantilever beam test geometry. The effect of the PVP block degree of polymerization and the areal density of block copolymer chains at the interface on the measured G c and on the fracture mechanisms was investigated quantitatively.

Remarks on Crack-Bridging Concepts

Abstract: The article draws upon recent work by us and our colleagues on metal and ceramic matrix composites for high temperature engines. The central theme here is to deduce mechanical properties, such as toughness, strength and notch-ductility, from bridging laws that characterize inelastic processes associated with fracture. A particular set of normalization is introduced to present the design charts, segregating the roles played by the shape, and the scale, of a bridging law. A single material length, {gamma}{sub 0}E/{sigma}{sub 0}, emerges, where {gamma}{sub 0} is the limiting-separation, {sigma}{sub 0} the bridging-strength, and E the Young`s modulus of the solid. It is the huge variation of this length-from a few manometers for atomic bond, to a meter for cross-over fibers - that underlies the richness in material behaviors. Under small-scale bridging conditions, {gamma}{sub 0}E/{sigma}{sub 0} is the only basic length scale in the mechanics problem and represents, with a pre-factor about 0.4, the bridging zone size. A catalog of small-scale bridging solutions is compiled for idealized bridging laws. Large-scale bridging introduces a dimensionless group, a/({gamma}{sub 0}E/{sigma}{sub 0}), where a is a length characterizing the component. The group plays a major role in all phenomena associated with bridging, and provides a focus ofmore » discussion in this article. For example, it quantifies the bridging scale when a is the unbridged crack length, and notch-sensitivity when a is hole radius. The difference and the connection between Irwin`s fracture mechanics and crack bridging concepts are discussed. It is demonstrated that fracture toughness and resistance curve are meaningful only when small-scale bridging conditions prevail, and therefore of limited use in design with composites. Many other mechanical properties of composites, such as strength and notch-sensitivity, can be simulated by invoking large-scale bridging concepts. 37 refs., 21 figs., 3 tabs.« less

Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy

Abstract: A two-phase gamma titanium aluminide alloy, Ti-47Al-1Cr-1V-2.5Nb (in at.%), was studied under forged and various subsequent heat treatment conditions, to investigate the microstructural evolution and the effect of microstructure on room temperature (RT) tensile properties and fracture toughness behavior. Four classes of microstructure and three types of lamellar formation were identified, and their formation mechanisms were analyzed using various analytical techniques including metallography, electron optics, differential thermal analysis (DTA), and crystallography. It was found that both tensile and toughness behavior were profoundly affected by the microstructural variations.

Conditions for Pseudo Strain-Hardening in Fiber Reinforced Brittle Matrix Composites

Abstract: Apart from imparting increased fracture toughness, one of the useful purposes of reinforcing brittle matrices with fibers is to create enhanced composite strain capacity. This paper reviews the conditions underwhich such a composite will exhibit the pseudo strain-hardening phenomenon. The presentation is given in a unified manner for both continuous aligned and discontinuous random fiber composites. It is demonstrated that pseudo strain hardening can be practically designed for both gills of composites by proper tailoring of material structures. 18 refs., 8 figs., 2 tabs.

Rock fracture mechanics : principles, design, and applications

Abstract: Preface. Contents. List of Notations. 1. Introduction. 2. Some Fundamental Aspects of Mechanics. 3. The Griffith Theory and the Evolution of Modern Fracture Mechanics. 4. Linear Elastic Fracture Mechanics and Fracture Initiation Theories. 5. Determination of Stress Intensity Factors. 6. Aspects of Non-linear Elastic Fracture Mechanics. 7. Some Aspects of Statistical Fracture Mechanics. 8. Mode I Fracture Toughness Testing. 9. Mode II and Mixed Mode I-II Fracture Toughness Testing. 10. Interrelationships Between Fracture Toughness, Hardness Index and Physico-Mechanical Properties of Rocks. 11. Fracture Mechanics Applied to Hydraulic Fracture Propagation and In Situ Stress Determinations. 12. Fracture Mechanics Applied to Rock Fragmentation by Cutting Action. 13. Fracture Mechanics Applied to Rock Fragmentation due to Blasting. 14. Fracture Mechanics Applied to Analysis of Rockbursts. 15. Fracture Mechanics Applied to Design and Stability of Rock Slopes Engineering Problems. Appendix: A Compilation of Mode I Fracture Toughness Values of Rocks. References. Supplementary List of Recommended References. Index.

Asymmetric Shielding in Interfacial Fracture Under In-Plane Shear

Abstract: The toughness of a glass/'epoxy interface was measured over a wide range of mode mixes. A toughening effect was associated with increasing positive and negative in-plane shear components. Optical interference measurements of normal crack opening displacements near the crack front and complementary finite element analyses were used to examine near-front behavior during crack initiation. Estimates of the tough­ening based on plastic dissipation, bulk viscoelastic dissipation, and interface asperity shielding did not fully account for the measured values. The results suggest that the inelastic behavior of the epoxy, frictional, and, perhaps, three-dimensional effects should be considered. 1 Introduction Interfacial crack growth occurs in a number of applications of technological importance. Because of the fact that the frac­ture path is constrained irrespective of the orientation of the globally applied loads and also because of the mismatch of material properties across the interface, crack growth is in­herently mixed mode. Critical and subcritical crack growth must then be governed by some combination of mode I, II, and III fracture parameters. The simplest approach, using one parameter, seeks to determine an effective parameter that can account for all mode mixes in a unifying manner. This is particularly useful for subcritical crack growth where corre­lations of crack growth rates fall on one curve for all mode mixes when the proper parameter is found. An alternative is to consider a two-parameter approach where one parameter represents the mode mix or direction and the other a magni­tude. For critical crack growth, the magnitude will generally be a function of mode mix. The form the function is usually determined experimentally but may, as mechanisms are better understood, even be predicted. The most common approach for examining interfacial crack initiation has been to consider the interfacial fracture tough­ness, G

Fracture energy and fracture process zone

Abstract: The fracture energy Gf can be determined following a RILEM recommendation. However, it has been found that fracture energy depends on both size and geometry of the test specimen. The underlying fictitious crack model postulates that fracture energy, tensile strength, the critical opening of the fictitious crack, and the shape of the softening curve (softening factor) are constants for a given type of concrete. Here it is shown that a local fracture energy ccan be introduced. This local fracture energy varies with the width of the fracture process zone. As the crack approaches the back end of a specimen the fracture process zone becomes more and more confined and hence the local fracture energy decreases. Theoretical predictions are compared with experimental results obtained with the wedge splitting technique described earlier. It is shown that a local variation of the fracture energy leads to a size dependence of the global specific fracture energy.

Microstructural Development During Gas‐Pressure Sintering of α‐Silicon Nitride

Abstract: Gas-pressure sintering of α-Si3N4 was carried out at 1850 ° to 2000°C in 980-kPa N2. The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si3N4 powder was similar to that in the materials from β-Si3N4 powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si3N4 ceramics.

Solvent Degradation and Reduced Fracture Toughness in Aged Composites

Abstract: Qartz- and barium-glass-filled composites aged for more than one year in ethanol experienced a significant reduction in fracture toughness (K1c), essentially identical to that experienced after two months of aging This reduction is mainly attributed to a softening of the resin matrix, but cracking within the resin and at the filler/matrix interface, as revealed by SEM microscopy, may also have contributed No significant cracking could be seen in the composites aged in water Composites post-cured at temperatures approaching their glass-transition temperature also experienced a reduction in K1c after alcohol storage Storage in water for one year had little effect on the K1c of composites cured at oral temperatures, but a significant increase was observed for those post-cured at elevated temperatures This increase is difficult to explain, but appears to involve a filler/matrix interfacial phenomenon, because it was not observed in the unfilled resin The results of this study demonstrate that an alteration in the fracture resistance and some degradation of the filler/matrix interface, as has been observed clinically, occur after long-term exposure of dental composites to certain solvents used as food-simulating liquids

Estimation procedure for the Weibull parameters used in the local approach

Abstract: The local approach was recently proposed by Beremin and Mudry for evaluating the statistical behaviour of toughness results of materials. This approach introduces a stress parameter σ w , termed the Weibull stress, as a measure of the fracture resistance of materials instead of the conventional toughness parameters such as K c, δ c and J cl (critical stress intensity factor, CTOD and J-integral, respectively). The Weibull stress σ w obeys the Weibull distribution with the two parameters m and σ u (the shape and the scale parameter, respectively). The first parameter m is normally estimated to be 22 irrespective of the kind of material. In this paper a procedure for the determination of the Weibull parameters m and σ u is developed. This procedure consists of the determination of the plastic zone ahead of the crack tip, from which cleavage fracture originates, and of the maximum likelihood estimation of the parameters m and σ u based on the stress distribution in the plastic zone. Calculations using this procedure confirm that the distribution of the Weibull stress σ w is a material property independent of specimen thickness, and in particular that the shape parameter m depends on the material, e.g. m≃12 for a German reactor pressure vessel steel (20 Mn Mo Ni 5 5). Using these parameters for the distribution of the Weibull stress the size effect in fracture toughness values is predicted and an improved agreement between theory and experiments is obtained compared to the Weakest Link model.

Influence of microstructure on crack-tip micromechanics and fracture behaviors of a two-phase TiAl alloy

Abstract: The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α2) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma + lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range.In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the process zone.

The fabrication and failure of laminar ceramic composites

Abstract: Rather than using fibres or other dense inclusions as a means of introducing weak interfaces to deflect cracks, a much simpler route has been developed where composite elements, in this case sheets, are made from silicon carbide powder. After coating, these sheets are stacked, compacted together and sintered without any pressure. It is shown that the graphite layers will deflect cracks preventing catastrophic failure whilst raising the apparent fracture toughness from 3.6 MPa✓m to 17.7 MPa✓m and the work required to break the sample from 28 J m −2 . Further crack growth through the sample occurs when the unbroken part reaches is failure strees as determined from an unnothched beam. This allows the apparent fracture toughness to be simply related to the failure stress and this has been confirmed by experiments where the strength of the beam has been controlled by adding artificial flaws. Crack growth is not much affected by the thickness of the interface except at low thicknesses where small gaps in the interface are more likely to occur, allowing the delamination crack to kink out of the interface and cross the next lamina. The thermal stability of the laminate up to 1500°C in air is limited only by the oxidation resistance of the graphite. The potential for the process is also discussed.

Fiber-Matrix Adhesion and Its Effect on Composite Mechanical Properties: IV. Mode I and Mode II Fracture Toughness of Graphite/Epoxy Composites

Abstract: To optimize the level of fiber-matrix adhesion an understanding of the relationship between fiber-matrix interfacial bond strength and the mechanical and frac ture behavior of composites is essential. This study establishes the relationship between fiber-matrix interfacial shear strength (ISS) and interlaminar fracture toughness (both Mode I and Mode II) and failure modes for graphite/epoxy composites. A well defined and characterized graphite fiber/epoxy system was chosen in which the level of adhesion be tween fiber and matrix was changed by using the same graphite fibers with different sur face treatments. These surface treatments changed the level of adhesion between the fiber and matrix thus resulting in an increase of the fiber-matrix ISS by over a factor of two while the fiber and matrix properties remained unchanged. The Mode I and Mode II tests were conducted by the double cantilever beam (DCB) and end-notch flexure (ENF) tests methods, respectively. The Mode I fracture toughness (GIC) of composi...

Cyclic Fatigue-Crack Growth in a SiC-Whisker-Reinforced Alumina Ceramic Composite: Long- and Small-Crack Behavior

Abstract: The ambient-temperature subcritical growth behavior of both long and microstructurally small cracks is investigated during cyclic-fatigue loading in a SiC-whisker-reinforced alumina (Al2O3–SiCw) ceramic composite (fracture toughness, Kc∼ 4.5 MPa · m1/2). Based on long-crack experiments using compact C(T) specimens, cyclic fatigue-crack growth rates (over the range 10−11 to 10−5 m/cycle) are found to be sensitive to the applied stress-intensity range and load ratio, and to show evidence of fatigue crack closure. Similar to other ceramic materials, under tension–tension loading the “long”(>3 mm) crack fatigue threshold, ΔKTH, was found to be on the order of 60% of Kc. Conversely, “small”(1 to 300 μm) cracks grown from micro-indents on the surface of cantilever-beam specimens were observed to grow at applied ΔK levels some 2 to 3 times smaller than ΔKTH. similar to behavior widely reported for metallic materials. The observed small-crack behavior is rationalized in terms of the residual stress field associated with the indent, and the restricted role of crack-tip shielding (from mechanisms such as crack bridging, closure and deflection) with cracks of limited wake, analogous to closure effects with small fatigue cracks in metals. Consistent with the lack of zone-shielding mechanisms in Al2O3–SiCw, under variable-amplitude cyclic loading crack-growth rates do not exhibit the marked transient response following block overload sequences as do transformation-toughened ceramics or ductile metallic materials. Possible mechanisms for cyclic crack advance in reinforced ceramic-matrix composite materials are discussed.

A Modified DCB Specimen for Mode I Testing of Multidirectional Laminates

Abstract: A detailed study of the Mode I interlaminar fracture toughness measure ment of multidirectional laminates is presented. The use of a novel DCB specimen is pro posed in which the conventional DCB specimen is delaminated along its edges by the in corporation of inserts in addition to the usual delamination at the loaded end. Investigations using this specimen to examine the fracture toughness at +45°/+45° and +45°/-45° interfaces indicate that the appropriate width of edge delamination can sup press the crack jumping and fibre bridging which occur in the conventional DCB speci men. Preliminary tests show that GIC for +45°/+45° and +45°/-45° interfaces in multidirectional laminates are significantly higher than the GIc measured for a 0°/0° inter face in a unidirectional laminate.

Strength and fracture of glass and ceramics

Abstract: 1. Strength and Fracture of Glass and Ceramics. Structure. Glass. Ceramics. Glass-ceramics. Mechanical properties. Elasticity. Non-elastic phenomena. The strength of glass and ceramics. Glass. Ceramics. Theoretical strength of brittle materials. The causes of low strength in glass and ceramics. Glass. Ceramics. Conditions for crack growth. Fracture behaviour with time. Subcritical crack growth, influence of the environment. Maximum fracture velocity. Crack trajectory. Failure due to various types of load. Concentrated load. Impact loading. Thermal stresses. Statistical aspects of strength and fracture. 2. The Principles of Fracture Mechanics. Stresses in a body with cracks. Basic terms, stress intensity factor. Determination of the stress intensity factor. The criterion for crack growth. The Irwin criterion for brittle fracture. The Griffith criterion for brittle fracture. The energy release rate. Practical use of fracture criteria. Fracture toughness of glass and ceramics. Velocity of crack growth. Glass. Ceramics. Determination of strength and lifetime of a body with a crack. Materials without subcritical crack growth. Materials with subcritical crack growth. Materials at high temperatures. Methods for ensuring service safety and lifetime. Non-destructive testing. Statistical methods. Proof tests. 3. Determination of Mechanical Properties. Determination of elastic constants. Young's modulus. Shear modulus. Determination of inelastic properties. Hardness. Viscosity. Creep. Strength tests. Tensile strength. Compressive strength. Bending strength. Shear strength. Strength under multiaxial stress. Inert and impact strength. Determination of fracture mechanics parameters. Test specimens. Determination of fracture toughness. Determination of the relationship v(K1). Determination of constants A, N. Other kinds of tests. Tests of resistance to sudden changes of temperature. Tests of impact resistance. Tests of resistance to pressure. Proof tests. Non-destructive tests, acoustic emission. Wear tests. Statistical methods. Introduction. Principal types of probability distribution. Determination of failure probability and allowable stress. The influence of body size. Errors due to scatter of measured values. 4. Fracture Analysis. Fracture pattern. General features. Practical examples. Morphology of fracture surfaces. General features. Fracture: origin, mirror, drawing. Rough surface. Shell-like fracture. General principles of fracture analysis. 5. Strengthening of Glass and Ceramics. General methods of increasing failure resistance. Strengthening of glass. Protection against surface defect generation. Surface treatment to remove or reduce surface flaws. Formation of a compressive prestress in surface layer. Controlled crystallization. Combination with other materials. Increasing the strength of ceramics. Reducing the size of critical flaws. Reducing unfavourable inner stresses.

Cracking of brittle films on elastic substrates

Abstract: An analysis is presented that relates the crack spacing in a brittle film on an elastic substrate to the stress in the film, its thickness and fracture toughness. This analysis differs from an earlier one presented by one of the authors in that it considers the effect of a sequential, rather than a concerted, propagation of cracks, and predicts a larger crack spacing. The validity of the present analysis was confirmed by experimental results from a model system consisting of epitaxial PrBa2Cu3O7−x films on SrTiO3 substrates. The experimentally observed relationship between the crack spacing and the film thickness was in excellent agreement with the theory.

Effect of Loading Path and Porosity on the Failure Mode of Porous Rocks

Abstract: Grain crushing and pore collapse are the dominant compaction mechanisms in high porosity clastic rocks. These micromechanical processes control the evolution of strain hardening during cataclastic flow, and they can also result in embrittlement of the rock. The mechanics of the transition from brittle fracture to homogeneous cataclastic flow for the Berea and Kayenta sandstones were investigated in the laboratory. The mechanical data show that the transition is sensitively dependent on the stress state as well as the porosity. In the stress space, the complete locus for brittle failure by shear localization can be determined by tests on normally consolidated and overconsolidated samples along different loading paths. Using porosity as the hardening parameter, the evolution of the inelastic yield locus with strain hardening can be mapped out in the stress space. This yield locus expands with decreasing porosity. Scanning electron microscope and acoustic emission measurements were used to elucidate the micromechanics. The onset of grain crushing and pore collapse was marked by a surge in acoustic emission activity. A Hertzian fracture mechanics model was formulated to analyze the roles of porosity, grain size and fracture toughness in controlling the onset of hydrostatic and shear-enhanced compaction. Stereological measurements of the microcrack density show that significant stress-induced anisotropy was induced by shear-enhanced compaction, with preferred orientations of the stress-induced microcracks subparallel to the maximum compression direction.

Development and characterization of electroconductive Si3N4-TiN composites

Abstract: Dense Si3N4-TiN composites, with the second phase ranging from 20 to 40 vol.%, were produced by hot pressing, gas-pressure sintering and pressureless sintering under nitrogen gas atmosphere. The influence of densification technique and parameters and of amount and grain size of TiN particles on microstructure, mechanical properties, electrical resistivity and oxidation resistance was evaluated. The addition of TiN particles increases the stiffness and the fracture toughness of the base material. For TiN content higher than 30 vol.% the electrical resistivity of the composites is less than 10−3 ωcm. An evident effect of the grain size distribution of TiN powders on some mechanical properties was ascertained. The thermal stability of the composites is strongly affected by the amount of the second phase.

Brittle fracture of adhesive joints

Abstract: The effect of mode mix upon crack path is examined for a brittle epoxy adhesive. The mode mix is varied over wide limits by using asymmetrical double cantilever beam and Brazil nut specimen configurations. The observed crack paths are interfacial, in-layer, and alternating from one interface to the other in a periodic fashion. The probability of occurrence and toughness of each crack path is measured, and is compared with existing theory. Interfacial fracture is the most common path for all values of mode mix; the interfacial toughness increases dramatically with increasing mode II component of remote load. Under remote mode I loading the alternating crack path covers approximately 20 percent of the fracture surface of the specimens and has the highest macroscopic toughness. The effect of residual stress magnitude, layer thickness and mode mix upon the wavelength of the alternating crack mode is measured. Predictions based on a previous analysis by Akisanya and Fleck [1] compare favourably with the measured values.

Design of a Laminated Ceramic Composite for Improved Strength and Toughness

Abstract: Adding aluminum titanate to alumina can result in dramatic improvements in toughness and R-curve properties. However, the improved toughness is offset by a significant reduction in strength at small flaw sizes. This problem can be overcome through the use of a laminated composite construction. By placing a thin layer of high-strength material on the surface of a high-toughness body, the toughness and flaw tolerance of the body material can be maintained without sacrificing small flaw strength. In this study, alumina + 20 vol% aluminum titanate (AAT20) was used for both the surface layer and the bulk material. The surface material was a homogeneous, fine-grained mixture of the two phases, while the bulk was an inhomogeneous mixture having a bimodal grain structure. In monolithic form, the homogeneous AAT20 displays a nearly P−1/3 indentation strength response, and the inhomogeneous material displays a flat strength response, indicative of R-curve behavior. The trilayer material shows a composite indentation strength response, with high strength throughout the entire range of starting flaw sizes. A method for predetermining the optimum surface layer thickness is presented. The processing and mechanical properties of these materials will be discussed.

The fracture resistance of metal-ceramic interfaces

Abstract: The effects of interface structure and microstructure on the fracture energy, Γi, of metal-ceramic interfaces are reviewed. Some systems exhibit a ductile fracture mechanism and others fail by brittle mechanisms. In the absence of either interphases or reaction products, Γi is dominated by plastic dissipation (for both fracture mechanisms), leading to important effects of metal thickness, h, and yield strength, σ0. Additionally, Γi is larger when fracture occurs by ductile void growth (for the same h and σ0). A fundamental understanding now exists for the ductile fracture mechanism. However, some basic issues remain to be understood when fracture occurs by brittle bond rupture, particularly with regard to the role of the work of adhesion, Wad. Interphases and reaction products have been shown to have an important effect on Γi. A general trend found by experiment is that Γi scales with the fracture energy of the interphase itself, wherein Γi tends to increase for the interphase sequence: amorphous oxides > crystalline oxides > intermetallics. However, there also appear to be important effects of the residual stresses in the interface (which influence the fracture mechanism and the layer thickness.

Structure/property relationships in sintered and thermally sprayed WC-Co

Abstract: Thermally sprayed WC-Co is widely used as a wear-resistant coating for a variety of applications. Although it is well established that thermal spray processes significantly affect chemistry, microstructure, and the phase distribution of WC-Co coatings, little is known about how these changes influence wear resistance. In this study, the microstructure and wear behavior of sintered and thermally sprayed WC-Co materials are examined. Powders of WC-12 wt% Co and WC-17 wt% Co were pressed and sintered, as well as thermally sprayed by high-velocity oxy-fuel (HVOF), air plasma spray (APS), and vacuum plasma spray (VPS) techniques. Results indicated considerable differences in the resulting microstructures, mechanical properties, and wear resistance. The thermally sprayed coatings showed anisotropic fracture toughness, whereas the sintered materials did not. It was also shown that a combined mechanical property/microstructure parameter, based on considerations of indentation fracture mechanisms, can be used in most cases to describe abrasive and erosive wear resistance of thermally sprayed WC-Co materials as follows: Wear resistance a % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiFj0de9sqqrpepC0xbbL8F4rrpm0dbba9-u0ddr-df9% qqFn0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-xfr-x% b9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcLbsacqaHXoqyju% aGdaqadaGcbaqcLbsacaWGlbqcfa4aa0baaSqaaGqaaKqzGeGaiaiG% aaab--xsaiacaciaaqW--neaaSqaaKqbaoacaciaaWX-liaameacac% iaaWX-jugibiacaciaaWX-iodaaWqaiaiGaaah-NqzGeGaiaiGaaah% --hoaaaaaaGaamisaKqbaoacaciaaWR-CaaaleqcaciaaWR-bGaGac% aa86Fcfa4aiaiGaaae-VGaaWqaiaiGaaae-NqzGeGaiaiGaaae-Jym% aaadbGaGacaaq8FcLbsacGaGacaaq8-FYaaaaaaaaOGaayjkaiaawM% caaKqbaoaabmaakeaajuaGdaWcaaGcbaqcLbsacGaGacaaKdWGwbqc% fa4aiaiGaaa5a0baaSqaiaiGaaa5aKqzGeGaamOzaaWcbGaGacaaKd% qcLbsacGaGS-3qaiacaY+FVbaaaaGcbaqcLbsacGaG48xmaiacaIZF% TaGaiaiodAfajuaGdGaG40baaSqaiaioieGajugibiacaciaaWO-+z% gaaSqaiaiojugibiacaciaaaX--neacGaGacaaq8-FVbaaaaaaaOGa% ayjkaiaawMcaaaaa!8ABD! $$\alpha \left( {K_{IC}^{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 8}}\right.\kern- ulldelimiterspace}\!\lower0.7ex\hbox{$8$}}} H^{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern- ulldelimiterspace}\!\lower0.7ex\hbox{$2$}}} } \right)\left( {\frac{{V_f^{Co} }}{{1 - V_f^{Co} }}} \right)$$ whereK ic is the indentation fracture toughness,H is hardness, andV Co f is the volume fraction of cobalt. This relationship provides a means for assessing wear resistance of WC-Co coatings intended for industrial applications requiring abrasion and/or erosion resistance.

Small-punch and TEM-disc testing techniques and their application to characterization of radiation damage

Abstract: The present paper summarizes the development of miniaturized small-punch (SP) and TEM-disc (TD) testing techniques and shows their applicability in characterizing the mechanical properties of irradiated materials. The yield strength, ductility and fracture toughness,J lc, were empirically estimated by analysing the deformation and fracture behaviour observed in the miniaturized specimen tests. The ductile-brittle transition temperature (DBTT) was determined from the variation of the SP or TD fracture energy with temperature. A linear correlation between the DBTT obtained from the SP and Charpy V-notched specimen tests has been theoretically and experimentally established. The problems of cracking detection and data scattering often observed in the SP or TD specimen tests are discussed in terms of heterogeneous embrittlement behaviour of materials. It has been demonstrated that these miniaturized testing techniques are capable of evaluating hardening, DBTT shifts andJ lc decreases caused by neutron irradiation.