To design the microstructure of materials by suitable synthesis and processing for optimal performance, it is necessary to relate the microstructure to the properties of the material. This requires recovery experiments with time-resolved measurements. Dynamic recovery experiments, until recently, have been a challenge to researchers. However, through a series of innovations, several techniques have been developed and perfected at the Center of Excellence for Advanced Materials (CEAM), at the University of California, San Diego (UCSD), which provide powerful tools for performing dynamic recovery tests on a broad class of materials, from metals and metal composites to ceramics and their composites, over a broad range of strain rates. This paper summarizes these new techniques. In addition, a brief account of a recent breakthrough in computational plasticity is given.

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Various methods are used in inhomogeneous functional graded technology to control the constituent elements of a composite material. Functional graded technology includes chemical and physical processes for such con-tro1. The vapor deposition method can be used to fabricate a film or a platelike meterial directly without need of melting or sintering. Furthermore, composition control of dispersion is comparatively easy in the direction perpendicular to the deposition surface. Fabrication of compositional graded materials (so called functionally gradient materials, FGMs) has been attempted using the vapor deposition method. For instance, Ti-TiC and Ti-TiN FGMs have been fabricated using physical vapor deposition (PVD), and SiC-TiC multilayer film and SiC/CFGM has been fabricated using chemical vapor deposition (CVD). In this paper; functional graded technology employing the vapor deposition method is reviewed.

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We consider an axisymmetric contact problem of a transversely isotropic layer indented by a circular rigid punch, with the elastic layer resting on a rigid foundation. This problem is equivalent to a mixed boundary-value problem in which the displacement is prescribed within the contact region of the punch and surface stresses are zero outside of the contact region. The problem is reduced to an infinite simultaneous equation using the technique of expressing the normal stress under the punch as an appropriate series. Numerical results are obtained with the indication of the effect of material anisotropy and layer thickness on the stress fields in comparison with those for the isotropic material.

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The shear properties of composite laminates are expected to degrade significantly due to interlaminar delaminations as the compressive properties. The shear buckling and postbuckling behaviors of delaminated composite plates are analyzed, and the effects of delamination on the shear properties are studied. A theory developed based on the Rayleigh-Ritz approximation is used. It can deal with the contact problem of delaminated portions with the introduction of constraints on the delamination surface. A significant drop of shear properties is shown. The shear behaviors of delaminated laminates are found to be composed of a complex combination of local delamination buckling and global instability. The buckling and postbuckling behaviors, particularly local deformations, depend strongly on the initial imperfections. The estimation proposed in the present paper is useful to obtain rough estimates of local and total unstable loads.

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An analytical investigation of the difference in the constitutive behavior of SiC-Al composites under uniaxial compressive and tensile loading was carried out and the associated changes of the residual stresses were studied. It is suggested that the observed asymmetric response of the constitutive behavior is primarily due to the existence of residual stresses in the composite, and the residual stresses are initially introduced due to the differences in the coefficients of thermal expansion (ΔCTE), and subsequently changed after the external load is applied. A two-dimensional finite element analysis of a hexagonal array of SiC whiskers in an Al matrix was performed, the result of which testifies to the appropriateness of the proposed explanation. It is concluded that the 2-D finite element analysis is an economical way of adequately reproducing the most prominent features of the material's constitutive behavior. Based upon the theory of mechanics of composite material, simplified analytical models were developed which can be used to investigate the influences of the whisker-matrix interface shear load transfer and the volume mismatch on the residual stresses. It is concluded that the mechanism of load transfer in terms of normal stress at the whisker tip, which is governed by the volume (bulk) mismatch between the matrix and the whisker, is predominantly responsible for altering the specific pattern of the residual stresses under the applied load. In contrast, the effect of shear load transfer at the longitudinal whisker-matrix interface is only restricted to a very small region in the vicinity of the whisker and does not significantly influence the overall proposed asymmetric constitutive behavior.

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Porcelain-fused-to-metal (PFM) restorations may develop cracks during processing or in-mouth service if the relative physico-mechanical properties of the porcelain and alloy are highly mismatched. The precise conditions when it might occur are not known. Many processing and property variations can affect the stresses developed throughout a porcelain-alloy system. To understand this, a computer simulation of stress in a PFM strip was conducted. The simulation considers cooling from temperatures higher than the porcelain sag point. The following temperature-dependent factors were incorporated : the elastic modulus, shear viscosity (porcelain), and coefficient of thermal expansion. Cooling rates during tempering of porcelain were measured. The cooling rate dependencies of the glass transition temperature and temperature distribution during cooling were also included in the simulation. The results suggest that tempering with a high cooling rate make up for compressive residual stress in body porcelain and the high curvature of the composite beam.

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Development of reliable bonding techniques for ceramics and metals is considered very important for more useful applications of ceramics. Generally, however, it is a serious problem that ceramics have a high thermal stess concentration at the bonded interface, caused by the different thermal properties of the two materials. The authors have been investigating various methods for effectively decreasing the stress concentration. In this paper, we present a fundamental consideration of the thermal stress distribution characteristics of the interface of a dissimilar material joint by using thermoleastic-plastic finite-element analysis. Through this analysis, it is found that the high thermal stress is induced only in a very narrow region, and that the value of the high stress is approximately estimated by simplified theoretical analysis. Further, we found that the stress concentaration can be reduced by changing the shape of the ceramics of the joint.

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The present paper treats a minimum weight design of laminated composite panels under in-plane loading. Constraints consist of strength and displacement conditions. Layer orientation angles as well as layer thicknesses are used as design variables. An approximation method is applied to generate a high-quality approximate optimization problem. Stress components are evaluated using a linear approximation of stress resultants while displacements are approximated in a linear form. Transformed design variables with respect to the layer angles are also introduced to reduce the nonlinearity between the strength constraints and the layer angles. It is shown that optimal laminate configurations can be obtained with fewer than ten iterations.

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A previously proposed lamination tailoring technique for precise control of a coefficient of thermal expansion (CTE) under a wide range of temperature is examined by detailed measurements of a CTE of two types of specimens : rectangular plates and tubes of carbon/epoxy composite. The theoretical basis of the technique consists of two concepts of the thermoelastic invariants and the lamination parameters. Experiments for verification are conducted with a specially designed measurement system. The results of 40% 0°and 60% ±45°plates fall between ±1×10^-7°C^-1 within the temperature range from -50°C to 40°C. Quite similar results are obtained for tubular specimens. It is also uncovered that the axial CTE is susceptible to variation of an off-axis angle. Precise control of the angle is a key point of the present technology .

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When a particle/metal matrix composite is subjected to a large deformation such as plastic deformation, some particles in the composite are expected to be damaged. In this paper, the effect of the damaged particles on the stiffness of the composite has been studied theoretically. We consider that there exist three cases of damage modes associated with particles : the shattering of particles producing complete voids, cracking within particles and debonding at the interface between the matrix and particles. The analytical technique used in our study is a combination of Eshelby's equivalent inclusion method and Mori-Tanaka's back stress analysis. Numerical results are presented to demonstrate the effect of the fraction of damaged particles on the Young's modulus of the Al₂O₃ particle/6061 Al composite. It was found from the theoretical study that the shattering mode induces the largest reduction in the composite Young's modulus.

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Using a megahertz-frequency ultrasonic method, we measured the elastic constants of a composite consisting of seventy-volume-percent tungsten particles in a copper matrix. We report the Young, shear, and bulk moduli, and the Poisson ratio. Tungsten is much stiffer than copper: the Young-modulus ratio equals 410/129 = 3.2. Thus, from a linear rule-of-mixture, we expected large departures, which we found.(The Poisson ratio comes close to a linear rule-of-mixture.) Unexpectedly, we found a substantial elastic anisotropy: eighteen percent in the shear modulus. To model the system, we used a scattered-plane-wave ensemble-average approach. For the four elastic constants listed above, we found reasonable model-measurement agreement, one-to-seven percent for the averaged-over-direction measurements. Assuming oblate-spheroidal (c/a = 0.5) particles improves the agreement: two percent or less for the four cases.

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The nonuniform distribution of fillers in a composite is studied both experimentally and analytically. A set of three parameters are proposed to characterize the distribution patterns of fillers in a particulate composite: homogeneity distribution parameterξ, anisotropy parameter of the first kindηand that of the second kindφ. These parameters are used to examine the distribution patterns of l0 % and 20 % SiC particulate/6061 Al (SiCp/Al) composite. This led us to believe application of the three parameters to actual composites provides useful information about the micromorphology of particulates. The distribution pattern of SiCp/Al composite is also examined from the viewpoint of fractal analysis, resulting in some correlation between the fractal analysis and one of the proposed three parameters.

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Interaction between delamination and local deformation on a metal-polymer laminated material subjected to uniaxial tension under a plane strain condition is investigated by using a finite-element method. The metal used in this work is aluminum foil assumed as a plastic and the polymer is PET assumed as a viscoplastic. The delamination process is represented as destruction of an adhesive between laminated materials under the specified stress criteria in which the allowable shear stress and normal tensile stress along the interface are prescribed. The results show that delamination at the early deformation is mainly caused by stress concentration around the void modeled by diminishing the stiffness of the element. Local deformation appears in aluminum foil after delamination is rapidly propagated in the relatively large strain field. The failure process induced by the delamination of the adhesive obtained from numerical analyses is found to be in reasonable agreement with the experimental observations.

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Damage ahead of notch tips in continuous fiber-reinforced cross-ply laminates consists of a group of microcracks extending parallel to fibers in each ply. In this paper, we try to simulate the damage extension by applying the concept of fracture mechanics directly to the extending microcracks. Elastic strain energy released by the extension of the microcracks was measured by using a compliance method with quick unloading, which is separated from inelastically dissipated energy due to the viscoelastic property of matrix resin. The experimental result suggests that the microcracks extend on the condition of constant released elastic strain energy per unit area of microcracks. Acoording to this result, we have derived the governing equation of damage extension, which is the modified Griffith's criterion for local microcrack extension. This equation enables us to simulate the damage extension, and the simulation results appear to be in perfect agreement with the experimental results.

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Metal matrix composites (MMCs) are generally considered attractive as structural materials because of their superiority over polymer matrix composites (PMCs) for high-temperature application as well as their specific strength and modulus. It has been the most important and urgent problem to characterize the damage tolerance behavior for a wide use of advanced MMCs for primary aerospace and automotive structural applications. In this paper, fatigue crack growth tests were conducted on silicon-carbide-whisker-reinforced high-strength aluminium alloy matrix composite SiC_w/2025 Al over a wide range of fatigue crack growth rates covering the threshold stress intensity factor range ΔK_th. The effects of whisker orientation and stress ratio on the fatigue crack growth characteristics were investigated on the basis of fracture mechanics and fractography. The whisker reinforcements of the high strength 2025 Al alloy are found to be effective in improving the fatigue crack growth resistance, especially in the near-threshold stress intensity factor range.

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A new probabilistic model has been proposed to predict the tension-softening relation for short-fiber-reinforced SMC composites by using the measured debond and pull-out lengths of fiber bundles. The relationship between cohesive stress and crack opening displacement, namely the tension-softening relation, is determined statistically by the probabilistic expectation of loads acting on the bridging fiber bundles at a fictitious crack. It is found that the area below the tension-softening curve is nearly equal to the apparent fracture energy of the composite, while the maximum value of the tension-softening curve is somewhat higher than the unnotched tensile strength of the composite. Comparison of the tension-softening relation with that estimated by a technique based on the J-integral is also made. Furthermore, the influences of fiber bundle-matrix interfacial conditions on the tension-softening relation are clarified.

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The objectives of this paper are to develop and standardize the plane-strain fracture toughness K_IC testing of whisker- and short-fiber-reinforced metal matrix composites. Fracture toughness tests using chevron-notched bend-type specimen were conducted on silicon carbide whisker-reinforced high-strength aluminium alloy matrix composite. Some problems in determining a valid K_IC were discussed on the basis of linear-elastic fracture mechanics and fractography. It is reconfirmed that the chevron-notched (CN) method is useful for whisker-reinforced aluminium alloy matrix composite to evaluate K_IC value. The effects of specimen orientation and loading type on K_IC were investigated, and a statistical analysis of the K_IC distribution was also performed.

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AFRP laminated plates, CFRP laminated plates and CF/AF hybrid laminated plates which were composed of epoxy resin and plane woven carbon cloth or aramid cloth or both were used as specimens. Flexural fatigue tests were carried out in a completely pulsating constant maximum load using a four-point bending method. At each stage during the testing, surface strains of the specimen, compliances, residual deflections and absorbed energies were measured and the damages were observed. It was shown that the fracture phenomena of the hybrid plates were similar to those of CFRP laminated plates and that the fracture mechanism of the hybrid plates was mainly dominated by the damage in the outermost CF layer of the compression side. Also a hybrid effect wherein the hybrid plates had a longer fatigue life than CFRP laminated plates under the same condition in the outermost CF layer of the compression side was clarified.

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The shrink fit between a ceramic shaft and a metal ring with the use of a bimetal shrink fitter has been developed in this paper. The fitting strengths of the shrink-fitted assemblies were measured from room temperature to 600°C. The contact pressures between the shafts and the shrink fitters of bimetal were also calculated. The ceramic shaft was Al₂0₃ or Si₃N₄. The shrink fitter was a bimetal of SUS304 and SUS403. The outer ring was SUS403. It was shown from the calculations that the shrink-fitted assembly using the bimetal shrink fitter was somewhat weaker than that using the monolithic SUS304 shrink fitter when the thickness of the shrink fitter in the radial direction was over 6 mm. The shrink-fitted assembly using the bimetal shrink fitter was approximately 3MPa stronger than that using the monolithic SUS304 shrink fitter at 600°C when the thickness of the shrink fitter in the radial direction was 4 mm. The fitting conditions under which the shrink-fitted assembly using the bimetal shrink fitter was stronger than that using the monolithic SUS304 shrink fitter were discussed.

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This paper is concerned with a criterion of the low-temperature strength for metal-FRP bonded joints. The authors have developed a system of thermal stress analysis of the bonded joints. The results of the analysis for the AL-CFRP bonded joint show remarkable stress concentration caused by the different coefficients of thermal expansion. Experiments on low-temperature strength have been well explained by the criterion "σ^-_<max>=σ_f" where σ^-_<max> is the maximum von Mises equivalent stress due to external load and thermal stress while σ_f is the fracture stress for the specific temperature. The above criterion also gives the characteristic low-temperature strength for AL-CFRP and AL-GFRP bonded joints between 23°C and-160°C.

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