A national project for the development of functionally gradient materials (FGMs), which is directed toward exploiting basic technologies required for the development of super-heat-resistant materials capable of space plane applications, is in progress. The FGM is a new composite material whose composition and microstructure vary continuously from place to place in ways designed to provide it with the maximum function of mitigating the induced thermal stress. The research activities of the program are divided among three groups: the Material Design Group, Material Synthesis Group, and Material Evaluation Group. This paper presents an overview of the current status of the FGM program with special focus on the research activities in the Material Evaluation Group. In order to provide material design data and evaluate the mechanical and thermal barrier functions of FGMs supplied from the Material Synthesis Group, the following testing methods are being developed : ( 1 ) small punch test, ( 2 ) laser heating thermal shock test, ( 3 ) thermal shielding performance test. Brief decriptions for these testing procedures and the experimental results obtained to date are given. Finally, the significance of the standardization of the testing procedures and the establishment of a material property data base are discussed for the interdisciplinary interaction among the three research groups.

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A resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks emanating from the notch root under an arbitrary stress ratio. The resistance-curve was determined from the experimental result obtained from the fatigue tests of single-edge-notched plates of low-carbon steel under several stress ratios. The relation between the crack opening stress intensity factor at threshold, K_opth, and the nonpropagating crack length, c_np, was approximated by K_opth=K_opth∞ [nemerical formula] c₁≤c_np≤c₂ K_opth=K_opth∞ c₂≤c_np, where K_opth∞ was the value for long cracks, c₂ was a linear function of K_opth∞, and c₁ was constant. Under compressive mean stresses, the effect of notch plasticity on crack closure was superposed on the above-mentioned closure. Good agreement between prediction and experiment was obtained for the fatigue limit for fracture and the nonpropagating crack length. The fatigue limit for crack initiation of a Stage II crack was almost constant and equal to the value predicted for the case of R = -1.

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Penetration tests on thin circular sheets were carried out in order to clarify the dependence of the energy-absorbing capacity on mechanical properties involving fracture toughness. The sheet materials, all with a thickness of 1.0 mm, used in this study were extra super duralumin for airplanes, corrosion-resisting aluminum alloy for super express trains and cold-rolled carbon steel for motorcars. Moreover, the dynamic factor which caused the effect on the energy-absorbing capacity of a thin plate under penetration was considered. As a result, it was clarified that fracture toughness is as predominant as the yield point and ultimate strength for energy of the ballistic limit.

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An approach to the description of the yield-point phenomenon of carbon steel was presented by the considering the behavior of the moving dislocations in the resisting force field of the random barriers theory. By reflecting the catastrophic increase of movable dislocations freed from any locking atmospheres in the internal structure variable in the theory, the equations expressing the stress decrement at the upper yield point and the yield-point elongation were derived. Through the examination of the test results for several annealed steels, it was clarified that the upper and lower yield points as well as the yield-point elongation of these metals could be described systematically with the area fraction of pearlite in a ferrite matrix.

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In this paper, we have attempted to precisely formulate a generalized acoustoelastic theory by extending the theory proposed by Kobayashi [1987], which was derived to evaluate acoustoelastic effects of ultrasonic waves induced by slight orthotropy in elasticity and plastic anisotropy growth in plastically deformed solids. Moreover, in order to examine the accuracy of the proposed theory, numerical calculations of the transverse wave velocity changes in the elastoplastic range were carried out and compared with the experimental data on a commercially available pure aluminum specimen subjected to uniaxial tension. The good agreement between theoretical and experimental results indicates that the plastic anisotropy is closely related to the acoustoelastic effects through the elastoplastic coupling strain rate induced by the vertexlike yield effects at the corner of the yield surface.

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The constitutive relation between the viscosity, temperature and hydrostatic pressure of liquid materials is investigated by the inelastic deformation theory (combined mechanism model) proposed in the previous paper in order to explain the inelastic behavior of solid materials. The constitutive relation of water and some other liquid materials is well explained quantitatively by the theory. The following are obtained : liquid flow due to several mechanisms, which are thermal activation processes similar to those of solid materials; the liquid region in a pressure-temperature diagram divided into several (mainly three) subregions classified by flow mechanism; a proposed constitutive equation containing hydrostatic pressure.

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The numerical analysis of the elasto/visco-plastic dynamic response of general thin shells to arbitrary loads is carried out by means of the elasto/visco-plastic overlay model, which is able to express the Bauschinger effect and the strain rate dependence. Namely, Perzyna's equation is used for the constitutive relation of each layer of the overlay model and as a whole, the Bauschinger effect and the influence of viscosity in the plastic range of the material are taken into account. The equations of motion derived from Sanders' nonlinear theory for thin shells by adding the inertia terms are used, and the relationships between strains and displacements where rotations of the shell are considered for the membrane strains are employed. The basic differential equations derived are numerically solved by the finite difference method. As a numerical example, a cylindrical shell under a semisinusoidaI external load with respect to time is analyzed, and the results are compared with those in the case of isotropic hardening.

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Parametric study on the thermal deformation and the thermal stress of a resinmolded package LSI has been performed by a boundary element analysis code specially developed for this study. The length and the position of the Si-chip in the LSI package are employed as the parameters in this study. Boundary element analysis of heat conduction and thermal stress in a steady state is carried out under the uniform temperature condition and the heat-generating condition with the heated chip. It is found that the position to minimize the package deformation is located just below the center of the package; however the stress concentration near the Si-chip corner becomes nearly maximum on the position of the chip under both conditions. In the case of the heated chip, both the thermal stress near the chip corner and the package deformation increase as the chip length becomes large.

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This study was carried out to decrease the residual stress that developed during the cooling of a ceramic-metal bonded joint. The analysis connected with the thermal stresses and the residual stresses was performed using the finite-element method and the boundary element method. The analysis clarified the effect of some factors (E^B/E^A, v^A, v^B, h^A/b, h^B/b) on the residual stress in ceramics. Several kinds of test pieces that differed in the shape of the joint were devised for decreasing the residual stress in ceramics in the neighborhood of the bond line. The experimental results of bonding of Si₃N₄ to Cr-Mo steel using a copper insert layer showed that changing of the joint shape was effective in decreasing the residual stress.

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The strength of a stepped-lap bonded joint under tensile shear loading is investigated both analytically and experimentally. The joint used in this study consists of adherends of carbon steel and an adhesive of epoxy resin. The strain and stress distributions were analyzed by applying the finite element method on the assumption of elastic deformation. The stress distributions in butt and lap sections varied remarkably on both edges of the steps. The joint strength was calculated by applying strength laws of three kinds of equation to the stress distributions of corresponding parts in the joint. It was predicted that the strength was smaller at the edge of the butt part than at the lap part in every step corner. The calculated strain distribution and strength values of the stepped-lap joint coincided approximately with the experimental ones. On experimental stress-strain curved of the joint, however, it was observed that the joint was completely broken with the increase of load after the initial cracks occurred at the edges of the butt part.

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Structural materials used in such products as airplanes and pressure vessels in nuclear reactors are usually subjected to cyclic loadings and to abrupt accidental overloadings. An accurate estimate of strength properties of cyclic prestrained materials under accidental overloading can be very useful for the purposes of design safety and cost reduction. In this report, viscoplastic behavior of Type 304 stainless steel after axial cyclic preloading of various types was investigated experimentally at room temperature. The difference between flow stress levels depending on strain rate decreases considerably after cyclic preloading. However, the relaxation stress drop increases rather than decreasing with an increase of flow stress even after cyclic preloading. No consistency in viscoplastic behavior, which is shown by the customary viscoplastic theories, can be found. Plastic modulus Ep depends on the flow stress level for the case of comparatively small cyclic preloading. Ep becomes much smaller and constant after strong cyclic preloading. Thus much larger plastic strain may occur when an abrupt overload is applied. A very strong linear relationship between Ep and plastic work per cycle is found.

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The tensile test of an aged Al-Li-Cu-Mg-Zr alloy was carried out to clarify the relationship between the detected acoustic emission (AE) activities and the dynamic behaviour of the microstructures including dislocations, precipitates, and secondary phases. The AE activities had the maximum value in the underaged condition near the yield point of the stress-strain curves. Afterward, the AE activities decreased progressively with aging. The AE did not occur in the peak- and overaged conditions. Both the continuous-type and the burst-type signals which depend upon the avalanche of the dislocation motion and the shear of precipitates, respectively, have been detected at the same time. By transmission electron microscope, superlattice dislocations, metastable δ'phases and the increase of dislocation density were observed in the underaged condition where the AE activities showed a peak. According to these results, it was assumed that the AE sources of this alloy during tensile deformation were the shear of metastable δ'phases and the avalanche of dislocation motion.

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In order to clarify the fatigue strength characteristics of lowalloy steels tempered at low temperature (150-200°C) or carburized in generated gas, rotating bending fatigue tests were conducted in laboratory air and dry air. The influence of moisture in air was only observed for the carburized steel. The low- temperature-tempered steel revealed two knees on the S-N diagram, and no fatigue limit was found up to 10⁹cycles (9 months). The carburized steel revealed three knees and the fatigue limit was found over lO⁸ cycles. When the fatigue test of carburized steel was carried out in dry air or the surface structure anomalies were removed, the fatigue strengths were improved, but neither knees nor fatigue limits were found up to 10⁹ cycles. The reasons for this were discovered from the results of fractography and the equations proposed by Murakami et al. for the prediction of the fatigue limit of steel with microdefects.

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The corrosionfatigue strength of notched high-tension steel (HT 50 and HT 80)specimens was investigated under cathodic protection in synthetic seawater. For slight notches (stress concentration factor, α_k≤2), the fatigue strength under cathodic protection decreases with an increase in α_k and is almost the same as that in laboratory air, because the cathodic protection retards the formation of corrosion pits. When α_k is further increased, the corrosion fatigue strength reaches a constant value which is probably the fatigue strength of the circular-cracked specimen. The value is higher than that in laboratory air due to the formation of nonpropagating cracks. The fatigue strength of the welded specimen corresponds well to that of the notched specimens. It should be noted that welded structures have a strong notch effect on the corrosion-fatigue strength.

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Cavitation erosion of sintered aluminum oxide and silicon carbide was investigated by a vibratory test. The erosion of aluminum oxide originated at defects such as pores and holes, where intergranular fractures occurred. Particles were removed granule by granule. Better erosion resistance is obtained for an aluminum oxide with a smaller grain size. The erosion of silicon carbide originated around the pores. Cracklike grooves propagating through pores were observed on the eroded surface and massive particles were removed. Better erosion resistance is obtained for a silicon carbide with a smaller number and size of pores. The erosion resistance was compared with that of metals.

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The fiber bridging effect, which affects the crack extension resistance in fibrous composites, is discussed in the case of in-plane-shear mode crack extension parallel to fibers in unidirectional fiber-reinforced composites. We first make a model of the bridging of a single fiber and estimate the force acting on the crack surface through a bridging fiber by using this model. Then, introducing the stochastic process of fiber breakage, we obtain the quantitative relationship between the relative crack surface displacement and the equivalent cohesive stress which is the probabilistic expectation of forces acting on the crack surface due to a large number of bridging fibers. We numerically simulate the crack extension behavior with the equivalent cohesive stress acting on the crack surface. Then the simulated results are in good agreement with the experimental results. We finally conclude that the in-plane-shear mode crack extension is greatly affected by the stochastic process of fiber breakage.

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An attempt is made to constitute an integrated consultation system for structural analysis. The objective at present is to furnish a prototype consultation system with the ability to recognize a structure and its properties in terms of the physical model and to predict the model response qualitatively. Elastic beams and plates are taken as the physical model. The system consists of the analysis knowledge base and structure-domain base of the physical model linked mutually by message communication and supported by the inference engine. The knowledge is stated in the form of the frame model. A symbolic processor is provided for the manipulation of equations and formulae. The inference engine is described in Prolog language. The system is implemented on a personal computer PC 9801 VX 2. The case studies of beam problems confirm the potential of the prototype system to evolve into sophisticated consultation systems for large computers in the future.

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First, the authors proposed a method for detecting cracks embedded in the structural members by an infrared thermal video system. The device used in this study, TVS (thermal video system)-3300, enables the distribution of temperature on the surface of the specimen to be measured and processed, thus producing real-time pictures. Various kinds of heating techniques snd the resulting thermal pictures were compared and the applicability of the proposed method was discussed. Then, the authors showed that the stress intensity factor, K, can be analyzed by measuring the distribution of the sum of the principal stresses around the crack tip by an infrared stress measuring device, SPATE-8000. The errors of the measured K-values for CT-specimens are at most l0-20%, and they decrease with decreasing amplitude of the applied cyclic stresses. The method has been applied to the measurement of K for a surface crack, and of K_I and K_II for a mixed mode crack.

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