The statistical properties of crack growth fatigue life are often needed in a reliability-based design of fatigue-critical structures. However, recent research revealed that a decrease in specimen thickness caused an increase in the variability of crack growth fatigue life. In this paper, the dependence of the statistical properties of random crack propagation resistance on specimen thickness is investigated. ConstantΔK fatigue crack growth tests are conducted on 2024-T3 aluminum alloy in four cases of specimen thickness. From the experimental data, there is no negligible influence of thickness on the statistical properties of random crack propagation resistance. Using the random crack growth model proposed in the previous paper, the thickness effect on the distribution of crack growth fatigue life is also investigated, and satisfactory agreement with the experiment is obtained.

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Creep crack growth rate of Hastelloy XR showed a dependence on thermally activated energy at very high temperatures. The Q* parameter was then applied to estimation of the creep cracking of the material. It was found that the Q* parameter well described the initial stage of crack growth as well as the steady-state cracking. The coefficients of the Q* parameter derived here for Hastelloy XR were compared with those of SUS 304 stainless steel and Cr-Mo-V steel and each of the coefficients was explained physically by the creep ductilities.

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This paper deals with the relations between the continuous distribution of circular twist disclination loops and the penny-shaped crack of type III. Based on the stress and displacement field of a circular twist disclination loop in an infinitely extended medium and those of a penny-shaped crack of type III given by the potential theory, the distribution function of disclination can be obtained by solving the Abel-type integral equation generated by the equivalency of the shear stress field over the crack surface. Furthermore, the distribution function thus derived can also be confirmed by the equivalency of the displacement field over the crack surface. Consequently, it can be concluded that a penny-shaped crack of type III can be represented by a continuous distribution of a circular twist disclination loops.

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In the present paper, the relationship between the discrete distribution of circular twist disclination loops and a penny-shaped crack of type IIIis dealt with in connection with the computational treatment of the engineering problem such as the modelling of the defect in terms of the discrete distribution of disclinations. The coplanar equilibrium array of discrete disclination loops is formulated from the viewpoint that the total potential energy of the system containing disclination loops takes the absolute minimum value when all the loops are in stable equilibrium. The pile-up properties such as the equilibrium position and the total number of the loops piled up are investigated and the accuracy of the modelling of a penny-shaped crack of type III by the discrete distribution of disclinations is also evaluated.

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In this paper, mixed mode stress intensity factors for one or more symmetric and asymmetric axial cracks in pressurized cylinders are calculated using a simple approach which we proposed in our previous work. Effects of parameters on both K_I and K_II are investigated systematically concerning ( 1 ) diameter ratios, ( 2 ) relative crack depths, ( 3 ) the number of cracks, ( 4 ) pressure acting on crack face, ( 5 ) the presence of cracks with unequal depth, and ( 6 ) the asymmetric arrangement of cracks. Then, the results obtained are summarized as follows : ( a ) K_I for two symmetric internal axial cracks (=K_I) is the largest of all that for other axially cracked configurations : ( b ) K_II is always negligible in unstable fracture prediction ; ( c ) the differences in K_I for various cracked configurations in thin-walled cylinders are so small that K_I is not so conservative in fracture evaluation ; and ( d ) accurate closed-form expression of K_I is obtained for practical use.

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Since a fiber volume fraction and a fiber orientation angle have an important effect on elastic coefficients and strengths of carbon fiber reinforced plastics (CFRP), they are main design variables of unidirectional and angle-ply CFRP. However, it is hard to determine the correct values of these variables in the case of designing CFRP plates having specific values of elastic coefficients and strengths. This decision becomes, so to speak, an inverse problem. This paper presents an application of a neural network to the design of elastic coefficients of CFRP and gives the unique solution of this inverse problem by use of the neural network.

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This paper presents a double-tandem-type substructuring method as an efficient calculation method of a microcomputer/personal computer-aided finite element method (FEM). This method uses the intermediate substructures between both basic and final ones in order to decrease the time for composition of stiffness submatrices in the tandem-type substructuring method. This method needs to utilize the disk memory storage unit such as floppy and hard disks in order to store the data of the stiffness submatrices and/or their decomposed ones. The stiffness submatrices of the storage unit are divided from a structure stiffness matrix within the limit of the machine memory capacity to be used. The data of the LDU submatrices are placed in the data set of the stiffness submatrices in order to save storage space in the unit. The FEM using the method is applied to an analysis of a three-dimensional elastic block with semicylindrical notches under tension. Therefore, the FEM is applicable for analysis of structures of a large-size matrix required for a memory capacity larger than that of the machine itself. This method also contributes to the decrease in memory capacity and central processing unit time in the conventional FEM.

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In general, a very thin metal with low yield strength is inserted into a ceramic/metal joint as an interlayer to reduce the residual stresses. A two-dimensional finite-element analysis was performed to study the effect of the thickness of the interlayer on an elastic-plastic singularity of residual stresses in the ceramic/metal joint. The yielding of the interlayer caused the change of the singularity. In the case of the joint with a relatively thick interlayer, the exponent of the elastic-plastic singularity depends on only the elastic-plastic properties of the materials. When the interlayer is very thin, however, the exponent depends on not only the elastic-plastic properties but also the thickness of the interlayer. This elastic-plastic singularity can be approximately estimated by a simplified elastic-plastic analysis.

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This paper presents a unified analysis of in-plane problems containing elliptic cavity or elliptic rigid inclusion in an anisotropic two-dimensional elastic medium under singular point forces and single-and dipole-type dis1ocations, as well as applied stresses and rotation at infinity. The analysis is based on the complex variable method using a conformal mapping technique. The analysis also covers the problems of stress intensity (or stress singularity) factors of a crack (or rigid line inclusion) under several singular forces and dis1ocations at arbitrary finite locations.

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This paper deals with an inverse problem analysis of thermal stresses in a transient state for a cylinder with nonhomogeneous material properties in a radial direction. Assuming that the solid circular cylinder is heated asymmetrically by the unknown temperature function of the surrounding medium, we have analyzed the inverse problem to determine the temperature function of the surrounding medium satisfying the prescribed transient thermal stress component in the axial direction of the cylinder. Coupling between the temperature and associated asymmetric thermoelastic fields is taken into consideration, and then the solutions of the temperature function for the surrounding media, the temperature change and the associated thermal stresses, thermal deformations for the cylinder are evaluated by means of the Laplace transform method. Some numerical results for the temperature change and the stress distributions are shown and discussed.

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This paper is concerned with the dynamic treatment of a transient thermoelastic problem for an infinite plate which is exposed to ramp-type heating on one of the plane boundaries. The material of the structure is continuously nonhomogeneous with mechanical properties varying linearly along the thickness of plate. The curvilinear characteristics in the space-time plane are transformed into straight lines of equal s1ope so that the numerical errors can be minimized. The problem is then solved using appropriate characteristic equations on boundaries while using more convenient explicit finite-difference approximations at all other points in the transformed space-time plane. The numerical results show clearly the propagation and reflections of discontinuities in stress due to a sudden heating on the boundary.

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A numerical study is made of the transient, uncoupled dynamic thermal stresses in a hollow circular cylinder subjected to a sudden change in temperature on its internal boundary. The material of the structure is orthotropic with cylindrical anisotropy and, in addition, is continuously in-homogeneous with mechanical properties varying along the radius. The curvilinear characteristics in the space-time plane are transformed into straight lines of equal slope so that the numerical errors can be minimized. The problem is then solved using appropriate characteristic relations on boundaries while using more convenient explicit finite-difference approximations at all other points in the transformed space-time plane. The numerical results show clearly the propagation of discontinuities in stresses due to step heating on the internal boundary. The effects of the anisotropic non-homogeneous material properties on the dynamic stress distributions are discussed briefly.

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To investigate the constitutive relation between the stress, strain rate and temperature, longitudinal compressive impact and quasi-static tests are made at various temperatures with cylindrical specimens of annealed copper. Experimental results of impact tests are compared with the analytical results calculated by using the Malvern-type constitutive equation proposed previously by the authors. The results in the temperature range examined in this study are discussed on the basis of the theory of constitutive relation proposed previously by the first author. Consequently, the constitutive relation of copper in the region from quasi-static strain rate to high strain rate can be explained by dividing the fields into three, each of which satisfies our constitutive equations, and two boundaries among the three fields measured on the quasi-static strain rate region extend continuously to the high strain rate region.

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A concept of thermopolar materials considered nonlocalities of thermodynamical quantities, was proposed in the previous report in order to express nonequilibrium processes. In the present paper, mechanical and thermodynamical balance laws are discussed as preliminary to the construction of a generalized continuum mecharlics for thermopolar materials. Defining the fluctuations of thermodynamical quantities by their microscopic gradient, we introduce the nonlocal effects. Moreover, one more balance law, which expresses the balance of the fluctuations of thermodynamical quantities, is formulated in order to make the balance equation system a complete set. Furhermore, the Clausius-Duhem inequality, which is important to discussion of the thermodynamical construction of the constitutive equations for the thermopolar materials, is derived. As a result, a part of the entropy production rate is expressed by fluctuations of thermodynamical quantities. Consequently, it is suggested that the microscopic gradient of temperature has to be included in the state variables under nonequilibrium conditions.

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Delamination or interfacial cracking in a two-layer laminated beam subjected to three-point bending is analyzed. The initial partial debonding is assumed to locate at the center of the beam and to symmetrically spread toward both ends along the bond-line. The compliance and energy release rate are derived on the basis of an elementary beam theory. The results are shown to compare well with those based on the finite-element method. It is also shown that the delamination process is generally of a mixed-mode type; i. e., both mode I and II deformation near the crack tips contribute to the cracking process.

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Thermodynamic quantities of state are defined under the nonequilibrium processes by the thermopolar theory. Constitutive equations for thermopolar materials are constructed on the basis of the results of the previous reports. As preliminary data, the arguments of the quasi-conservative part of the constitutive equations are restricted by the Clausius-Duhem inequality derived in the previous report. Moreover, the thermodynamic potentials are discussed ; these are not only Helmho1z's free energy, but also newly defined thermodynamic potential characterizing thermopolar materials. Then dissipative parts of the constitutive equations are restricted to satisfy the principle of maximal dissipation rate. Furthermore, the constitutive equations for thermopolar materials are constructed thermodynamically on the basis of the above discussion. Consequently, considering the physical interpretations of the linear constitutive equations, it is indicated that both the quasi-conservative internal force and the dissipative internal forces depend on the microscopic gradient of temperature.

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The present paper dealts with the mechanical analysis of fiber-reinforced composite material having matrix cracking and/or interface sliding between a fiber and a matrix, say, the problem of a bridging fiber, by the method of micromechanics. In the field of micromechanics, inhomogeneity is generally treated as an inclusion with eigenstrain, and the quivalent inclusion method, Eshelby's method, is applied to simulate the elastic stress disturbance due to the inhomogeneity. However, in the case where there are many kinds of inhomogeneities, the interaction between the inhomogeneities, which were neglected in Eshelby's method, must be taken into consideration. In doing this, the method developed by Taya is usefu1, and the present method is the extension of Taya's method to the analysis of the fiber-reinforced composites with interfacial sliding.

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In order to investigate a constitutive equation at finite strains for crystalline polymers of polyethylene (PE) and polypropylene (PP), stress responses were investigated under various strain paths such as proportional, cruciform, circular paths and cyclic ones under combined tension-torsion. It was shown that ( 1 ) for creep tests at relatively low stress, creep strain increases with an increase in time, and the ratio of torsional to axial strain is nearly equal to the ratio of torsional to axial stress, ( 2 ) the degree of a cross hardening or softening caused due to a change in the principal deformation direction is much less than that in steel and ( 3 ) the strain history greatly influences to the stress-strain behavior of PP, but not PE.

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In this study, the dependence of fracture stress on the crack length is examined at high temperature of 1100°C and 1200°C, and the availability of the process zone size fracture criterion for structural ceramics is studied. The mechanical properties of Si₃ N₄ at high temperature are compared with those of room temperature. It is shown that the process zone size fracture criterion is useful not only at room temperature, but also at high temperature. Furthermore, the effects of grinding depth and direction on the bending strength of ceramics are investigated. Various sizes of surface crack caused by the grinding process are estimated based on the critical process zone size fracture criterion by using the relation betweenσ_c and a_e, and the Newman-Raju formula.

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By using an ultrasonic mode conversion method, phase velocities are measured for three kinds of bulk waves (first and second compressional waves and shear wave) propagating through a water-saturated sintered glass bead plate immersed in water. The observation of the second compressional wave confirms Biot's prediction of its existence. It is also shown that its measured velocities are in good agreement with those predicted by the Biot theory and Berryman's self-consistent model for effective moduli, and that the phase velocities obtained for the first compressional and shear waves fall between the two curves predicted based on the two self-consistent models, that is, Wu's (static)one and Berryman's (dynamic) one. In addition to the velocities, values of a dynamic tortuosity parameter are also obtained.

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Fatigue crack growth and the fracture resistance curve (R-curve) were investigated in a polycrystalline alumina (AD90) and an alumina silicon carbide composite (Al₂ O₃-SiC_w) using a combined loading technique for stabilizing crack growth and a surface film technique for monitoring crack length. Fatigue crack growth was evaluated successfully with those experimental techniques. Load shedding tests were performed until the crack became dormant, in order to determine the threshold stress intensity factor K_th. Then, the specimens were used for the quasi-static tests under monotonic loading condition. R-curves were determined in this experiment ; however, the fracture resistance did not increase markedly with the crack growth. Detailed observation of the crack growth behavior revealed that the flat R-curve was attributed to the shielding effect of the wake of the fatigue crack tip. Thus, the fatigue precrack introduced by the load shedding test was not regarded as an ideal crack for determining the R-curve. Fractographic observations were performed to investigate the mechanistic difference between fatigue and quasi-static crack growths. It was found that the cyclic loading produced fretting damage on the wake region and it reduced the shielding effect of the fatigue cracks. Based on the experimental results, the relationship between the fatigue crack growth and the R-curve is discussed, and the significance of the K_th as a material parameter is suggested.

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Uniaxial stress, strain, strain rate and temperature relations for polymers are formulated and generalized to the multiaxial stress system. A full axisylnmetric finite-element method with thermo-elasto-viscoplastic constitutive model is employed to analyze the deformation behavior of polymeric bars subjected to tension. The effects of environmental temperature, deformation-induced heating and strain rate sensitivity on neck propagation are clarified. The results include load-elongation behavior, triaxiality factor, evolution of specimen profiles and temperature distributions.

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The tensile tests were performed for the sintered silicon nitride ceramic at displacement rates, δ=0.6, 0.06 and 0.006 mm/min in the temperature range from room temperature to 1400°C. According to the measurements of tensile displacement, the load P vs. displacementδrelationship was markedly influenced by test temperature and displacement rate. Young's modulus decreased with an increase in test temperature. The tensile strength was also reduced with increase in test temperature and decrease in displacement rate. The nominal stress-strain relationship was calculated from the measured P-δrelationship, under each test condition. Moreover, these stress-strain relationships were corrected so that the P-δcurve calculated using the elastic-plastic finite-element method agreed with the measured P-δcurve. The corrected stress-strain response seems to give the intrinsic response of the sintered silicon nitride ceramic at elevated temperatures. Furthermore, the development of the inelastic deformation zone was calculated through the elastic-plastic finite-element method on the basis of the corrected stress-strain curve at elevated temperatures.

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Spheroidal graphite cast irons with different microstructures were heat-treated using a CO₂ laser. The microstructures of these specimens were examined by optical microscopy. Wear, Charpy impact and rotating-bending fatigue tests were then conducted on untreated and laser treated specimens. The results obtained were as follows: In the heat-treated surface layer of ferritic matrix cast iron, spheroidal graphites were surrounded by the ringlike martensite phase. The wear resistance of this structure was superior to that of an untreated structure. This wear resistance increased as the amount of martensite phase in the matrix increased. It was observed that the wear resistance of laser treated cast iron with 20% pearlitic phase was equal to that of untreated cast iron with 80% pearlitic phase. On the other hand, the impact strength of laser treated cast iron was inferior to that of untreated cast iron under the Charpy impact test. However, the impact strength of the laser treated cast iron with ferritic matrix was superior to that of untreated cast iron with pearlitic matrix. The fatigue limit of the laser treated cast irons was slightly decreased.

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