The corrosion fatigue process under cyclic torsion was investigated in annealed carbon steel. The results obtained were as follows. (1) During the fatigue process in laboratory air and salt water at the high stress amplitude region, slip bands appeared on the smooth specimen surface and cracks initiated from some of these slip bands. These cracks propagated in the direction of the specimen axis at mode II and thereafter branched and propagated in the direction inclined 45° to the specimen axis at mode I. (2) During the corrosion fatigue process at the low stress amplitude region, cracks initiated at corrosion pits and directly propagated in the direction inclined 45° to the specimen axis at mode I. (3) In torsional fatigue tests, the decreasing rate of fatigue strength due to the corrosive environment was smaller than that of rotating bending tests. (4) Differences in the initiations and growth behavior of corrosion pits between the torsional corrosion fatigue test and the rotating bending corrosion fatigue tests were not observed.

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For CFRTP of short carbon fibers in a resin matrix of polyacetal, the effects of carbon fiber content, temperature and notch radius on fatigue behavior have been investigated. Fatigue strength increases with increasing carbon fiber content and decreases with increasing test temperature. Logarithm of fatigue strength for CFRTP with 30% fiber content is proportional to the reciprocal of temperature, 1/T. However, the fatigue strength of CFRTP depends little on the notch radius.

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In this paper, an attempt to develop a thermal shock and fatigue test of ceramic thermal barrier coating (TBC) for gas turbine blades is presented, using a laser heating technique. This method uses a cylindrical specimen which simulates the curvature of the leading edge of a gas turbine blade. A plasma-sprayed ZrO₂-Y₂O₃/NiCrAlY TBC system was tested. The cylindrical specimens used consisted of ZrO₂-8wt%Y₂O₃ thermal barrier layers (0.35 mm thick) deposited on a stainless steel substrate, with a NiCrAlY bond coat (0.15 mm thick). A CO₂ laser (maximum output=50 W) was used for the thermal shock and fatigue tests, where a laser beam with a diameter of 4 mm was irradiated onto the coating surfaces. The specimen inner surfaces were air-cooled or water-cooled to achieve various temperature drops within the ZrO₂ coating. Concurrently, laser experiments on coating surface temperature (T_s) and back-surface temperature (T_b) were measured, and acoustic emission (AE) monitoring was carried out to detect the onset of the coating damage. Using the experimental results, a thermal shock fracture mechanism map was constructed for the ZrO₂ TBC as a T_s-T_b diagram. The T_s-T_b diagram indicated that the most probable damage of the TBC system under the advanced gas turbine environments was delamination growth at the ZrO₂/NiCrAlY interface which leads to the spalling of the ZrO₂ layer. In order to improve the thermal shock/fatigue resistance of the ZrO₂ coating, vertical cracks were induced within the ZrO₂ coating by laser heating, and the effect of the laser heat processing on the thermal resistance was examined. It is demonstrated that the thermal resistance of the TBC is significantly enhanced by the laser precrack processing.

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A fatigue testing method of actual car body structures under concentrated load instead of uniformly distributed load is studied. (1) The test under a concentrated load is able to simulate a state of uniformly distributed load by applying a four-point bending moment to a car body. (2) The stresses of side plates and carrying loads of spot welds near the supporting part of a car body are in fair accordance with those under uniformly distributed load. Judging from the above results, fatigue test of a car body can be carried out by making use of the concentrated load.

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This paper is concerned with the torsional impact response of an infinite body with two annular cracks. The method of analysis is the body force method extended to elastodynamic problems, which was developed previously by the anthors. In this method, the solutions are obtained by superposing the elasto-dynamic stress fields of point force so as to satisfy the given boundary conditions. The dynamic stress intensity factors are obtained numerically, and the effects of the interactions between two cracks on the dynamic stress intensity factor are shown graphically.

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The interfacial structure formed by silane coupling agents influences interfacial properties between the reinforcement and matrix in GFRP. However, the evaluation of interfacial structure at the molecule level has not obtained sufficient experimental results yet. Therefore, a numerical analysis method is expected to presume evaluation in molecule levels. In this study, we proposed forming a model of interfacial structure, and tried to evaluate the model with fractal dimensions that can classify complex figures. We proposed a model of interfacial structure for the finite-element method, and investigated the relationship between interfacial structure and the stress transmission mechanism. Moreover, we compared the results of numerical analysis with interfacial transmissibility in order to verify this analysis method. It was clear that the difference in interfacial structure influenced interfacial properties. The above result suggests that this analysis method is useful in investigation of the effects of interfacial structure on interfacial properties.

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This paper represents the 3D BEM analyses of the stress distribution and the stress singularity on the interface between dissimilar materials, and also the stress intensity factors for an interface crack. The results obtained are compared with 2D results under the plane strain or plane stress state, and the validity of the plane approximation for the interface problems is discussed. The condition for the stress singularity at the edge of the interface to disappear is clarified in the 3D case. The distribution of the stress intensity factors (K₁, K₂, K_III) for the through interface crack along the crack front is presented for various material combinations. It is found that the plane strain state exists near the center of a dissimilar material plate if the thickness is large enough. But the plane stress state does not exist even if the plate is very thin.

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An influence function method has been developed to analyze stress intensity factors, K, for cracks which are subjected to arbitrarily distributed surface stresses. The method has been applied to the analysis of a series of influence coefficients for flat plates with an embedded elliptical crack. It has been shown, based upon the data of the influence coefficients, how easily the K-values for an embedded elliptical crack in a flat plate can be evaluated against arbitrarily distributed surface stresses.

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In the previous papers, it was shown through the experiments of stable crack growth and corresponding generation phase simulations that the CTED (crack tip energy density) can express the fracture resistance of a material throughout the life of a crack consistently. In this paper, the simulation method of crack growth in the discontinuous model is studied first, and then, it is demonstrated that the crack growth behavior can be simulated well through the application phase simulation, if only the CTED value at the onset of crack growth, ε_IC(≃J_IC), is given. Moreover, it is also shown that the so-called J-R curve, of which the evaluation has been possible only based on crack growth experiments, can be obtained through the above simulation.

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Dynamic crack propagation in Homalite-100 was studied by moire interferometry in combination with high-speed photography. Crack opening displacements (COD) were measured as a function of distance from the crack tip by using moire interferometry fringe patterns. Crack velocities were also measured to correlate with COD. The results showed that the crack opening shape approaches a parabolic crack tip opening displacement with increasing crack velocity.

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The plane elastic problem of an interface crack of a rectangular inclusion is considered. The inclusion is assumed to be completely bonded to the interior of an elastic infinite medium, except for a portion which is regarded as an interface crack. Muskhelishvili's stress function is determined for m terms of finite series of the mapping function, by which the inclusion is mapped into an unit circle. Then, the stress intensity factors for the interface crack are determined under the equal bi-axial loading condition.

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In order to establish the methodology of hydraulic fracturing tectonic stress measurements, the relation between the so-called reopening pressure of longitudinal cracks and tectonic stresses is studied. To this end, the coupled problem of fluid flow in the crack and the crack reopening behavior in an impermeable rock is analyzed based on the linear theory of elasticity and also on the linear fracture mechanics, where it is taken into account that the crack does not reclose perfectly upon deflation due to crack surface damage and some minor shear displacement. The results show that, contrary to the conventional theory, the apparent reopening pressure, i. e., the borehole pressure at which the pressure-time history apparently deviates from a linear behavior, is hardly affected by the maximum horizontal tectonic stress acting in the direction parallel to the crack line. It decreases and approaches the minimum horizontal tectonic stress normal to the crack plane with decreasing injection flow rate. Moreover, it is shown that the results of laboratory hydraulic fracturing experiments conducted in the present work support those conclusions.

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The mechanistic condition of delayed fracture was studied on the basis of fractographic analysis in the SEM photograph of the fractured surface of cathodically charged quenched/tempered SNCM439 and 0.36%C plain carbon steels. A typical feature of the fracture process of both materials can be characterized as the quasi-cleavage facet in the vicinity of the crack origin followed by the intergranular facet and dimple pattern resulting in a brittle fracture. The fracture condition of this brittle fracture can be characterized on the basis of two parameters, the nominal applied stress and the depth of the crack origin. Discussion has been made on the estimation of the threshold stress of the delayed fracture test from the application of the above brittle fracture condition.

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The present paper is concerned with the theoretical treatment of volume constraint arising from nearly incompressible response of rubberlike material in the previous hyperelastic modeling of Green, as well as the hyperelastic one using rotationless strain proposed by the present author. The constitutive equation is based on the modification of the deformation gradient tensor of Flory, which can decompose volumetric and dilatational parts of the utilized strain precisely. By applying the method of Lagrange multipliers with respect to internal work related to volumetric change, three-field Hu-Washizu and two-field Hellinger-Reissner variational principles are systematically derived. The mixed variational principle proposed here is proven to hold exactly the condition of equilibrium in rate form, which has been bypassed in previous works.

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This paper shows Green's functions for torsional body force problems of a bonded elastic solid with a bonded layer. Green's functions are defined as a solution to the problem of a bonded solid with a bonded layer subjected to a torsional body force acting on a circle. A simple example of application of Green's functions shown is presented for the stress concentration problem of a bonded elastic solid with a spherical cavity in the bonded layer under torsion. A method of solution is developed for the problem mentioned above. Influences of the shear moduli of elasticity for the layer and the bonded elastic solids on the stress concentrations around the cavity and on the bonded planes are investigated.

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The CED (crack energy density) in an arbitrary direction, ε_ψ, is expected to be a parameter to describe not only elastic fracture but also elastic-plastic fracture of a mixed mode crack. This ε_ψ is related to load-displacement curves of specimens and it was shown in the elastic region that ε_ψ can be evaluated from experimentally obtained load-displacement curves with practically sufficient accuracy based on the relation. In this paper, the applicability of the method to elastic-plastic fracture problems is examined. That is, the load-displacement curves of specimens with a notch (or crack) inclined to the loading axis are obtained by experiments in the elastic-plastic region, and ε_ψ is evaluated from those curves. The results are compared with those of finite element analyses, and the availability of evaluation method based on experimentally obtained load-displacement curves is demonstrated. Moreover, the method is applied to a fracture problem and it is shown that the method can be a powerful tool to evaluate the critical value of ε_ψ (fracture toughness).

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In this paper, an inverse analytical technique to obtain contact stress distributions between two elastic bodies is proposed. The technique uses a strain datum distant from the contact region. The finite-element method and the numerical optimizing technique (DFP method) are applied to formulate the method. Moreover, the formulation of strain sensitivity analysis for the DFP method is proposed. By using this inverse technique, two-dimensional contact problems are analyzed. The reasonable contact area and stress distribution are obtained from the strain datum measured experimentally. The results are compared with an inverse analytical result using isochromatics.

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Rate dependence becomes more remarkable with increasing temperature in the inelastic behavior of engineering materials. When we deal with a stress/strain response in a structure, a sophisticated constitutive relationship must be applied to the stress analysis since the materials exhibit complicated characteristics under multiaxial stress states. The aim of this paper is to apply a unified constitutive model to finite element analysis of a notched cylinder. Experimental verification is carried out for a PCA material (Primary Candidate Alloy for a fusion reactor component). The local strain response near the notch root is measured by an electric-capacitance type strain gauge.

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Taylor series approximation (TSA) has been used to form an approximate subproblem to the actual design problem for the efficiency. A new intermediate variable approximation method for stiffened plate and shell structures is suggested and minimum weight design under displacement and stress constraints is discussed. The approximation method of intermediate variables is considered for the thickness and geometric dimensions of the plate or shell and the stiffeners to construct a wide-range approximation model. For the displacement and stress constraints, it is found that the Taylor series approximation using reciprocal variables of thickness and geometric dimensions (RTSA) gives good approximations for the typical stiffened plate structures. The approximation method is also applied to solve the minimum weight design problems of stiffened plate and shell structures, and the results are compared with those of direct variable Taylor series approximation (DTSA) to illustrate the efficiency of optimization.

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The cyclic stress-strain response of copper under conditions where tensile deformation was followed by cyclic deformation and the change in strain range occurred during cyclic deformation was estimated, extending the analysis proposed by authors in the earlier paper. At that time it was assumed that copper had dislocation density inherent in the controlled strain range at the stable stage in the cyclic deformation process, independent of the strain history which copper was subjected to. The cyclic softening occurring in copper cyclically deformed following the tensile deformation was successfully described through the present analysis. Moreover, the cyclic softening and hardening behaviors induced by the change in strain range controlled in the cyclic deformation test were also satisfactorily calculated through the proposed method.

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In the thin plate theory, shear deformation is neglected. This theory is unreliable for plates of considerable thickness in the vicinity of the point of application of load, and sandwich plates with shear rigidity which is very low compared with bending rigidity. A widely accepted theory which includes the effects of shear deformation was developed by Reissner and Mindlin. In recent years, composite materials have been widely employed as structure elements, and it is important to understand their characteristics for designing structures. Plates of composite material are characterized by strong anisotropy and low out-of-plane shear rigidity. This paper provides a convenient representation for the stiffness matrix of the finite element in order to analyze a sandwich plate with an anisotropic face plate and core. The formulation is based on the non-conforming element of Zienkiewicz and is obtained with a modified stiffness matrix in the condition in which the out-of-plane shear strain is constant in two directions within an element.

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This paper formulates a volume minimization problem of a 3-D truss structure with the constraints on stresses and natural frequencies of specified modes. Cross-sectional areas of truss members are optimized by sequential linear programming. When the object mode is in the same direction as that of the static load considered in the strength problem, the minimization can be carried out effectively. Moreover, it is observed that the obtained structure has a good controllability in the context of the optimal regulator theory.

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