In this study, an attempt was made to clarify the detrimental effect of nitriding on the fatigue strength of titanium alloy through some critical experiments concerning the fatigue properties of gas-nitrided Ti-6Al-4V alloy including the material with a small artificial surface pit. The results are summarized as follows: (1) The results of the plane bending fatigue test showed that the fatigue strength of Ti-6Al-4V alloy is reduced greatly due to nitriding. (2) Fractography of the fracture surface revealed that the existence of a nitrided layer at the surface affects the behavior of a fatigue crack in an early growth stage. (3) All parts of the nitrided layer affect the reduction of fatigue strength. (4) A small pit (d = 82μm) has little effect on reducing the fatigue strength of nitrided alloy while such a pit gives a large reduction of the fatigue strength of unnitrided alloy.

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The rotating bending fatigue test and the measurement of inclusion size distribution in a material using the quantitative metallography technique were carried out on a quenched and tempered SAE9254 steel. The relationship between the size of inclusions which act as nucleation sites for actual fatigue failures and the inclusion size distribution was discussed, and an attempt was also made to estimate the effect of the size distribution of inclusions on the scatter of fatigue lives of the steel. The results obtained were summarised as follows. (1) Almost all the sizes of inclusions, Dfs, which were observed at crack nucleation sites on the fracture surfaces are larger than the maximum size of inclusions, Dic, expected from the inclusion size distribution in the material. indicating that the inclusion which acts as a nucleation site for actual fatigue failure is the largest one in the present microstructure. (2) The scatter of fatigue life is closely related to the inclusion size distribution in the critical volume for fatigue failure and can be estimated using a new parameter of Dfs/Dic.

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Fatigue tests of a push-pull loading were carried out on the smooth specimens of polycarbonate at the speed of 2Hz at room temperature. The roles of very small inclusions and shear bands in the initiation and growth of fatigue crack on the surface of specimen were investigated. This is accomplished by careful successive observations by using the replica of cellulose methyl ether. It was found that the initiation of the surface crack appeared from the fine inclusions near the surface of specimens at 20-30% of the fatigue life, and that the surface crack from these inclusions was hindered by the shear bands that formed at their ends. The final length of surface crack is strongly influenced by the stress amplitude. The characteristic behaviors of the crack initiation and growth on the surface of the smooth specimens of polycarbonate were discussed in terms of the mechanical properties and yield behavior of the material.

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The J-R curve, fatigue crack growth rate and aharacteristics of ductile unstable fracture under monotonic and cyclic load were investigated using 1TCT test specimens which were cut out from A508 steel for reactor pressure vessels. All the tests were carried out at 100°C. The main results obtained were as follows. (1) The J-R curve under the cyclic load is not a material constant but is dependent on the test conditions. (2) da/dN from typical fatigue data cannot be extrapolated by ΔJ only if the value of da/dN is above 5×10^-4 mm/cycles. However, it can be extrapolated by using the following equation in which J_max is used: da/dN=C{√(ΔJ)/(B-√J_max)}^m. (3) The J values at instability obtained from the ductile unstable fracture test carried out under the cyclic load of stress ratio R=0, 0.1 and -1.0 were compared with those from the monotonically increasing load. These J values at instability were almost the same as that for the monotonically increasing load.

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The S-N curves of plain specimens seem to be controlled directly by the small-crack growth law, because the fatigue life of plain specimens is occupied mainly by the propagation life of a small crack. In this paper, the relation between the small-crack growth law, dl/dN=C_σⁿ_al, and S-N curves is investigated, and it is shown that the constants n and C₁ of the small-crack growth law are determined approximately by the equations n=β and C₁=[ln(d/5)+6]/C₀, if the S-N curves can be approximated by the relation σ^β_aN_f=C₀ (d: specimen diameter). The validity of this method is checked through its application to the present experiments and other researchers' S-N curves.

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The authors deal with fracture criteria on kinking of an interface crack out of the interface in dissimilar materials based on the maximum stress (σ_θmax) concept similar to the mixed-mode fracture criterion for a crack in a homogeneous material. If the bimaterial constant ε of dissimilar materials is zero or small, the kinking angles can easily be determined independent of the distance r from a crack tip. However, if ε becomes large, the kinking angles cannot be determined and the angles strongly depend on the distance r due to the oscillation singularity. This criterion is compared with the energy release rate criterion of He and Hutchinson.

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This paper presents some results of laser heating thermal shock tests performed on plasma-sprayed thermal barrier coatings. The coatings tested were single-layer ZrO₂ (stabilized with 8% Y₂O₃), and functionally gradient materials (FGMs) which consisted of multi-layers with varying mixture ratios of ZrO₂ and Ni-based superalloy (NiCrAlY). FGMs have recently been proposed as coating materials which aim at mitigating thermally induced stresses in the coating. The substrate was a stainless steel (SUS304), and its thickness was 3.4mm. A CO₂ laser (maximum output=50W) was used for the thermal shock tests, where a laser beam with a preset spot size, duration and intensity was irradiated onto the coating surfaces. Concurrently with the laser experiments, acoustic-emission (AE) monitoring was carried out to detect the onset of the thermal shock fracture. In this investigation, critical laser power density at the onset of coating failure was defined in order to characterize the thermal shock resistance of the coatings. It was shown that the critical power density of the FGMs is significantly higher than that of the single-layer coating, indicating the effectiveness of the FGMs to reduce the thermal expansion mismatch stress. The single-layer coating was also subjected to thermal cycling using the laser method, either in the as-sprayed condition or after preoxidation treatment (1000°C, 10days in air). Two different fracture patterns were identified which led to coating spallation or exposure of the substrate; one is the extensive coalescence of vertical cracks, and the other the delamination growth. In paticular, the oxidation at the ceramicmetal interface was shown to control the thermal cycling life of the ceramic coating.

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In some experiments conducted by other authors, axially compressive cylindrical shells with a large ratio of radius to thickness were observed to buckle with circumferential waves. The finite element method is utilized to study this buckling phenomenon. The bifurcation mode, as well as the axisymmetric one, is considered in the analysis as the mode of creep buckling. The number of circumferential waves obtained from the present analysis agrees well with that of other experiments. This implies that the circumferential waves observed in the creep buckling experiments are due to bifurcation.

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The commercially supplied pure iron was cathodically hydrogen charged at various current densities in sulfuric acid aqueous solution in order to investigate the blister growth behavior, and the computer simulation was carried out to predict the blister formation characteristics. The relationship between maximum blister diameter observed on the specimen surface d_m(mm), hydrogen charging current density i_c(A/m³), and charging time t_c(ks) was expressable as: d_m=3.6×10^-3i^0.61_ct^0.61_c. By means of the computer simulation obtained by assuming random walk of hydrogen in metal, the relation, d_m=5.1×10^-3i^0.56_ct^0.63_c, was obtained; this relation could predict well the blister diameter experimentally obtained. Moreover, the simulation could display the three-dimensional distribution of blisters inside the specimen.

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This paper deals with numerical simulations for nonuniform yielding processes in mild steel by use of the finite element method. The yield drop is taken into account in the simulation by means of Hahn's constitutive equation. The yielding processes in straight and notched specimens are discussed based on the simulation. The simulated result agrees well with the actual behavior of nonuniform yielding observed in the annealed mild steel. The simulation also shows that the nonuniform yielding phenomenon is relevant to the yield drop behavior of the material element.

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A complicated internal residual stress occurs in a structural member due to machining, welding or the like. To estimate the unknown residual stress in an nondestructive manner an inverse analysis based on the boundary element method is considered useful. It is reported in this paper that the unknown residual stress induced by weld or spot heating a part of the plate was estimated by the inverse analysis method in which the known boundary displacements and tractions around the plate. It is assumed that the residual stress was replaced with uniformly distributed body forces which direct to the center of the circle of the spot. Numerical computation of some example problems reveals that it is possible to effectively estimate the residual stress in a plate with sufficient accuracy by using this inverse analysis method.

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A frictionless, axisymmetric contact problem is considered for an elastic half-space indented by a ring-shaped rigid punch. The shape of the contact region under the punch is a circular annulus, and both the width of the region and the depth of penetration vary with the magnitude of the indentational force applied to the punch. The problem is reduced to a solution of infinite systems of simultaneous equations using Papkovich-Neuber stress functions in oblate spheroidal coordinates. Numerical results are given for the variation of the width of the contact region and the depth of penetration. The distribution of the contact stress and the surface deformation of the half-space are also shown graphically.

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In studying the dynamic behavior of a body under impulsive loading, it may be fundamentally important to analyze elastic/plastic deformation at a contact part and to analyze the impulsive force generated at that part. In this study, the deformed shape at the contact part and the generated impulsive force have been measured when an aluminum bar, whose end is a truncated cone, collides axially, perpendicularly against a devised sensing plate. The length of the bar was selected in the range from 0.2m to 2m. It was found through the experiments that a new truncated cone was formed at the contact end, and that the conical angle of the cone depended only on the conical angle of the former cone before the collision. It was also confirmed that the generated mean stress on the boundary plane between the deformed part and the undeformed part at the impact end was constant and independent of the shape of the cone and of the length of the bar.

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For nondestructive evaluation of the structure anisotropy in the stressed state, a new method to adjust the isotropic component of the elastic stiffness of anisotropic materials is presented. The new method is applied to the measurement of welding residual stresses of a welded joint of carbon steel. The results obtained are as follows; (1) As the sum of the square of velocities in three directions becomes constant, the texture model is applicable. (2) The coefficient of adjustment for the isotropic component of the elastic stiffness of anisotropic materials, K, is unique to the measurement system. Nondestructive evaluation of the structure anisotropy in the stressed state is possible using longitudinal and transverse wave velocities. (3) Applying the new method to measurement of welding residual stresses of a welded joint of carbon steel, it is made clear that the precision of measurement is improved compared with that of the conventional method.

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Uni-lateral and bi-lateral elastic-plastic compressions of a circular cylinder with three different wall thicknesses by flat plates are numerically analysed by the Finite Element Method (FEM). J₂-flow theory (J2F), and J₂-Gotoh's corner theory (J2G) which was previously proposed by one of the authors are used as the constitutive equations. In the case of uni-lateral compression, the cylinder is compressed up to a completely flattened shape, which is considered a kind of plastic forming process. The deformed shapes and the compressive force are predicted better by J2G than by J2F. The spring-back behaviours are also analysed by imposing unloading process during deformation. The deformation process in the compression of a ring (plane stress state) and a spherical shell (axi-symmetric state) is also analysed. In the case of bi-lateral compression, the process is considered a kind of square-shell forming. In its final stage, the cylinder deforms into a completely unexpected shape which could be thought of as a square shell reinforced with ribs. The J2G allows the process to proceed at a lower compressive force than that for J2F. The effect of n-value (the strain-hardedning exponent) on the deformation pattern is also discussed.

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Stress analysis of blood vessels is performed by means of a transversely isotropic viscoplastic constitutive model. This model is based on the concept that the hysteresis loops of stress-strain relationships under cyclic loading conditions are caused by loading path dependence and loading rate dependence. Then the method of analysis is discussed in detail on the basis of finite deformation theory for the case of axisymmetric deformation of a long circular cylindrical tube under the condition of constant axial stretch and repeated pressurization. Simulation results show that the model can describe the experimental results by Takamizawa et al. with a relatively high accuracy. The stress histories at some points in the wall and the stress distribution across the wall are also discussed with a special emphasis on the difference between viscoplastic and elastic predictions. The results show that the inelastic properties of blood vessels reduce the stress concentration at the inner wall.

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A photoelastic method was used for a new device to measure contact pressure distribution between soles and the floor surface. The size of a first fringe was linearly correlated with weight loaded upon the point. Its error ranges were within 3%. When 17 individuals with Down syndrome (ages 1-15 years) and 27 controls (ages 1-50 years) were analysed by this method, it was revealed that, in the former, the number of points with a first fringe was increased and, in the latter, an average loading weight for each point with a first fringe was increased. From the patterns of contact pressure distribution of soles, the center of gravity for each sole and that of a whole body were determined. In the Down syndrome group, the position was shifted forward at 50% the length of the sole from the ankle; meanwhile, it was only 35% in the control.

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In order to understand the mechanical properties of human patella, regional differences of viscoelasticity and compressive strength have been investgaited in normal and pathological human patella by viscoelastic spectrometer and a testing machine of the Instron type. In the normal human patella cancellous bone, the averaged compressive elastic modulus at the lateral side was 417.2 MPa and that at the medial side was 243.3 MPa. Comparing normal and pathological patella, we find that the normal patella stiffness and tan δ value were higher than the pathological ones. This fact implies that the normal patella is stiffened as a result of functional adaption.

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This paper concerns boundary element formulation of a class of two-dimensional quasi-harmonic differential equation, which governs several practical problems with fluid flow and solid mechanics. Boundary integral equation is derived with the aid of the fundamental solution in terms of the elliptic integrals of the first and second kinds. This is applied to solve the stress distribution of helical spring with arbitrary cross-section profile under static compression only with boundary discretization. The efficient numerical approach is employed to search the design synthesis of optimal shape of the spring wire under minimum weight restriction and stress criterion.

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The new adaptive boundary element method (BEM) is developed in this paper by using the concept "sample point error analysis". The analysis relies on estimation of the discretization error over each boundary element appearing in the boundary integral equation through the magnitude of inconsistency of intermediate solution except at the boundary nodes for the prescribed discretization. The extended error indicator is defined and employed for boundary mesh refinement. This paper deals with the h-adaptive scheme with some simple numerical examples associated with two dimensional Laplace equation.

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The dynamic strength of industrial materials at low temperatures, especially lower than the temperature of liquid nitrogen, has attracted special interest. In this paper, as an approach to examining the dynamic properties of materials at these low temperatures, a modified split Hopkinson bar method is proposed. In the method, a narrow gap is made between a pressure bar and a specimen. By this method, it is possible to shut off heat conduction between them and to control the temperature of the specimen accurately. Theoretical conditions for good measurement by this method have been examined thoroughly. By carrying out an experiment under these conditions, the effectiveness of this method has been confirmed.

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