Kenichi Oshita

Experimental Study on Transformation Plasticity in Terms of Three-Point Bending System

Three-point bending system with one end simple support and the other end fix support has been proposed to analyze the transformation plasticity (TP) behavior and obtain transformation plasticity coefficient. In this investigation two types of materials SCM440 steel and S45C steel have been studied. The specimens were heated to austenite temperature and the temperature kept constant for several minutes, then cooling and loading processes were performed. Austenite to martensite phase transformation with forced cooling for SCM440 steel and austenite to pearlite phase transformation with natural cooling for S45C steel due to bending stresses have been occurred. The deflections of specimen were measured during loading process. By obtaining the maximum deflection due to transformation plasticity, the transformation plasticity coefficient was determined.

Experimental Investigation of Transformation Plastic Behavior under Tensile-Compressive-Torsional Stress in Steel

A tensile/compressive-torsional biaxial testing system was employed and tensile/ compressive-torsional tests were performed for the hollow specimen, which was loaded and the austenized specimen was cooled so that pealrite transformation accompanied by transformation plasticity occurred and axial and torsional strain were measured. Furthermore, the elastic-plastic constitutive equation due to phase transformation based on the hydrostatic pressure dependent model was proposed, and the validity of this equation was discussed experimentally. The test results showed the transformation plasticity coefficient due to pearlitic transformation of S45C depends on the loading direction, and these behaviour can be appropriately expressed by the hydrostatic pressure dependent model than the isotropic model.

Anisotropic Damage Evolution for Perforated Sheet under Tensile Deformation

It is important to formulate a constitutive equation which represents the growth of voids during plastic deformation in order to predict ductile fracture of metallic materials. For this purpose, we proposed an anisotropic Gurson’s yield function with the damage tensor, which represents the anisotropy due to the void distribution and the damage evolution was assumed isotropic for simplicity. Then we also proposed an anisotropic void growth law derived from the anisotropic Gurson’s yield function based on thermodynamic consideration. In this study we carried out the uniaxial tensile test of perforated sheets of stainless steel and aluminum alloy as the ideal two dimensional model of the damaged material and investigate the damage growth during plastic deformation. As a result, we obtained a good agreement between the experimental and the calculated void growth for both materials and it is also found that material parameters for damage evolutions are almost the same for both materials and are hardly affected by the work-hardening exponent.

Thermo-Elasto-Plastic Analysis of Pearlitic Transformation Plasticity under Combined Bending-Tensile Loading

In a previous study, we showed the anisotropy of plastic strain due to the pearlitic transformation and proposed a hydrostatic pressure-dependent constitutive equation to describe this phenomenon. In the present study, we assess the validity of this model using a bending-tensile loading system to experimentally and numerically analyze and characterize the pearlitic transformation plasticity. First, the maximum bending deflections due to the austenite-pearlite transformation were measured under different loadings and then transformation-plasticity coefficients were determined. Furthermore, as was done for bending-tensile loading tests, the pearlitic transformation plasticity was simulated using Abaqus Standard under the same austenitization and loading conditions as in experiments, and the calculated results for pearlitic-transformation plastic deformation are compared with the experimental results. The results show that the transformation plastic deflection due to the pearlitic transformation decreases with increasing applied tensile stress. In addition, this behavior can be described by a hydrostatic pressure-dependent model in large-deformation theory.

Dynamic Creep in Sintered Silicon Nitride Ceramics at Elevated Temperature.

Dynamic creep tests were performed for the specimen with projections of sintered silicon nitride ceramics at test temperature, T=1300°C, where triangular stress wave was imposed at stress rates σ=0.1, 0.5 and 20 MPa/s. Then dynamic creep displacement was measured between the two projections by means of the laser beam type displacement measuring system. Cyclic strain range decreased monotonously with increase in the number of stress cycles in dynamic creep test and this attenuation was much more enhanced at the higher cyclic stress, suggesting that cyclic deformation resistance increases during dynamic creep at 1300°C. The equivalent stresses σep.1 and σep.2 were assessed to examine the relationships between the dynamic and static creep strain rates, and the dynamic and static creep life times to failure. Then minimum creep strain rate ecmln and creep life time to failure tf were plotted against the equivalent stresses in the dynamic creep. It was shown that the relationships between ecmln and σep.1, and tf, and σep.2 were in quite good agreement with those in the static creep.

Optimum Design of Sintered Silicon Nitride Ceramics Specimen with Projections for High-Temperature Creep Test.

A tensile creep specimen with projections was designed to measure the creep displacement by means of the laser beam displacement meter at temperatures elevated beyond 1000°C. Stress/strain concentrations, however, occur around the root of projections in this specimen. The high-temperature creep strain was determined form the measurement of tensile displacement between the two projections by correcting influence of the projections. The procedure used to determine the steadystate creep constitutive equation of Si3N4 ceramics was proposed by combining a high-temperature creep test using the speciment with projections and the finite-element method (FEM) calculations for the specimen. Furthermore, the method for designing the optimum creep specimen with projections was proposed via the FEM calculations using the steady-state creep constitutive equation.