Papers by Author: Tsuyoshi Mayama

Abstract: The present paper investigates relationships between the macroscopic viscoplasticity and the surface morphological changes at room temperature for commercially pure titanium (CP-Ti) and austenitic stainless steel (SUS316L). Pure tensile test and tension-intermittent creep test are conducted. Both CP-Ti and SUS316L are deformed up to 16% of inelastic strain with a few unloading, and surface conditions are observed during pure tension test and tension-intermittent creep test. Qualitative surface observations and quantitative surface roughness measurements are made for the unloaded specimens. The surface roughness measurement shows that the curves plotted between surface roughness and inelastic strains are almost linear for all the present experiments. The slopes of curves depend, however, on material and type of tests, and this tendency agrees well with the qualitative surface observations by an optical microscope. The experimental results for CP-Ti suggest that different deformation mechanisms during tensile loading and creep contribute to different surface morphological changes.

Abstract: In the present study, a new approach is conducted to evaluate dislocation structure induced by cyclic plasticity. First, cyclic plastic loading tests are carried out up to 100 cycles with three different small strain amplitudes on SUS316L stainless steel at room temperature. The test result presents the dependence of the strain amplitude on cyclic hardening and softening behaviors. Specifically, it is found that the cyclic loading test with strain amplitude of 0.25% shows both cyclic hardening and cyclic softening, while the cyclic loading tests with strain amplitudes of 0.75% and 1.0% show no cyclic softening. Secondly, the dislocation structures of the specimens after cyclic loading are observed by using a transmission electron microscope (TEM), and this observation reveals that the dislocation structure after cyclic loading test depends on the strain amplitude. Finally, a quantitative evaluation method of the dislocation structure is also proposed. The TEM images are converted into binary images and the resolution dependence of the generated binary image is used to visualize the characteristics of the dislocation structure. The relationship between strain amplitudes of cyclic plasticity and dislocation structure organization is clarified by the evaluation method. Finally, the heterogeneity of the dislocation structure is discussed.