Papers by Author: Yuji Nakasone

Abstract: The present study investigates the relations between distributions of strain with those of martensitic volume fraction around fatigue cracks in SUS304 stainless steel. Specimens having random pattern created by spraying on their surfaces were fatigued at room temperature (297K) in air. The pictures around fatigue crack were taken during fatigue crack propagation experiments. Distributions of strain were calculated by the digital image correlation (DIC) method with these pictures. Distributions of martensitic volume fraction Vα’ were measured around fatigue cracks with ferrite scope. The distributions of strain resembled those of Vα’. The higher the strain becomes, the more the α’ martensitic phase was induced by the plasticity-induced transformation around fatigue cracks. It was revealed that the same value of Vα’max is induced by strain regardless of static or fatigue.

Abstract: The present paper describes the FEM code the present authors have developed based on the theory of the polycrystal plasticity with dislocation distributions taken into account and the simulations of tensile deformation behavior in FCC polycrystalline materials having bimodal structures by using the developed FEM code. In order to simulate the deformation behavior of materials having bimodal structures, it is necessary for the code to simulate the mesoscopic deformation behavior with the size effect of the initial yield strength, or the 0.2% proof strength. The present study has attempted to simulate the size effect of 0.2% proof strength by modifying the Bailey-Hirsch relation. By using the modified relation, the size effect of the initial plastic yield is successfully reproduced by FE polycrystal plasticity analysis. The results also showed that the 0.2% yield strength is decreased as the volume fraction of coarse grains is increased in the bimodal structure. As the ratio of the average diameter of fine grains to that of coarse grains is increased, the yield strength of the bimodal structure is decreased. The yield strength and work hardening rate of the bimodal structure, however, is not so much decreased as that of fine grain models. It was also revealed that the reason why materials having bimodal structures show higher ductility is that coarse grains yield in earlier stage of deformation and lower the maximum stress in the materials.

Abstract: The present paper investigates distributions of α’ martensitic phase on fracture surfaces of plate specimens of SUS 304 stainless steel having through- and part-through-thickness fatigue cracks. The volume fraction Vα’ of the α’ phase transformed on the fracture surfaces including the two types of cracks was measured after specimens had been fractured due to fatigue at room temperature in air. The measurements were made with ferrite scope. The contour maps of Vα’ revealed crack shapes imprinted on the fracture surfaces of the specimens; i.e., nearly rectangular shapes on the fracture surfaces of through-thickness-cracked specimens, whereas semi-elliptical shapes on the fracture surfaces of part-through-thickness-cracked specimens. The values of Vα’ measured on crack surfaces can be correlated with the maximum stress intensity factor values Kmax calculated at the points of Vα’ measurements. The resultant Vα’ vs. Kmax plots fall within narrow bands whose upper and lower bounds are expressed by exponential curves. These results imply that quantitative post-fracture analysis can be made by measuring Vα’ values on the fracture surfaces of structural components with ferrite scope.

Abstract: The present study has investigated plasticity-induced martensitic phase transformation in fatigue of unnotched SUS304 plates. Martensitic phase transformation occurred in uunotched SUS304 plate specimens fatigued at room temperature in air. Volume fraction Va’ of a’ martensite in the uunotched portion of fatigued specimens was measured by ferrite scope. The relations between the maximum value of Va’, Va’max, and the number of load cycles N were represented by reverse sigmoidal curves for all the applied stress range Ds levels tested in this study. For the most portion of fatigue life, the value of Va’max remained almost constant. This value was increased with increase in the value of Ds. The spatial distribution of Va’ in the specimens varied with N: i.e., continued cycling of stress made a’ transformation localized near the central portion of specimens where the Va’ value reached as high as 35-40%. This value is more than doubled compared to the highest Va’ value found in the tensile tests of SUS304 at room temperature in air. Invisible cracks of 200µm in length were found in the high Va’ value region. These results imply that the measurement of Va’ in fatigued SUS304 components may detect crack initiation sites and may predict residual fatigue life.