オーステナイト系ステンレス鋼の散逸エネルギ評価

抄録

In recent years, fatigue limit estimation based on dissipated energy is being introduced in various industries because of its time and cost effectiveness. However, the energy dissipation mechanism and the relationship between energy dissipation and fatigue damage have not been well investigated. In the 1st report, the present authors conducted fatigue limit estimation based on dissipated energy measurement and conventional fatigue test for JIS type 304 and 316L austenitic stainless steel. We found that the fatigue limit of the type 316L steel could be precisely estimated, although estimated fatigue limit of the type 304 steel was conservative values compared with that obtained by conventional fatigue tests. It was considered that the fatigue limit estimation method based on dissipated energy could not evaluate the microstructure change, such as dislocation and phase transformation during short cyclic loading. In this study, fatigue tests under the constant stress amplitude were conducted for austenitic stainless steel to investigate the relationship between dissipated energy and microstructure change during fatigue tests. It was found for the type 316L steel that tendency of the observed dissipated energy during the fatigue tests was similar as that of plastic strain energy. On the other hand, for the type 304 steel, the dissipated energy decreased in spite of increasing plastic strain energy around N = 10⁵cycles, when the applied stress amplitude was close to the actual fatigue limit. It is considered from the observation of the change in martensitic volume fraction, the decrease of dissipated energy was caused by the martensitic transformation ; some amount of irreversible plastic strain energy was consumed for phase transformation. It was found from the experimental studies that dissipated energy reflects the microstructure changes, such as material softening, hardening and martensitic phase transformation. Therefore, the condition of the number of cycles in the fatigue limit estimation based on the dissipated energy should be changed depending on the behavior of microstructure change in material under cyclic loading.