Estimation of Kinematic Hardening of Sheet Metals Based on Stress-Relaxation Behavior

This study investigates the relationship between stress relaxation behavior and kinematic hardening of sheet metals. Stress relaxation tests and tension-compression tests for a high-strength steel sheet and an aluminum sheet were carried out to discuss whether it is possible to estimate the back stress as the amount of kinematic hardening based on the results of the stress-relaxation tests. In the stress relaxation after uniaxial tension, stress decreases with time. On the other hand, in the stress relaxation after uniaxial tension followed by full unloading, stress increases. It was found that in the stress relaxation after uniaxial tension followed by unloading to a certain stress level, stress does not change during the relaxation test. As for the high-strength steel sheet, the unloading stress level where stress does not change in the relaxation test was found to be relatively high and its Bauschinger effect is significant. On the contrary, as for the aluminum sheet, the unloading stress level is found to be relatively low and its Bauschinger effect is weak. These results suggest that the externally applied stress and the internal back stress is balanced at a certain unloading stress level where stress does not change in the relaxation test, and the unloading stress level is correlated with the amount of kinematic hardening.