Description and simulation of cyclic stress-strain response during residual stress relaxation under cyclic load

Compressive residual stresses are often induced by mechanical surface treatment such as shot peening for the purpose of improving fatigue life of components. Cyclic relaxation of the induced compressive residual stress, however, reduces the benefit. For component design and life management purposes it is important to be able to assess the effect of residual stresses and their relaxation on fatigue life. The problem, however, is that the phenomenon of residual stress relaxation under cyclic mechanical loads is not well understood. Furthermore, there is the technical challenge of accurately quantifying residual stress relaxation during component operation. The difficulties in measuring residual stress relaxation in real-time impede the consideration of tracking and assessing its effect on remaining fatigue life.In this paper a previously proposed cyclic hardening/softening model is adapted and modified to account for residual stress relaxation. Considering residual stress as virtual mean stress the cyclic stress-strain response during residual stress relaxation is described and simulated using the Matlab/Simulink Program. The virtual mean stress has a corresponding mean strain which conditions result in biased stress/strain limits under cyclic load. It is shown that residual stress relaxation is associated with mean stress shift in stress-axis direction and mean strain shift in the strain axis direction. The significance of this mean strain shift is briefly discussed in terms of tracking and quantitatively determining residual stress relaxation in real-time.