Repeat stress relaxation of notched bars and the dependence of creep damage on relaxation rate

This paper aims to investigate whether creep strain accumulated at different strain rates is equally damaging. Previous research has suggested that creep strain is more damaging when accumulated more slowly in creep of notched bars. The research presented here seeks to address this question by considering the accumulation of creep strain during stress relaxation of notched bars, i.e. specimens which support an initial load in a displacement-controlled test which then relaxes through creep deformation. Repeat stress relaxation tests with varying dwell lengths were conducted so that the relative damaging effects of the early, rapid accumulation and later, slower accumulation of creep strains could be compared. The material used was an ex-service powerplant stainless steel Type 316H. The stresses and strains in the specimens were then assessed using finite element analysis; a user subroutine was implemented so the onset and propagation of creep damage could be simulated throughout the specimens’ creep life. The research showed that creep strain accumulated rapidly at the start of a dwell is significantly less damaging than the same creep strain accumulated more slowly towards the end of the dwell. Consistent with this, the creep strain which accumulated very rapidly during the reloading period appears to produce negligible damage.