The effects of long range residual stress, elastic follow-up and applied load on creep crack incubation and material toughness

Creep crack incubation of Type 316H stainless steel at 550C is explored in this paper. Fracture mechanics specimens, subjected to combinations of residual and applied loads and in the presence of elastic follow-up, are tested. The design of two new test rigs is described. The rigs introduce planned levels of elastic follow-up together with combined residual and applied loading conditions to the specimens. A series of high temperature elastic-plastic and elastic-plastic-creep experiments is undertaken to compare experimentally determined values of elastic follow-up with theoretical values. A further series of fracture mechanics tests is performed to measure creep crack incubation and material toughness for samples subjected to constant load and for tests under combined loading with elastic follow-up. It is demonstrated that for tests subjected to the same initial reference stresses longer incubation times are attained for elastic follow-up tests compared to constant load tests. Also, combined loading tests exhibit longer creep crack incubation times based on the same measured material toughness obtained from constant load tests. This suggests that not all the available strain energy provided by combined loading to a specimen at high temperature contributes to creep crack incubation.