Fatigue-creep and plastic collapse of notched bars

The paper describes a study of the application of a new assessment method, based on the linear matching method (LMM), to the creep fatigue of notched bars of Udimet 720Li at 650degreesC. This high strength nickel based alloy is taken as typical of alloys used in high temperature gas turbine applications. The primary purpose of the study is to see whether it is possible to predict the failure modes for such alloys in terms of standard materials data by the evaluation of a sequence of simplified calculations corresponding to the steady state cyclic stress history. These calculations involve the evaluation of limit load, shakedown limit and ratchet limit for perfect plasticity, rapid cycle creep solutions and the evaluation of an elastic follow-up factor. Within the limitations of the tests, the correlation between predicted failure mode and observed mode is very good and the calculations clearly show up the differences between the two types of notched bars discussed. This implies that LM methods are well suited to the evaluation of failure modes in materials of this type.