Equivalent static stress for comparison to creep-dominated cyclic tests using a triangular waveform

Abstract

Fatigue of ceramic matrix composites during cyclic loading has been a topic of recent interest [1–5]. Creep is one mechanism that could limit lifetime in hightemperature environments [6–15] and could be expected to occur under both cyclic and static loading conditions. A comparison of creep lifetime in both cyclic and static loading could be used to provide insight into failure mechanisms. To ensure that materials failing by the same mechanism in static and cyclic loading exhibit the same lifetime, the tests must be designed for the same creep rates. The comparability of stress states in both types of loading has been recently addressed for a sinusoidal waveform [16]. This letter examines the selection of an appropriate static stress using equivalent time average creep rates for direct comparison with triangular waveform cyclic results. Using the assumption that steady-state conditions occur during the majority of the creep lifetime, the time average creep rate in static loading has been recently reported as [16]: