PREDICTION OF FATIGUE LIFE BASED ON LEVEL CROSSINGS AND A STATE VARIABLE

Abstract The differences between the cycle count approach and the level crossing approach are discussed from the general fatigue life estimation assumptions as formulated by Holm and de Maré. It is concluded that the differences are related to the interpretation of the assumption of order independence, i.e. the neglecting of sequential effects. A revised level crossing model is proposed where damage accumulation depends on the level crossing and the stress history condensed in a state variable. In order to formulate a mean fatigue life the stationarity and ergodicity conditions on the involved processes are outlined. In this revised model sequential effects can be taken into account and in an example the state variable is chosen as the opening stress of a fatigue loaded crack. The dynamics of the opening stress is described by a simple two parameter auto-regressive model. The entering parameters are estimated from published experimental results and the model is applied to different variable amplitude results from the literature. Calculated results for fatigue life are promising compared to experimental results.