Low-Cycle Fatigue of a Medium Carbon Steel under Variable Strain Cycling (Application of the Rain Flow Method to the Prediction of Microcrack Propagation Life)

The rain flow method for counting fatigue damage was applied to predict microcrack propagation life under varying strain amplitudes. Low-cycle fatigue tests were conducted on a medium carbon steel under block-random and pseudo-random strain cycling as well as constant strain cycling. Through continual observation of the microcrack initiation and propagation behaviours of plain specimens and holed specimens with a small drilled hole, the results obtained are summarized as follows : (1) For a microcrack propagating under varying strain amplitudes, a fatigue damage calculated by means of the rain flow method can be fairly well correlated with a cycle ratio under constant strain amplitude. (2) The rain flow method is shown to be suitable in predicting microcrack propagation life as well as in evaluating fatigue failure life. (3) The essential conditions for Miner's rule to hold under variable strain cycling conditions are that the microcrack propagation law shows a relation of the linear type, dl/dN = Cl, and that prior fatigue damage hardly influences the subsequent microcrack growth rate.