Fatigue crack propagation rate and crack closure behavior under repeated two-step tests composed of loads above and below the threshold condition were studied on a medium carbon steel and an aluminum alloy.
By means of electron fractography, striations corresponding to high and low level loadings were found on the fractured surface of both materials, which confirmed that crack propagation occurred even below the threshold level under varying loading conditions. And such behavior of crack propagation was observed even under the pulsating loading test as well as the reversed loading test.
The acceleration of crack propagation was found to be contributed by both reasons of the acceleration under high level loading due to the increase in the effective stress intensity range and the above mentioned crack propagation below the threshold level. Furthermore, the effect of the cycle ratio of high level load to low level one on crack propagation rate was investigated.
