An analysis of multiple two-step fatigue loading

Murakami and Matsuda have carried out multiple two-step tests using axially loaded cylindrical specimens of a 0.46% carbon steel. The specimens were pre-cracked and heat-treated to develop closure-free starter cracks ranging from 100 to 1000 μm in length. S/N curves were developed under axial loading at R=−1 for each starter crack length. In the multiple two-step tests, one stress level was above the run-out stress level for that particular initial crack length, and the other level was below the run-out stress level. The loading pattern involved the application of one high-stress cycle followed by 40 low-stress cycles. This pattern was repeated until the cracks propagated to failure. Damage summations based upon an extrapolation of the upper portions of the S/N curves to the lower stress level were of the order of 0.2. In the present paper, these data are analyzed using a modified LEFM approach. The modifications include a correction for elastic-plastic behavior, consideration of the endurance limit-threshold interrelation (Kitagawa effect), and consideration of the development of crack closure in the wake of an initially closure-free crack. The method is able to predict crack growth behavior under multiple two-step loading conditions fairly well. It is found that a principal role of the repeated application of the lower stress is not to propagate the crack, but to reduce the crack closure level and thereby facilitate an accelerated crack growth at the higher stress level.