Abstract
A simple method to predict fatigue life is the linear cumulative damage rule. It is sometimes difficult to predict precisely the crack growth life under random loading in this method, because there is an effect of crack closure behavior on the fatigue life. In order to predict precisely, a new prediction method of fatigue life under random loading has been proposed. In the proposed method, the fatigue life was divided into 2 stages: the crack initiation stage and the crack propagation stage. The initiation stage was calculated by using the linear cumulative damage rule. The propagation stage was calculated by using the effective stress range to consider the crack closure effect. When the fatigue stage changes from the crack initiation stage to the crack propagation stage, there is a period where only the larger stress amplitude propagate's the crack. This study also proposed a method to consider that period hereafter, to the propagation stage, called the transition stage, by using the degree of fatigue and diving the fatigue life into 3 stages: the initiation stage, the transition stage and the propagation stage for brevity. To evaluate the validity of the proposed methods, a series of fatigue tests were carried out under several two step loading waves changed a ratio of high load amplitude to low amplitude. From obtained test results, it was found that the proposed life prediction became more accurate compared to the linear cumulative damage rule.
