FATIGUE-CRACK CLOSURE AND GROWTH-BEHAVIOR UNDER RANDOM LOADING

Fatigue crack closure and growth behavior under random loading is investigated by performing various random loading tests. The effects of random load spectrum and history length on the behavior of crack closure and growth are discussed. Moreover, single overloading, periodic single overloading, and Hi-Lo (high-low) block loading tests are performed to compare the crack closure behavior under random loading and under simple variable amplitude loadings. The crack opening load is nearly constant during a random loading block of history length ranging from 500 cycles to 2000 cycles and is mainly governed by the largest load cycle in a random load history, irrespective of random load spectrum and history length. The crack opening load under random loading is different from that under constant amplitude loading, indicating that the crack closure behavior under random loading cannot be estimated from constant amplitude tests. The crack opening stress intensity factor under random loading is linearly proportional to the maximum stress intensity factor of the largest load cycle in a random load history. Fatigue crack growth under random loading can be successfully described by the crack closure concept. It is found that the crack closure behavior under random loading can be estimated from single overloading or periodic single overloading tests. Based on the results obtained, a relatively simple method is proposed to predict crack growth under random loading.