Alternating two-load level fatigue tests were performed using annealed S40C steel to investigate the influence of overloads on the fatigue damage in crack initiation and propagation stages. The base block consisted of axial reversed strain cycling Δεp=1.1% as overloads and constant stress cycling σa=22kgf/mm2 as a baseline loading. Load levels were changed carefully so that no macroscopic residual strain could remain in the specimen.
The load level at which the microcrack initiated in the first block determined the future crack growth behavior: In the case of large number of overloads in one block, microcracks initiated during overload cycling and the damage was cumulated linearly in the next base loading until cracks grew 0.1mm at the specimen surfaces. On the other hand, in the case of small number of overloads, microcracks initiated during next base load cycling and grew at higher rate than the normal; 0.1mm crack initiation life was reduced about 1/3 of the life for no overload and was about the same as that of the pre-strained materials. In the crack propagation stage, overloads caused crack growth retardation and a greater number of overloads resulted in a longer propagation life.
