The evolution of the surface crack distribution in single-step and two-step plastic strain amplitude controlled cycling was studied. The frequency of crack occurrence and total crack density were evaluated together with crack growth rates of the largest cracks. The crack density increases, reaches a maximum, and then decreases during fatigue life. The maximum value of crack density partitions the whole life into a "crack generation period" and a "crack propagation period". In the first period the crack growth rate is approximately constant and is determined by the applied plastic strain amplitude. The crack growth rate increases with crack length in the second period, but is basically determined by the crack growth rate in the "crack generation" period. Hence the crack growth rate in the "crack generation" period is the life determining factor.

An experimental study of the surface evolution during controlled plastic strain amplitude single-step and two-step loading tests reveals the important damage mechanisms for 316L stainless steel. In the first stage, the cyclic plastic strain is concentrated into persistent slip bands (PSBs) and a surface relief is formed consisting of extrusions and intrusions. The frequency of occurrence and the total density of PSBs has been assessed using systematic observations in a scanning electron microscope. The relative volume occupied by PSBs determines the fatigue damage in this stage. Two-step loading has only a small effect on the PSB damage evolution and nearly equal saturated values (corresponding to the applied plastic strain amplitude) were achieved in the single-step and the two-step loading investigations.