Short crack growth behaviour under variable amplitude loading of shot peened surfaces

The effect of variable amplitude loading on the initiation and propagation of fatigue cracks from shot peened and unpeened surfaces was investigated. The results confirm the well-known effect of shot peening, i.e. the increase of fatigue life in comparative tests under constant amplitude loading. Shot peening increases the time to crack initiation and also decreases crack propagation rate, particularly in the earlier stages of growth. Variable amplitude tests were of various kinds: (i) half-life at one stress level, followed by testing at the second stress level until failure, (ii) sequence loading at two stress levels, and (iii) a different number of overload cycles (1, 10, 100) following a given number of base amplitude cycles. The results of the two amplitude tests showed different behaviour depending on whether the first half-life cycles were of higher or lower stress levels. These results are explained in terms of crack density and crack coalescence. The extent of damage accumulation during block loading depends on the stress levels employed. The benefits of shot peening are compromised by high stress levels. Results from the overload tests showed that the interpretation of results is not straight forward. A combination of the theories of crack retardation and microstructural-fracture mechanics seems a necessary prerequisite in an explanation of the resultant behaviour.