SHORT CRACK-GROWTH BEHAVIOR UNDER VARIABLE AMPLITUDE LOADING OF SHOT PEENED SURFACES

The effect of variable amplitude loading on the initiation and propaga tion of fatigue cracks from shot peened and unpeened surfaces was inve stigated. The results confirm the well-known effect of shot peening, i .e. the increase of fatigue life in comparative tests under constant a mplitude 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 stre ss level until failure, (ii) sequence loading at two stress levels, an d (iii) a different number of overload cycles (1, 10, 100) following a given number of base amplitude cycles. The results of the two amplitu de tests showed different behaviour depending on whether the first hal f-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 lev els employed. The benefits of shot peening are compromised by high str ess levels. Results from the overload tests showed that the interpreta tion of results is not straight forward. A combination of the theories of crack retardation and microstructural-fracture mechanics seems a n ecessary prerequisite in an explanation of the resultant behaviour.