Er. Delosrios et al., SHORT CRACK-GROWTH BEHAVIOR UNDER VARIABLE AMPLITUDE LOADING OF SHOT PEENED SURFACES, Fatigue & fracture of engineering materials & structures, 19(2-3), 1996, pp. 175-184
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.