For the analysis of the retardation behavior in 7075-T73 aluminum alloy, which is produced after a single overloading, the effects of the crack depth (a)W), the baseline stress intensity factor range (Δkb), and l overload (% O.L.) are examined. Test results showed that the amount of the retardation after a single overloading increased with a decrease in a/W and an increase in alb, and %O L. These phenomena are related to the change of the micro-crack growth direction and the crack branching in near-surface area of the specimen. It was difficult to explain the retardation after a single overloading by the plastic zone size only. Main mechanisms of these retardation phenomena are the crack closure and the relaxation of kat the crack tip, which are caused by the branching and deflection of the crack.
The retardation mechanisms of an engineering material after a single overloading can provide the bases of the fatigue life estimation of the material under variable amplitude loading. Authors (1989) examined the effect of the thickness variation on the retardation phenomena of the fatigue crack growth and the effect of the crack depth, a)W, on 7075 aluminium alloy after applying a 100% single overload. These results show that the effect of a/W, which has not been noticed, should be considered. Of the main parameters of the retardation 0 L. and alb, l 0 L has been known to increase the amount of retardation as it increases (Vardar, 1988), but, in case of Δkb, there have been inconsistent reports (Hills et al,1977; Vecchio et al, 1984) on its results and mechanisms. Therefore, to find the effect of the crack depth on the retardation phenomena, it is necessary to consider the effects of the above two main parameters at the same time.