The effect of the R value on the number of delay cycles following an overload

Compact tension (CT) specimens, 6 mm thick, of the aluminum alloy 6061-T6 were subjected to 100–300% overloads at R values ranging from 0.1 to 0.9. Both experimental and analytical results show that the delay phenomenon is found at all R values. For a given overload ratio, the number of delay cycles, Nd, at high R values was found to be specimen-type dependent. In tests using compact specimens, Nd increased with R, whereas in tests using center-cracked specimens Nd decreased with R. Over the complete range of R values, the delay phenomenon is shown to be caused by overload-induced crack closure.