Effect of Various Programmed Overloads on the Threshold for High-Frequency Fatigue Crack Growth
SourceA threshold value of crack tip stress intensity range (ΔKth) exists below which Mode I cracks do not propagate in high-frequency fatigue. The response of ΔKth, in a commercial nickel-base and titanium-base alloy, to single and multiple cycle overloads (that is, the behavior of an overload modified threshold, ΔKth*) has been defined for a variety of fatigue conditions. Both alloys responded similarly and showed an exponential increase of ΔKth* with increased magnitude of the overload. The effects of overload rate, temperature, cycle shape and number, and the fatigue conditions of mean stress and cyclic frequency have lesser, but significant, effects on ΔKth*. Metallography and scanning electron fractography have been used to define the changes in crack size and tip shape which contribute to the overload effects.