Fatigue Crack Growth Delays

The effects of single and multiple tensile overloads on precracked center-notched sheet specimens of Ti-8 Al-1 Mo-1V titanium alloy were investigated under laboratory conditions. Single tensile overloads always delayed subsequent fatigue crack growth at lower stress levels. Multiple tensile overloads caused a delay in fatigue crack growth greater than a single overload, with the amount depending on the number of overloads. The quantitative values of fatigue crack growth delay are reported for overloads ranging from 116-233%. Single intermittent overloads applied within the delay caused by previous overloads produced the greatest delays. Conditions subsequent to overloading and encountered in realistic environment-including time at zero load, temperature, and a seawater environment-were found to reduce the delay effects greatly and should be incorporated into theoretical models. These effects are explained in terms of fatigue crack closure concepts.