Effect of single and multiple overloading on the residual fatigue life of a structural steel

This work was carried out to determine the effect of overload cycles on the fatigue life of a structural steel used for offshore applications. Single and multiple overloads were adopted and the corresponding fatigue crack growth retardation was evaluated. Residual stress fields were measured in the vicinity of the crack tip using an X-ray diffraction technique and their size compared with that of the overload cyclic plastic zone. In regard to crack growth retardation, the results indicated that the extension in fatigue life increases with an increase in overload, as a consequence of the generation of higher compressive residual stress levels over a larger distance ahead of the crack tip. The effect of two equal and consecutive overloads, with the second one applied at different intervals of crack propagation from the first, was also considered. Larger intervals were shown to lead to a longer residual fatigue life.