Fatigue crack growth behavior under single-peak overloads is investigated for structural steel JIS SM52OB. Fatigue test is carried out on seven center prenotched specimens.Three parameters, i. e. overload ratio, overload application stage and stress ratio are introduced to investigate the retardation effect. It is verified that fatigue crack growth rate is retarded after overloading and then gradually recovers to the normal.It is also observed that the retardation effect is intensified when the overload ratio and the stress range between overload stress and the following minimum stress increase. From the test results the crack length increment in which retardation occurs is compared with Irwin's and Dugdale's plastic zone sizes.

In order to evaluate accurately fatigue life of weldments under variable amplitude loading, load interaction effect due to load sequence has to be clarified. A simple case is the fatigue crack growth retardation effect resulting from single or periodic overloads. In the present study, fatigue crack growth rates were measured under single and periodic overload conditions on center-cracked tension specimens of structural steel SS400, SM520B and HT780. Effects of several parameters including overload ratio, stress ratio, yield strength of material and interval between overloads on fatigue crack growth retardation were investigated.

Crack closure phenomenon is observed in center-cracked tension (CCT) specimens of structural steel JIS SM520B. The remaining plastic deformation in the wake of an advancing crack causing crack closure is a significant factor of the load interaction effects on fatigue crack growth rates. A prediction model based on crack closure concept is used to compute fatigue crack opening stress and consequently effective stress intensity factor range, LIKef f, and correlate it with the measured fatigue crack growth rates. The analytical results are compared with test results under overload conditions. Fatigue crack propagation lives are computed under various load sequences to investigate the load interaction effects.

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