Fatigue life assessment of welded joints by the equivalent crack length method

The fatigue life of welded structures is often dominated by crack initiation and growth from the weld toes, where the notch or an initial crack like flaw determines the fatigue endurance of the structure. In the present paper, the equivalent crack length method is proposed for predicting the crack propagation life of a welded joint from this initial flaw of length a0 to a final crack at fracture lc. The geometrical configuration of a welded structure with stress concentration is assumed approximately equivalent to an initial crack in an unwelded plate. The equivalent crack length a0 depends on the joint geometry and is determined from a single experimental data point under the low cycle fatigue region, where most of the fatigue life is spent on crack propagation and the crack initiation cycle can be ignored. Once a Paris law type equation is determined by the single experimental data point, the fatigue life of the same type welded joint can be calculated for other applied stress ranges. The critical crack propagation length lc for final fracture depends on the applied stress level in terms of ΔKfc; with higher stress levels the crack propagation length may be very short, whereas with very low stress levels the maximum propagation length is longer but it is limited for unstable fatigue crack growth. In this way, the final crack length is related to the critical stress intensity factor ΔKfc. The method gives conservative results in the high cycle fatigue region as only the fatigue crack propagation phase is taken into account in the fatigue life calculation. The proposed approach has been successfully applied to experimental data. The extension of the proposed method for fatigue under variable amplitude loading will be also discussed.