Combine S-N curve and fracture mechanics for fatigue life analysis of welded structures

Based on the evolution of fatigue cracks in welded structures, the fatigue life of welded structures was defined as the sum of the crack initiation life N_i and the crack propagation life N_p. Correspondingly, a fatigue-life analysis method combining S-N curves and fracture mechanics theory was proposed. The equivalent structural stress method and the lower 99% boundary of the master S-N curve were used to evaluate N_i, and cracks at the end of the initiation stage were considered as semi-elliptical surface cracks. Moreover, Paris equation and the stress intensity factor range of the cracks were used to evaluate N_p. Furthermore, the fatigue test results obtained from the running girder of cranes were used as a reference for comparison and verification of the results. The results revealed that the equivalent structural stress is a good indicator for the crack initiation behavior of complex welded structures. In addition, the predicted fatigue life corresponded closely to the testing life.