Consideration on Estimation of Welded Structure's Fatigue Life under Block Loading

Abstract

Almost all the fatigue design methods for welded structures are based by S-N diagrams, that is, crack initiation life criterion. Fatigue crack initiation lives deviate with definition of life and with local peak stress. The evaluation by the initiation criterion lacks in assessing structures' functional loss by crack propagation and effect of cracks on loads.Another fatigue design method that gives solution for the lack, is crack propagation life criterion. The method is based on fictitious initial crack hypothesis, and the change of structure's performance can be given every moment.Especially, in ships of large welded structures, the ratio of crack propagation life per fatigue life is a large extent. So to show the calculation methods for fatigue crack propagation lives is useful for evaluation of structures safety.The authors show simulation of fatigue crcak propagation originated from box weld toe end, making use of the results of structural fatigue tests by SR216. In the simulation the authors show the approximate stress intensity factor K in welded structure with attachment and crack propagation's retardation phenomena under block loadings.