Fatigue life assessment for welded structures without initial defects: An algorithm for predicting fatigue crack growth from a sound site

A new fatigue life evaluation methodology, which enables to predict fatigue crack growth from a stress concentration site without the initial defect is proposed. This method is based on the following scenario of fatigue crack growth: (i) a shear mode crack initiates and grows from the stress concentration site, (ii) a crack opening mode appears when the shear crack reaches the first grain boundary, (iii) the crack mode shifts from shear to opening/closing, (iv) opening/closing mode crack propagates. It is further assumed that cracked bodies which have the same stress intensity factor range (ΔK) show the same fatigue crack growth. On the basis of this hypothesis, an equivalent distributed stress (EDS) is defined. EDS corresponds to the distributed stress in an infinite plane with a straight crack for which the relationship between the crack length and the stress intensity factor (K) is the same as in the considered cracked body. Estimation of fatigue crack growth from a sound stress concentration site in corner boxing joints is performed by applying a fatigue crack closure simulation code FLARP developed by the authors and using the EDS concept. The stresses due to the external loading and welding are converted into EDS in order to apply FLARP. The estimation procedure is validated by comparing the predicted and experimental results.