On Fatigue Crack Paths under Biaxial Loading Conditions

Abstract

Considering welded thin plate structures such as ships, which are orthogonally stiffened by periodically arranged transverse and longitudinal frames, attentions should be focused to the fatigue strength at the orthogonal intersections of frames where severe stress concentration may occur due to the structural discontinuity and weldment. Stress states of frames at intersections are generally biaxial, because loads transmitted from the intersected members act perpendicularly to the frame direction. The biaxial stress ratio varies depending on the load effects of frames such as axial stress due to hull girder bending and local bending stress due to lateral pressure acting on shell plating.Formerly fatigue damage was observed around slots and web stiffeners of transverse girders. In order to prevent this type of fatigue damage, a lot of efforts had been made so that the certain refinement of structural details were accomplished. Recently, new type of fatigue damage is observed in longitudinals, where fatigue cracks extend toward shell plating. The change of the fracture modes may be caused by the change of the proportion of transeverse and longitudinal stresses, which is closely related to the biaxial stress ratio of the intersection.In the present paper an analysis model is introduced in order to investigate paths of fatigue cracks propagating under various biaxial stress conditions. The local symmetry criterion, which was proposed by Kitagawa et al. and confirmed by experiments for paths of fatigue crack growth, is applied to the computational crack path prediction. In order to avoid the complexities due to nonproportional loading, numerical simulations are performed for the purely in phase or out of phase biaxial stress conditions. Considering a fatigue crack whose growth length is within three to five times of the initial crack length, it extends approximately normal to the larger stress direction except that the biaxial stress ratio is close to unity.