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Stress intensity factors for cracked plates under out-of-plane bending

Stiffened plate structures such as ships and bridges often develop local stress concentrations which are a mixture of membrane tension and out of plane bending. Any cracks which initiate due to fatigue soon grow through the plating thickness. Modern steels are resistant to brittle fracture, but if cracks are left un-repaired, they may spread rapidly by fatigue. The rate of increase in fatigue crack length can be quantified using the elastic stress intensity factor, K. Routines for the determination of K are available in ABAQUS, but they have limitations for performing extensive parametric studies on fatigue crack growth. Multiple fatigue crack tip positions must be analysed, requiring frequent re-meshing; and the crack direction and through thickness profile must be anticipated in advance. Zentech have developed the ABAQUS compatible software Zencrack specifically to deal with these problems. This paper describes the problems of applying existing ABAQUS J integral methods to model cracks in structures under out-of-plane bending. Shell elements are shown to give path dependent and unreliable J values. Solid elements can be used, but the crack shape under fatigue cannot be easily predicted. Zencrack provides re-meshing techniques which allows the crack shape and speed of fatigue crack propagation to be efficiently predicted. It is shown that Zencrack analysis overcomes ambiguities over how to take account of variations in K along the crack front in fatigue crack growth predictions. A set of results are presented which suggest that an out-of-plane bending stress is very much less detrimental for the fatigue life of a through thickness crack than a membrane stress of the same magnitude.