Cracks and Structural Redundancy

Abstract

This paper proposes a way to use finite element models to determine the effect of fatigue cracks in ship structures. Cracks of different lengths are modeled and the maximum nodal stress at the crack tip is used to estimate the stress intensity level. The calculated stress intensity factor is substituted into fracture mechanics equations to calculate the rate of crack growth and the critical crack length. The main advantage to calculating the stress intensity factor from an appropriate finite element model is that load redistribution effects are included. Since the ship structure is redundant, load shedding and multiple load paths can significantly reduce the propagation rate and thereby improve the safety of the structure. Including the crack in the finite element model also makes it possible to study the crack's impact on the stress levels in surrounding structure. The method is independent of the cause of structural failure, hence this kind of analysis can be used to investigate the effects of cracks created by fatigue, collision, or grounding damage.