Nonlinear Fatigue Crack Growth Predictions for Simple Specimens Subject to Time-Dependent Ship Structural Loading Sequences

Abstract

The typical classification rules-based fatigue assessment of ship structures is inherently limited and must be rethought as new designs push the boundary of past experience. To this end, the majority of research over the past decade has focused on providing more accurate experimental and/or numerical predictions of ship motions and hence fatigue loading. However, comparatively little attention has been paid to the accompanying linear damage accumulation models originally contrived for spectral-based approaches. Here, a nonlinear crack growth model is proposed based on the finite element analysis of plastically-induced crack closure. In contrast to previous studies, this approach is readily generalized as it relies solely on experimentally measured fatigue crack growth rates (da/dN vs. ΔK) and a full material constitutive model. To extend this existing area of research to marine engineering, it will be shown that simple instances of variable amplitude loading are analogous to typical ship loading sequences through so-called “storm model” loading. Thus, the proposed model facilitates ship fatigue life predictions (crack propagation phase) in which both the order of the loading (load sequence effects) and material hysteresis (load interaction effects) are included. Using this model, it will be shown that these load sequence and load interactions effects are first-order phenomena and must be considered in fatigue life predictions. Finally, by including these effects, we challenge the presumption that loading nonlinearities (e.g., combined midship wave bending and whipping) are insignificant within the context of lifetime accumulated fatigue damage and/or that they can be included without being explicitly evaluated.