The Use of High-Fidelity Numerical Models in Ship Structural Fatigue Predictions

This dissertation advances the current state-of-the-art with respect to fatigue fracture in the marine industry by considering three interrelated issues in concert. The first involves the time-dependent nature of the fatigue inducing loads which, given a linear damage hypothesis, is typically a mute point. Intuitively, however, the order of the loading does matter and extreme overloads are not randomly dispersed throughout, but clustered together during physical storms. Therefore, the present work addresses the simulation of long, time-dependent (storm model) stress sequences which are stationary at one timescale (i.e., on the order of hours), yet decidedly non-stationary over longer intervals. Two categories of nonlinearities are simultaneously addressed. They are considered to arise from nonlinear ship motions and responses, and from the conversion of the resultant structural loading to equivalent fatigue damage and/or crack growth. In the present work, the former is taken to include the contribution of nonlinear wave-induced bending and whipping responses, whereas the latter encompasses the material hysteresis or load interactions inherent to variable amplitude loading.;In accounting for this material hysteresis, focus is shifted from a hypothetical fatigue damage criterion (i.e., applicable to the crack initiation and early crack growth phases), to macroscopic fatigue crack growth behavior within the context of a damage tolerant design. By considering the time-dependent nature of the fatigue inducing loads, a novel modeling approach is proposed which extends the finite element analysis of plasticity-induced crack closure to variable amplitude, high-cycle fatigue predictions. In doing so, cycle-by-cycle material hysteresis is included through a time-dependent crack opening level. This approach is demonstrated to be both consistent and convergent and, in contrast to previous numerical studies, permits the incorporation of a material constitutive model suited to cyclic plasticity in structural steels. Implementing this model within the context of storm model loading, several aspects of the fatigue fracture process are explored. Elucidated behaviors include the influence of nonlinear ship responses, the significance of physical storms, and the random nature of the fatigue process over a finite interval of ship operation.