Page 505 - Marine Structural Design

Part IV

Structural Reliability

Chapter 27 Fatigue Reliability

27.1 Introduction

In conventional strength assessments, the safety of the marine structure is considered under a
static maximum design load. However, marine structures are to a large extent affected by
stresses that vary over time. The causes of these stresses are the forces generated by a seaway
and the propulsion plant, but also changes in the cargo loading.
The failure behavior of a structure subjected to fatigue loading deviates markedly from that of
a structure subjected only to static loading. Static loading can give rise to various forms of
failure such as yielding, instability or brittle fracture. Such failure occurs under a single
extreme load. The damage caused by fatigue loading can be outlined as follows: In the crack
initiation phase, microscopic fatigue cracks are formed as the result of an accumulation of
alternating plastic deformations. Here local structural changes, precipitation, microstructure
changes, etc. can occur. In the further course of the damage, the fatigue crack develops out of
one or more microcracks running along slip bands.
Fatigue is a typical failure mode for in-service structures. Proper prediction of the fatigue
behavior is of vital importance to maintain a sufficient level of reliability and integrity in
structures.
High cycle fatigue is a governing design criterion for certain welded components in marine
structures with large dynamic loading, high stress concentration and high stress level due to
use of high strength steel, notably braces and brace to deck connections. Fatigue may be of
concern for the primary strength of ships. However, most fatigue cracks have been
experienced in secondary members, such as transverse fi-ames, especially in joints between
longitudinal stiffeners and frames.
A large uncertainty is introduced in fatigue assessment due to various assumptions and
hypotheses. Additional uncertainties are due to the lack of the data and inherent random
nature exists in the analysis. This necessitates the use of statistical and reliability approaches.
The hndamentals of fatigue strength assessment for ships and other marine structures are
explained, e.g. Part 4 of this book, Almar-Naess (1985), Rice et a1 (1988), Maddox (1991),
among others.

27.2 Uncertainty io Fatigue Stress Model
27.2.1 Stress Modeling
The process of computing stresses in a component includes the following steps: