Page 341 - Marine Structural Design

Part I11

Fatigue and Fracture

Chapter 16 Mechanism of Fatigue and Fracture

16.1 Introduction

Fatigue is the cumulative material damage caused by cyclic loading. Many structural
members must withstand numerous stress reversals during their service life. Examples of this
type of loading in marine structures include alternating stresses associated with the wave
induced loading, vortex-induced-vibration (VIV) and load fluctuations due to the wind and
other environmental effects. In the following Sections, the basic fatigue mechanism will be
reviewed. A detailed theoretical background for fatigue analysis is given by Almar-Naess
(1985), Gurney (1979), Maddox (1991), Suresh (1991), Dover and Madhav Rao (1996). An
extensive list of recently published papers may be found from the proceedings of ISSC (1988,
1991, 1994, 1997, 2000). AWS (1985) can be considered as a representative code for fatigue
strength design. Recent developments in ship fatigue research may be found in Xu (1997) and
Xu and Bea (1997).
As part of the limit-state design criteria, Part III of this book covers the following aspects:
Chapter 16 Basic mechanism of fatigue and fracture
Chapter 17 Fatigue criteria such as S-N curves, stress concentration factors
Chapter 18 Fatigue loads and stresses determined based on deterministic methods,
stochastical methods and Weibull distribution.
Chapter 19 Simplified fatigue assessment based on a Weibull distribution of long-term
stress range
Chapter 20 Spectral fatigue analysis and time-domain fatigue analysis and their
applications to structural design
Chapter 21 Fracture mechanics and its applications to the assessment of crack propagation,
final fracture and calibration of fatigue design S-N curves.
Chapter 22 Material selection and damage tolerance criteria

16.2 Fatigue Overview
Generally, the load amplitude of each cycle is not large enough to cause the structural failure
by itself. But failure could occur when the accumulated damage experienced by the structure
reaches a critical level. The fatigue life of a structural detail is directly linked to the fatigue
process, which can be grouped into the following three stages:
Crack initiation