Page 342 - Marine Structural Design

318 Part 111 Fatigue and Fracture

High-cycle (low stress) fatigue
Low-cycle (high stress) fatigue
Typically, a fatigue failure is called “low-cycle fatigue” if the number of cycles to failure is
less than lo4. The number of cycles in a high-cycle fatigue is usually several millions. For
marine structures, the latter has been of real concern.
Methods for Fatigue Analysis: In general, there are two methods for fatigue analysis, namely
S-N approach (based on fatigue tests, see Chapters 17) and fracture mechanics approach (see
Chapter 21). For fatigue design purpose, the S-N curve approach is widely used and is the
most suitable one. Fracture mechanics method is used to determine acceptable flaw size;
assessing the fatigue crack growth; planning inspection and repair strategy, etc. For the S-N
curve approach, there are three methodologies for fatigue damage calculations, depending on
the methods of determining fatigue loads (see Chapter 18):
Simplified Fatigue Analysis (see Chapter 19)
Spectral Fatigue Analysis (see Chapter 20)
Time Domain Fatigue Analysis (see Chapter 20)
In order to study the fatigue and fracture damage mechanism, numerous experiments have
been conducted to investigate the material characteristics. These experiments can be divided
into two categories: stress-controlled fatigue and strain-controlled fatigue.

16.3 Stress-Controlled Fatigue

Stress-controlled fatigue is generally related to high cycle (low stress) fatigue, in which a
major part of the material behaves elastically. Even though the material immediately adjacent
to the notch may become plastic, both the extent of plastic zone, and the stress in it are limited.
Since stress is directly proportional to strains, conventionally, the fatigue strength is expressed
in terms of stress.