Page 353 - Marine Structural Design

Part I11

Fatigue and Fracture

Chapter 17 Fatigue Capacity

17.1 S-N Curves

17.1.1 General
In Part I11 Chapter 16, it is stated that the relationship between the stress range and the number
of cycles to failure is a function of the type of joint, the environment, and the plate thickness.
In this Chapter, factors affecting S-N curves will be discussed in Section 17.1, while the
determination of the stress range at the critical location (hot spot) of the joint will be discussed
in Section 17.2. Methods for determining stress concentration factors will be presented in
Section 17.3. In Part 111, tubularjoints and plated connections are also termed “critical details”,
or “details”.
For fatigue analysis based on the nominal stress approach, welded joints are divided into
several classes. Each class has a designated S-N curve. The classification of S-N curves
depends on the geometry of the detail, the direction of the fluctuating stress relative to the
detail, and the method of fabrication and inspection of the detail. The types of joint, including
plate-to-plate, tube-to-plate, and tube-to-tube connections have alphabetical classification
types, where each type relates to a particular S-N relationship as determined by experimental
fatigue tests. The design S-N curves are based on the mean-minus-two-standard-deviation
curves for relevant experimental data. The S-N curves are thus associated with a 97.6%
probability of survival.
For example, Norwegian and British codes reference the D curve for simple plate connections
with the load transverse to the direction of the weld and the T curve for tubular brace to chord
connections, see Figure 17.1.
In the American codes (e.g. API RPZA), fatigue has been relatively less of concern.
Consequently, the number of joint classifications is less than that recommended in Europe.
Each construction detail, at which fatigue cracks may potentially develop, should be placed in
its relevant joint class in accordance with criteria given in the codes. Fatigue cracks may
develop in several locations, e.g. at the weld toe in each of the parts joined, at the weld ends,
and in the weld itself. Each location should be classified separately.
The basic design S-N curve is given as:
logN = logK - m logs (17.1)
where,
S = Stress range