TABLE OF CONTENTS When designing a structure against the fatigue-limit state, the aim is to ensure, to an acceptable level of probability, that the integrity of the structure is satisfactory throughout its planned service life. The structure should be unlikely to fail from fatigue or to require extensive repair of damage caused by fatigue (Ref [1]). To carry out this type of safe life analysis, the designer must verify the fatigue life for each critical welded detail in the structure. Fortunately, even large, complex structures may often be divided into a relatively low number of elementary joints. The fatigue strength for these elementary joints must be known from experience or laboratory testing. Based on laboratory tests, the most common type of analysis for fatigue life prediction is based on the S-N method. For each critical joint it is verified that the predicted fatigue life (PFL) is superior to the target service life (TSL) of the structure, and there should be with a required safety margin. The stress spectrum must be estimated for the planned service life and the S-N curves characterizing the fatigue strength of the various critical details of the structure must be at hand.
As outlined in Chapter 2, the S-N curves are based on a simple relationship between applied stress range ?S and the number of cycles N to failure. For constant amplitude loading, the curves are assumed to be linear for a log-log scale until the curves reach a fatigue-limit. For a stress...
