Fatigue assessment of welded joints taking into account effects of residual stress

Residual stress may have a significant effect on the fatigue strength of welded joints. As a non-fluctuating stress, it has an effect similar to that of the mean stress. Recently the International Association of Ship Classification Societies (IACS) has issued Common Structural Rules (CSR) for respectively tankers (IACS 2006a) and bulk carriers (IACS 2006b). The effect of mean stress in fatigue design is taken into account in both sets of rules. However, the treatment is quite different, in particular with regard to residual stress and shakedown effects. In the present paper a comparative study of fatigue design procedures of the IACS rules is reported, with emphasis on residual stress effects. Testing was carried out with longitudinal attachment welds in the as-welded condition. The initial residual stress was measured by a sectioning method using strain gages. Hot spot stress was determined experimentally by strain gauges and numerically by finite element analysis using different types of elements. Fatigue testing was carried out and SN-curves were plotted according to the relevant stress as specified by the rules. In order to investigate the shake-down effect of residual stress, testing was performed for several pre-load conditions which could be taken to represent maximum load levels in a load history. The aim of the study is to contribute towards better understanding of the effect of residual stress and shakedown on fatigue strength of welded joints.