Influence of a preload on the fatigue behaviour of S355 and S700 cruciform joints

Preload application techniques are often used to verify the structural integrity of mechanical systems. Generally, these procedures are performed after fabrication but before commissioning of such equipment, without any assessment of the impact of the preload on fatigue behaviour. These approaches are performed according to standards such as ASTM or DNV, which define applicable preload conditions, but neither technical justification nor quantification of the impacts is usually given. The aim of this study is to evaluate the effect of a preload on structural steel S355 or high-strength steel S700 cruciform joints for various levels of plastic strain. Fatigue tests were performed under tension (R = 0.1) without a preload as a reference and after two different levels of preload were applied: one corresponding to a local stress close to the weld toe equal to 1.25 times the yield stress and the other corresponding to a local stress equal to 1.56 times the yield stress. The fatigue results show an increase in the fatigue strength, depending on the configuration (load-bearing weld beads or non-load-bearing weld beads), of between two and seven fatigue (FAT) classes for the highest level of preload.