Fatigue performance of welded T-joints between S690 circular hollow sections under cyclic in-plane bending actions

Abstract

This paper presents a comprehensive experimental investigation into the fatigue behaviour of welded T-joints between high strength S690 circular hollow sections under cyclic in-plane bending actions. A total of 10 fatigue tests were carried out on the S690 welded T-joints over a wide range from 1 × 10⁴–5 × 10⁶ cycles through displacement control, and five different target displacements were selected. With proper instrumentation, the deformation characteristics of these welded T-joints in terms of reductions in strength and stiffness, and surface crack propagation were captured, and careful data analyses on their strength and stiffness degradation against the numbers of cycles were obtained. It was demonstrated that, depending on the magnitudes of the target displacements, a 5 % reduction in the resistances of these welded T-joints only happened after completion of 60–80 % of the number of the cycles to fatigue failure. There are at least 10–20 % more cycles to be completed with significant crack propagation before the end of the fatigue tests. Both nominal stress and hot spot stress approaches are adopted to assess the fatigue performance of these S690 welded T-joints. All the S-N data of these welded T-joints are found to lie above the design S-N curves given in EN 1993–1–9, CIDECT-8 and API RP 2A-WSD, especially for the design fatigue life exceeding 10⁶ cycles. Moreover, the fatigue life of these S690 welded T-joints over a practical range of cyclic actions may be conservatively related with the (S - N)꜀ curve of coupons of S690 welded sections after considering presence of welding-induced residual stresses, effects of stress concentration due to geometry, and different stress ratios in fatigue tests.