Ultra low-cycle fatigue performance of S420 and S700 steel welded tubular X-joints

The present study is motivated by the need for improving the fatigue performance of offshore wind energy structural systems. In particular, the ultra low-cycle fatigue performance of welded tubular X-joints is examined, motivated by the need of safeguarding the integrity of offshore platforms under extreme loading conditions. The welded specimens are manufactured using hot-rolled tubes of steel grade S420 and S700, and represent X-brace joints of a bottom-fixed offshore wind tubular jacket, with scaling factor of 1:3. Seven specimens are tested under strong fully-reversed cyclic in-plane bending, leading to through-thickness fatigue cracking within less than 100 cycles, simulating extreme loading conditions. The experimental results indicate that X-joints manufactured from both steel grades exhibit similar structural response, in terms of ultra low-cycle fatigue. Rigorous finite element models are also developed, with emphasis on constitutive modeling, to simulate the cyclic loading procedure, providing very good comparisons in terms of load-displacement response and local strain predictions during the initial loading cycles. The experimental data are compared with a large dataset of low-cycle fatigue experiments on welded components, reported in the literature for mild and high-strength steel materials, as well as with existing design provisions. The results indicate similar performance of high-strength steel and mild steel welded connections, and are compared with stress-based and strain-based design methodologies in predicting the number of cycles to failure in the ultra low-cycle fatigue regime.