EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The present paper examines the fatigue performance of eleven (11) 90-degree welded X-joint scaled-down specimens, commonly used in offshore tubular steel structures. The braces and the chord are 8-inch-diameter tubes made of S355J2 steel, with a brace-to-chord-thickness ratio of 0.6. The research work is part of an extensive research program, with acronym JABACO (2015-2018), sponsored by the European Commission. Three different welding/post-welding processes for the brace-to-chord connection have been employed: (a) semi automatic GMAW; (b) fully-automatic GMAW; (c) semi automatic GMAW with High Frequency Mechanical Impact (HFMI) post-welding treatment. The main objective is to study the fatigue resistance and the localized strains near the weld-toe, mainly responsible for the development of fatigue cracks under repeated loading, for the different welding/post-welding techniques. The implementation of automation in welding procedures improves the quality and reduce the cost, whereas the HFMI weld treatment can affect favorably the fatigue life of welded connections. The X-joint specimens tested in the present study have been subjected to in-plane bending under four different constant load-amplitudes until through-thickness crack failure. Experimental observations, supported by metallography, MPI and post-fatigue fractography, have identified two possible critical locations of the weld-toe where fatigue cracking may initiate: the chord “crown” and an “in-between” location. Moreover, all specimens failed at a number of loading cycles significantly higher than the fatigue life predicted by existing design standards, indicating a conservativeness of the existing design tools. Moreover, the HFMI post-welding treatment led to significant improvement of fatigue performance.

REFERENCES
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