Fatigue and fracture strength of ship block joints welded with large gaps

Abstract

The welding of block joints in shipbuilding often requires much effort as the fabrication without margins sometimes leads to large gaps between them. The high constraint during the welding process can lead to unfavourable high residual stresses. For this reason, a project was undertaken to investigate whether butt joints welded with larger gaps fulfill the requirements. The objective was to determine the sufficient strength of welds performed with gaps of up to 30 mm. For this purpose, butt joints of 250 mm-wide and 15 mm-thick plates made of mild and highertensile steel, welded with different gaps, were investigated with respect to their fracture toughness and fatigue strength. Welding was performed under definite, elastic restraints corresponding to the stiffness of the surrounding ship structure. Apart from the width of the gap, further parameters were varied including the welding method (string-bead, weaving technique). From the fracture mechanics point of view, no objections exist regarding the large gap of 30 mm welded by weaving. The specimens welded with the string-bead technique achieved relatively low, partly uniform critical fracture toughness values. The fatigue tests confirm the results of FITNET. Despite a small decrease of fatigue strength, the weaving technique agrees with the existing fatigue class. The string-bead technique results in a fatigue class which is consistent with the nominal stress approach. However, the local approaches result in non-conservative fatigue assessments and therefore do not fulfill the requirements. Generally, fatigue as well as fracture investigations indicated a clear influence of welding technique on the fatigue and fracture strength of a weld.