SAFETY AGAINST FRACTURE OF BLOCK JOINTS WELDED WITH LARGE GAPS IN SHIPBUILDING

摘要

The welding of block joints in shipbuilding often creates much effort as the fabrication without margins leads sometimes to large gaps between them. Unfavourable may be the high stresses occurring due to constraints. For this reason a research project was performed to investigate whether butt joints welded with larger gaps fulfill the requirements. The objective was the proof of 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 of mild and higher-tensile steel, welded with different gaps, were investigated with respect to their fracture toughness and fracture strength. The welding was performed under definite constraints corresponding to the stiffness of the surrounding ship structure. Apart from the width of the gap, further parameters were varied, among others the welding method (string-bead, weaving technique). The fracture mechanics investigations in the preliminary part of the project included extensive hardness measurements and tensile tests of the parent and weld metal. Within the main part, crack resistance curves (CTOD) were determined for several variants using small-scale specimens. The safety against fracture of the butt joints was investigated experimentally by tensile tests of the butt-welded plates containing artificial defects. The verification was performed with the FITNET procedure (options 1, 2 and 3) using the obtained fracture toughness parameters from the small-scale specimens. From the fracture mechanics point of view, no objections exist regarding the large gap of 30 mm welded by weaving. The tensile tests as well as the FITNET procedure even showed a positive effect of the large gap with regard to the material properties of the weld metal. However, the results for the string-bead technique are critical with respect to the CTOD values as well as to the critical crack lengths. Generally it can be stated that the FITNET assessment shows a quite good agreement with the test results and can evaluate effects of the welding method on the safety against fracture.