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Safety against Fracture of Block Joints Welded with Large Gaps in Shipbuilding
Abstract
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.
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 project was performed to investigate if butt joints welded with larger gaps fulfil the requirements. The objective was the proof of a sufficient strength of welds performed with gaps of up to 30 mm. For this purpose, butt joints at 250 mm wide and 15 mm thick plates made of mild and higher-tensile steel, welded with different gaps under restraint conditions, were investigated with respect to their fatigue strength. Apart from the width of the gap, further parameters were varied, among others the welding technique (string-bead, weaving technique). The fatigue tests indicated a relatively small influence of the gap width, but partly a significant effect of the mean stress. The evaluation of the life cycles obtained from the fatigue tests was done with the nominal stress concept as well as with local approaches. Both procedures showed different results. According to the nominal stress concept the investigated variants of butt-joints fulfil the requirements. With regard to the local approaches a small decrease of the fatigue strength can be stated for the weaving-technique, but the results agree with the existing fatigue class. Welding by the string-bead technique however leads to a relatively low fatigue class for the local approaches. Effects of the material and the local weld geometry as well as unfavourable residual stresses resulted in cracks at unexpected locations for that welding technique, for which the local approaches are non-conservative. Only the crack propagation approach yields reasonable results. Generally the fatigue investigations indicated a clear influence of welding technique on the fatigue strength of a weld.
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