The Significance of Local Brittle Zones to the Integrity of Large Welded Structures

Abstract

Abstract An extensive group-sponsored research program has been carried out within the UK, which, supplemented by additional work by British Steel, was designed to study the structural significance of local brittle zones. Charpy impact, CTOD and wide plate tests have been examined in order to compare and contrast their fracture behavior under similar conditions. In order to permit this comparison, two welding conditions were studied giving nominally 30 and 10 percent of coarse-grained HAZ through the thickness of a 50-mm-thick plate. The high percentage of coarse grain condition was tested in the as-welded and PWHT state and the low percentage condition as-welded only. It was shown that low toughness (<0.1 mm CTOD) measurements could be obtained using both through thickness and surface-notched test pieces over a temperature range of −30 to +10°C. Similar minimum toughness values were obtained for all the test conditions studied, but the frequency of occurrence varied. Twenty-two wide plate tests were carried out and nine fractured, four in the as-welded condition (two each for the two welding variables studied) and five in the PWHT condition. A further plate initiated a fracture from a bevel weld procedure which arrested in the parent plate. It is postulated that the requirement for an extreme combination of stress level, defect size and coarse-grained microstructure at the crack tip plus the potential for crack arrest is the reason for the absence of fractures in offshore structures.