SOME ISSUES IN LIFE ASSESSMENT OF LONGITUDINAL SEAM WELDS BASED ON CREEP TESTS WITH CROSS-WELD SPECIMENS

In order to reduce production costs, it is of great interest to use lo ngitudinal seam welds when manufacturing large diameter pipes. The cos t reduction can be as high as 30%. However, severe inservice accidents for this type of pipes working in the creep regime have occurred main ly due to mismatch match in weldment creep properties. In many cases, creep tests of cross-weld specimens, taken from the seam weld, are use d to predict the behaviour of the seam weld assuming that the creep be haviour of specimen and weldment is equivalent. Experiences of this pr ocedure indicate that further knowledge is required before translation between specimen and component can be made. In the present paper, bot h full scale seam welded pipes and cross-weld specimens are studied wi th the damage mechanics concept using finite element, FE, technique. T he same mechanical model of multiple material zones is used for the tw o components. Both the influence of differences in creep properties be tween the weldment constituents and the size effect of the cross-weld specimen, are studied. It is found that the cross-weld test results ca n not directly be translated to the full scale component. Factors such as the creep properties and the relative geometry of the weldment con stituents and the size of the cross-weld specimen have to be considere d when performing creep life assessment.