Significance of Fracture Toughness Test Results of Beam Welds in Evaluation of Brittle Fracture Performance of Girth Welded Pipe Joints

Abstract

High power beam welds, such as electron beam welds or laser welds, sometimes provide fracture path deviation (FPD) in standardized Charpy V-notch fracture toughness testing due to narrow bead profile together with higher overmatching in strength between weld metal and base metal. Moreover, it should be noted that these typical features of beam welds might result in a plastic constraint loss around both the notch and crack tip in fracture toughness test specimens. Even in the temperature range where FPD would not occur, the fracture toughness test results could not necessarily be an intrinsic value of such beam welds. These fracture properties make it difficult to evaluate fracture performance of girth welded pipe joints. In this paper, the estimation method of intrinsic fracture toughness of beam weld metal itself using standard toughness test specimens is proposed on the basis of “Weibull stress criterion.” The predicted intrinsic fracture toughness was found to be lower than the test results both in standard Charpy specimen and in three-point bend specimen with fatigue precrack. The assessment of brittle fracture performance of girth welded pipeline was conducted from the estimated intrinsic fracture toughness of girth welds by means of Weibull stress criterion. It was demonstrated that the low intrinsic fracture toughness of beam welds could not directly lead to the low fracture performance of a pipe joint under tensile loading. This is because of a lower plastic constraint compared to a three-point bend specimen due to difference in loading mode together with constraint loss in pipe joints and shielding effect of straining in weld metal due to highly overmatched narrow welds.