Crack Opening Displacements in Pipes Containing a Part-Through Circumferential Flaw

Abstract

Three-dimensional, nonlinear finite element analyses were performed for three sizes of circumferential surface cracks in a 36 inch diameter (X70) pipe subjected to ben-ding. Two crack sizes (a/t=O.S, L/t=4 and a/t=0.25, L/D=0.4) represent the maximum length deep and shallow flaws permitted in API-1104: Appendix A. A third, very shallow flaw (a/t=0.125, L/D=0.4) was also analyzed. CTOD values were obtained from the finite element analyses using the 90° intercept method. Applied CTOD values were related to the wall stress, wall strain, bending moment, and extent of plastic deformation. Finite element CTOD estimates were compared to line-spring and stripyield values and to values implied by the use of Fig. AS in Appendix A. The Welding Institute of Canada conducted a full-scale pipe test which provided experimental validation of the finite element analysis. The analyses show that the maximum length deep flaw and maximum length shallow flaw produce nearly equal CTOD values under loading. The very shallow flaw (a/t=O.l25) produced very low CTOD values (<0. 005 in.) at wall strains of 2*cy. The Net Ligament Yield condition is reached at CTOD values < 0. 005 in. in each case. The finite element and experimental results confirm the fundamental approach for flaw assessment in Appendix A for both long and ~ow flaws. The application of Fig. A5 with critical CTOD values obtained from deeply notched bend bars is conservative. The degree of conservatism varies with crack size.