Piecewise Polynomial Stress Distribution for DHC Crack Initiation Evaluation in Candu Pressure Tubes

Abstract The CSA-N285.8 standard [1] describes an Engineering Process Zone method for evaluating volumetric flaws for Delayed Hydride Cracking (DHC) initiation in Zr-2.5Nb Pressure Tubes. Clause A.6 of the CSA-N285.8 standard outlines the procedures and criteria for assessing volumetric flaws in these tubes. This section includes stress calculation and hydride precipitation evaluation at the flaw-tip. It also details multiple approaches for evaluating DHC initiation, including the threshold peak stress procedure, the explicit process-zone procedure, and the process-zone procedure with weight functions. For DHC initiation analysis using the explicit process-zone procedure, a crack is postulated ahead of the flaw tip and the flaw tip stress distribution needs to be approximated to determine the stress intensity factor of the crack. A cubic polynomial stress distribution specified in the standard allows closed-form expressions in calculating coefficients for the stress intensity factor. However, the cubic polynomial function has limitations in how accurately the stress distribution can be described, particularly in cases involving non-linear material behaviour ahead of a sharp flaw. To improve the representation of the stress distribution, a novel piecewise polynomial approach is proposed. This approach provides a more flexible and accurate representation of stress distribution. Thus, well-fitted functions can be used in calculating the process zone length and process-zone displacement when assessing DHC initiation in Pressure Tube Fitness-for-Service assessments.