J-Based Failure Assessment Diagrams for Axially Cracked Pipes Under Pressure

Abstract

This paper describes the results of performing a comprehensive matrix of J-based finite element analyses (FEA) of external and internal surface axial cracks in pressurized pipes. The computations were performed for a range of stress-strain behaviors and pipe sizes typical of gas and liquid transmission pipelines. The J results for the deepest and surface points on flaws are presented in the form of failure assessment curves (FACs) plotted on failure assessment diagrams (FADs). The FACs are consistent with the Level 3C and material specific Level 2B FADs used in BS 7910. Yield pressures derived from the FEA results are used to evaluate the FAD parameter Lr (= pressure/yield pressure) for the Level 3C FACs. This choice of yield pressures facilitates the collapse of the Level 3C FACs in the fully plastic region of the FAD onto material specific curves relatively independent of geometric features. The Level 3C FACs generated from the FEA J results show a strong dependence on crack size in the elastic-plastic part of the FADs, particularly for long flaws. In these cases, the Level 3C FACs fall inside the corresponding material specific FACs derived according to Level 2B procedures. The reason for this is thought related to the development of local and global yield mechanisms and the incorporation in Level 3C FACs of geometry dependencies which are ignored in Level 2B. It is concluded that the Level 2B procedures may not always be conservative for long deep axial flaws in pressurized pipes when used in conjunction with accurate (e.g. determined from FEA) global yield pressures. Similarly, account has to be taken of the transition from local to global yielding in J estimation schemes that are formulated as the sum of elastic and plastic components if these are not to be non-conservative in the transition from elastic to fully plastic behavior. A method is suggested for incorporating the local to global yielding transition in J estimation schemes that also reduces the geometry dependence of Level 3C FADs to facilitate their representation by approximately material specific (Level 2B-type) FADs applicable to axially flawed pipelines.