Efficient J-Based Failure Assessment Diagrams for Engineering Critical Assessments of Circumferentially Cracked Pipes Subjected to Axial Force, Pressure, and Bending

Abstract

Engineering critical assessments (ECAs) of cracked pipes increasingly involve situations of high strains (e.g., reeling and ratcheting fatigue), multiple loads (combined bending, axial forces, and internal pressure), and multi-axial stressing (due to pressure). In this paper, some of the implications of these loading conditions on ECAs are investigated by generating BS 7910 Level 3C Failure Assessment Diagrams (FADs) from the results of a large matrix of finite element analysis (FEA) J computations for circumferentially cracked pipes. The Level 3C (J-based) FADs (which provide the most accurate FAD approach to ECAs) are compared with the corresponding and more widely employed (but less accurate) Level 2B (material dependent) FADs in order to assess the accuracy of the latter. Use of FEA J solutions in a Level 3C FAD ensures that the effects of material behavior, load type, crack type, crack geometry, and pipe geometry are accurately captured whereas a Level 2B FAD only attempts to accurately capture the effects of material stress-strain behavior. It is demonstrated that under some circumstances a Level 2B assessment will result in non-conservative results compared to the corresponding Level 3C assessment. The current comparison between Levels 3C and 2B addresses the mechanics involved in these approaches and does not take into account the possible differing treatments of material property uncertainties on ECAs within the two approaches. Based on the current results, an efficient J formulation is described that facilitates the practical implementation of a J-based ECA. The novel approach used is based on determining material dependent shift factors that transform Level 3C FADs derived from the fully plastic components of J solutions into Level 3C FADs that represent J behaviors in the linear elastic and fully plastic regimes, and the transition region in-between. This new J formulation treats combined axial forces, pressure, and bending when applied proportionally or non-proportionally and forms the basis of the monotonic and cyclic crack tip driving forces employed in the program FlawPRO. This program performs comprehensive conventional and high strain J-based ECAs that involve reeling, arbitrary strain cycling, ratcheting fatigue, and ductile tearing that are equivalent to a Level 3C FAD approach.