Constraint Analysis and Failure Prediction of Line Pipes

The crack tip stress fields in X80 pipeline steels were analyzed with the J-A₂ constraint theory in fracture mechanics. The relationship between the constrain parameter, A₂, and the ratio of crack length to specimen width (a/W) or pipe wall thickness (a/t) was obtained, respectively, for three-point bend, single edge-notched tension, and a part-through wall axial crack in a line pipe subject to internal pressure. For engineering purposes, the equivalent values of a/W (or a/t) between these geometries at the same constraint level can be calculated by a conversion formula based on curve fitting. In addition, an empirical factor, which is a function of a/t and is defined as the ratio of plastic limit load to the fracture load based on J-A₂ theory, was shown to be an effective method to estimate the pipeline failure stress without complicated elastic-plastic stress analysis.