Failure assessment of steel pipes using non-standard ring specimens.

Abstract

In recent years, several attempts have been made by research groups worldwide to propose and verify new laboratory specimens for measurement of pipeline materials fracture resistance. The main reason for these efforts is the requirement of standard fracture mechanics procedures for relatively large specimen thickness, i.e. plane strain conditions. It is almost impossible, and also too conservative, to apply such an approach to the thin-walled pipes, which form the majority of industrial pipelines. Failure resistance of pipeline steel is examined by testing the recently proposed Pipe Ring Notched Bend (PRNB) specimens with different dimensions. The seam, i.e. longitudinal weld, is analysed as a source of material heterogeneity; however, seamless pipes are also examined. The results are repeatable, and the dependence on the specimen geometry is not significant (it is even lower than it could be expected from standard thick fracture mechanics specimens). The seam materials (weld metals) have shown different fracture behaviour for different batches/sizes of the pipes; they have either lower or similar fracture resistance in comparison with the base material. Briefly, it can be said that fracture testing of a relatively small amount of specimens revealed a lot of important data about the resistance of the pipes to ductile failure. Fracture is modelled by application of the micromechanical approach in software package Abaqus, in particular the Complete Gurson model. Additionally, the model is used for prediction of fracture resistance in plane strain conditions. This can be useful for comparison of the examined pipe materials and others with known plane strain fracture properties. Currently, the authors are working on a new geometry: pipe rings with a sharp notch exposed to tensile loading. This will form a comprehensive approach to pipeline testing, where two geometries and two loading regimes (bending and tension) will be available in the framework of a unified testing procedure.