Testing Techniques for Establishing Fracture Resistance of Steel in a Sour Environment

Abstract

It is well established that sour operating environments can give rise to significantly reduced fracture toughness of pipelines made of carbon manganese steel. Fracture resistance of a material is usually defined in terms of a fracture resistance curve, commonly known as an R-curve which is determined by testing pre-cracked specimens under a rising load. Fracture resistance data can be derived by the single specimen method, where crack extension is determined using unloading compliance or the multiple specimen method, where crack extension is measured from the fracture face of each specimen and each specimen is taken to a different load level. The fracture resistance behaviour of API 5L X65 grade pipeline steel determined by testing single edge notched bend specimens in a specific sour environment using both single and multiple specimen test methods is reported. The fracture resistance of the steel was found to be highly sensitive to the loading rates (described by the initial rate of increase of stress intensity factor in the elastic range) applied during the fracture resistance tests. It was possible to identify a loading rate slow enough to provide fracture initiation toughness reasonably close to the expected lower bound toughness. It is possible to produce similar R-curves from single and multiple specimen testing methods (if conditions are otherwise the same). Under comparable loading rates and environmental conditions, side grooved specimens resulted in lower fracture toughness as compared to the toughness determined from the plane sided specimens. It was also noticed that there was a weaker correlation between side grooving and toughness at slower loading rates.