Improved Fracture Toughness Test Method for Single Edge Notched Tension Specimens in Clamped-End Conditions

Abstract

Abstract The oil and gas industry has favored to use fracture toughness measured using single edge-notched tension (SENT) specimen in the clamped-end conditions for the strain-based design or fitness for service analysis of pipelines. Different SENT fracture test methods have been developed, including DNV, CANMET and ExxonMobil procedures in terms of the J-integral or crack tip opening displacement (CTOD). These efforts led to the first SENT test standard BS 8571 published. However, these SENT methods may determine inconsistent test results on fracture toughness for the same specimen because no consensus is reached for any of the methods, and different estimate equations are used for an experimental evaluation. This paper aims to develop an improved SENT fracture toughness test method after a comparative study on the primary SENT fracture test methods. Comparisons are performed on available solutions of the stress intensity factor K, the geometrical eta factor, the elastic compliance inverse equation, and the m factor for converting the J-integral to CTOD that all were obtained for the clamped SENT specimens. Through comparisons, this work identified a set of more accurate solutions for the K factor, the eta factor, the compliance equation, the m factor, and those used for weld testing. The difference between the J-converted CTOD and double clip gage measured CTOD is also discussed. On these bases, an improved SENT test method is proposed for determining more reliable fracture toughness or resistance curves for ductile pipeline steels.