Abstract

Crack tip opening angle (CTOA) has been used for decades as a reliable fracture toughness parameter to characterize stable ductile crack growth for thin-walled aerospace structures in the low-constraint conditions. Recently, a constant critical CTOA was also utilized as a fracture parameter to describe fracture toughness required to arrest ductile crack propagation for a gas transmission pipeline. This greatly improves the dynamic fracture control technology for gas pipelines, where a Charpy-V notched (CVN) impact energy-based Battelle two-curve model (BTCM) was developed to determine arrest toughness for a gas pipeline. Practice showed that the CVN-based BTCM is not applicable to high-strength, high-toughness modern pipeline steels of Grade X70 and above. By contrast, a CTOA-based BTCM works well for modern pipeline steels, but a constant critical CTOA is required against stable ductile crack growth. This article studies four methods for determining the constant critical CTOA using single edge notched bend (SENB) specimens, including three load-displacement methods and a J-differential method. Experimental data for an A285 carbon steel show that these CTOA methods can determine constraint-independent, constant critical CTOA values over stable ductile crack growth using SENB specimens with shallow to deep cracks. The constraint independent, constant CTOA demonstrates its transferability from laboratory to a dynamic crack propagating in a gas pipeline.

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