CONSTANT CTOA DETERMINATION FOR STABLE DUCTILE CRACK GROWTH AND ITS APPLICATION TO RUNNING FRACTURE CONTROL FOR GAS TRANSMISSION PIPELINE

The crack tip opening angle (CTOA) has been used as a reliable fracture toughness to characterize stable crack growth for thin-wall structures in low-constraint conditions. Recently, it has been found that CTOA can be also utilized as a robust fracture parameter to describe arrest fracture toughness for gas transmission pipelines in modern ductile steels. This is a great improvement of the traditional fracture control technology for gas pipelines, where a Charpy-vee notch (CVN) impact energy based two curve model developed at Battelle (BTCM) was used to determine the arrest toughness. While the CVN-based BCTM is not applicable to modern pipeline steels with grades X70 and above, the CTOA-based BTCM works well for these high grades, but requires constant CTOA. This work develops four methods to determine constant CTOA using the single edge notched bend (SENB) specimens, including a load-displacement linear fit method, a logarithmic load-displacement linear fit method, a stable tearing energy method, and a J-differentiation method. The test results for A285 carbon steel show that these CTOA methods can determine nearly identical critical CTOA values over stable ductile crack growth using the SENB specimens.
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