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Research on the fracture toughness test methods for high grade pipeline steel

In order to overcome the insufficiency in the existing standard methods of fracture toughness, which are not generally suitable for the steel pipeline, this paper tries to seek a palatable solution to the problem. The relevant tests were carried out by using four kinds of casing pipes (APIX70, APIX80, APIX100 and 4145 drill collar steel), and the parameters of plane-strain fracture toughness KIC, J-integral and CTOD values were measured by three-point-bending tests. Through the observation of the fracture morphology of the specimen, the paper had analyzed the experimental results in the microscope scale. The results show that the CTOD values drop successively from X70 steel, X80 steel to X100 steel. As the thickness is limited and the plane strain condition for measuring KIC couldn't be meeted, KIC fracture toughness test for X70 steel, X80 steel and X100 steel cannot obtain the effective experimental result. The J integral test result indicate that only the X100 steel can obtain the effective J value. Therefore, comparing with KIC or J integral, CTOD is a more reasonable and easier way to discribe the fracture toughness of high grade pipeline steel, although CTOD theory is imperfect.