Crack growth resistance curves of API X65 steel in sour environment through normalization technique.

Based on the material mechanical behavior, the experimental fracture toughness assessment of modern API steels needs to be done through elastic-plastic fracture mechanics (EPFM) methodologies. Nowadays there is an increasing necessity to obtain J-R and/or CTOD-R curves from API steels exposed to sour environments. Laboratory tests are often performed with fracture specimens immersed in aggressive environments, where traditional instrumentation for crack propagation measurements by elastic compliance becomes complicated or even impossible to be implemented. An alternative solution is to evaluate crack growth resistance curves through normalization techniques, where no special instrumentation is required. These techniques, based on the unique relation between crack length, load and load-line displacement, requires a very accurate data acquisition during the tests, as well as numerical solutions for the theoretical equations. On the other hand, at development stages the Direct Current Potential Drop technique (DCPD), which requires some degree of instrumentation, can be also applied to the test specimen in order to compare the results. Fracture mechanics tests were performed on pre-cracked SE(B) specimen of API X65 steel immersed in a simulated seawater solution, pH 5.4 ± 0.1, ppH2S 250 mbar/CO2 balance at standard conditions. J-R curves were experimentally evaluated through Normalization Data Reduction technique and DCPD method. The experimental results indicates that J-R curves from both methodologies are similar, validating the experimental setup and the mathematical solution of the normalization equations.