CTOA Measurement of Pipe Steels Using DWTT Specimen

A major issue of applying the crack-tip opening angle (CTOA) approach to fast ductile fracture control is to characterize material toughness (CTOA_c) using laboratory-scale specimens and to recommend a mill test. In this report, existing direct and indirect CTOA measurement methods are comprehendsively reviewed. Direct surface measurement methods as standardized in ASTM E2472-06 are mainly based on experience with sheet materials, and the measurement distance from the crack tip recommended in the ASTM standard should be increased for thicknesses typical of pipe steels. Methods using data on load (P) vs. load-line displacement (LLD) from instrumented drop-weight tear test (DWTT) specimens are given special attention, including original and modified two-specimen methods, original and modified single-specimen methods (SSM) and a recent simplified single-specimen method (S-SSM). The results of DWTT/CTOA tests at CANMET-MTL on six pipe steels (five modern high-strength steels and one older X52 steel) are briefly presented. The simplified single-specimen method (S-SSM) is repeatable and may be suitable for a mill test. However, there remains a discrepancy in CTOA values between surface measurements and those from the S-SSM, and it was found that the CTOA decreased somewhat with increasing ligament size from 66 mm to 95 mm. Transferability of results from lab to field will be discussed briefly.