The crack-tip opening angle (CTOA) has been investigated as a fracture propagation resistance parameter for prevention of fast ductile fracture in gas pipelines. A CANMET simplified single specimen CTOA method has been proposed as a mill test and is being applied to characterize critical CTOA (CTOAc) of typical pipe steels to develop a toughness database and improve the test method.
In this work, tests using standard machined V-notch and API pressed notch (PN) drop-weight tear test (DWTT) specimens at quasi-static and impact rates were performed on modern X65 and X70 pipe steels typical of those used for CO2 pipelines. The rotation factor of the X65 steel deduced from the deformed ligament geometry is equivalent to those of X70 to X100 steels. Pressed-notch DWTT specimens were successfully fractured in impact and yielded CTOAc values equivalent to those of V-notched specimens following the CANMET recommended practice for determination of CTOAc. The effect of loading rate on CTOAc between the quasi-static and impact rates (covering five orders of magnitude) is small or negligible, being within experimental scatter. This is in agreement with data in the CANMET database, except for a low-toughness X52 steel that showed an increase in CTOAc between quasi-static and impact loading rates. The effect of flattening on CTOAc was also investigated and is small or negligible for the large-diameter (at least 508 mm) pipes tested in this work. The results may be used to support and improve a proposed test method for determination of CTOAc being considered by an ASTM Task Group and currently being evaluated in a round-robin testing program.