Effects of Notch Type and Loading Rate on CTOA of Modern X65 and X70 Pipe Steels

Abstract

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.

Characterization of stable ductile crack propagation by CTOA: Review of theory and applications

“…41 It has been found that loading rate has negligible effect on CTOA B/2 from quasi-static to impact (tup velocity of 5.1 m/s), equivalent to crack propagation rates from quasi-static to approximately 15 m/s. 41 In the S-SSM, the only adjustable parameter is the plastic rotation factor r p . Its value may be determined experimentally or numerically.…”

“…The starter notch may be either machined or pressed; the latter is commonly used in to the pipeline industry. The notch type as well as sample flattening are found to have negligible effect on measured CTOA B/2 . It has been found that loading rate has negligible effect on CTOA B/2 from quasi‐static to impact (tup velocity of 5.1 m/s), equivalent to crack propagation rates from quasi‐static to approximately 15 m/s .…”

“…The notch type as well as sample flattening are found to have negligible effect on measured CTOA B/2 . It has been found that loading rate has negligible effect on CTOA B/2 from quasi‐static to impact (tup velocity of 5.1 m/s), equivalent to crack propagation rates from quasi‐static to approximately 15 m/s . In the S‐SSM, the only adjustable parameter is the plastic rotation factor r p .…”

Measuring J‐R curve under dynamic loading conditions using digital image correlation

“…The authors quantify the difference in the maximum principal stress among specimens containing V-shaped chevron notch and straight pressed notch. On the other hand, Xu et al 18 demonstrate insignificant effects caused by the notch shape on the ductile fracture resistance during the stable dynamic tearing process. Shim et al 19 fabricate back-slot on the DWTT specimen to generate faster fracture speed than the standard ones.…”

Simulations of Ductile Fracture in Pipeline Steels Under Quasi-Static and Dynamic Loading Conditions

“…The results did not display any significant difference between the CTOA for the two loading rates for either material. The effect of loading rate was further investigated in the work of (Xu, Sollen, Liang, Zavadil, & Tyson, 2014) with C7-X65 and C8-X70 steels. The results did not present any significant difference between the CTOA of the two loading rates for both steels.…”