Evaluation of Fracture Toughness of X100 Pipe Steel Using SE(B) and Clamped SE(T) Single Specimens

J-resistance testing using a single-specimen unloading compliance technique has been performed on single-edge-notched tension (SE(T)) specimens of X100 pipe steel base material at room temperature and at −20°C, using a procedure developed at CANMET. J-resistance testing using single-edge-notched bend (SE(B)) specimens according to ASTM E1820 was also conducted for comparison. The specimens included two nominal through-thickness pre-crack aspect ratios (a/W = 0.25 and 0.5). The results show that shallow-cracked (a/W∼0.25) bend and tension specimens produce higher resistance curves than deeply-cracked (a/W∼0.5) specimens; ductile propagation was observed at both temperatures. Resistance curves are slightly higher at −20°C than at room temperature for both bending and tension, especially for shallow-cracked specimens. Crack length predicted from unloading compliance of crack mouth opening displacement for the SE(T) specimens was validated by optical measurement of initial crack length (ao) and final crack extension (Δa>1.0 mm) after heat-tinting, as per ASTM E1820. Predicted crack growths show acceptable agreement with measured values in all cases. The effect of side-groove depth on the resistance curve and straightness of the crack front was briefly investigated. For both bending and tension, resistance curves for 10% (total) side-grooved specimens were close to those from plain-sided specimens when other testing conditions, such as precrack and testing temperature, were the same, whereas 20% (total) side-grooved specimens showed lower toughness. It was occasionally observed that the crack grew faster at the side for 20% side-grooved bend and tension specimens, resulting in a crack front of concave curvature. For 10% side-grooved specimens a rather straight crack front or slightly faster crack growth in the middle of the specimen (convex curvature) was observed.