Effects of Notch Acuity on Fracture Toughness Measurements

Effects of initial crack-tip acuity on toughness measurements has been evaluated through extensive small-scale toughness testing — single-edge notched tension and bend (SE(T) and SE(B)) — at room temperature using two X70 pipeline steels and girth welds. In order to investigate effects of different crack tip radii on toughness, some specimens were notched to the target initial crack size using electrical discharge machining (EDM), and no further fatigue precracking was made. On the other hand, other specimens were EDM-notched and then fatigue-precracked to the target crack size according to the current practice of ASTM E1820. The results show that effects of crack tip acuity on toughness measurement are significant especially at the crack-tip blunting stage. The EDM precrack increased toughness measurements by a factor of up to 1.6 and 2 for SE(B) and SE(T), respectively, at 0.2 mm of crack extension, compared to the fatigue precrack.