Fracture toughness determination from load-line displacement of 3-point bend specimen using 3D digital image correlation method for CLF-1 steel

Abstract Load-line displacement (LLD) is essential for determining fracture toughness in terms of J-integral for nuclear structure materials. To obtain the accurate LLD, using a three-dimensional (3D) digital image correlation (DIC) method, the different LLD corresponding to different load points were measured on the miniature 3-point bend (3 PB) specimens of Chinese low activation ferritic/martensitic (CLF-1) steel in unloading compliance testing, and the load-displacement curves and J-integral determination were analyzed. The results show a higher LLD value generally derives a higher J-integral value. The J-integral values increase and then decrease with the distance from the top of specimen along crack direction, which is similar to the variation of LLD. It is interesting that the LLD of the load point almost at the neutral axis derive the peak J0.2(B) value (447 and 471 kJ/m2 at 250°C, 270 and 331 kJ/m2 at 450°C), close to the values (441 and 470 at 250°C, 269 and 332 kJ/m2 at 450°C) from the corresponding crack mouth opening displacement (CMOD). And the corresponding J0.2(B) values from LLD of points near the notch tip are much lower. The LLD of the load point at the neutral axis was recommended the accurate LLD to determine J-integral of the 3 PB specimens.