ASTM E1820-11 and ISO 12135-2002 are the main two testing standards of fracture toughness. There are some obvious differences between these two standards, such as blunting line, effective data and J–R curve equation, which cause discrepancy of the fracture toughness results. The bases of the blunting line of the two standards are studied and the differences of the two standards are compared. The ASTM blunting line is based on elastic-perfectly plastic model of material, and the ISO blunting line is from the...