Fracture toughness tests were performed in the transition region on ASTM A508 Cl.3 and A470 Cl.6 steels using about 300 specimens of 0.5T, 1T, 2T and 4T compact tension (CT) specimens. The K_J values which are converted from J_c of the smaller specimens indicated a wide scatter ranging from below the K_Ic value to much higher toughness. The fracture behavior was investigated by a scanning electron microscope in detail and the sources of scatter of the toughness in small specimens were studied. The fast brittle fracture behavior in the transition region can be deviled into two regions: (1) the region where fracture occurs on a blunting line (Region I) and (2) the region where fracture occurs on an R-curve (Region II). The scatter of the K_J values is caused by the amount of crack extension contained in the specimens.
The methods to obtain the fracture toughness equivalent to the K_Ic per ASTM Method from the K_J value were presented. In Region II a modified R-curve method is proposed. In Region I the lowest bound of K_J values obtained using specimens of which number meets the requirement of NB≥3000J_c/σ_Y is recommendable, and this method may also be used in Region II.