Fracture toughness tests were performed in the transition region for ASTM A508 Class 3 and A470 nickel-chromium-molybdenum-vanadium 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 with 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 regime can be divided 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 in each region is caused by the amount of crack extension contained in the specimens.

Methods to obtain the fracture toughness equivalent to the K_Ic from the K_J-values are presented. In Region I the lowest bound of K_Jmin-values obtained using specimens that meet the requirement NB ⩾ 3000 Jc_min/σ_Y is recommended, and this method may also be used in Region II. In Region II a modified R-curve method is proposed.