Abstract
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 KJ-values which are converted from Jc of the smaller specimens indicated a wide scatter ranging from below the KIc-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 KJ-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 KIc from the KJ-values are presented. In Region I the lowest bound of KJmin-values obtained using specimens that meet the requirement NB ⩾ 3000 Jcmin/σY is recommended, and this method may also be used in Region II. In Region II a modified R-curve method is proposed.
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