シャルピー衝撃試験における遷移挙動および破壊靭性の評価について

Abstract

Analysis of the transition behaviour of mild steel in the Charpy impact test was made over a wide temperature range (100°∼−196°C) by observing the relationship between load and time during impact. As a result of this study, the transition behaviour in the Charpy impact test may be divided into six temperature regions. And it appears that usual transition temperatures such as Charpy-V 15 ft-lb energy. Charpy-V 0 percent shear and Charpy-V 15 mil lateral expansion are the yield point fracture criteria. On the other hand, Charpy-V 30∼35 ft-lb energy or Charpy-V 25 percent shear is the initiation (ductility) transition temperature at which no ductile crack can propagate after che maximum bending load is attained and rapid transition to brittle fracture occurs. It seems that this temperature correlates with NRL drop weight test NDT. Another important transition temperature is the cleavage initiation temperature at which first cleavage fracture appears in the load/time curve and this temperature coincides with nearly 100 percent cleavage fracture appearance temperature in the brittle boundary specimens (i.e. carburized specimens). And it appears that this temperature is a measure of estimating the brittle crack propagation arresting temperature of a large scale test. Furthermore we made attempt to obtain fracture toughness Gc values proposed by Irwin et al. from a small scale test such as the Charpy impact test in this case. For this purpose, we used brittle boundary specimens prepared by carburizing the free surface layers of Charpy specimens and measured load/time curves during fracture of these specimens. By these procedures we obtained some agreement with temperature gradient type ESSO test results. And estimates of Gc values relative to onset and arrest of brittle crack propagation were made and discussed.