Kic Transition-temperature behavior of A517-F Steel

Abstract

Linear-elastic fracture mechanics has been widely used to obtain K ic values on very-high-strength steels (yield strengths > 200 ksi) that generally do not exhibit a ductue-to-brittie transition in failure mode as a function of temperature. However, as the use of the K ic test approach is extended to those steels that do exhibit a ductile-to-brittle transition, information on the K ic transition-temperature behavior of steels is required. Therefore, to establish general relations between K ic and Charpy test results, slow-bend K ic fracture tests and various Charpy tests were conducted on A517-F steel at temperatures between −320 and +80°F. The results indicated that a plane-strain K ic température transition does exist for A517-F steel. Furthermore, this transition occurred in the same temperature range (−150 to −50°F) as the transition denned by slow-bend Charpy test results for fatigue-cracked specimens. In both the K ic tests and the Charpy tests, the transition-temperature behavior appeared to be related to a gradual change in the microscopic fracture mechanism. The upper shelf, as denned by slow-bend Charpy tests, appeared to be a region in which K ic values cannot be obtained, regardless of specimen geometry, because of general yielding and crack blunting. A procedure is proposed in which the dynamic K ic behavior of a material can be predicted from static K ic test data by shifting the static K ic values along the temperature axis by the same amount as the static Charpy energy values are shifted by impact testing. In general, the results of this investigation have demonstrated that a transition in K ic behavior of A517-F steel does exist as a fution of temperature and that that transition is independent of the K ic to K c stress-state transition.