Kic Transition-temperature behavior of A517-F Steel

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.