Failure Mode Transition

Whereas in the Charpy V-notch test, ductile failure occurs at low loading rates or high temperatures and brittle failure at high loading rates or low temperatures, in a prenotched plate impact loaded on the notched side, brittle failure ensues at low impact speeds and ductile failure in the form of an adiabatic shear band occurs at high impact speeds. The latter transition in the failure mode was experimentally observed by Kalthoff and Winkler (1987) and has been numerically scrutinized by several investigators. Recently Batra and Lear (2003) have simulated the initiation and propagation of a crack from a point on the upper surface of a notch-tip by using the node splitting technique. The Johnson-Cook viscoplastic relation is used to account for the strain hardening, strain-rate hardening and thermal softening of the material. A schematic sketch of the problem is shown in Fig. 1a and the path followed by the crack due to brittle failure is depicted in Fig. 1b. For the steel plate, the crack propagation speed was found to be 1.6 km/s or 28% of the longitudinal wave speed for this material.

Figure 1a:  Sketch of a prenotched plate dynamically loaded on the notched side

Figure 1b.  Deformed shape of the plate after a crack due to brittle failure has propagated through it.