Fractographic aspects of blunting at high loading rate

The results of an experimental study are presented to illustrate the effect of high loading rate on the initial propagation of the crack tip (blunting) of a rate-sensitive steel. Dynamic fracture tests were performed on precracked three-point bend specimens at 15, 29 and 43 m/s impact velocities. Fracture mechanisms in the initial fracture zone (blunted zone) were examined using scanning electron microscopy. All impact velocities resulted in blunting by ductile fracture mechanisms. The blunted zone consisted of a ''flat'' region containing considerable local stretching, and an ''inclined'' region containing a combination of intergranular and transgranular separation by localized stretching and microvoid coalescence. The extent of each region was increased with increased impact velocity. The propagation mechanisms in the inclined region depended on the impact velocity. Large void coalescence dominated the inclined region at low impact velocity (smooth blunting), whereas stretch and shear fracture between small voids dominated at high impact velocity (sharp blunting). Subsequent to blunting, the main fracture continued on the flat surface by microvoid coalescence or quasi-cleavage crack growth mechanisms depending on the impact velocity.