LOAD RATE INFLUENCE ON THE FRACTURE MORPHOLOGY OF A DUCTILE STEEL

Precracked three-point bend specimens of a ductile ferritic-pearlitic microalloyed steel (E355 ISO 4950/2) were impact tested in the as-rece ived condition. The tests were performed at 15, 29 and 43 m/sec impact velocities. The fracture surfaces were investigated visually and by o ptical microscopy and scanning electron microscopy. The results from t hese techniques are brought together to obtain a detailed description of the influence of impact velocities on the fracture modes in this ma terial. All impact velocities result in three different fracture modes : intergranular dimple rupture, transgranular cleavage and transgranul ar shear-rupture dimple. The intergranular dimple rupture and the tran sgranular cleavage are caused mainly by splitting, while the transgran ular shear-rupture is believed to result from a mixed state of stress: shear and tension. The transgranular cleavage mode may be described a s quasi-cleavage fracture to emphasize the presence of the observed te ar ridges within the cleavage fracture areas. The combination of these three fracture mechanisms changes with the impact velocity and, in pa rticular, the brittle mechanism increases with increasing impact veloc ity. The initial propagation of the main crack is triggered by the sam e mechanism for all the studied impact velocities (intergranular dimpl e rupture), but this zone decreases with increasing impact velocity.