Dynamic Fracture Toughness of 4340 VAR Steel under Conditions of Plane Strain

Plate impact experiments are conducted to study the dynamic fracture processes in 4340 VAR steel which occur on submicrosecond timescales. These experiments involve the plane strain loading of a planar crack by a plane tensile pulse with a duration of approximately 1 /xs. The loading is achieved by impacting a precracked, disk-shaped specimen by a thin flyer plate. Motion of the rear surface of the specimen, caused by waves diffracted from the stationary crack and by waves emitted from the running crack, is monitored at four points ahead of the crack tip using a laser interferometric system. The measured rear surface motion is compared with the calculated motion using the finite element method to gain nnderstanding of the dynamic fields that occur near the crack tip during crack initiation and propagation. For low temperature experiments, the measured rear surface particle velocity fields are in good agreement with the computed profiles obtained for a constant velocity crack propagation model. For the room temperature experiments, the experimental free surface particle velocity vs time profiles show a sharp spike, with a duration of less than 100 ns at the moment of crack initiation. The spike, which is not predicted by the inverse square root singular stress fields of linear elastic fracture mechanics, is understood to be related to the onset of crack growth. Critical values of the fracture toughness are estimated from the crack initiation times determined both from the velocity time profiles and the elastodynamic modeling of crack advance. The toughness values obtained increase with increasing impact velocity and are as large as 170 MPa'x/-m at the highest impact velocity. Such relatively high values appear to be consistent with the ductile mode of crack initiation observed at all impact velocities used in the present study.