DYNAMIC FRACTURE-TOUGHNESS OF 4340-VAR-STEEL UNDER CONDITIONS OF PLANE-STRAIN

Plate impact experiments are conducted to study the dynamic fracture p rocesses in 4340 VAR steel which occur on submicrosecond timescales. T hese experiments involve the plane strain loading of a planar crack by a plane tensile pulse with a duration of approximately 1 mu s. The lo ading 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 fro m the running crack, is monitored at four points ahead of the crack ti p using a laser interferometric system. The measured rear surface moti on is compared with the calculated motion using the finite element met hod to gain understanding of the dynamic fields that occur near the cr ack tip during crack initiation and propagation. For low temperature e xperiments, the measured rear surface particle velocity fields are in good agreement with the computed profiles obtained for a constant velo city crack propagation model. For the room temperature experiments, th e experimental free surface particle velocity vs time profiles show a sharp spike, with a duration of less than 100 ns at the moment of crac k initiation. The spike, which is not predicted by the inverse square root singular stress fields of linear elastic fracture mechanics, is u nderstood to be related to the onset of crack growth. Critical values of the fracture toughness are estimated from the crack initiation time s determined both from the velocity time profiles and the elastodynami c modeling of crack advance. The toughness values obtained increase wi th increasing impact velocity and are as large as 170 MPa root m at th e highest impact velocity. Such relatively high values appear to be co nsistent with the ductile mode of crack initiation observed at all imp act velocities used in the present study.