EFFECT OF PLANAR SIZE AND DYNAMIC LOADING RATE ON INITIATION AND PROPAGATION TOUGHNESS OF A MODERATE-TOUGHNESS STEEL

The effects of specimen planar size and dynamic loading rate on the dynamic initiation and propagation toughness properties of a high-strength, rate-insensitive 4340 steel at room temperature were investigated. Simultaneous dynamic initiation and propagation testing of prefatigue-cracked, compact specimens was accomplished with a coupled pressure bars technique which enabled the specimen planar size to be varied from 38 to 89 mm. Dynamic crack initiation was characterized by stress intensity loading rates varying from 1 x 10(6) to 3 x 10(6) MPa root m sec(-1) while dynamic crack propagation was characterized by load-line displacement rates varying from 13 to 27 msec-(1). These experiments were simulated with a viscoplastic dynamic finite element code and toughness data were extracted from the dynamically calculated J'-integral. Dynamic initiation and propagation toughnesses were found to be independent of planar size down to a size of 38 mm. Dynamic initiation toughness was found to increase with increasing dynamic loading rate. These results are related to a decrease in transition temperature with increasing loading rate of rate-insensitive 4340 steels. Dynamic propagation toughness was found to increase with increasing crack velocity. For a constant local strain rate, a higher toughness was measured with a propagating crack when compared to a stationary crack. The higher propagation toughness was interpreted based on increased adiabatic heating at the tip of the propagating crack.