Dislocation Mechanics of Extremely High Rate Deformations in Iron and Tantalum

Abstract

High strain rate simulations were performed using the multiscale dislocation dynamic plasticity (MDDP) method to calculate different rise times and load durations in mimicking high deformation rate shock or isentropic (ramp) testing of a-iron and tantalum crystals. Focus for both types of loading on both materials was on the inter-relationship between the (dislocation-velocity-related) strain rate sensitivity and the (time-dependent) evolution of dislocation density. The computations are compared with model thermal activation strain rate analysis (TASRA), phonon drag and dislocation generation predictions. The overall comparison of simulated tests and previous experimental measurements shows that the imposition of a rise time even as small as 0.2 ns preceding plastic relaxation via the MDDP method is indicative of relatively weak shock behavior.

High-Rate Crystal/Polycrystal Dislocation Dynamics

“…Additionally, the flow stresses are substantially lower than those interpreted as due to phonon drag in other gas-gun-induced shock measurements [14,18]. Temperature-dependent increase in shock-induced flow stresses obtained for annealed copper plate impact tests [20], as compared with Swegle and Grady plate impact measurements [5] and, at lower stresses, SHPB measurements of Follansbee et al [1] fitted with a Z-A type fcc crystal constitutive relationship [6].…”

“…It might be noted that the value of m = m T k B T/v*σ, as related to Equation (3) and Figure 2, is a relatively complicated parameter. Computed strain rate sensitivity, m = [∆lnσ/∆ln(dε/dt)] T , measurements and calculations for copper and tantalum materials covering a range of strain rates from SHPB to shock deformations [1,5,7,14,17,20].…”

“…In Figure 4, the open-triangle, Follansbee et al [1], and filled circle, DDP, calculations of Agnihroti and Van der Giessen [17] are compared with crossed Z-A TASRA [7] and open-triangle, Z-A shock [14], as well as with Swegle and Grady [5] copper measurements and added open-square, MDDP calculations made for tantalum crystals [20]. The values of m were computed in accordance with the Equation (2) Z-A shock relationship as…”

“…Figure5. Temperature-dependent increase in shock-induced flow stresses obtained for annealed copper plate impact tests[20], as compared with Swegle and Grady plate impact measurements[5] and, at lower stresses, SHPB measurements of Follansbee et al[1] fitted with a Z-A type fcc crystal constitutive relationship[6].…”

“…Figure 4. Computed strain rate sensitivity, m = [∆lnσ/∆ln(dε/dt)] T , measurements and calculations for copper and tantalum materials covering a range of strain rates from SHPB to shock deformations[1,5,7,14,17,20].…”