Atomistic simulation of the shock wave in copper single crystals with pre-existing dislocation network

IA Bryukhanov, INTERNATIONAL JOURNAL OF PLASTICITY, 151, 103171 (2022).

DOI: 10.1016/j.ijplas.2021.103171

We present molecular dynamics simulations of shock compression and spall fracture in 111 copper single crystals with pre-existing dislocations. Shock waves are simulated for an impact velocity of 50-900 m/s, which is below the Hugoniot elastic limit for pure 111 crystals. A dislocation network is created by carving out interstitial dislocation loops, followed by their interaction and relaxation to the dislocation network. Samples with different dislocation densities are considered. We find that, behind the shock wave, shear stress is relaxed by multiplication of moving dislocations, which increases with piston velocity. Positively oriented dislocation segments move in the direction of the shock wave, while negatively oriented segments move in the opposite direction. Dislocation segments of different orientation interact with and intersect each other behind the shock wave. The dislocation segments moving at supersonic speeds are observed already at the impact velocity of 300 m/s. When the shock pulse ends, the subsequent rarefaction wave creates tensile shear stresses which promote the negatively oriented dislocation segments to move along it. Where incident and reflected rarefaction waves contact, the tensile longitudinal stresses cause dislocation multiplication, which results in an increased dislocation density. The time series of dislocation density and stress along the samples are computed during the shock waves propagation and release. An increase in dislocation density behind the shock wave is found to vary quadratically with shock stress, and does not depend on the initial dislocation density. In the region of tensile stresses, an increase in dislocation density is also approximated by a power function of the maximal tensile stresses. The elastic precursor decay is analyzed for different impact velocities and initial dislocation densities. The decay power increases with the impact velocity and the initial dislocation density in the sample. The plastic strain rate is estimated from the decay of elastic precursor, compared to experimental values, and discussed in terms of dislocation dynamics. Spall fracture is observed in samples of various dislocation density at impact velocities of at least 500 m/s, corresponding to tensile stresses of about 12 GPa, in agreement with the previously published simulation data. The number of voids, their volumes and distribution are studied using the alpha-shape algorithm for free surface extraction. We show that pre-existing dislocations almost do not affect the spall strength, but significantly slow down the spall process compared to defect-free crystals, resulting in a more ductile fracture.

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