V.M. Chavan

Integrated experimental and computational studies of deformation of single crystal copper at high strain rates

Quasi-static (0.0033u2009s−1) and dynamic (103u2009s−1) compression experiments were performed on single crystal copper along ⟨100⟩ and ⟨110⟩ directions and best-fit parameters for the Johnson-Cook (JC) material model, which is an important input to hydrodynamic simulations for shock induced fracture, have been obtained. The deformation of single crystal copper along the ⟨110⟩ direction showed high yield strength, more strain hardening, and less strain rate sensitivity as compared to the ⟨100⟩ direction. Although the JC model at the macro-scale is easy to apply and describes a general response of material deformation, it lacks physical mechanisms that describe the influence of texture and initial orientation on the material response. Hence, a crystal plasticity model based on the theory of thermally activated motion of dislocations was used at the meso-scale, in which the evolution equations permit one to study and quantify the influence of initial orientation on the material response. Hardening parameters of the...

Temperature sensitivity of void nucleation and growth parameters for single crystal copper: a molecular dynamics study

The effect of temperature on the void nucleation and growth is studied using the molecular dynamics (MD) code LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator). Single crystal copper is triaxially expanded at 5 × 109u2009s−1 strain rate keeping the temperature constant. It is shown that the nucleation and growth of voids at these atomistic scales follows a macroscopic nucleation and growth (NAG) model. As the temperature increases there is a steady decrease in the nucleation and growth thresholds. As the melting point of copper is approached, a double-dip in the pressure–time profile is observed. Analysis of this double-dip shows that the first minimum corresponds to the disappearance of the long-range order due to the creation of stacking faults and the system no longer has a FCC structure. There is no nucleation of voids at this juncture. The second minimum corresponds to the nucleation and incipient growth of voids. We present the sensitivity of NAG parameters to temperature and the analysis of double-dip in the pressure–time profile for single crystal copper at 1250u2009K.

Effect of material damage on the spallation threshold of single crystal copper: a molecular dynamics study

High velocity impact of copper plates using molecular dynamics has been performed to study the spallation of single crystal copper at impact velocities of 1100 and 1000 m s−1. The molecular dynamics code LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator) with the embedded atom method potential is used for this study. It is found that for an impact velocity of 1100 m s−1, nucleation and growth of multiple voids take place which lead to the spallation of the material. For the impact at 1000 m s−1 in the 1 0 0 impact direction, the material does not undergo spallation but gives a spall-like signal in the free surface velocity of the target. We show that the tension developed by first traversal of the shock wave creates various kinds of defects in the target. These become void nucleation sites during the subsequent traversal of the shock wave. The presence of void nucleation sites due to the first traversal of the shock leads to the nucleation of the voids at a lower tensile pressure. We also show that the spall-like signal in the free surface velocity of the target at 1000 m s−1 impact along the 1 0 0 direction occurs due to stress relaxation resulting from the nucleation and growth of the voids without physical separation of scab from the target.

Edge cracks in nickel and aluminium single crystals: A molecular dynamics study

A molecular dynamics study of edge cracks in Ni and Al single crystals under mode-I loading conditions is presented. Simulations are performed using embedded-atom method potentials for Ni and Al at a temperature of 0.5K. The results reveal that Ni and Al show different fracture mechanisms. Overall failure behavior of Ni is brittle, while fracture in Al proceeds through void nucleation and coalescence with a zig-zag pattern of crack growth. The qualitative nature of results is discussed in the context of vacancy-formation energies and surface energies of the two FCC metals.

Numerical modeling of copper single crystal specimens subjected to mechanical loading at elevated strain rates

This paper describes the simulation methodology adopted to model the mechanical response of Copper single crystals subjected to dynamic loading in different crystallographic orientations. Crystal Plasticity Finite Element Simulations are performed to explain the observed plastic anisotropy in terms of stress-strain response and deformed shapes in [100] and [110] directions. The crystal plasticity model used is based on the thermally activated theory for plastic flow and an evolution equation for slip system deformation resistances incorporating self and latent hardening. The hardening parameters of the model are obtained by calibrating against the previous experiments. The computed stress-strain responses and the deformed shapes agree well with the experimental data.

Excitation of characteristic modes of a crystal during solid fracture at high tensile pressure

We have performed uniform triaxial deformation of single crystal copper at high strain rate using molecular dynamics code LAMMPS. The best-fit void nucleation and growth parameters are obtained using a macroscopic nucleation and growth (NAG) model. The detailed analysis of the data shows that voids nucleate at specific locations in the domain, and subsequently grow and coalesce. We explain the spatial location of first void nucleation in terms of the excitation and interaction of characteristic modes in the crystal domain using singular value decomposition analysis.

Fracture during high-velocity impact of copper plates: a molecular dynamics study

High velocity impact of copper plates has been studied using the molecular dynamics code LAMMPS. The impact of copper plates at 1100 m/s impact velocity shows that spall of the material takes place at 160 kbar tensile pressure. Void nucleation is not observed at lower impact velocities where the peak tensile pressure stays below 160 kbar. No significant void nucleation occurs in the neighbouring regions where the peak tensile pressure stays below 160 kbar. For impact velocities close to the threshold, we observe stochastic behaviour of the spall process with respect to small changes in the initial atomic coordinates.