Abstract
We performed a series of systematic spall experiments on single-crystal copper in an effort to determine and isolate the effects of crystal orientation, peak stress, and unloading strain rate on the tensile spall strength. Strain rates ranging from 0.62 to 2.2 × 106 s−1 and peak shock stresses in the 5–14 GPa range, with one additional experiment near 50 GPa, were explored as part of this work. Gun-driven impactors, called flyer plates, generated flat top shocks followed by spall. This work highlights the effect of crystal anisotropy on the spall strength by showing that the spall strength decreases in the following order: [100], [110], and [111]. Over the range of stresses and strain rates explored, the spall strength of [100] copper depends strongly on both the strain rate and shock stress. Except at the very highest shock stress, the results for the [100] orientation show linear relationships between the spall strength and both the applied compressive stress and the strain rate. In addition, hydrodynamic computer code simulations of the spall experiments were performed to calculate the relationship between the strain rate near the spall plane in the target and the rate of free surface velocity release during the pullback. As expected, strain rates at the spall plane are much higher than the strain rates estimated from the free surface velocity release rate. We have begun soft recovery experiments and molecular dynamics calculations to understand the unusual recompression observed in the spall signature for [100] crystals.
Highlights
A material that is compressively shocked and subjected to tensile strain can incur damage, including spallation and fragmentation
We performed a series of systematic spall experiments on single-crystal copper in an effort to determine and isolate the effects of crystal orientation, peak stress, and unloading strain rate on the tensile spall strength
Hydrodynamic computer code simulations of the spall experiments were performed to calculate the relationship between the strain rate near the spall plane in the target and the rate of free surface velocity release during the pullback
Summary
A material that is compressively shocked and subjected to tensile strain can incur damage, including spallation and fragmentation. Kanel observed high spall strength with the [111] crystal orientation and reported values significantly higher than Minich, perhaps because Kanel’s impactors were significantly thinner and resultant release rates were high. In those studies, too, the compressive shock strength and tensile strain rate were varied simultaneously, and their individual effects were not isolated. The Minich results show an increase in spall strength in single crystals relative to polycrystalline materials, with the highest spall strength observed for the [100] orientation This can be attributed to the lack of grain boundaries in single-crystal samples, which can serve as damage nucleation sites. We present results of molecular dynamics (MD) calculations and propose a model based on crystal orientation plasticity to explain these results
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