Several miniature Kolsky bars are used to obtain stress–strain curves for nanocrystalline Cu–10Ta over a range of high strain-rates. The smallest bar (steel) has a 305 μm diameter, and achieved rates up to 700 × 103/s. Different sample sizes are needed to obtain different strain-rates, and it is shown that there is no appreciable sample size effect when different sizes are tested at similar strain-rates, even though the sample sizes vary by over an order of magnitude. No significant increase in strain-rate sensitivity is noted over the strain-rate range studied, i.e., the strength increases linearly with the logarithm of strain-rate from 0.001/s to 700 × 103/s.
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One can also use a bar with a smaller Poisson’s ratio, e.g., Bateman et al. .
The exact theory assumes an infinitely long bar, but is assumed to apply to the free-end measurements made here. However there does not seem to be much error at least up to 7.5 MHz.
This is not due to any equilibrium problems in the sample, which may also be the case, i.e., a stress–strain curve may not be valid for low strains due to the need for the sample to “ring-up” to an adequate state of equilibrium.
Note the use of a low-impedance striker results in it rebounding back into the barrel—this may pose a hazard in some gun systems.
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Casem, D., Ligda, J., Walter, T. et al. Strain-Rate Sensitivity of Nanocrystalline Cu–10Ta to 700,000/s. J. dynamic behavior mater. 6, 24–33 (2020). https://doi.org/10.1007/s40870-019-00223-w
- High Rate Testing
- Kolsky Bar
- Metal Plasticity