Constant strain rate nanoindentation testing up to 10⁴ s⁻¹: Experimental considerations, calibrations and data analysis for reliable hardness measurements

Abstract

High strain rate testing at micrometer length scale is crucial for understanding the fundamental deformation mechanisms and identifying potential changes associated with varying strain rates. However, the technique is not well established due to instrumental limitations and the lack of standardized experimental protocols. This hinders the investigation of deformation mechanisms of materials at high strain rates and limits its incorporation into computational models. This work focuses on the experimental considerations in designing constant strain rate (CSR) nanoindentation measurements up to 104 s−1 strain rate. An extensive study was conducted on three materials – fused silica (FS), nanocrystalline nickel (nc Ni), single crystal nickel (sx Ni) – to validate the experimental approach, explore potential artefacts, study strain rate sensitivity of hardness, and understand the underlying deformation mechanisms at high strain rates. For all materials, the hardness increased with strain rate and the strain rate sensitivity remained constant. For sx Ni, the formation of sub-grain boundaries was found to be responsible for the observed increased strain rate sensitivity exponent. However, the results indicate no change in the deformation mechanisms across seven orders of strain rates from 10−2 s−1 to 104 s−1 in the tested materials.