Fig. 2: Geometrical reconstruction of impressions and comparison of plastic work per unit impression volume across different strain rates, from nanoindentation to microprojectile impact.

Laser confocal microscopy reconstructions of four impressions produced by spherical nanoindentation at strain rates of a 1 s⁻¹, b 10 s⁻¹, c 100 s⁻¹, and d LIPIT at strain rate of 6 × 10⁸ s⁻¹ (impact velocity of 593 m/s). e Plastic work normalized by impression volume as a function of effective strain rate (\(\dot{\varepsilon }={v}_{{{\rm{i}}}}/h,\) with \({v}_{{{\rm{i}}}}\) being the impact velocity and \(h\) being the indentation depth). The indentation and impact data were compared only at comparable impression strain levels (\(\varepsilon =h/2.4a=\)0.21 ± 0.02, with \(a\) being the half-width of the impression⁵⁹) to exclude possible strain hardening effects from the data. The error bars for the nanoindentation data points represent the standard deviation from multiple repeated measurements. For the LIPIT data points, the error bars were calculated based on the resolution of the LIPIT snapshots and laser confocal microscopy.