Nanoindentation at High Strain Rates: Challenges and recent advances

Nanoindentation is typically performed at slow strain rates < ~0.1/s, which precludes it from ballistic applications. Recent years have seen the development of nano-impact testing, which produces much higher deformation rates. However, data from such experiments are challenging to interpret, because the high strain rates are not sustained throughout the experiment and the classical definition of hardness is not applicable.

Constant strain rate nanoindentation yields more meaningful data, albeit at the expense of the deformation velocity. Here, we show that the current strain rate limitation to ~0.1/s primarily derives from the so-called plasticity error related to the continuous stiffness measurements (CSM) and we explore ways to push this limit. In a first step, we demonstrate that increasing the CSM harmonic frequency of the system is feasible and sufficient to perform valid measurements one order of magnitude faster than usual.

In order to access high deformation rates, we take the more drastic step of rewriting the standard Oliver-Pharr evaluation method, so as to avoid the need for a measurement of the contact stiffness, e.g. by CSM. With this improvement, the experimental upper strain rate limit is mostly determined by the time constants of the hardware components and lies around 100 /s with most current commercial systems.

References:

[1] B. Merle, V. Maier-Kiener, G.M. Pharr. Influence of modulus-to-hardness ratio and harmonic parameters on continuous stiffness measurement during nanoindentation. (2017) Acta Materialia, 134, pp. 167-176.

[2] B. Merle, W.H. Higgins, G.M. Pharr. Critical Issues in Conducting Constant Strain Rate Nanoindentation Tests at Higher Strain Rates. (2019) Journal of Materials Research, 34(20), pp. 3495-3503

[3] B. Merle, W.H. Higgins, G.M. Pharr. Extending the Range of Constant Strain Rate Nanoindentation Testing. (2020) Journal of Materials Research, 35(4), pp. 343-352.