Progress in the Development of High Strain Rate Nanoindentation Testing

Over the past three years, we have developed new hardware, software and testing methods to conduct nanoindentation testing at the very high strain rates. To date, indentation strain rates as high as 10⁴/s have been achieved during the initial stages of indentation contact with a Berkovich indenter, with the potential to go even higher. At the heart of the new testing system is a laser interferometer that measures indenter displacements with sub-nanometer resolution at data acquisition rates in excess of 1 MHz. High data rates are essential since the loading of the indenter usually lasts no longer than a few hundred microseconds. The new high strain rate system also incorporates a very high stiffness hexapod for precise sample positioning and alignment, and various hardware modifications that provide for the measurement of indentation load at speeds commensurate with the displacement measurements. Various testing methods have been explored, including impact tests in which the indenter is accelerated to velocities up to 0.5 m/s before contacting the specimen surface, and step load tests in which the indenter starts in contact with the specimen and is then step loaded to a high value in a relatively short period of time. Results demonstrating the capabilities and limitations of the system are presented and discussed based on experiments conducted in fused silica (as a model hard material) and aluminum (as a model soft metal). New methods for analyzing the nanoindentation data to extract hardness as a function of strain rate are also discussed.