Browsing > By author > Bellón Bárbara

The Fourth Industrial Revolution has ushered in rapid advancements in high-speed transportation, machining, and miniaturized electronics, exposing materials to extreme conditions, including high strain rates. While macroscale techniques like split-Hopkinson pressure bars, Kolsky bars, and gas gun experiments can reach strain rates of up to 10⁴ s⁻¹, maintaining a constant strain rate during testing remains challenging. These methods require extreme actuation speeds, often in the range of several hundred meters per second, which can introduce shock propagation and complicate post-deformation analysis. Instrumented nanoindentation offers a promising alternative, in which one could achieve very high strain rates of up to 10⁵ s⁻¹ with actuation speeds as low as ~100 mm/s over indentation depths upto ~1 µm. Additionally, nanoindentation using exponential displacement profiles allows for a constant strain rate, which is advantageous for post-deformation analysis and generating reliable load-displacement curves. Despite these benefits, current limitations in hardware and protocols restrict the highest reported constant strain rate nanoindentation to only 10² s⁻¹.

This presentation will provide an overview of a highly specialized, high-speed piezoelectric-based micromechanical setup designed to achieve constant strain rate nanoindentation up to an unprecedented strain rate of 10⁵ s⁻¹. Key aspects of the custom-modified electronic hardware and essential experimental protocols for capturing precise load-displacement data during high-strain-rate indentations and accurately extracting hardness values will be discussed. The presentation will also examine the material behavior of two different materials: single-crystalline BCC molybdenum and nanocrystalline FCC nickel. Notably, a significant hardness upturn was observed in these materials beyond strain rates of 10³ s⁻¹ and 10⁴ s⁻¹ for nanocrystalline nickel and molybdenum, respectively. This upturn, linked to strain-rate-dependent microstructural changes, resulted in a threefold increase in strain rate sensitivity for both materials. The observed hardness variations and underlying deformation mechanisms will be supported by transmission electron microscopy (TEM) analysis and thermal activation studies.