Investigation on dynamic hardness and high strain rate indentation size effects in aluminium (110) using nano-impact

Nano-impact, a high strain rate pendulum-based indentation technique, has been widely used to extract the local dynamic mechanical response of materials in repetitive contact. In contrast the analysis of single high strain rate impacts to obtain quantitative information on strain rate sensitivity is less well developed. A robust and reliable method for determining dynamic hardness at the nano- and micro-scale is highly desirable. In the present work, nano-impacts with different accelerating force and distance were carried out on a sample of single crystal Al (110) to study the dynamic hardness and its size effects. The common energy-based approach for dynamic hardness has been compared with an approach using Meyer's hardness corresponding to the ratio of the recorded peak force and the projected area of the indentation. It was found that both methods showed broadly similar values at larger penetration depth, but the energy approach became less reliable at the low impacting energy due to unavoidable energy dissipation in the system. Dynamic indentation size effects have been identified which were more pronounced than those in quasi-static indentation tests. This dynamic effect has been attributed to the localized strain caused by the high strain rate and its rapid change during contact. At the highest impact energies studied a transition into strain softening was also identified which is probably due to the adiabatic effects caused by the localized strain under severe impacting conditions.