A new long-term indentation relaxation method to measure creep properties at the micro-scale with application to fused silica and PMMA

A new procedure is developed to measure strain rate sensitivity m and apparent activation volume V* using nanoindentation relaxation experiments. This procedure is based on the control of the dynamic contact stiffness as measured by the continuous stiffness measurement module (CSM). Load decrease is monitored while maintaining the contact stiffness constant. It allows for very stable measurements of load relaxation (up to 10 h) since the contact area between the tip and the sample surface is kept constant. This improvement is significant as it increases by two orders of magnitude the measurement time compared to classical constant displacement experiments. The load relaxation data are interpreted using an analogy to uniaxial tests in order to extract representative material's parameters. An excellent agreement with literature data is found for fused silica and PMMA. Nanoindentation relaxation experiment is proved to be an accurate procedure to extract viscoplastic parameters of materials under very low strain rates.