A new index to estimate the strain rate sensitivity of glassy polymers using conical/pyramidal indentation

The increasing use of polymeric materials as thin coatings requires appropriate mechanical characterization methods taking account of time-dependent effects. A well-known result is that a constant indentation strain rate allows us to determine the mechanical properties of such solids from indentation tests. To do that, most people assume a power-law creep behaviour and use the viscoplasticity index to determine the strain rate sensitivity. In the first part of this paper, we investigate the influence of the indentation strain rate on the hardness of power-law creep solids and elastic-viscoplastic solids. It is shown that the assumption of a power-law creep solid is not sufficient to describe the behaviour of polymeric materials in indentation. In the second part, we propose a new index, the EVP index, which gives a better approximation of the strain rate sensitivity than the viscoplasticity index. In the last part, we apply this method to nanoindentation tests on PMMA and polycarbonate samples.