Effect of loading strain rate on nano-indentation response of an aerospace grade epoxy polymer

Depth sensing indentation holds the promise of local mechanical properties determination. However, the number of studies on materials that exhibit time-dependent and hydrostatic pressure-dependent behavior is still scarce. This paper aims to understand the effect of loading strain rate on the nano-indentation response of an aerospace epoxy resin by combining physical measurements and numerical simulations. Physical nano-indentation tests were carried out using a Berkovich in-denter at different constant loading strain rates (0.01 to 1.25 s-1). It was observed that as the loading strain rate increases, the penetration displacement at which the target load is reached decreases while the maximum displacement attained at the end of the load-hold period increases. The indentation response was numerically simulated to get insight into the phenomenon by finite element analysis. The polymer behavior was described by a nine-parameter elastic-viscoplastic constitutive model (EVP-9). Constitutive parameters were calibrated from uniaxial tensile and compression experimental data. Simulations agreed reasonably well with physical experiments being able to reproduce the loading strain rate effect observed in physical nano-indentation tests.