Size effects have also been observed in nanoindentations on metals. The size effect is typically more significant for soft materials with low E/H. Epoxy is a soft material which was observed to exhibit size effect in beam bending. In this work, the size effect of the viscoelastic behavior of epoxy is experimentally investigated under constant strain rate loading and dynamic loading in nanoindentation. Prior to data collection, benchmarking experiments were conducted to ensure that the experimental errors at both large and small indentation spurious rate effects are minimized and that the results are reproducible....[ Read more ]

Size effects have also been observed in nanoindentations on metals. The size effect is typically more significant for soft materials with low E/H. Epoxy is a soft material which was observed to exhibit size effect in beam bending. In this work, the size effect of the viscoelastic behavior of epoxy is experimentally investigated under constant strain rate loading and dynamic loading in nanoindentation. Prior to data collection, benchmarking experiments were conducted to ensure that the experimental errors at both large and small indentation spurious rate effects are minimized and that the results are reproducible.

After benchmarking, nanoindentation experiments showed that the load-indent depth behaviors are strain rate dependent. The contact moduli were extracted from the peak portion of the unloading curves and the results indicated that the moduli increased with strain rate. More significantly, the contact moduli were shown to have significant size dependence on the indent size. The full unloading curves were also analyzed using a Maxwell model developed by Cook et al. (2001). The analysis indicated that the elastic constant and viscosity increased with decrease in indent depths.

The elastic constant and the viscosity of a Maxwell material is directly related to the storage (E’) and loss (E”) modulus of viscoelastic materials. Dynamic nanoindentations were conducted to determine the E’ and E”, and they were compared to the E’ and E” determined from constant strain rate loading experiments. The comparison indicated that the results from both loading modes are in excellent agreement and that the side dependence is independent of strain rate. As in the case of plasticity where strain gradients are used to describe size dependence, strain gradients and strain gradient rate maybe needed to fully characterize viscoelastic materials such as biological and polymeric materials.