Surface energy in impact testing of unnotched specimens: Influence of carbon nano-fillers in epoxy matrix

Abstract

This work focuses on the mechanical response of an epoxy-based system submitted to Charpy low-velocity impacts with unnotched specimens. The use of samples without notch provokes scattered results on the accumulated energy of the impact. The dispersion is reduced if the energy involved in the failure is calculated per unit of the fractured area. This leads to the estimation of the surface energy in the impact of the material, which is defined as the energy required to create a new free surface. Therefore, the crack paths of epoxy unnotched specimens after breakage are analysed and measured with normalization purposes. A possible correlation of the failure pattern with the action of the positioning tab of the Charpy support is pointed out. As well, the non-negligible effect of modifying the post-curing process of the neat resin is highlighted. The influence of introducing a low percentage of carbon-based nanofillers in the polymeric system is reviewed in terms of the surface energy and its elastic response. To assess the stiffness and strength of the material, the results of quasi-static three-point bending tests are analysed combining the so-called Homogenised Section Technique and the full strain fields acquired by Digital Image Correlation.