Instrumented indentation of an elastomeric material, protocol and application to vulcanization gradient

Instrumented Indentation Testing is applied on a fluoroelastomer so as to get information about the local mechanical response of such a rubbery material. Due to their viscoelasticity, elastomers exhibit a time-dependence, responsible for a variation in their behavior as a function of the stress or strain rate. A few studies highlight modifications of the indentation hardness H and modulus Er when the load function varies. However, these works concern polymers and, to our knowledge, such an investigation has not yet been carried out on elastomeric materials. Results about the influence of the loading-unloading procedure on Er are presented here. They show elastic modulus' variations depending on the chosen protocol's time scale and reported evolutions are consistent with the expected behavior of viscoelastic materials. Furthermore, in preparation of this main study, preliminary verifications were necessary since sources of error are known to distort indentation results. A focus on the microstructure and roughness is presented in this paper and reveals, in particular, a non-negligible improvement of indentation curves (load-displacement hystereses) with an appropriate sample preparation and surface finish. These results have finally been used as a basis for the study of local gradients within a rubber sample. This study revealed a gradient (i.e. an increasing modulus) from the outer part of the sample to the inner one, attributed to thermal effects during the molding.