In-situ characterization of the local mechanical behaviour of polymer matrix in 3D carbon fiber composites by cyclic indentation test

In this work, a cyclic indentation test is employed for the in-situ characterisation of the local mechanical properties of a PR520 epoxy resin matrix in a 3D carbon fiber composite. Since the evolution of indentation response with time is studied, the cyclic loading allows to characterise both the elastic and the time dependent, viscoelastic, mechanical response of the material simultaneously. The 3D carbon fiber composite used in this study contains a number of large resin pockets between fiber bundles (mesoscopic scale) with a characteristic dimension ranging from several hundred of micrometers to several millimeters. The mechanical behaviour of the polymer matrix is investigated on the external surface and in volume of the composite and compared with the neat polymer. The instantaneous elastic modulus from reloading, the energy ratio and the residual depth are determined from the cyclic material response and compared through a Student t-test based statistical analysis. Results show that there is at least 95% of probability that the neat and in-situ polymer matrix material belong to different populations. However, this difference is rather small (between 1 and 2.5%) and almost constant with cycles. Moreover, a difference between composite core and surface was measured.