Nanoindentation characterisation of carbon fibre reinforced plastic microstructures

The increasing use of fibrous composite materials in the automotive and aerospace industries has led to increased demand for predictive analysis tools that require careful measurement of the fibre and matrix constituent properties in order to accurately predict the macroscopic deformation and failure response of the materials. Recent developments in nanoindentation testing have enabled accurate property measurements to be carried out at the scale required to probe the individual fibrous composite constituents in situ. However, quantitative characterisation of the matrix constituent has proven challenging due to the inhomogeneity of the composite microstructure and the non-linear time-dependent polymer matrix properties. Hence, a detailed experimental and numerical investigation was carried out into the use of nanoindentation to determine the in situ properties of the polymer matrix constituent in an aerospace grade carbon fibre reinforced epoxy composite (Hexcel HTA/6376). An experimental comparison of the bulk and in situ 6376 matrix properties was carried out using a state-of-the-art Nanoindenter G200, supplied by Agilent. The bulk matrix and composite material were co-cured to produce specimens ideal for matrix characterisation. It is found that the unconstrained in situ indentation modulus increased with a decrease in matrix pocket size, and was up to 19% greater than the bulk matrix modulus. However, a clear discrepancy was noted between the bulk matrix elastic modulus values derived from indentation testing and those determined using traditional macroscopic tensile tests, with the indentation modulus being consistently larger by approximately 40%. Thus, an investigation into the effects of pile-up, viscoelasticity and hydrostatic stress on the polymer indentation modulus was carried out and experimental and analytical techniques were developed to account for viscous and hydrostatic stress effects that systematically reduced the calculated modulus to within 3% of the macroscopic value. The proposed techniques move towards addressing the well-known disparity between nanoindentation and macroscopic measured moduli. Numerical studies using non-linear Finite Element Analysis were carried out to provide insight into the experimental observations and to help decouple highly interdependent variables, such as fibre constraint and residual stress effects on matrix properties. The apparent indentation modulus was found to increase by up to 47% due to the added constraint of the surrounding discrete fibre regions. A two-step modelling approach was used to determine the effect of residual stress states in two regions of interest, namely, the pocket and interfacial matrix regions following thermal cool-down of the composite from cure temperature. It is found that the hardness property decreased for the majority of the residual stress states, whereas the modulus property was found to be insensitive to residual stress.