Fibrous composite matrix characterisation using nanoindentation: The effect of fibre constraint and the evolution from bulk to in-situ matrix properties

The variation of the in-situ matrix properties of a carbon-fibre composite has been investigated using nanoindentation. The aerospace carbon-fibre composite material (HTA/6376) and the bulk matrix (6376) have been co-cured to produce specimens ideal for matrix characterisation. The in-situ matrix has been characterised using fifty indentations in matrix pockets of many different sizes. The fibre constraint effect on in-situ matrix indentations has been characterised experimentally using the continuous stiffness measurement (CSM) technique, showing good correlation with finite element results from a previous study. The co-cured specimens allow the evolution of property change in the matrix material to be observed. The in-situ matrix modulus increases with decreasing matrix pocket size, and is up to 19% greater than the bulk matrix. This property change occurs outside the normal range of the interphase region for CFRP materials, and is statistically significant relative to the experimental scatter associated with the nanoindentation technique.