The effect of fibre constraint in the nanoindentation of fibrous composite microstructures: A finite element investigation

In this paper a thorough investigation of the fibre constraint on the nanoindentation response of the matrix region of fibrous composites has been carried out. Both 2D and 3D finite element models of the indentation process were developed, defining the matrix material using the Mohr-Coulomb yield criterion. The results show that using a 2D model with an encastre boundary to represent the fibre constraint leads to an overestimation of the constraint on the nanoindentation response. Moreover, the full 3D models containing actual cylindrical fibre sections provide a useful insight into the mechanics of a nanoindentation experiment carried out in matrix regions in the vicinity of fibres. All finite element results show a gradient in modulus as indentations are carried out closer to the fibre-matrix interface which suggests an inherent difficulty when attempting to determine the true 'interphase' properties experimentally using the nanoindentation technique. It is also shown that using the Oliver and Pharr method to determine the contact area for constrained indentations can underestimate the contact area due to the occurrence of pile-up, which is induced by the constraining fibres.