Different response under tension and compression of unidirectional carbon fibre laminates in a three-point bending test

The apparent flexural elastic properties of laminates are traditionally obtained from experimental testing under the supposition of same behaviour under tension and compression. However, any composite generally presents different response when it is submitted to these loading cases. This work is focused on the three-point bending test parallel to the fibres, in laminates of unidirectional and continuous carbon fibre-reinforced epoxy. The purpose is to evaluate the influence on the test results of the dissimilar composite behaviour under tension and compression in the fibres direction. Analytical expressions for obtaining the normal and shear stress distributions in the most loaded section are derived using the technique of the homogenised section. Subsequently, a relation between the span and the thickness of the beam is determined for estimating whether the sample will present flexural or shear failure in the experiments. Parallelepiped specimens with two different thicknesses have been tested for reviewing both failure modes by means of optical microscopy. The experimental evidences are used to evaluate the correctness of the analytical model.