An experimental and numerical study of the influence of local effects on the application of the fibre push-in test

Several methods, such as pull-out, microbond and push-in/push-out tests, have been developed to test interfacial adhesion in composite materials. Some of them can only be applied to single-fibre matrix composites, like the microbond test, and others are difficult to perform on brittle fibres due to premature fracture of the fibre. Push-in tests, consisting of pushing the fibre with a micro- or nanoindenter on a bulk specimen, constitute a powerful technique that can be applied directly to composite laminates. However, the interfacial adhesion values obtained from different tests (microbond, push-in) often differ and even the results from one type of test are subjected to a large scatter. This might be due to the fact that the existing analytical solutions that are typically used to interpret the experimental data take into account the constraining effect of the surrounding fibres on a simplified manner. To study interfacial adhesion and the effect of the constraint of the neighbouring fibres, a micromechanical model of the push-in test was developed, coupled with experimental adhesion testing in a glass fibre-reinforced epoxy matrix composite. The model takes into account the interfacial fracture process by means of interface cohesive elements at the fibre-matrix interface and focusses on the study of the constraining effects due to the local configuration of the surrounding fibres.