Effect of surface roughness on friction in fibre-bundle pull-out tests

The fracture behaviour of ceramic–matrix composites is highly influenced by frictional conditions in the interface. In the present paper the influence of interfacial roughness and geometric misfit on the clamping stress is investigated by a specific fibre bundle pull-out test: this test is performed by the superposition of an external load perpendicular to the plies of a 2D endless fibre reinforced C/C composite. The superposition is achieved by means of a specially designed load frame, which acts as a load cell. With this principle of measurement the evolution of external and internal stresses during pull-out is obsessed and the Coulomb friction coefficient μ is calculated. A distinct decrease of the Coulomb friction coefficient in the course of the pull-out test was observed. This indicates a change in the surface roughness due to interfacial wear. The friction coefficient is furthermore dependent on the pull-out velocity: its increase leads to an increase of the friction coefficient.