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.