A study of the drilling of advanced carbon fibre composites
Carbon fibre composites are increasingly being used in aircraft structures due to their superior physical and mechanical properties. The process of drilling of carbon fibre composites in aircraft manufacture is economically important since the extremely abrasive nature of the fibres limits drill life. The hole quality produced by drilling in terms of fibre pullout and matrix cracking affects the notch sensitivity of the hole. The present thesis describes an experimental and analytical study of drilling of the carbon fibre composites carried out with the support of British Aerospace (Military Aircraft Division). Full drill life testing was carried out using four low cost commercial cemented carbide drills, three of which had brazed inserts, and drill life was determined by measuring the outer drill corner wear. Hole quality was measured in terms of diametrical tolerance using accurate plug gauges. Drill forces were measured using a two component Kistler dynamometer and attempts were made to measure residual stress in the workpiece using the birefringent photoelastic technique. The hole quality was related to drill wear, cutting forces and heat generated during drilling. Independent tasks were carried out to relate cemented carbide physical and mechanical properties to wear using several standard sliding wear experiments. Three different cemented carbide tool materials were investigated in terms of cobalt layer thickness, carbide distribution and physical properties including hardness and fracture toughness. Independent sliding wear tests were performed using a Pin-on-Disc machine, lathe and machining centre. These tests allowed the materials to be ranked in terms of wear resistance when rubbing against carbon fibre composite. The fracture toughness was measured using the techniques developed by Palmqvist. The wear resistance was correlated to the physical and mechanical properties of the tool materials. Hole quality was studied experimentally using scanning electron microscopy and fibre pullout shown to be primarily dependent on the fibre-matrix interface bond strength and the intrinsic strength of the fibres. The surface morphology of the fractured fibres in areas of fibre pullout showed inultimode damage due to anisotropy of the carbon fibre composite and the dynamics of drilling. The degree and pattern of damage developed in the drilled holes was found to be highly directionally dependent. The experimental results and theoretical analysis showed that the degree of hole damage depends not only on drilling parameters but also on the material composition and the manufacturing process of the carbon fibre composite.