Tool Geometry Evaluation for Carbon Reinforced Composite Laminates

SUMMARY The use of composite laminates in complex structures has increased significantly. However, there are still some issues when considering their use, mainly related with machining, leading to some difficulties and lack of acceptance. In this work, a methodology to evaluate drill geometry and feed rate based on thrust force and delamination extension is presented. INTRODUCTION For the past decades composite materials are increasing their importance as one of the most interesting groups of materials, due to their unique properties of low weight, high strength and stiffness. One of the main machining operations needed in composite structures is drilling, as it is required when bolts, rivets or screw are used to join different parts. Usually, it is accepted that drilling can be carried out using conventional machinery with suitable adaptations. However, as composites are non-homogeneous, drilling causes some difficulties. In fact, some damage in the region around the hole boundary is evident after a drilling operation is accomplished, being delamination the most serious one as it can reduce the load carrying capacity of the joint. The main mechanism responsible for delamination is the indentation effect caused by the quasi-stationary drill chisel edge (Hocheng et al. 2005). This effect can be diminished by a correct choice of tool geometry and/or cutting parameters (Hocheng et al 2006). In general, it is accepted that a drilling process that reduces the thrust force exerted by the drill chisel edge can prevent delamination occurrence (Dharan et al 2000, Davim et al 2003, Durão et al. 2008). Another possibility for delamination reduction is an adequate design of the cutting tool – drill – which, combined with proper machining parameters, can help in delamination prevention (Persson et al 1997, Piquet et al 2000, Hocheng et al 2006, Tsao et al 2007).