Damage analysis of carbon/epoxy plates after drilling

The use of laminate composites in structures is under a considerable increase. The advantages of their use are related with a considerable weight reduction and consequent improvement of dynamic characteristics, in the case of aeronautics, automobile, railway and naval industries. Some applications of these materials in sports are already well known, like cycling, motor sports or golf, just to mention some. Drilling is a widely used technique as it is usually needed to assemble components in more complex structures. According to Stone and Krishnamurthy (1996), in the aircraft industry, about 60% of part rejections come from drilling-associated delamination. Drilling is a complex process characterized by the existence of " extrusion " and cutting mechanisms, the former performed by drill chisel edge that has null or very small linear speed and the latter by the existence of rotating cutting lips at a certain speed. As composites are non-homogeneous and anisotropic, drilling raises specific problems that can affect parts strength and fatigue life, Persson (1997), Hamdoun (2004). Typical damages after drilling are push-out delamination at exit side, peel-up at entrance side, intralaminar cracking, fibre/matrix debonding and thermal damage. Due to their abrasiveness, composites drilling cause high tool wear, leading to the need of frequent tool changes that affect the production cycle. From all these problems, delamination is the most serious as it severely reduces the load carrying capacity of laminated composite structures and must be avoided, Bader (1994). Delamination is a defect that occurs in interlaminar regions, that is to say, in the contact plan between adjacent layers,