A study of hole drilling in Kevlar composites

Because of the high toughness and flexibility of the fibres, Kevlar-reinforced polymer composites are always difficult to machine, characterised by poor, fuzzy surface finish with partially pulled-out and crushed fibres. For drilling, tungsten-carbide-reinforced fooling in conjunction with very high rotational speeds are normally suggested. The present paper describes a simple technique of drilling Kevlar composites by using slightly modified high-speed steel (HSS) drill bits at commonly available rotational speeds. The investigation has been carried out at both ambient and cryogenic temperatures, the latter obtained through the application of liquid nitrogen at the drill site. Although there is an increase in drill thrust under cryogenic conditions enhancing the probability of delamination, this shortcoming is more than offset by the improved surface finish, hole quality and far superior fool life. Furthermore, delamination can be minimised by using a back support, a commonly followed practice in the industry. Interestingly, the poor surface quality and edge damage at the entry and particularly at the exit faces of the drilled holes can be minimised by the application of liquid nitrogen and are totally eliminated by the introduction of very thin resin-rich layers on the laminate surfaces. The delamination results are shown to agree reasonably well with the predictions made by a linear-elastic fracture mechanics model and ave also investigated by using a three-dimensional, orthotropic, finite-element model. It is shown through finite-element analysis that any major surface delamination is unlikely to happen when a back support is used; however, crack propagation is still possible in the shearing mode. (C) 1998 Elsevier Science Ltd. All rights reserved.