Experimental Investigations on Drilling of Woven CFRP Epoxy Laminates: The Effect of Pilot-hole or Drill Chisel Edge on Delamination

Abstract

The demand for carbon fibre reinforced plastics (CFRPs) in lightweight-engineering applications is rapidly growing. Drilling of CFRP composites is a challenging task due to their inhomogeneous, anisotropic and abrasive material characteristics. Delamination in drilling CFRPs is encountered as one of their major material-quality related defects. Drilling is usually performed as a post-manufacturing operation for components and subassemblies; consequently, its better control becomes essential. Many researchers have, hitherto, stressed the need for a pilot-hole prior to performing final drilling. And this is in order to minimise the hole exit delamination by negating the chisel edge effect of a drill bit. This article compares the results of experimental investigations on two different drilling operation-modes: Normal drilling and subsequent drilling conducted on some predrilled pilot-holes in woven CFRP epoxy laminates. Two uncoated cemented carbide drills were used. The thrust force during the drilling operation and the quantitative results (via a non-dimensional delamination factor) of the induced hole entry and exit delamination, respectively, for both operation-modes are compared and discussed. The main objective of this work is to analyse the practical impact of a pilot-hole or conversely that of the chisel edge on the observed hole exit delamination. The obtained results are interestingly different to what is found generally in literature. This research highlights the need to concentrate more on the quality and the design of the cutting edges of a drill bit, rather than to concentrate mainly on the role of its chisel edge.