Initiation and progression of drilling damages on composite laminates

Making high quality drill holes on composite laminates is one of the challenging machining processes. The machining process of drilling applies thrust load to the out of plane direction on the composite plate, where composites have weak mechanical characteristics on that direction. This out of plane thrust load by drill chisel tips cause the initiation of drilling damage during the machining process, and then the initiated damages are further progressed depend on the geometry of drill cutting edges. The initial damages are inevitable in the nature of composite drilling process, even though the failed size and characteristics are different. This drilling damage initiation by thrust load was investigated by Park et al. [1]. Once a drill chisel edge penetrates the last laminate, a second damage step, progression starts. The initiated damages are either removed or developed with additional cracks or delamination. The characteristic, commonly assumed, is mostly affected by the geometry of drill cutting edge. To investigate the damage progression process numerically, element deletion method is implemented into the previously developed FE analysis. Thrust load distribution along the drill cutting lip was calculated, we assumed the load as cutting force, and then we can adapt the value as element deletion criteria. The FE code performs progressive failure analysis and element deletion method together, so the code calculates drilling damages from initiation to progression by removing FE elements, simulating cutting process. Finally, we discussed the final hole quality by comparing two different thrust load distribution cases. The simulation results showed small variation of thrust load distribution on the cutting lip region cause different results in both how damages are progressed and what final failure configuration be formulated.