Effect of induced bending moment (IBM) on critical thrust force for delamination in step drilling of composites

During final assembly operations, hole quality is a key aspect when making holes in composite-based components, which can affect the in-service life under fatigue loads. The step drill, however, was widely used in the aircraft, automotive and machine tool industries to produce a step, a countersink or counterbore holes in a single operation. On the other hand, delamination caused by drilling thrust has been recognized as one of the most problematic defects after drilling. The present study presents a comprehensive model of critical thrust force for step drill with and without the effect of induced bending moment (IBM) considered. In this analysis, the critical thrust force associated with the effect of IBM, which causes the onset of delamination when using the step drill, is predicted and discussed.

► Effect of induced bending moment (IBM) in step drilling composites is investigated.
► An analysis is compared for step drill with and without the effect of IBM considered.
► Thrust factor can be increased by 30% at high feed rate when the IBM is considered.
► Stage ratio is an important factor in step drilling composites.