Investigation of hole quality during drilling of KFRP based on the interaction between collars and cutter

Abstract

Kevlar fiber-reinforced plastic (KFRP) composites are difficult to machine and more likely to produce severe drilling-induced damage. Due to high toughness, ductile fracture of Kevlar fiber occurs after a certain extent of plastic deformation. Therefore, the mechanism of drilling-induced damages of KFRP composite cannot be accurately explained by previous damage mechanisms of carbon fiber-reinforced plastic. In this study, the mechanism of the typical drilling-induced defects of KFRP, such as delamination, tearing, and fuzzing, was systematically investigated. On this basis, a new approach (combined drilling) based on the interaction between collar (a ring-shaped device fitted on laminate) and cutter was presented to prevent damages. To investigate the effects of combined drilling on reducing drilling-induced defects, the influences of collar inner diameter and feed speed on the thrust force, delamination factor, tearing length, and fuzzing area were studied through systematic drilling experiments. Experimental results showed that delamination, tearing, and fuzzing defects were reduced with the decrease in collar inner diameter and feed speed. A reduction in defects was observed for combined drilling with collars of 6.1 mm when compared to drilling without collars at a feed rate of 20 mm/min: 26.3% for the delamination factor, 52.7% for the tearing length ratio, and 73.1% for the fuzzing area. Furthermore, a delamination model for combined drilling was proposed to predict the critical thrust force for the onset of delamination at the exit layer. Compared with experimental results, the safe limit of feed speed without causing delamination is 20 mm/min in combined drilling.