The influence of sequential drilling and machining parameters on uncut fiber formation and delamination damage in fiber-reinforced composites

Drilling is crucial in composite manufacturing, but often induces delamination and uncut fibers, degrading structural integrity. This study examines the combined influence of sequential drilling (using drills of progressively increasing diameter) and machining parameters (feed rate and cutting speed) on damage in fiberglass composites. Sequential drilling was performed at a feed rate of 1000 mm min⁻¹ and spindle speed of 500 rpm, while the effects of machining parameters were investigated over feed rates ranging from 50 to 2000 mm min⁻¹ and spindle speeds from 500 to 3000 rpm. Results show that sequential drilling effectively reduced delamination at both the entrance and exit of drilled holes, while simultaneously decreasing exit-side uncut fibers and increasing entrance-side uncut fibers. Furthermore, increasing cutting speed significantly mitigated damage, while higher feed rates worsened it. Three-point bending tests revealed that modified drilling conditions increased the fracture load of drilled specimens by up to approximately 18%, with fracture loads rising from about 956 N to 1130 N depending on feed rate and spindle speed. Unlike previous studies that primarily examine drilling strategies or machining parameters in isolation, this work provides a unified experimental framework that links sequential drilling methodology, quantitative damage characterization (via image processing), and mechanical performance (via three-point bending tests). Quantitative damage assessment reveals the complex interplay between drilling method and parameters, offering valuable insights for optimizing composite drilling processes.