Thermomechanical Properties of Carbon and Glass Fiber-Reinforced Epoxy Composites as Function of Drilling Parameters

Dry drilling experiments were carried out on pure and hybrid carbon/glass fiber-reinforced epoxy laminates, viz., all carbon (C8), all glass (G8), glass-faced hybrid (G2C4G2), and carbon-faced hybrid (C2G4C2), using the newly designed high-speed steel (HSS) flow step drill. Drilling operations were conducted at three drill diameters (4, 6, 8 mm), three spindle speeds (125, 250, 535 rpm), and a constant feed rate of 0.1 mm/rev. The thermomechanical behavior and hole-quality assessments were done by means of in-process thrust force monitoring, infrared thermography, 3D optical surface roughness, and scanning electron microscopy (SEM). Drill diameter had a non-linear impact on the thrust force and peak temperature, the highest values of which were obtained with the 6 mm diameter drill, due to its characteristic cutting engagement stages. Both mechanical and thermal behaviors depended on the composite material composition and stacking sequence: the G8 laminate had the lowest thrust force and the most dimensionally stable holes, the C8 laminate had a low thrust force and minimal thermal reaction, but created exit burrs, whereas hybrid laminates had the maximum thrust force and peak temperature (reaching 103.5 °C) along with the maximum entry/exit asymmetry caused by dissimilar carbon/glass ply interfaces. Spindle speed had little effect on the thrust force and was the main parameter affecting the peak temperature (having a 51.78% contribution). Analysis of variance (ANOVA) and multivariate analysis of variance (MANOVA) revealed the drill diameter, material structure, and their interaction as the significant parameters (p < 0.0001) of the coupled thermomechanical drilling response.