High-frequency vibration effects on the hole integrity in rotary ultrasonic drilling of carbon fiber-reinforced plastic composites

This investigation represented a fundamental research on the potential effects of the high-frequency vibration on the hole integrity involved in rotary ultrasonic drilling (RUD) of carbon fiber-reinforced plastic (CFRP) composites. It was found the increased thickness of the CFRP plate shrunk the flowing velocity of the coolant, which brought about the residual chippings gradually accumulated at the radial clearance between the tool and the material. Furthermore, the chipping accumulation at the clearance seriously increased the friction effects and the resultant thermal load, thus leading to the chipping adhesions on the tool surface and machined cylinder jamming at the central hole of the tool. The mutual constrain between two vertical bundles brought the delamination around the holes generated in conventional drilling (CD) process to a termination at the bundle interface. The ultrasonic superimposition reduced the thrust force of the diamond tool which provided inadequate energy for the delaminated fibers reaching the bundle interface. Moreover, hole position on the two-dimensional orthogonal fabrics significantly influenced the propagation of the delaminated fibers, which weakened the effects of the drilling parameters on the delamination dimensions. Additionally, superimposing an ultrasonic vibration prolonged the abrasive trajectories and increased their overlapping probability, and the induced smoothing effects resulted in the obvious reduction of the surface roughness. The tensile stress exerted on the margin of the machined surface was responsible for the initiation of the CD delamination. After the delaminated fibers reached the bundle interface, the further extrusion of tool brought about the margin suffered from the shear stress, thus leading to the collapse of the machined cylinder. Considering the thrust force of the diamond tool and the undrilled thickness of the machined surface, the critical conditions of the delamination initiation were developed, which revealing that the decreasing of the thrust force caused the reduction of the critical undrilled thickness.