Mechanics of thrust force on chisel edge in carbon fiber reinforced polymer (CFRP) drilling based on bending failure theory

CFRP (Carbon Fiber Reinforced Polymer) is widely used in the aerospace, automobile and construction industries. Its non-homogeneous and anisotropic nature often results in unfavorable hole quality during the drilling process, and exit delamination is a resulting phenomenon that has received much attention. Exit delamination occurs when the drilling thrust force exceeds the critical thrust force. It adversely affects product quality and may even result in rejection. The thrust force on the chisel edge is an important component of the total thrust force, and it helps to predict the total thrust force so that delamination can be reduced or avoided. In this study, noticing that the chisel edge has a certain length, the author puts forward the novel concept of the azimuth angle on the basis of the micro analysis of the contact state between the chisel edge and the carbon fiber. In addition to the orientation angle of the carbon fiber, the azimuth angle of the cutting edge is also an important factor that affects the thrust force. A single carbon fiber may be modeled as a beam on an elastic foundation, and the prediction model of thrust force on the chisel edge is established based on the theory of bending fracture. The model can reflect the influence of the material properties, cutting tool geometry parameters (including the length of chisel edge, rake angle and blunt radius of cutting edge), drilling process parameters, and ultrasonic vibration parameters on the thrust force on the chisel edge and quantitatively explain the reduction of thrust force and improvement of drilling quality during the longitudinal ultrasonic vibration drilling. The prediction model of the thrust force presented in this work can make up for the shortages of qualitative interpretation. The law of periodic variation of the chisel edge thrust force with the azimuth angle helps in developing a deep understanding of the CFRP drilling process and the material removal mechanism. The basis is provided in order to reduce and avoid the exit delamination defects in drilling, and to reasonably and quantitatively determine the spindle speed, feed rate, and drill bit geometry parameters. The reliability of the prediction model is verified by a drilling test.