Investigation On Generation Mechanism And Control Strategy Of Defect In Drilling Of Resin-based Composite Materials

Carbon fiber reinforced polymer (CFRP) composites are widely used in aerospace due to its excellent comprehensive performance, and the amount of advanced composite materials has been an important indicator for the advance of aircraft. When the CFRP workpiece is assembled with other aerospace structural components, machining operations such as drilling and milling are still necessary to give the CFRP parts their final shape, which the drilling operation accounted for more than 50% of the subsequent machining. However, due to high hardness, low bonding strength and anisotropic, CFRP is difficult to machine. The defects in drilling restricted the overall performance and service life of CFRP components severely, such as delamination, burrs and tearing. In the current study, few investigations have focused on the theoretical support and systematic approach in the formation and suppression of CFRP drilling. Therefore, the study of defects formation, theoretical support for defect suppression, and establish the control strategy of defects have become worthy of attention for CFRP processing.In this paper, the carbon fiber reinforced polymer composites was used as research object, and the extended mechanism and control strategy of cutting defects with the effect of cutting force and cutting temperature was studied. As the representative defect of CFRP machining, three-dimensional morphology of delamination was innovative obtained by the grinding method. The developing process of delamination was investigated through the different drilling depth experiments, and the existence of "hidden delamination zone" was obtained finally. Then the micro-scale finite element modeling of CFRP was established with the cohesive element, and fiber breakage under the condition of different ply angles was studied. Finally, the cutting defects of CFRP were analyzed from the theoretical point, and the effects of force and temperature on generation and expansion of the defect were obtained, and the foundation for the subsequent analysis of cutting force and cutting temperature was laid.The cutting temperature has a direct influence on the performance of CFRP. Due to the effect of cutting temperature on CFRP cutting defects, the distribution of temperature and the influence of it on the surface quality in the cutting process were analyzed. The heat transfer theory was used to study the line heat source temperature field of high-speed motion. The thermal performance of CFRP under the condition of different ply angles was researched. The temperature experiments were designed using the tool-workpiece thermocouple technique, and the influence of glass-transition temperature on the process quality of 0° and 90°plies was analyzed by SEM. Finally, the influence of cutting force on the processing quality under the condition of different cutting temperature was studied, and it was proposed that the glass-transition temperature had a significant effect on the CFRP cutting quality.The precise theoretical prediction of cutting force is a prerequisite for the surpression of defects in CFRP cutting. Based on the mechanical properties of CFRP, the fracture characteristics under the condition of different ply angles were researched by experiments. and the fracture processes were divided into were bending-cutting and bending-breaking. With contact mechanics, mechanics of composite materials, fracture mechanics and other related theories, the force of fiber deformation zone, the slipping region of rake face, and the bounced back region of flank face was analyzed, and the cutting force models for different regions were established. The prediction of cutting forces was realized in orthogonal cutting of CFRP, and the experimental verification of force model was carried out. By constructing the theoretical cutting force prediction model, the theoretical support for defects surpression in CFRP cutting was providedBased on the analysis of cutting temperature and cutting force in CFRP machining, the control strategy of processing defects was studied. From the structure of drilling tool, the rake angle and clearance angle were optimized according to the theoretical model of cutting force, and the sharpness of drilling tool was enhanced. The burrs and tearing defect could be surpressed effectively, and the corresponding verification tests were conducted. Then according to the three-dimensional structure of the drill bit, the influence of point angle on thrust force for per unit length of the cutting edge was studied. The effective increase of tool life was realized by the optimization of point angle. For the drill bit of structure improved, combined with the effects of cutting force and cutting temperature, the processing parameter optimization for thrust force and drilling temperature was completed by response surface method. The tool life comparison test used the optimal parameter was conducted to verify the reliability for increased life of optimization tool.