Research Of Key Technologies For Low-damage Drilling CFRP Composites

Carbon Fiber Reinforced Polymer/Plastic(CFRP)has been characterized by high specific strength and stiffness,and feasibility of integrated manufacturing to near net shape.Consequently,it has been the first choice for high-end equipment in fields of aviation,aerospace,etc.pursuing weight reduction as well as structural efficiency enhancement.A large quantity of holes are needed on CFRP components in assembling processes,however,with extremely strict damage tolerances.The traditional drilling is the principal way to complete the holes.However,CFRP exhibits multiphase mixed morphology,at micro level,consisting of high strength carbon fibers phase and low strength epoxy phase as well as interface.At macro level,it has specific features such as heterogeneity,anisotropy,laminated structure,etc.as well as sensitivity to temperature.Consequently,the material failure behavior and removal mechanism of CFRP induced by cutting force and heat in drilling are totally different from homogeneous materials like metals.Serious drilling damages such as delaminations,burrs,etc.frequently occur at drill-exit area which restrict the application of CFRP in high-end equipment.Therefore,low-damage drilling CFRP has been a great challenge to exiting drilling technologies and is of great significance to carry out deep research.A great deal of valuable investigations have been done at home and abroad.However,the available cutting theories and drilling technologies for CFRP cannot fundamentally reveal and solve the problem of the frequent damages induced by cutting force under supportless condition at drill-exit area.Consequently,it is difficult to develop appropriate drilling tools.On the other hand,there also lack comprehensive studies on temperature effects on cutting performance as well as temperature features at drill-exit area,which makes it difficult to develop reasonable processes to inhibit high temperature effect in drilling CFRP.Therefore,serious drilling damages still always occur.This doctoral dissertation has studied on fundamental cutting theory of CFRP with weak constraint as well as temperature influence mechanism,and proposed reasonable cutting force control and temperature effect reduction method and strategy.Further novel drill bits and processing technology have been developed to achieve low-damage drilling.Four parts of specific contents are as follows:Firstly,in order to reveal the cutting mechanism and damage mechanism of CFRP at drill-exit area,the constraint condition at drill exit has been analyzed.A micro-mechanical model has been built for cutting a single fiber with one side weak constraint to represent for cutting behavior at drill-exit area.Meanwhile,considering the anisotropic properties of fibers,an anisotropic contact model has been established to characterize cutting action between cutting edge of the cutting tool and fiber.The fracture behavior of fiber and the debonding law between fiber and epoxy as well as interface have been solved analytically based on those two models.Effects of cutting edge radius,cutting direction and tool-workpiece contact point on material removal and damage have been analyzed,and as a result,the cutting direction is the most influential factor.Cutting from outside to inside has been recognized as the effective way to avoid debonding and cut off fiber at the same time,and is preferred to suppress cutting damages of CFRP.The results provide guidance for drilling damage suppression at drill-exit area.Secondly,epoxy resin is characterized by softening under high temperature and embrittlement under low temperature,which affect cutting performance of CFRP.Under different temperature conditions,chips formation processes in cutting unidirectional CFRP have been studied using high-speed and micro-observation photogrammetry.Meanwhile,temperature effects on the cutting performance have been revealed under typical fiber cutting angles.On the other hand,considering combined temperature effects on cutting multidirectional CFRP with laminates in various laying directions,cutting experiments on multidirectional CFRP have been carried out under various temperature conditions.Comprehensive assessments have been done according to the cutting performance of multidirectional CFRP,and the effects of the material removal volume and tool structure have also been discussed.It could concluded that the cutting temperature should be controlled as much as lower than the glass transition temperature to obtain low-damage cutting performance,and however,it should not be too low in order to avoid the sharply increasing cutting force as well as edge chipping.Thirdly,temperature features in drilling CFRP are quite complex and difficult to describe accurately due to the anisotropic mechanical and thermal properties of CFRP.A comprehensive study on drill-exit temperature features in drilling unidirectional and multidirectional CFRP has been carried out using high-frequency and microscopy infrared thermography technology.Close relationships between temperature feature and cutting condition have been revealed under typical fiber cutting angles.Further,the time-dependence temperature features on the normal diameter as well as the highest temperature features at drill-exit area have been analyzed.Moreover,effects of multidirectional laminates on drill-exit temperature and drilling qualities have been revealed.The results would provide valuable guidance for developing drilling process to suppress temperature effects on drilling qualities.Fourthly,based on the fundamental study on the removal behavior,the principle of"reverse cutting" has been proposed and a kind of sawtooth structure with the large right-hand helix angle has been invented on drill bits.In normal drilling process,the sawtooth structure is able to perform a relatively cutting motion in the reverse direction of feeding,and implement the "reverse cutting" from outside to inside at drill-exit area.Then comparison experiments have been conducted.Burrs and delaminations have been effectively reduced,and tool life has also been extended due to the sawtooth structure.The positive results have verified the validity of the principle of "reverse cutting" on damage reduction in cutting CFRP and the effectiveness of sawtooth structure implementing "reverse cutting".Consequently,a series of drill bits with sawtooth structure have been developed and the original cutting mode at drill-exit area,which only generates cutting forces in feeding and rotating direction,has been changed.With the developed drill bits,the low-damage drilling performance has been achieved.Besides,based on constraint effects and temperature effects on cutting CFRP,a cooling process of reverse suctioning air,namely suctioning air from drill entrance,has been proposed in drilling CFRP.Compared with drilling in dry,the cooling process has controlled and reduced the drilling temperature,and effectively suppressed drilling damages.Through the above researches,the cutting theory of CFRP in weak constraint area has been innovated,and temperature effects on the cutting performance as well as the complex temperature features at drill exit have been revealed.Furthermore,key technologies of drill bits with sawtooth structure and cooling process of reverse suctioning air have been developed.Based on laboratory experiments and drilling tests in companies,the technologies have been verified to be effective and have provided new paths for low-damage drilling CFRP composites.