Study Of Mechanical Behavior Of Material In Cutting Carbon Fiber Reinforced Plastic

Carbon fiber reinforced plastic(CFRP)exhibits excellent mechanical properties,such as the high specific stiffness,specific strength and designability.Manufacturers of the high-end equipment always try to use more CFRP in the main structures for the light-weight design.The main structure consists of the CFRP and metal components.To satisfy the dimensional tolerance and assembly requirements,a large number of machining processes such as milling and drilling are inevitable.However,CFRP is a multiphase material at the micro level,consisted of the high-strength carbon fiber and the low-strength epoxy and interface.The low-strength epoxy and interface easily fracture when the fiber is cut in cutting CFRP.Meanwhile,it has the anisotropy and laminated feature at macro level,and this causes the various chip formations in cutting CFRP.The fracture and propagation of the epoxy or the interface easily leads to the serious cutting damage,so that the CFRP cutting is a challenge.Therefore,this dissertation studies the mechanical behavior of material in cutting CFRP,aiming to give some methods for damage control and tool design.The achievements can provide the basic conditions for high-quality and efficient machining of CFRP.Furthermore,it is important for the promotion of the use of CFRP in high-end equipment.The theoretical achievements can be a supplement to the theoretical system of cutting CFRP.The main works of this research are as follows:Firstly,this work is aimed to control the initial stage of the fracture at the micro level.For this purpose,the micro-mechanical model for cutting a single fiber is proposed considering the normal and shear effect on the fiber deformation.The quasi-static processes of the fiber under the cutting and pressure load are analyzed,and the mechanical behavior of the fiber and the interface also are investigated.Finally,the effect of the constraint of the epoxy on the fiber deformation is analyzed,as well as the effect of the cutting force on the fracture of interface.The results reveal that the fiber with strong constraint may be cut off at the contact point with the cutting edge,and the fiber does not bend obviously.When the fiber is cut with small depth of cut,the fracture of the interface is difficult to happen.So that,the damage is controlled from the root by controlling the fracture initiation of the interfaceSecondly,in order to control the propagation of the fracture in chip formation at the macro level under the cutting force,the cutting model is built with the cutting of fiber,the pressure of the flank face on the fiber,and the push of the rake face on the chip.In this model,the cutting forces are predicted.The experimental equipment for cutting CFRP is developed with functions of high-speed cutting and high-speed micro observation.The cutting force predictions are validated based on this equipment.The chipping behavior and fracture propagation of the interface are investigated via the simulation and micro observation on cutting processes.The results reveal the epoxy and interface easily fracture due to the opening load in bending-shearing and bending-domain chip formations.Meanwhile,the fracture extends easily along the fiber and into the material.The fracture propagation decreases with the decrease of the rake angle since the direction of cutting force becomes closer the fiber direction.Consequently,the fracture propagation of the matrix and interface is controlled during the chip formation.Thirdly,the effect of the cutting speed on the cutting process is studied on the basis that the fiber with a strong constraint could be cut off without the interface fracture.The micro model with considering the strain-rate property of epoxy is established to simulate the failure of the material in cutting CFRP under different cutting speeds.In addition,the effect of the cutting speed on the mechanical behavior and the tool deterioration is analyzed combined with the experiment method.The results reveal that the enhancement on the constraint of the epoxy to fiber conducts the reduction of the deformation area,and it is the main reason for the significant reduction of cutting forces.The specific energy almost decreases to the same level under the cutting depth of 20?m and the cutting speed of 3.0m/s.In this case the fracture of epoxy and interface hardly ever happens.The major mechanism of tool wear is the extreme crashing and ploughing of the uncut fibers on the flank face.Under the high pressure of the fibers constrained,the micro chipping of a mass of diamond crystals occurs due to the bending failure with large deformation on the cutting edge and the rake face.The frequency of the crashing and pressure increases due to an increase of the cutting speed.In this case,the tool deterioration is more likely to happen.The above results can provide the beneficial reference for the reasonable selection of the cutting speed in cutting CFRP.Finally,the methods that locating the deformation area affected by cutting and reducing the opening load are proposed on the basis of the above works in this research.The specific cutting tool with the multi-micro-tooth structure is designed based on these methods.The milling experiments are carried out via the cutting tool,and the results reveal that the machining quality is eligible,and the feed rate increases four times than the former parameters used by the manufacturer.Furthermore,the continuous cutting of the large hole is performed with high quality via the specific tool.As a result,the machining quality and efficiency both meet the industrical requirements.