Mechanism Of Damage Formation And Damage Evaluation Method In Machining Of CFRP Composites

Carbon fiber reinforced polymer(CFRP)has outstanding advantages such as lightweight,high strength,and performance design.It is easy to achieve the integral manufacture of structural parts and is the preferred choice for weight reduction and efficiency enhancement of the key components of high-end aerospace equipment.The amount of materials used has become an important indicator of the advancedness of equipment and international competitiveness.However,for large-scale CFRP structural parts,which have characteristics such as large size and complicated structure,a large amount of machining is still required to meet the requirements of connection and assembly.Once uncontrollable cutting-induced damage occurs,the service performance and reliability of the components will be seriously affected,and even the entire structural parts will be directly rejected and the loss will be huge.Compared with homogeneous materials such as metal,the cutting process of CFRP includes fiber breakage and fiber/resin interface cracking.The mechanism of damage formation under the cutting force and cutting heat is more complex than traditional metal cutting.In addition.during the machining,it is easy to cause the resin to soften due to the excessive temperature rise,resulting in a decrease in the bonding strength of the fiber/resin interface,thereby aggravating the damage.In addition,in the process of using ttraditional cutting technology,it is easy to produce multi-morphological,multi-scale,random distribution of burrs and spalling damages,and the accurate evaluation of such cutting-induced damage is an important part of ensuring the machining quality.Thererfore,it is necessary to explore the basic research of fiber destruction behavior,macroscopic subsurface damage formation process,cutting heat distribution mechanism in the cutting zone,and the characterization method of the cutting-induced damage zone,so as to understand the formation mechanism of the cutting-induced damage and accurately evaluate the damage extent.Boost CFRP large area applications.Therefore,this paper develops the causes of the damage formation,the method of determining the amount of heat dissipation,and study of multi-dimensional evaluation methods for cutting-induced damage.First of all,taking the orthogonal cutting as the object of research,which has the relatively simple interaction between tool and workpiece.A micro-scale mechanical model considering the cutting edge radius on the fiber cutting is established based on the representative unit of the single fiber.The effect of cutting edge radius on the fiber deformation is studied,and clarify the influence of fiber orientation and cutting edge radius on the fiber bending deformation and failure mechanism.At the same time,in order to study the sub-surface damage formation process,a two-dimensional macroscopic cutting simulation analysis model is established based on the assumption of macroscopic anisotropy of equivalent homogenization.By introducing the maximum stiffness degradation coefficient and material failure evolution criterion.the continuous dynamic simulation of CFRP cutting process is realized.The influence of different fiber orientations,tool geometrical parameters and cutting parameters on the depth of sub-surface damage is studied.The complete process of sub-surface damage form initial formation to extended evolution is obtained.The results provide the basis for revealing the formation mechanism of burrs and spalling damage.In the drilling or milling of CFRP process,a large number of burrs and spalling damage occur on the machined edge.Taking the edge trimming of unidirectional CFRP as an example,by introducing the concept of fiber cutting angle,the tool rotation process is simplified to a number of orthogonal cutting.Afterward,the effects of fiber orientation,radial cutting depth and up-milling/down-milling modes on the change of fiber cutting angle range during tool rotation.At the same time,a series of edge trimming experiments are carried out by double-edge straight flute milling cutter.The influence of the fiber cutting angle range and the cutting edge radius on the cutting-induced damage is studied.The corresponding fiber cutting angle range is obtained when burrs and spalling occurred.Furthermore.based on the research findings in Chapter 2 of this paper,the mechanisms of burrs and spalling damage during the milling process are revealed.In addition.the drilling tool rotation process is divided into four independent edge trimming processes,the burrs and spalling damage formation mechanism is revealed.The results provide the basis for the establishment of the method to suppress burrs and spalling damage caused by drilling and milling of CFRP.Due to the resin matrix of CFRP composites is sensitive to temperature,it has the characteristics of poor thermal conductivity,high temperature softening,low temperature embrittlement and so on.Moreover,the cutting fluid is not allowed to be used in the cutting process.Therefore,it is easy to be damaged by heat accumulation and excessive temperature rise,thereby causing frequent change of the cutter.For solving these engineering problems,air cooling is used to take the excess heat energy out of the cutting zone,making the temperature in a suitable range.In order to realize the reasonable control of cutting heat.the calculation method of heat distribution ratio in cutting zone of CFRP is established.The influence of fiber orientation,tool parameters and cutting paramters on heat distribution in cutting zone is clarified.The heat flows into the CFRP workpiece in the traditional cutting process and its impacr on the damage extent is obtained.On this basis,by establishing the temperature field distribution equation of the CFRP cutting process considering the frictional heat source of the tool flank face,the heat energy of CFRP workpiece required for cutting at different temperatures is obtained,and then the heat dissipation of optimum temperature cutting is determined.Based on the Dittus-Boelter equation,the effect of initial air temperature on the air flow required to remove the same heat energy is studied.The experimental results are also validated.The results provided an important basis for future development of new technology and new method to minimize the cutting-induced damage.The multi-dimensional and accurate characterization of milling-induced damage is very difficult due to the multi-shape and random distribution of damage.The traditional one-dimensional maximum damage length method often has the problem of misjudgment.A method of milling-induced damage evaluation method based on the damaged area is proposed.The method of automatic image characterization in the damaged area of "Visual inspection imaging+image preprocessing+area segmentation and extraction" has made up for the deficiency of human eye measurement judgement.Taking edge trimming of CFRP as an example,the damage extent is quantified by the ratio of the area of the damaged area to the area of the workpiece to be removed,and the effect of different milling parameters on the area-based damage factor is studied.In addition,for the CFRP drilling process,all layers of materials may have damage under the action of axial force,and the evaluation based on the damage extent at the exit of the drilling will inevitably lead to incomplete damage information.Based on the damage volume cumulative evaluation method.a three-dimensional volume damage factor calculation model is established,and the multi-dimensional characterization of the damage is realized by introducing the nanometer CT tomography imaging technology.Finally,based on the damage area evaluation method proposed in this paper,the edge of the typical CFRP test pieces and damage area evaluation method validation is carried out.The research results provide technical support for the manufacture of the CFRP components.