Study On Quantitative Characterization And Controlling Technology Of Sub-surface Damage In Orthogonal Cutting Of CFRP

Owing to the excellent mechanical properties and high efficient fabrication technology,carbon fiber reinforced polymers(CFRPs)have been used for structural parts in aviation and aerospace industry and are widely considered as new high-performance materials to replace traditional metal and alloy.In order to reduce internal structural defects and ensure final mechanical properties and fatigue resistance,CFRP are generally fabricated in a near-net-shape.Meanwhile,CFRP structural parts are often assembled with metal materials in engineering application then a secondary operation for post processing after CFRP fabrication can hardly be avoided.However,due to the heterogeneity and anisotropy,CFRP machinability is completely different from metal of alloy materials.Neither of processing technology and empirical formula for traditional metal materials is yet fully applicable for CFRP.When machining CFRP,there are various failure modes appear on its machined surface and sub-surface.These microstructural defects induced by cutting force can seriously reduce mechanical properties and fatigue resistance of CFRP structural parts.Identical to the significance of studying CFRP processing performance,it is also important to scientifically characterize the micro defects in order to master the method of controlling sub-surface damage in machined CFRP.In this paper,the experimental and numerical study based on a CFRP orthogonal cutting process are both carried out to study CFRP machinability and the CFRP sub-surface damage characterization influenced by machining parameters to improve the processing efficiency,the surface quality,the forming dimension precision and reduce the micro cutting induced defects of machined CFRP structural parts.In CFRP orthogonal cutting experiment,two classic types of CFRP,unidirectional CFRP(UD-CFRP)and 2-D plane orthogonal woven-fabric CFRP laminate(Woven CFRP),are used to fabricate CFRP cutting workpieces then taken into cutting test.In order to characterize the cutting performance of CFRP,the cutting force(main cutting force and thrust force)in the orthogonal cutting process and the surface topography of machined surface are both the key indicators.The orthogonal cutting experiment is carried out as a single-factor experiment with factors of fiber orientation,cutting speed and depth of cut which have several levels.A cutting force measuring system based on a quartz three-component dynamometer is set up for capturing the real-time values of main cutting force and thrust force in CFRP cutting process.A metallurgical microscopy and a surface profilometer are utilized to observe and characterize the damage and the surface roughness of machined CFRP.The results of the cutting experiment show that the fiber orientation angle of CFRP has a significant effect on the cutting force and the surface quality,in addition,the increase of cutting speed and the decrease of depth of cut can effectively improve the cutting performance of CFRP: for UD-CFRP,the increase of cutting speed can maximally reduce the cutting force,thrust force and surface roughness by 142.29 N,97.12 N and 5.9068?m respectively,and the increase of depth of cut can maximally increase the cutting force,thrust force and surface roughness by 143.67 N,80.54 N and 10.3689?m respectively;for Woven CFRP,the increase of cutting speed can maximally reduce the cutting force,thrust force and surface roughness by 66.06 N,37.16 N and 3.9580?m respectively,and the increase of depth of cut can maximally increase the cutting force,thrust force and surface roughness by 55.06 N,33.29 N and 4.4107?m respectively.In addition,for analyzing the influencing factors of sub-surface damage in CFRP orthogonal cutting experiments,a scanning acoustic microscopy(SAM)is used to scan and detect the sub-surface damage of machined CFRP.Meanwhile,in order to quantitatively analyze the cutting induced sub-surface damage in the CFRP workpiece,a one-dimensional damage factor Fdep and a two-dimensional damage factor Fa are established and utilized to quantitatively characterize the sub-surface damage in scanning images of machined CFRP workpieces with a method of digital image analysis(DIA).In the finite element(FE)simulation of CFRP orthogonal cutting,a 3D equivalent homogeneous FE model of CFRP orthogonal cutting process is successfully established with the user subroutine VUMAT of 3D composite failure criterion.In order to improve the simulation accuracy and verify the applicability of FE model,the FE model based on four different failure criteria are developed compared with the experimental results of the parameters including cutting forces and sub-surface damage factors.The damage factors are computed with coordinate geometries of the elements that are captured by a Python script.The Hashin & Puck failure criterion in this paper is the optimal choice of the four failure criteria after verification with the experimental results.UD-CFRP FE model based on Hashin & Puck failure criterion has an average error for four target parameters of 16.44% with the verification of the experimental data.Both Hashin failure criterion and Hashin & Puck failure criterion show a well performance in Woven CFRP FE model,and the Woven CFRP FE model based on Hashin & Puck failure criterion has an average error of only 8.03%.The validated FE model is used to predict the cutting force and damage factors of CFRP under two different machining parameters,and thus provide a reliable optimization range for machining parameters,in order to reduce the degree of sub-surface damage of machined CFRP workpiece,and improve the cutting efficiency of CFRP.Then,the optimal machining parameters of UD-CFRP orthogonal cutting are as follows: cutting speed equals to 309.5m/min and depth of cut equals to 0.1mm.The optimal machining parameters of Woven CFRP are given as ranges: cutting speed is greater than 200m/min and depth of cut is less than 0.35 mm.The cutting experiment and finite element numerical model established in this paper can effectively characterize the cutting performance and sub-surface damage degree of CFRP.At the same time,optimizing the machining parameters of CFRP with the damage factors that can quantify the sub-surface damage is effective and has a good prospect for engineering application.