Mechanistic Modeling Of Vibration Assisted Drilling Of CFRP Components And Its Stability

Carbon Fiber-reinforced Plastic/Polymer(CFRP)is more and more widely used in the fields of aeronautics and astronautics because of its high specific and modulus strength,and good corrosion resistance.Machining processes such as turning,milling and drilling are often used to shape FRP parts with required tolerances and surface finish.Therefore,the quality of CFRP holes has become an important factor affecting the product assembly process.CFRP,consisting of carbon fiber and epoxy resin,is characterized as heterogeneous and anisotropic,and thus the material removal mechanism is more complex compared to metal cutting.Drillinginduced damage such as fiber pull-out,delamination at the exit and circularity defects,which results in undisirable hole-making quality and high rejection rate of products,and thus seriously hinders the quality and efficiency of aircraft assembly.In order to resolve the aforementioned problems,this work focuses on the mechanistic modeling for vibration assisted drilling of CFRP components and its stability,built the 3D micro-scale Finite Element(FE)model for orthogonal cutting of CFRP to reveal the behavior of CFRP during orthogonal cutting,proposed the approach for force coefficients prediction during the CFRP machining process,established the mechanistic model for prediction of fluctuating thrust forces and torque,and conducted simulations and mechanistic analysis of ultrasonic vibration drilling so as to reduce the drilling-induced damage and to control the stability of CFRP drilling process.1.A 3D micro-scale FE model for orthogonal cutting of CFRP was presented to reveal the behavior of CFRP during orthogonal cutting.Fiber,matrix and fiber-matrix interface were modeled in ABAQUS.The constitutive behavior of fiber was developed by the user subroutine VUMAT.Johnson-Cook model was implemented to describe the strain rate and temperaturedependent behavior of matrix,and the J-C parameters were calibrated by experimental data conducted using pure matrix at different strain rates and temperatures.Zero-thickness cohesive elements based on the traction separation law were established to model the interfacial debonding between the fiber and the matrix.Orthogonal cutting simulations for various fiber orientations was carried out,and cutting force,chip morphology,surface roughness and cutting temperature were compared and verified by experimental results.Based on the results of FE simulations,energy dissipation mechanisms through the orthogonal cutting process were quantified by energy analysis method.The dominating fiber damage modes at four typical fiber orientations were evaluated by Python post-processing method.The cutting and failure mechanisms beneath the dynamic cutting force,chip formation and cutting temperature are quantitatively explained,which well reveals the micro-scale cutting behavior.2.The approach to predict force coefficients during the CFRP machining process were proposed.The drilling parameters and geometric characteristics for drilling CFRP components were analyzed with mathematic equations.The cutting edge was divided into infinitesimal elements conducting orthogonal cutting,and cutting speed and normal tool rake angle were predicted by energy analysis method during the orthogonal cutting process.Based on the energy analysis results,the cutting and edge coefficients were proposed to correlate the relationship between cutting force and depth of cut.The power law function was used to describe the nonlinear relationship between cutting force and tool rake angle.The BP neural network model was constructed and trained by data obtained from orthogonal cutting simulations.Thus,the mapping relationship between cutting force and parameters for the entire range of fiber orientation during orthogonal cutting of CFRP was revealed3.A mechanistic model was established to predict the fluctuating thrust forces and torque.The CFRP drilling process is divided into three stages: entrance stage,stable stage and exit stage.During the stable stage,the correlation between elemental tangential and feed forces of an infinitesimal element and force coefficients was established.The resultant thrust forcs and torque were obtained by the summation of elemental tangential and feed forces along the whole cutting edge and orthogonal-oblique cutting transformation.At the entry stage,for a given time,the time-dependent number of infinitesimal elements was calculated the instantaneous drilling forces were obtained for the effective cutting length of the cutting edge.The force at the exit stage were assumed as equivalent to the difference between the drilling force at the stable stage and the drilling force at the entry stage.The influence of fiber orientation,tool geometry,process parameters and effective cutting length of cutting edge during the CFRP drilling process was taken into account during the drilling forces modeling,the model was capable of well describing the fluctuating features of thrust forces and torque.Compared to results of drilling experiments with various feed speeds,the mechanistic model can accurately predict the maximum,the minimum,the change frequency and the average of drilling forces.4.Simulations and mechanistic analysis of ultrasonic vibration drilling were conducted to reduce the drilling-induced damage and to control the stability of CFRP drilling process.The micro-scale FE simulations of ultrasonic vibration orthogonal cutting were conducted,and the influence of vibration frequency and amplitude on the fiber damage modes,chip morphology,fluctuating cutting force and percentages of energy dissipation mechanisms were analyzed.Based on the sensitivity analysis,the optimal combination of ultrasonic vibration parameters was evaluated to avoid serious machined surface damage.The novel cohesive element model was proposed to avoid element destortion and to model the interfacial debonding.The macroscale FE simulations of conventional drilling and ultrasonic vibration drilling were carried out,and drilling force,exit delamination and hole circularity from the different machining process were compared and the cutting mechanisms revealed.The kinematic model for ultrasonic vibration cutting process was established,and the relationship between instantaneous chip thickness/cutting force and ultrasonic vibration parameters/machining parameters was established.It is proved that this approach can predict the influence of different ultrasonic vibration parameters and processing parameters on the processing stability.