In Situ Imaging Of Cutting Process And Prediction Of Strain/Stress Fields

Excessive plastic deformation occurs due to the thermo-mechanical loads during the cutting process.It is well known that the plastic deformation has a great influence on the surface integrity and the service performance of the high-end products.Currently,there exist two problems in the traditional cutting predictive models.One is that the predicted plastic deformation is rarely verified using direct and effective measurement methods.The other one is that the shear angle,tool-chip contact length,cutting strain and other internal variables could not be accurately predicted due to the errors with the workpiece 's constitutive model and friction model.In order to solve these two problems,the high-speed imaging technique is adopted in this thesis to in-situ observe the workpiece deformation during the cutting process.The dig-ital image correlation technique and streamline technique are used to derive the deformation field and the strain field,respectively.Predictive cutting models are developed to predict the strain/stress fields and subsurface microhardness.During the developments of the predictive models of the cutting strain and stress,the following innovative results have been achieved,1.A new method based on the measured workpiece deformation field is proposed to identify the material constitutive parameters.The shear angle,tool-chip contact height,and workpiece deformation field are in-situ measured during the cutting process.Finite element modeling of the cutting process is conducted until the predicted results are close to the measurements.Low speed cutting experiments of aluminum alloy and conventional speed cutting tests of titanium alloy are carried out to identify the plastic constitutive parameters and the thermo-viscoplastic constitutive parameters,respectively.2.A novel method based on the experimentally measured workpiece deformation field and elastoplastic constitutive model is proposed to predict the strain/stress fields of the work-piece.Firstly,the measured equivalent strain and strain rate fields are used to calculate the workpiece equivalent stress field according to the plastic constitutive model.Secondly,the elastic/plastic strain fields and stress field of the workpiece are computed using the incre-mental analysis theory,and the modification of the hydrostatic pressure is performed to recover the stress tensor in the main deformation region according to the equilibrium equa-tion.The average error of the predicted tangential cutting force through integration of stress tensor along the shear plane is Finally,the relationship between the residual unbalanced node force and the deviatoric stress is established thanks to the finite element method.Combined with the plastic flow rule,the velocity field of the workpiece is modi-fied to derive a cutting strain/stress fields that satisfy both the boundary conditions and the equilibrium equation.The largest difference between the predicted tangential forces and the measurements is 5%.3.An experimental-numerical hybrid cutting simulation model is developed to predict the cutting strain and stress fields.A steady-state cutting process is achieved in the short stroke cutting process by gradually updating the workpiece 's initial state of the equivalent plastic strain field,temperature field,and tool-chip friction coefficient.The experimental measurement results are preserved in the proposed method.Moreover,this method could not only obtain the stress field that satisfies both the boundary condition and the equilibrium equation but also yield a steady-state deformation field of the workpiece.Experimental verification shows that the shear angle,the tool-chip contact height and the cutting force predicted by this method are very close to the measured values.4.A semi-analytical model is established to predict the subsurface plastic deformation of the machined workpiece.The cutting models proposed by Oxley and Fang are used to calculate the mechanical and thermal loads on the subsurface.Then the stress and temperature fields of the workpiece are obtained based on the contact mechanics and the moving heat source theory,respectively.According to the elastic-plastic mechanics and a blending function,the plastic strain of the subsurface is calculated.Finally,the subsurface hardness is predicted based on the relationship between plastic deformation and micro-hardness.It is found that the relationship between the subsurface deformation between the cutting speed and uncut chip thickness and the relationship between the microhard-ness and the uncut chip thickness are well predicted when compared with the measurements.