Saturation Limit Theory Of Adiabatic Shear Localization Fracture And Its Prediction In High Speed Machining

High speed cutting technology is a key technology in advanced manufacturing field. With cutting speed increasing, adiabatic shear evolution is going to occur in primary shear zone of workpiece, the chip tansforms from ribbon chip to serrated chip with adiabatic shear band (ASB), and finally to adiabatic shear localization fracture (ASLF) resluting in complete fracture along ASB in serrated chip, which is an inevitable reslut in high speed machining. Although this fracture phnomenone is helpful for cutting efficiency increasing and chip breaking, the fluctuation of cutting force and temperature and the failure of cutting tool are difficult to avoid. The recent recognization and study on ASLF resulting from adiabatic shear evolution are deficiency. Therefore, in this work, experimental and theoritical researches on characteristics and mechanics condition of ASLF are carried out with three typical materials of medium carbon steel, stainless steel and titanium alloy. The primary items in this work are as follows:Firstly, through high speed turining experiment, chip-tool quick seperation experiment and metallography experiment, rate-ralating characterastics of ASLF in high speed machining is studied. The physical models of ASLF in high speed machining are built. The results show that with the increase of cutting speed, adiabatic shear evolution mainly experiments the occurrence of adiabatic shear, deformed band, transformed band and ASLF. ASLF in high speed machining is a periodic cycle process from energy convergence to energy release of primary shear zone. Inhomogeneity of thermal-mechanical distribution in primary shear zone causes ASLF to be composite mechanism of "ductile-brittle" fracture.Secondly, on the basis of propagation theory of stress wave, mechanics and deformation characterastic of primary shear zone in high speed machining, propagation model of thermo-plastic shear wave are established. The expressions of saturation limit and saturation degree are derived through energy euqation and minimal energy principle, and the criterion of ASLF in high speed cutting is porposed. The influnces of material characteristics and deformation conditions on the occurrence of ASLF are anlayized quantitatively. The results show that under higher strain, strain rate, adiabatic temperature rise and peak stress, a material with higher density, heat capacity, heat conductivity coefficient, and larger effects of strain hardening, strain rate strengthening and thermal softening is apt to ASLF in serrated chip.Thirdly, dynamic mechanical characteristics of material are studied through Split Hopkinson Pressure Bar (SHPB) under high strain rate. The strain rate hardening term of Johnson-Cook constitutive model is modified by using Power function and dynamic constitutive relation under high strain rate is obtained. According to high speed cutting experiment and tool-chip quick seqarating experiment, adiabatic shear deformation model and mechanical model of serrated chip is built. Combining with dynamic constitutive relation, adiabatic shear deformation conditions and shear band energy are calculated, and the influence of shear band energy on crack propagation is analysed. The results show that the strain and strain rate of shear band are considerably larger than those of rigid-plastic reion. Crack propagation degree in shear band increases with the increase of shear band energy.Finally, based on saturation limit theory of ASLF, dynamic constitutive relation and coupling relation between adiabatic shear deformation conditions and cutting conditions, prediction model of ASLF in high speed machining is proposed. And then critical cutting conditions of ASLF of medium carbon steel, stainless steel and titanium alloy are predicted theoritically and verified experimentally. The influence law of cutting speed, feed and rake angle on ASLF are revealed. The relation between material characteristics and ASLF sensibility is analysed. The results show that the critical cutting speed of ASLF decreases with the decrease of rake angle and the increase of feed. A material with higher heat capacity, larger effects of strain hardening, strain rate strengthening and compressive stress sensitivity has better ASLF sensitivity.The experiments verify the correctness of saturation limit theory and prediction model of ASLF.