Analysis Of Cutting Process Based On Slip Line Field And Fracture Mechanics

The cutting process of plastic material is a complex process including elastic deformation,plastic deformation,fracture,friction and thermal field.At present,the main view of plastic material cutting process is shear slip theory.Shear slip theory can better explain and predict the cutting force,contact length,deformation coefficient and other parameters in the cutting process.Fracture mechanics can effectively explain the phenomena such as split point,axe effect and fracture in the cutting process.The cutting process is dynamic complexity.When the shear slip theory or fracture mechanics are used to explain the cutting process,the two theories have some limitations due to the different application conditions and observation scales.Therefore,it is very necessary to reasonably explain the physical phenomena in different stages of cutting process by combining shear slip theory and fracture mechanics theory.The organic combination of the two theories complements each other in the analysis of the cutting process,and then comprehensively reveals the mechanical law of the cutting process,further deeply understands the cutting process,and provides a more reasonable theoretical for improving the tool design and optimizing the process parameters.Taking typical plastic materials such as Copper and Aluminum alloy 211 Z as research objects,it is established which include the mechanical model of elastic stage,the slip line field model of plastic stage,the fracture model of crack cracking and the chip beam model and plastic hinge model of crack dynamic propagation(unstable and stable).The contact length of sliding-line field,chip shape,chip sliding-band and fracture phenomenon in the cutting process are analyzed by means of orthogonal experiment and high-speed camera.The evolution law of the sliding field,the cracking mechanism in the front of the blunt and the transformation law of three chip shapes are revealed.By studying the different stages of the cutting process,the bridge between the theory of shear slip and the fracture mechanics is built with the chip beam model,which can reasonably explain the physical phenomenon of the cutting process.First of all,based on the contact state of tool and workpiece,the elastic theoretical solutions of point contact,blunt contact and blunt-rake contact are analyzed by using the elastic mechanics and Hertz contact theory,and the change rules of tool displacement and contact length are constructed;the theoretical solutions of cutting force in elastic state are established,The theoretical solution of cutting force in elastic state is established,and the nonlinear relationship between cutting force and tool geometric parameters and process parameters is revealed.those theory are the basis to analysis the plastic analysis,slip line field construction and fracture.Secondly,according to the distribution of the force field on the rake face and the blunt,the slip line field of point contact,blunt contact and blunt-rake face contact was established.As the tool feeds,The slip line field expands continuously and experiences four stages: blunt contact,uniform feed,material accumulation and stable state.After the contact area between the cutter and the workpiece is stable,the slip line field formed will not change.The influence law of tool geometry parameters and process parameters on slip line field was analyzed.Then,based on the theory of maximum elongation linear strain,the discrimination conditions of dead zone and failure zone are constructed.The crack generation is judged by the theory of crack opening displacement.After the crack was cracked,the chip was taken as the beam model.According to the boundary of upper and lower plastic zone,the conditions of forming plastic hinge was analyzed.Combined with the fracture theory and plastic hinge,the causes and critical conditions of saw-tooth chip formation was analyzed.The results show that the dynamic equilibrium state of "tool feed-crack open-crack propagation" or "tool feed-crack open-crack opening-plastic hinge" was formed in the cutting process.Finally,the orthogonal cutting experiment was carried out on the planer,and the cutting process was photographed with high-speed camera,and the chips are collected and then the samples are made.The morphology and slip band of chips were photographed by metallographic microscope.C# and Emgu CV are used to extract the contour of chip.The contour coordinates of chip were input into Solid Works to fit the contour.By means of high-speed camera technology,the change rule of slip line field was analyzed from the beginning contact of tool-workpiece to form stable cutting in the process of Copper,and the four stages of slip line field development are verified;the horizontal and oblique cracks in the cutting stage are captured,and the existence of open cracks in the front of the blunt is verified.Through the graphic analysis technology,the chip morphology of aluminum alloy is analyzed,and the corresponding relationship between the five segments of chip morphology and the six stages of cutting is revealed.In order to better explain the physical phenomena in the cutting process,slip line field analysis is used in the whole cutting area,and fracture mechanics is used in the failure area in the front of the tool blunt.In the cutting process of plastic materials,which are not only shear sliding phenomenon,but also two kinds of cracks: sliding crack and opening crack.The two kinds of fracture play different roles in the different materials phenomena.Tool blunt also plays a very important role in cutting,which has an important influence on cutting force,chip deformation and chip morphology.The analysis of the wrinkle from the chip beam model can well explain the open crack on the chip’s inner surface,and the criterion of plastic hinge and crack growth speed are used as the rule of the transformation of chip’s three morphologies.The research content of this paper combines the shear slip theory and fracture mechanics,which can provide a deeper and more comprehensive understanding of cutting mechanism,and provide a new perspective for tool design and process parameter optimization.