Surface Topography Characterization And Stress Concentration Coefficient Of Ball Milling For Fatigue Performance

Ball-end milling is widely used in the processing of curved surface key parts in the fields of aviation,shipbuilding and automobile.The appearance of milling surface is closely related to the stress distribution of parts during service,and the stress concentration is easily caused by the uneven stress distribution,which leads to the deterioration of fatigue performance and the reduction of fatigue life of parts.At present,due to the lack of analysis methods and evaluation parameters of the machined surface topography oriented to fatigue performance,the relationship between the geometric characteristics of the milled surface and fatigue is still unclear,and the performance can only be improved by increasing the surface roughness.The influence of surface topography on fatigue performance needs to be revealed.Therefore,the surface topography of ball-end milling is taken as the research object in this paper,and the stress concentration coefficient which has a significant influence on the surface fatigue performance and is determined by the surface topography is taken as the research link.The internal influence relationship between surface topography-stress concentration coefficient-fatigue performance of ball-end milling is studied by numerical simulation,fatigue test and DIC(Digital Image Correlation)detection.On the basis of realizing the characterization of milling topography,the solution method of stress concentration factor is proposed,a method for solving stress concentration coefficient is proposed to explore the influence mechanism of surface topography on fatigue properties,and further improve the evaluation indexes of surface fatigue properties related to processing.It has important engineering significance and guiding role for improving the processing technology level,service performance and fatigue life of key parts of major equipment.In order to show the influence of surface topography on the stress concentration coefficient,the numerical simulation of fatigue life of thin plates with different morphological characteristics was carried out,and the relationship between surface topography and stress concentration coefficient was studied.The influence of stress concentration coefficient on surface fatigue under different loading conditions was analyzed,and the correlation between stress concentration coefficient and fatigue performance was explored,which proved the effectiveness and feasibility of using stress concentration coefficient to describe surface topography characteristics and study the influence of surface topography on fatigue life,which provided a basis for subsequent research.The formation process of ball-end milling surface is analyzed,and the spiral edge line equation of ball-end milling cutter considering tool wear is established.Combined with milling parameters,motion trajectory and tool vibration,the mathematical model of dynamic spiral edge line is obtained by using spatial coordinate transformation.According to the established model,the point cloud data of the topography are solved,and the relationship between the point cloud and the topography is determined.The modeling method of the ball-end milling topography is proposed to realize the characterization of the ball-end milling surface topography considering the milling parameters,tool inclination angle,tool wear,tool vibration and other factors.The reference plane of the spatial discrete points is fitted,and the theoretical calculation of the morphological characterization parameters based on the point cloud data is carried out.Using the white light interferometer detection data of ball-end milling surface,aiming at the maximum height S_z and the arithmetic average height S_athat can express the peak and valley characteristics of surface topography,the accuracy of ball-end milling surface topography characterization is verified by comparing the theoretical calculation value with the experimental detection value,which lays a foundation for elucidating the relationship between surface topography and stress concentration factor.Based on the similarity between the micro-disturbed surface and the ball-end milling surface,the first-order perturbation theory is applied to study the change and value of the stress on the micro-disturbed surface.The surface stress concentration coefficient solution equations are established for the two-dimensional and three-dimensional surfaces,respectively.The influence of surface topography on the stress concentration coefficient is revealed.For the three typical morphologies,the data of the surface profile are fitted,and the surface stress concentration factor field is calculated theoretically according to the established surface stress concentration factor equation.Three typical topography samples were prepared,and the DIC tensile strain detection of the samples was carried out.According to the transformation relationship between stress and strain,the extreme value and distribution data of the stress concentration factor of the whole field were obtained,and the accuracy of the theoretical calculation of the stress concentration factor of the milling surface was verified.The stress concentration coefficient of ball milling surface is correlated with the basic equation of fatigue life,and a fatigue life model considering the stress concentration coefficient is established.Tensile fatigue experiments and numerical simulation of fatigue life of different milling surface topography samples were carried out,fracture morphology and fatigue crack propagation path under different surface topography were studied,and the effectiveness of the surface topography characterization and stress concentration coefficient solution method for ball milling for fatigue life was verified through the comparative analysis of experimental and simulation data.According to the established milling topography characterization and stress concentration solving methods,the surface stress concentration coefficient under different milling parameters is calculated,the influence law of milling parameters on the stress concentration coefficient is studied,and the intrinsic influence relationship between milling parameters,surface topography,stress concentration coefficient and fatigue performance is obtained,which provides effective data reference for engineering analysts and designers.