Study On Stress Distribution And Influences On Wear Of Moving Friction Pairs

Friction and wear are common phenomena in daily life and engineering.Although they have been widely concerned and studied for a long time,however,due to their complex nature,many friction and wear problems have not been completely solved yet.The mechanism of friction and wear in the case of contact between flat surfaces plays a decisive role in the performance,reliability and longevity of the working surfaces.A deeper understanding of the contact stress field,wear distribution and the change of frictional properties under different external pressures in surface contact is very important.In this thesis,using a combination of experimental methods and simulations,the change of contact stress fields and the damage situation of the contact surface during the friction process were analyzed.At the same time,using theoretical analysis and experiments,the influence of environmental pressure on friction properties of the contact of rough surfaces was studied.Firstly,using photoelasticity experiments,the contact stress field was observed.Photoelasticity experiments applied in the analysis of plane contact stresses have shown many advantages such as visual and instantaneous stress analysis of direct contact between surfaces,and so on.Through photoelasticity experiments,the principal stress difference(isochromatic)and its directions(isoclinic)in the subsurface can be obtained.In this thesis,the photoelastic experimental method was analyzed in detail.Based on the in-depth study of the manufacturing methods of photoelasticity materials,the material stress fringe coefficient of epoxy resin was measured by three methods.Then,photoelasticity experiments were carried out to measure the stress distribution of the slider friction model under different working conditions.The results indicated that: in the static contact,the stress is concentrated around the edges of the slider;when sliding,the stress is gradually deflected towards the direction of motion;and the load and stress are basically proportional to each other.In the experimental process,it can be observed that the contact stress field has unstable changes over time,which was caused by the stick-slip effect in the sliding friction process.Photoelasticity experiments can easily observe the contact stress field in the subsurface,but it is difficult to analyze the stress fields on the contact surface accurately.Another part of this thesis analyzed the contact stress by combining photoelasticity experiments and finite element simulations.In the same conditions as the experiments,the thesis used Abaqus software to simulate the two-dimensional finite element model of the slider friction model to analyze the process and characteristics of the contact stress changing from static state to sliding state.The actual analysis indicated that the simulation results are consistent with the photoelastic experimental results,so we can use the simulation results to predict further the contact pressure and frictional stress distribution on the contact surface.The following significant laws have been proposed: the contact stress is concentrated in the front edge of the moving direction,so the most likely area of failure is the front region;the influence of load and friction coefficient on the distribution of contact stress is very significant;however,in the static state,the variation of loads and coefficient of frictions has no obvious influence on the position of the contact pressure average(x/a = ?0.77).Real contact is the contact between limited surfaces in three-dimensional space,varies over time and is very different from plane contact models.Therefore,in order to do a deeper study of the real contact stress field,the three-dimensional sliding friction model was used for further simulations.The results indicated that the effect of stress concentration on the contact between limited surfaces is more significant,which is consistent with the real situation.The results of photoelastic experiments and simulation analysis show that stress tends to concentrate at the front of motion,while the wear theory studies show that wear is proportional to the contact stress,so it can be predicted that wear will tend to be greater in the front region of motion.In order to further study the real contact surface wear and compare the results of theoretical research,a wear test device was developed,and experiments were carried out based on the slider friction model.The experimental results indicated that: at the beginning of the movement,wear almost occurs on the front edge of the slider,then extends backward and finally cover the whole contact surface.This phenomenon is consistent with the results of photoelasticity experiment and simulation analysis.Besides,as the load increases,the degree of wear increases.Therefore,attention should be paid to control the wear condition of the surface area with high contact stress,especially the area at the front of motion,so as to improve the wear resistance of the contact surface and help to improve the overall service life of mechanical parts.In addition,friction surfaces always have a certain roughness,so the actual contact of the surfaces usually includes point contacts and line contacts.There always exist gaps between the two contact surfaces,so the air can fill these gaps,changing the friction properties of contact surfaces.In order to study this problem,based on the analysis of the geometric characteristics of rough surfaces,a mathematical-physical model of a slider on the inclined plane was built.Based on this model,the experimental principle was built.Three friction pairs of bearing steel/bearing steel,polycarbonate/polycarbonate,and bearing steel/polycarbonate were used to study metal/metal,non-metal/non-metal and metal/non-metal friction pairs,respectively.Then the corresponding experimental devices were built.The experiment was conducted under different pressure environments and dry friction conditions.The change of friction properties of rough surfaces was measured.The experimental results indicated that: as the external pressure increases,the normal force and tangential force(frictional force)increase;for the friction pairs and component parts with small hardness,lightweight and large contact area,the external pressure has a greater effect on their friction properties;therefore,in order to ensure the measurement accuracy of friction coefficient,the measurement should be carried out in vacuum environment.At the same time,the thesis proposed a new method to determine the actual contact area between the contact surfaces by using the model of the slider on the inclined plane placed in the different pressure environments.