| By the end of 2020,China’s railway operating mileage has reached 146,000 kilometers,and the high-speed railway operating mileage has reached 37,900 kilometers,making it the country with the longest high-speed railway operating mileage in the world.In the long-term service of high-speed railway overhead lines under complex load conditions such as vibration,impact,tension and compression,fretting wear failure occurs in the key components of overhead lines,especially those of strands,such as slings,elastic slings and load-bearing cables.It becomes a potential hazard of safety accident in overhead line system of high-speed railway,which seriously affects the safe operation of high-speed railway.Copper-magnesium alloy(CuMg0.4)has high mechanical strength,corrosion resistance and good electrical conductivity,so it is an important material for preparing strand parts such as load-bearing cables,droppers and elastic slings in overhead-line system.However,due to the weak wear resistance and complex service environment,the failure problems of CuMg0.4 alloy such as contact loosening,corrosion and even fracture caused by fretting wear are very prominent in the operation of catenary system.According to the literature,the research on fretting damage of copper-magnesium alloy for high-speed railway overhead line structure is just in its infancy and it is limited to bending fretting fatigue,and there are few articles on fretting wear mechanism.In this thesis,copper-magnesium alloy,the structural material of high-speed railway overhead lines,was selected as the research object,and the fretting wear mechanism of copper-magnesium alloy was systematically studied to reveal the fretting wear mechanism.The research results provide basic data and theoretical support for improving and solving the failure of overhead line components of high-speed railway,and are of great significance to ensure the safe operation of high-speed railway.The main research work and conclusions obtained are listed as follows:1.A multi-functional tangential fretting wear tester was developed.A multi-functional tangential fretting wear tester with high-precision was developed independently.The tester uses piezoelectric ceramics as the fretting driving device to ensure the high control precision of displacement amplitude,and has the functions of dynamic alternating load,dynamic real-time electrochemical testing,high temperature and low temperature.It provides a research means for the experimental study of fretting wear mechanism under dynamic alternating load,corrosion medium environment and high and low temperature environment.The host machine,data acquisition and processing system of the tester run stably,it has a good repetition of test data and diversified functions.The successful research and development of the testing machine has laid a stable equipment foundation for the experimental research of this paper and related scientific research projects.2.Fretting wear mechanism of Copper-magnesium alloy under static load at room temperature.Adhesion and tangential force are the two main factors affecting the fretting process of copper-magnesium alloy under plastic condition.In partial slip regime,the tangential force is always at a low level,and the fretting is controlled by elastic coordination;in slip regime,the tangential force can overcome the adhesion bond strength in the contact zone,and the relative slip always occurs;in mixed regime,after a period of adhesion zone growth,the overall adhesive bond strength exceeds the external tangential force,the relative slip stagnates,and the fretting is coordinated controlled by the plastic deformation of the adhesion zone.The damage in the mixed regime goes through four evolution stages,slight adhesive wear stage,adhesive bonding and crack nucleation stage,crack propagation stage and damage accumulation stage.Adhesion is the main cause of fatigue crack initiation and fatigue spalling.The damage mechanism of the mixed regime is mainly adhesive wear and plastic deformation,and the adhesion will cause fatigue crack formation and fatigue spalling under large tangential force.The damage in the slip regime also goes through four evolution stages,namely the adhesive wear and plastic deformation stage,the third-body layer formation stage,the third-body layer thinning stage,and the steady-state wear stage.The nucleation and propagation of cracks in the subsurface strained layer is the main reason for delamination wear in the slip regime.The main damage in the slip regime is adhesive wear and fatigue wear(delamination),and oxidative wear and abrasive wear play a synergistic role.The third body layer formed under the synergistic effect of the above processes plays an important role in the wear mechanism.In the mixed regime,the increase of the displacement amplitude will directly cause the increase of the tangential force on the surface,which in turn promotes the faster initiation and propagation of cracks and increases the damage degree.In addition,by increasing the normal load will lead to a decrease in the area of the adhesive zone,resulting in severe fatigue spalling.In the slip regime,the increase of the tangential frequency in the slip regime will cause the plastic deformation of the wear debris to increase,and the formation speed of the third body layer will increase,but at the same time,it will also cause the peeling rate and transfer rate of the wear debris to increase,resulting in greater surface wear.3.Effect of dynamic load on fretting wear mechanism of Copper-magnesium AlloyThe dynamic load generates alternating compressive stress on the contact surface,which caused contact fatigue wear and accelerated fatigue crack growth at the edge of the wear scar in the mixed regime.Adhesive wear and plastic deformation are the main damage mechanisms in the mixed regime under dynamic loading conditions,accompanied by fatigue cracks and fatigue spalling.The increase of the dynamic load amplitude will lead to the increase of the tangential force of the contact surface in the mixed regime,which will lead to the increase of the plastic deformation,adhesive wear and fatigue wear.The increase of the dynamic load amplitude will lead to the increase of the stress amplitude in the contact area,resulting in more serious fatigue wear in the slip regime.The increase of the dynamic load frequency will cause the deterioration of the alternating stress condition on the contact surface,accelerate the initiation and propagation of fatigue cracks in the mixed regime,and cause more serious fatigue damage.The sliding friction force fluctuates at higher dynamic load frequency,which leads to aggravation of surface fatigue wear,which eventually leads to an increase in the wear volume.4.Effect of ambient temperature on fretting wear mechanism of copper-magnesium alloyAt the ambient temperature of 80℃,the material is prone to plastic deformation,which is beneficial to reduce the stress concentration in the mixed zone and weaken the fatigue crack propagation.At the ambient temperature of-10℃,solid or liquid water film is easy to be formed in the contact zone,which reduces the adhesive wear and fatigue wear in the mixed regime,and plays the role of interfacial lubrication and protection,and reduces the wear degree of the surface in the slip regime. |
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