| The brake pad,as one of the key components in the brake system of the high-speed train,is a vital link in the brake process,playing a significant part in determining the safe operation of the fast-running highspeed train.However,the brake pad will be excessively and eccentrically worn after a long time of service due to the uneven pressure acting on it,which may greatly reduce the brake torque and thereby make the train operation unreliable.In addition,friction-induced vibration and noise(FIVN)generated during the brake process generally relating to the poor wear situation at the brake interface can be extremely annoying.Such phenomenon can not only pollute the environment but also affect the ride comfort of the passengers.Furthermore,friction-induced vibration generated from the brake interface will affect the reliability of the brake system and aggressive the wear situation of the pad,causing the friction block to fall off form the brake pad.Therefore,for the sake of safe operations of the high-speed railway,it’s of great significance to comprehensively study the tribological behaviors at the brake interface,so as to find effective ways to improve wear and FIVN performance of the brake pad.In practical settings,the brake pad of a high-speed train consists of multiple friction blocks.For enhanced heat dissipation and pressure welding process,perforated structure is introduced to the friction block.However,the effective contact area is inevitably reduced because of the existence of the hole.If appropriate filling materials are filled into the hole,on the one hand,the contact area is compensated,on the other hand,the role of wear debris flow at the brake interface influencing the surface wear,thermal distribution and FIVN characteristics can be figured out through comparing the tribological behaviors of the perforated block and the blocks filled with different additive materials,which can provide new guidance to optimize the interfacial properties of the friction block.In this study,the wear mechanism of the brake pad used on the brake system of CRH380 A high-speed train has been fully revealed through a variety of micro analysis means in order to find ways to modify the properties of the brake interface.Brake drag tests using perforated friction block and blocks filled with magnet,body materials,composite material and Mn-Cu damping alloy material are performed on a selfdesigned small-scaled brake dynamometer.Conbined with finite element analysis,the effect of different filling materials on the tribological behavior of the brake interface is comprehensively studied.Relatively low operational speed condition of high-speed train is mainly concerned in this experimental study,considering the fact that the fast-running high-speed train is generally stopped by electro-pneumatic blended brake technique.At first the train is deaccelerated by electric brake technique(in most case is regenerative brake technique),and pneumatic brake technique is not applied until the speed is below 60 km/h.After that the regenerative brake will lose its efficacy and the power generation performance becomes poor when the speed of the train is lowered to 5-15 km/h,and consequently the electric brake will be cut off,leaving the pneumatic brake to complete the final stage of the brake process.Moreover,the brake noise is most likely to happen at low operational speed.The main works and conclusions of this study can be drawn as follows:(1)The failure features of the friction block used on the CRH380 A high-speed train is charcterized utilizing a variety of micro-observation methods,and the damage mechanism of the friction block is revealed.The relations between the damage characteristics and the contact stress distribution are established by analyzing the time-varying contact stress of the brake interface in finite element software.It is found that the main failure mechanism of the friction block is mainly characterized by peeling off and surface crack.The pores inside the friction block formed during the manufacturing process,the differences between the hardness and wear resistance of different components are the causes leading to the material peeling off.Stress concentration is likely to occur on the edge of the hole,resulting in small cracks on the block surface which then gradually expand into the block along the boundaries of the different components.The damage analysis and the finite analysis confirm that the eccentric wear is mainly caused by the contact stress concentration at the leading edge and Stress concentration is the cause of the crack initiation and expansion.(2)Three kind of friction blocks,which are perforated block,chamfered block and perforated block filled with magnet,are employed to conduct brake test and the experimental results of the different friction blocks are compared,aiming at figuring out the effect of wear debris flow on the brake performance of the high-speed train.It is found that both the hole structure and the filled magnet can affect the wear movement on the brake interface,thereby affecting the FIVN performance of the brake system.Compared with the perforated block,the chamfered block has a better performance to trap wear debris since the wear debris is more easily to be swept into the hole owing to its larger opening.Therefore,the wear situation and the FIVN performance is significantly superior to the original block.After adding the magnetic filler,however,most of the wear debris are attracted into the hole,which breaks the dynamic balance between the generation and discharge of the interface wear debris.The effect of the wear debris to bear the brake force and lubricate the interface is wekened,causing more surface crack initiation and peeling pits,as a result,high-intensity FIVN with complex frequency components is triggered from the brake interface.Therefore,the tribology behavior is worsened.(3)Friction blocks filled with powder metallurgy,cooper,graphite and cast iron are introduced to test their brake performance on the brake dynamometer.Complex eigenvalue analysis and contact stress analysis are performed on finite element software.The effect of different additive fillers on the brake tribology behavior is investigated,and the relationship between the wear situation,thermal distribution,and FIVN at the brake interface is discussed.The results find that the additive materials don’t have visible effect on the interfacial friction coefficient and the system vibration characteristic like vibration frequency and vibration mode.However,the tribology behaviors in terms of wear behavior,thermal distribution and FIVN characteristics at the brake interface are significantly affected by the fillers.The wear trapping performance of the block is reduced after adding powder metallurgy filler into the block,consequently the wear behavior is worsened and the FIVN of the brake system is intensified.Significant thermal concentration is observed on the brake interface when the friction block is filled with graphite.For the cast iron filler case,the number of large contact plateaus are significantly reduced,which lowers the contact stiffness and improves the wear situation and thermal distribution of the braek interface,and the FIVN of brake system is greatly reduced.The wear behaviors caused by the additive fillers are the key factor affecting the FIVN and the interfacial thermal distribution.Filling proper additive materials into the friction block contributes to reduce the FIVN and improve the interfacial wear behavior and thermal distribution.(4)Composite material and Mn-Cu damping alloy are chosen as fillers to add into the friction block,so as to investigate the performance of composite material and damping material in influencing the tribology behavior and FIVN.To be fairly persuasive,the experimental results are compared with those of the original block and the Powder metallurgy filled block.The results indicate that the wear components and the wear debris flow behavior exhibit visible difference after introducing these two fillers,so as the surface morphology and the thermal distribution characteristics.The FIVN of the original block is lower than that of the powder metallurgy filled block.While the FIVN of the composite material filled block is weaker than the other two blocks.The FIVN is significantly surpressed after introducing Mn-Cu damping alloy as the filler,which is the smallest among all the brake systems of different friction blocks.The finite element analysis shows that the physical properties(damping properties)of the Mn-Cu damping alloy have little effect on the mode coupling vibration and the interfacial contact stress distribution of the brake system,suggesting the improved interfacial tribology behavior is the only factor that influences the FIVN and the thermal distribution of the brake system. |
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