| Friction braking of high-speed trains is mainly realized by the friction between the friction block of the braking pad and the brake disc.The trains must be able to stop within a specified distance to guarantee train safety.According to the braking control strategy for high-speed trains in China,mechanical braking(i.e.,friction braking)begins to intervene and works together with the electric braking system when the train travels below 60 km/h.When the train travels below 20 km/h,the train speed is only controlled by mechanical braking until full parking can be completed.In this case,intensive pad/disc friction tends to induce the high-frequency and high-intensity friction-induced vibration and noise(FIVN),resulting in the friction block of the braking pad to undergo abnormal wear and even severe spalling.This endangers traffic safety and causes noise pollution along the railway(a harsh squeal noise can be heard when the train approaches the station at a low speed).Additionally,high-frequency and high-intensity friction-induced vibration(FIV)of the braking interface of the high-speed train contains considerable energy.Accordingly,it is possible to collect and store the energy of FIV to supply power to passive sensors and promote the development of the wireless condition monitoring system of the braking system.Therefore,investigating the mechanism of the FIVN of the high-speed train braking system and improving the tribological behavior of the braking interface to achieve FIV energy collection are of great significance for ensuring the safety of high-speed trains.Aimed at the braking FIVN of high-speed trains,the dynamic correlation of the contact state and dynamic response of the braking interface of high-speed trains was thoroughly investigated from the friction and dynamics perspectives based on advanced analysis tools for surface morphology,the signal processing method,finite element simulation,and dynamic modeling.Meanwhile,the interface feature mechanism of the friction block(e.g.,the shape and installation direction)influencing the braking friction behavior was clarified,and novel methods combining the collection of the braking FIV energy and interfacial friction behavior regulation,which are represented by piezoelectric cantilever beams(PZT beams)and sandwich damping structures,were proposed.The main contents and conclusions of this study can be summarized as follows.(1)The brake disc’s noise characteristics and interfacial tribological behavior at 200,300,and 400 r/min were clarified by braking friction tests and finite element(FE)simulations.Additionally,the correlation with the contact state exhibiting FIVN features was discussed from the perspectives of contact pressure distribution on the interface,block surface morphology,and temperature distribution on the disc surface.Additionally,a mathematical model for a two-degree-of-freedom friction system containing a contact inclination angle was established to analyze the influences of the coefficient of friction(COF)and contact inclination angle on the stability and vibration intensity of the system.The results showed that under interfacial friction force,the brake disc drags the friction block to move tangentially,resulting in a contact inclination angle between the friction block and the normal direction of the brake disc.In this case,the brake disc and friction block no longer perfectly fit with one another,and both the friction force and interfacial contact area exhibit time-varying characteristics.Furthermore,the two-degree-of-freedom friction model containing the contact inclination angle revealed that the contact inclination angle increased as the COF increased.As a result,the friction system changed from a stable state to an unstable one.Modal coupling was also observed,which increased vibration intensity and the probability of generating squeal noises.Therefore,the contact inclination angle between the brake disc and the friction block is a key factor influencing the vibration features of the braking system during friction.This should be reduced by all means to improve interfacial fitting and promote the homogeneous distribution of contact stress,thus relieving the frictional wear of the interface and optimizing the vibration features of the braking system.(2)Drag brake experiments using hexagonal,pentagonal,and circular friction blocks are conducted using a self-designed small-scale brake dynamometer to investigate the influence of the friction block shape on the tribological and dynamical behaviours of high-speed train brakes.A four-degree-of-freedom model that considers the interface contact behaviour is established to investigate the influence of the contact stiffness at the friction interface on the stability and vibration characteristics of the braking system.The experimental results show that the surface of the hexagonal friction block is slightly worn,and the contact plateaus is relatively uniform in size,whereas the pentagonal and circular friction blocks show visible ploughing and exfoliation and a significant proportion of large contact plateaus.Accordingly,the hexagonal friction block produces considerably less FIVN than the pentagonal block and the circular block.Moreover,analysis of the vibration characteristics of the system indicates that the interface of the hexagonal friction block is mainly subjected to normal impact,whereas the interfaces of the other two blocks exhibit significantly more ploughing and shearing action in the tangential direction.In general,higher contact stiffness results in stronger FIV,thereby enhancing the instability of the brake system.In this work,it is found that the friction block shape has a significant effect on the contact pressure distribution.The hexagonal friction block exhibits a more uniform distribution of contact pressure than the pentagonal and circular friction blocks,resulting in the lowest contact stiffness,as well as the least wear,vibration,and noise.(3)To investigate the effect of the hexagon friction block installation direction on the surface tribology and the FIVN,tribological tests were carried out on a self-manufactured high-speed train brake dynamometer under different installation directions.Accordingly,a FE model with real material parameters and boundary conditions was used to conduct complex eigenvalue analysis(CEA),and the Archard wear formula was employed to simultaneously study the wear and vibration.The results demonstrate that the installation direction has little effect on the frequency and mode shape of unstable modes,but the FIVN intensity,the phase-space characteristics of the friction noise,and the damping ratio of the braking system are changed.The contact stress and wear are found to be mainly distributed in the leading edge and the inner area of the friction block,leading to the severe eccentric wear phenomenon,and the friction heat is mainly concentrated in this area and the inner region of the disc friction area.The degree of uneven wear of the block is found to vary with the change of its installation direction,which also causes differences in the amount of wear debris flowing in the brake interface and the damage to the interface.Under the joint action of wear debris,friction heat,and contact stress,the brake interfaces of the brake systems with different block installation directions are found to exhibit different tribological behaviors,which affects the intensity of FIVN and the phase-space characteristics of friction noise.(4)A PZT beam was designed and fabricated.It was fixed in different directions of the friction block holder and exposed to a tribological test for the braking performance simulation of high-speed trains to achieve improved interface tribological behavior and collect FIV energy.The surface tribological behavior of the friction block was investigated using multiple microscopic analysis tools,and the wear surface contact platform of the friction block was characterized by the Otsu threshold segmentation method to quantitatively analyze the effects of the PZT beam on the tribological behavior of the braking interface.Based on that,FE models were established for braking systems with different PZT beam orientations.Furthermore,the complex eigenvalue and implicit dynamics of these braking systems were investigated.Numerical simulation models were established for different braking systems,and the stability and dynamic features of these braking systems were investigated.The results showed that the friction block of the original braking system is exposed to severe irregular wear.Upon PZT beam installation,the eccentric wear of the friction block is mitigated,the number of large contact platforms on the worn surface is significantly reduced,and the homogeneity of the temperature was distributed in the friction area of the brake disc.The PZT beam can tune the contact state of the block in real time to avoid interfacial contact stress concentrating in the leading edge of the block,reduce the eccentric wear angle of the block,and improve the tribological behavior of the braking interface.Meanwhile,an appropriate PZT beam can convert the FIV energy of the braking system into electric energy via its own deformation,thus reducing the FIV of the braking system and improving the contact state of the braking interface.The PZT beam can effectively reduce the real part of the complex eigenvalue without significantly altering the frequency or mode of their unstable vibration.Additionally,the mass of the PZT beam has a significant effect on the unstable vibration of braking systems.However,its stiffness and piezoelectric parameters have negligible effects on the stability of the braking systems.(5)A sandwich damping piezoelectric structure(SDPS)was designed to improve the tribological behavior,reduce the FIVN of the braking system of a high-speed train,and convert FIV energy into electrical energy.The SDPSs were perforated using different configurations to improve the performance.FE analysis was used to analyze the mechanical properties of the SDPS,and tribological tests were carried out to investigate the effect of the SDPS and the perforation configurations on the tribological behavior and the conversion efficiency of FIV energy into electrical energy.A FE model of the test rig was established to conduct a CEA and transient dynamics analysis to discuss the effect of the SDPS on the complex modal,dynamic behavior,and output voltage characteristics.The results show that the proposed SDPS can convert FIV energy into electrical energy due to deformation,and the perforated structure produces greater deformation and a higher output voltage.The SDPS significantly affects the generation and evolution of FIVN of the braking system,reducing its persistence and intensity.The SDPS efficiently transforms the FIV energy of the braking interface into electrical energy and can continuously power several LED lights;the SDPS with the largest deformation capacity produces the highest output voltage.The SDPS installed in the braking system significantly reduces the eccentric wear angle and improves the friction and wear properties of the surface.The FE analysis results show that modal coupling occurs when the COF is low,and the critical COF is higher with than without the SDPS.The SDPS installed in the braking system improves the surface contact stress distribution of the friction block,which reduces the FIV intensity,and can transform FIV energy into electrical energy. |
PDF Full Text Request