The Effect And Suppressive Method Of Uneven Wear Of Friction Interface On Friction Induced Vibration Under Flat-flat Configuration

As an inevitable phenomenon,friction widely exists in the operation of industrial equipment,and the existence of vibration and noise caused by friction is also common in mechanical systems,such as wheel-rail system,brake system,gear system and so on.Severe friction induced vibration(FIV)at the friction interface not only causes the noise pollution,but also deteriorate the contact status,elevate the risk of fatigue failure,and the initiation and propagation of cracks.Many scholars have studied the friction induced vibration by using dynamic theory and proposed a few theoretical models to explain its mechanism.However,due to the time-varying characteristics,randomness,and sensitivity to working conditions of FIV,these mechanisms can only explain part of the phenomenon but cannot form a unified conclusion.So,lots of researchers have considered the microscopic wear as an important factor to FIV.However,if the friction interfaces endure a heavy load or frequent friction for a long period,changes will not only occur in the microtopography,but also in the macroscopic surface morphology.These macroscopic surface morphology changes caused by wear are also common,but its effects on FIV are barely reported.Through investigating the friction induced vibration under different uneven wear conditions,and then build up the relationship between the microscopic wear,macroscopic uneven wear,and FIV.It can not only reveal the generation and evolution of FIV,but also be critical in mitigating FIV and afford a way to monitor the friction surface.The main works of this thesis are as follows:1.In this study,the influence mechanism of non-uniform wear on frictional self-excited vibration in the form of surface contact was systematically investigated from tribological perspective.The increasing of FIV in experiment accompanies with the changes of macroscopic surface morphology(uneven wear).The increased friction coefficient also contributes to FIV,but the macroscopic morphology(uneven wear)dominates the increasing of FIV.From a tribological perspective,the microscopic topography plays a transition role in the process of macroscopic morphology influencing the FIV.The changes in the macroscopic surface morphology resulted in contact plateaus with large theoretical contact length(TCL),especially at the pad’s leading edge.The contact pressure distribution was closely related to the macroscopic contact morphology,and the uneven distribution of the concentrated pressure produces extensive surface damage in different locations and deteriorates the wear status,consequently increase the unstable vibrations of the friction2.The numerical results of surface wear have the same tendency with the experimental results.Besides,the change of the macroscopic surface morphology directly affects the change of the real part of the complex eigenvalues of the system.With the increase of the non-uniform wear,the positive real part of the system also increases,which strengthens the friction selfexcited vibration phenomenon of the interface.A 2-Do F lumped parameter system is established which consider the tangential surface uneven wear.With the increase of tangential wear,the positive real part of the system increases,the critical friction coefficient decreases,and the system has a higher tendency to be unstable.In addition,the results show that the pressure distribution of the system decreases with the increase of non-uniform wear,but more places on the friction interface bear more contact pressure,which leads to more serious wear and damage of the interface.3.The friction experiment was carried out on the scaling braking device to investigate the effect of the perforated structure and the installation direction of friction block on the surface uneven wear.An appropriate installing angle can mitigate the uneven wear phenomenon,and the perforated structure can divide the origin contact surface into two distinctive surfaces to bear the contact pressure so that the uneven wear phenomenon is decreased.The wear simulation process of multi-contact surface was established and applied to the full-scale finite element model of high-speed train’s brake system.By,comparing the FE results of original and improved brake pad,the uneven wear phenomenon of the interface can be directly controlled by changing the contact state of friction blocks.4.On the basis of the two-degree-of-freedom model which takes the tangential uneven wear of the interface into account,a new two-degree-of-freedom model is proposed,considering the effects of normal damping and tangential damping,and the nonlinearity of surface contact.Based on this,the effect of normal and tangential damping on the stability and dynamic behavior of the friction system are investigated.The results show that when the friction system is in modal coupling instability,the damping ratio of normal direction and tangential direction should be considered in damping design.There is an optimal damping ratio of the system.When the system’s damping ratio is equal to this value,the system stability will be best.On the contrary,if the damping ratio of a certain direction is simply increased,the stability of the system may decrease.When the friction system is in stick-slip instability,the normal damping of the system should be increased to reduce the vibration limit cycle.