Study On The Temperature-Dependent And Frequency-Dependent Dynamic Characteristics Of Pads Used In Unballasted Track Of High-Speed Railway

Elastic pads of rail fastenings are the only components which possess both elastic and damping characteristics in the ballastless track and their dynamic mechanical properties paly an important role in the vibration of vehicle-track coupled system. Rail pads are made of polymer viscoelastic materials, whose dynamic mechanical properties are relevant to temperature, frequency, amplititude, preload, loading history and so on. In high speed railway, the two factors which affect properties of rail pads significantly are extreme weather and high frequency caused by high speed. The purpose of the paper is to investigate dynamic mechanical properties of rail pads used in the the ballastless track of high speed railway in extremely cold weather and analyze the influence of temperature-dependent and frequency-dependent dynamic parameters of pads on the random vibration of vehicle-track coupled system.Firstly, the elastic pads in WJ-7B, WJ-8B and Vossloh300 rail fastenings which are usually used in the non-ballasted track of Chinese high-speed railway were taken as the test subjects. Their dynamic mechanical characteristics at a frequency of 0.3Hz were measured by the universal testing machine equipped with temperature control box in the temperature of -60??+20?. Experimental results show that the hysteretic loop of these three rail pads is close to ellipse, so they are linear viscoelastic materials. In the temperature of -60?-+20 C, they are at glassy state or rubbery state.+67? is not high enough to make them be at viscous flow state. They are provided with the conspicuous low-temperature sensitivity in the temperature of -55?-10? (-20? or 0?)and high-temperature stability above -10? (-20? or 0?). The storage stiffiness and loss factor of elastic pads in WJ-8B increase with increasing preload at load amptitude of 4kN.And then, the frequency-dependent dynamic parameters of the elastic pads in WJ-7B, WJ-8B and Vossloh300 rail fastenings in the frequency of 0-1000Hz at different reference temperature can be obtained by applying Time-Temperature Superposition (TTS) of high polymer materials.The results indicate that The storage stiffiness and loss factor of these three pads increase with increasing frequency within 1000Hz. The change in dynamic mechanical properties of pads with frequency is smaller than that with temperature and preload. Only taking the time-variant dynamic parameters of pads into consideration, can the random vibration response of vehicle-track coupled system be calculated accurately.And then, dynamic behavior of elastic pads in WJ-8B rail fastenings is characterized by fractional derivative Kelvin-Voigt model. The model has good agreement with experimental data. The results demonstrate that fractional derivative Kelvin-Voigt model can describe the dynamic behavior of pads accurately over a wide frequency range (within 1000Hz).Finally, the fractional derivative Kelvin-Voigt model of WJ-8B rail fastenings is applied in vehicle-track vertical coupled moving model which is solved by pseudo-excitation method and pseudo-steady state approach. Based on the model, the infulences of vehicle velocity, temperature-dependent and frequency-dependent dynamic parameters of pads on system responses are discussed. The results show that random vibration responses of coupled system increase with increasing vehicle velocity in the whole frequency range, which have adverse impact on safety, comfort, rolling noise, track deterioration and environmental vibration. The temperature-dependent dynamic parameters of pads mainly affect system response in middle and high frequency and have little influence in low frequency. When temperature decreases from 20? to -30?, the vertical vibration accelerations of vehicle system decrease at frequencies of 50?200Hz and increase at frequencies above 200Hz. The vertical vibration accelerations of rail drop dramatically at frequencies above 50Hz. The dominat frequency of vertical wheel-rail interaction force increases while the amplititude decreases. The dominat frequency and amplititude of vertical fastening force increases. The frequency-dependent dynamic parameters of pads mainly affect system response in middle and high frequency and have little influence in low frequency. Compared to constant parameters of pads, the vertical vibration accelerations of vehicle system used frequency-dependent dynamic parameters decrease at frequencies of 80-200Hz and increase at frequencies above 200Hz. The vertical vibration accelerations of rail increase in the whole frequency range. The dominat frequency and amplititude of both vertical wheel-rail interaction force and vertical fastening force increases. Applying time-variant frequency-dependent dynamic parameters of pads has similar infucence to applying frequency-dependent dynamic parameters on responses on vehicle system and rail. Compared to frequency-dependent parameters, the dominat frequency of both vertical wheel-rail interaction force and vertical fastening force increases while the amplititude decrease significantly. Obviously, compared to consant and frequency-dependent parameters, time-variant frequency-dependent parameters can characterize the dynamic behavior of pads more precisely. Therefore, it is suggested that time-variant frequency-dependent parameters should be taken ino consideration in the vehicle-track coupled model in order to calculate system responses accurately.