Determination of dynamic properties of rail pads using an instrumented hammer impact technique

The repeated impact of train wheels over sleepers can reduce the lifetime of a sleeper and degrade ballast. In more extreme cases it can lead to the breakdown of the concrete sleeper. Concrete sleepers are rigid compared to steel and wooden sleepers and therefore it is necessary to provide impact attenuation to prevent premature breakdown of the concrete. One of the measures employed to attenuate the effect of the impact loads on concrete sleepers has been the use of the resilient rail seat pads. Numerous analytical and numerical models have been formulated to investigate the dynamic behaviour of railway track substructures. All models require careful selection of the track component properties to satisfactorily represent track vibration response. However, there is currently no standard method available that can be used to evaluate the dynamic properties of the rail pads. At the University of Wollongong, an instrumented hammer was used to excite an equivalent single degree-of-freedom system (SDOF), incorporating a rail pad as a resilient element, to determine the dynamic properties using methods of modal analysis. The analytical SDOF dynamic model was applied to best fit the experimental modal measurements that were performed in a frequency range of 0-500 Hz. The curve fitting gives such dynamic parameters as the effective mass, dynamic stiffness, and dynamic damping constant, all of which are required for numerical modelling of a railway track.