State dependent properties of rail pads

Rail pad is one of the main components in ballasted railway track systems. It is inserted between the rail and the sleeper to attenuate dynamic wheel/rail interaction forces, preventing the underlying railway track components from excessive stresses. Generally, the dynamic design of tracks relies on the available data, which are mostly focused on the structural condition at a specific toe load. Recent findings show that track irregularities could significantly amplify the loads on railway tracks. This phenomenon gives rise to a concern that the rail pads may experience higher deterioration rate than anticipated in the past. On this ground, an innovative test rig for estimating the dynamic properties of rail pads has been devised at the University of Wollongong. A non-destructive methodology for evaluating and monitoring the dynamic properties of the rail pads has been developed based on an instrumented hammer impact technique and an equivalent single degree-of-freedom system approximation. This investigation focuses on the state-dependent model of rail pads where the dependent effects of frequency and static load contents can be distinguished. Based on the impact-excitation responses, the analytical state-dependent 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 modern numerical modelling of a railway track.