3. Rail pad

The rail pads used were made of thermoplastic elastomer (TPE) with a nominal stiffness of 100 kN/mm. As a new track is subjected to the repetitive loads caused by the passage of the trains, it suffers plastic deformations that grow until the system reaches the steady state and the material responds in a true elastic manner.

It has been verified [6, 7] that a high value of the stiffness of the rail pad increases the dynamic overloads due to the nonsuspended masses, accelerating the deterioration of the track, while a low value causes an excessive displacement of the track with a significant increase in the rail stresses. Therefore, it is necessary to delimit the validity interval of the stiffness. In the technical specification for the supply of fasteners of the GIF [8], both static and dynamic stiffness values are delimited. The value of the static vertical stiffness, ks, must be included in the interval 80 ≤ ks ≤ 125 kN/mm, while the dynamic stiffness value, kd, must belong to the interval ks ≤ kd ≤ 2 ks. Due to the nature of the constituent material of the pads, stiffness may be modified by different environmental agents such as temperature, which can fluctuate in track between −20 and 80°C, humidity or deterioration suffered by the pads due to the continued mechanical stresses of compressive fatigue [9]. In this section, the influence of these variables on the stiffness of the pads has been measured, in order to identify the conditions leading to the nonfulfillment of the above-mentioned requirements.
