3.1 Influence of the temperature on the static behavior

The evaluation of the static behavior was determined from static stiffness tests (20/95 kN), applying a vertical load to the rail pad by means of the specific device. This device simulates the in-service working conditions, where the load was applied by means of a rail coupon, equipped with a ball joint that ensured the verticality of the loads. The mean vertical descent of the rail with respect to the support tool registered by four LVDTs (range: ˜ 5 mm), located in each of the corners of the support, was considered as the strain index of the rail pad. The load was applied by means of an actuator with a loading capacity of ˜250 kN. The influence of temperature on the behavior of the rail pad was evaluated by introducing the device into an environmental chamber adapted to the test machine. Measurements of Shore D

hardness [10] were carried out, aimed at finding a correlation of this parameter with the evolution of the mechanical behavior of the rail pad. The vertical static stiffness tests were carried out according to the provisions indicated in the technical specification [8] using a clamping force of 20 kN. The static behavior of rail pads at different temperatures (−10, 20, 50, and 80°C) was studied. In the graph of Figure 10, the evolution of the shortening of the rail pad in the third load-unload cycle at temperatures of −10, 20, 50, and 80°C can be seen.

The rail pad shortening ranges from maximum values of 0.93 mm at 80°C to a minimum of 0.59 mm at −10°C.

In Figure 11, the stiffness and the hardness of the rail pad versus temperature are represented in the double axis. An inverse linear correlation between static stiffness (20/95 kN) and the Shore D hardness with the rail pad temperature is shown; in summary, an increase in temperature causes the softening of the rail pad. An increase of 90°C in the test temperature, from −10°C, generates a 44% decrease in the stiffness of the rail pad (from 128 kN/mm to −10°C to 80 kN/mm at 80°C). The decrease in Shore D hardness is less noticeable (from 43.6 to 39.6) which is a decrease of 9.5% compared to the room temperature. The stiffness value is between

Figure 10. Static behavior of the rail pad at different temperatures.

Figure 11. Evolution of the stiffness and hardness of the rail pad with the temperature.
