# Influence of Aging Conditions on the Dynamic Stiffness of EPDM and EVA Rail Pads
## 5 Discussion  
This study has analyzed the influence of the main mechanical and chemical aging processes to which rail pads are subjected during their service life, demonstrating their effect on the dynamic stiffness of three different types of pads manufactured with EPDM and EVA, materials commonly used by rail pad manufacturers. To achieve this, an experimental program was carried out that included 864 dynamic stiffness tests under different degradation scenarios. The results allow for a more detailed understanding of how degradation mechanisms affect the dynamic stiffness of the pads under various aging conditions, which is fundamental for the proper design and maintenance of the railway superstructure. The importance of the degradation of polymers used in the railway sector has been demonstrated by [PERSON] et al. [12], who showed in their article how the mechanical properties of rail pads are affected by exposure to temperature, ultraviolet (UV) radiation, and degradation by chemical, biological, and mechanical agents. Mechanical degradation progressively increases the dynamic stiffness in all types of pads, with the most notable increase observed in the EVA pad (pad C in this study), which reached a 27% increase in the 2,000,000-cycle scenario. These results agree with those obtained by [PERSON] et al. [21], who observed significant increases in both static and dynamic stiffness for EPDM rail pads subjected to mechanical fatigue aging. One possible reason for these results is that the loads experienced by the rail pads may lead to a reduction or deformation of the protrusion area, resulting in an increase in stiffness. This hypothesis is consistent with the findings of [PERSON] et al. [22], who observed that the permanent deformation experienced by rail pads due to fatigue and repeated loading causes a significant increase in their dynamic stiffness. Specifically, they found that pads exhibiting greater permanent deformation after repeated loading cycles also showed greater increases in stiffness. UV exposure has different effects depending on the material and the pad, although a general trend is observed in all cases: for low exposures, the stiffness is lower than for high exposures. This is most evident in the case of pad C, manufactured in EVA, where a 13% decrease in dynamic stiffness is observed under the lowest exposure condition and a 9.5% increase under the highest exposure. The effect of UV exposure on polymers has been studied by other authors [12; 23], such as [PERSON] et al. [23], who demonstrated that exposure to UV light causes modifications in  
Figure 16: Scatterplot correlating rail pad stiffness with tow load and maximum applied load for UV exposition in pad C.  
the structure of the EVA polymer, affecting its mechanical properties so that short exposures improve mechanical behavior while prolonged exposures deteriorate it. These findings differ from those obtained in the present study, where it is observed that, in the case of rail pads, geometry plays a crucial role, as noted by other studies [10,24]. It is likely that, under low exposure conditions, material degradation reduces the load-bearing capacity of the protrusions, leading to a decrease in dynamic stiffness, while for higher exposure conditions, their load-bearing capacity is exhausted, which would induce an increase in stiffness, as seen in Figure 16 Furthermore, it should be noted that the evaluation of mechanical properties by [PERSON] et al. [23] is carried out through tensile tests, whereas in the present study, dynamic stiffness was obtained through compression fatigue tests at different frequencies. Hydrocarbon exposure also produces different effects depending on the material and the pad. In the case of pad A, a reduction in stiffness is observed under both conditions, with an 11.5% loss at the lower exposure level and a partial recovery, reaching only a 3% decrease at the higher exposure level. On the other hand, a slight 5% increase was recorded for pad C under prolonged exposure.