# Influence of Aging Conditions on the Dynamic Stiffness of EPDM and EVA Rail Pads
## 1 Introduction
The mechanical behavior of polymeric materials used to fabricate rail pads is nonlinear and dissipative and is affected by several in-service conditions such as temperature, frequency, tow load, and axle load. In a previous study by [PERSON] et al. [10], three types of rail pads fabricated using EPDM, TPE, and EVA were exhaustively characterized (720 tests) under different combinations of the aforementioned factors to determine their influence on the dynamic stiffness of the component. This experimental information was subsequently modeled using Machine Learning algorithms. The analysis of feature importance revealed that temperature and tow load are the most influential variables affecting the dynamic stiffness of EPDM and TPE rail pads, while tow load is the most significant feature for EVA rail pads. Finally, in [PERSON] et al. [11], a Finite Element model of the slab track was developed to generate synthetic samples representative of actual in-service conditions where relevant track parameters were varied within their usual ranges using a Monte Carlo procedure. The results were then used to train and validate a series of regression Machine Learning models, which showed that the properties of the soil, characteristics of the rail pads, and axle loads have the greatest influence on the railway infrastructure. Polymer materials utilized in the railway superstructure are susceptible to degradation under various environmental conditions [12]. Such conditions encompass low and/or high temperature, exposure to rain and/or sunlight, water and/or soil properties, inducing different kinds of degradation mechanisms such as oxidation, hydrolysis, ultra-violet (UV) irradiation, chemical influence, aging or fatigue, among others. A paper authored by [PERSON] et al. [7] outlines the degradation mechanisms (physical-chemical and mechanical) experienced by rail pads exposed to the railway environment. The authors reported that environmental factors can cause the pads to progressively increase in stiffness, with values reaching up to 33-41% for a service life of 1-3 years, respectively. There is a recent bibliography on the aging and changes in the mechanical behavior of the materials used in this study. In this sense, [PERSON] et al. [13] show that under accelerated thermal aging, EPDM undergoes oxidation at high temperatures (120-140 \({}^{\circ}\)C) that generates hydroxyl and carbonyl groups, reducing its crystallinity and deteriorating its mechanical properties. Likewise, [PERSON] et al. [14] investigate \(\gamma\)-ray irradiation, finding that it reduces the Mooney viscosity and molecular weight of EPDM, increases the gel content, and raises the glass transition temperature, which modifies its internal structure and processing performance; finally, [PERSON] et al. [15] examine the thermo-oxidative aging of EPDM, noting that crosslinking predominates during degradation, which translates into an increase in hardness and tensile strength along with a decrease in permanent deformation. In the case of EVA, [PERSON] et al. [16] present a review in which the degradation of the EVA encapsulant used in photovoltaic modules is described under the influence of environmental factors (temperature, humidity, and UV radiation), affecting its chemical, mechanical, optical, and electrical properties, which in turn reduces the system's efficiency and durability. Other authors report similar results; for example, [PERSON] et al. [17] show that during thermo-oxidative aging at 165 \({}^{\circ}\)C in air, EVA degrades mainly due to the decomposition of its acetate groups, leading to significant changes in its electrical and mechanical properties. This research aims to experimentally evaluate how mechanical and environmental deterioration conditions affect the dynamic stiffness of rail pads manufactured from EPDM and EVA materials.