# Evaluation of Impact Energy Absorption by Natural Fiber Composites in Motorcycle Helmets
## 1 Introduction  
The motorcycle industry has experienced unprecedented growth, demonstrated by a significant increase in the motorization rate, which reflects the relationship between the number of registered vehicles and the population. Between 2000 and 2018, this rate surged by 456.2%, rising from 23.3 to 129.6 motorcycles per 100,000 inhabitants. By 2019, the motorization rate had increased to 134.1 [1]. With this expansion comes heightened concern regarding the safety of motorcycle riders. In this context, the motorcycle helmet serves as the primary protective equipment utilized by users of this mode of transportation. According to the Pan American Health Organization, wearing a helmet reduces the risk of death by 42% and the likelihood ofmajor injuries by 69%. In 2022, the number of motorcyclist fatalities in crashes reached 6222, marking the highest figure ever recorded. These statistics underscore the helmet's critical role in preventing fatalities and serious injuries in motorcycle accidents [2]. A motorcycle helmet is a vital piece of safety equipment composed of several components, including the shell, inner liner, comfort liner, chin strap, and visor, as illustrated in Figure 1[3]. This investigation will focus on the inner lining, commonly called the protective padding. This component is typically made of expanded polystyrene (EPS) due to its soft, cushioning properties and robust structure designed to absorb impacts and minimize injuries [4]. However, technological advancements, growing global demand for resources, and evolving consumer desires and expectations have underscored significant material availability and environmental sustainability challenges. In this context, there is an increasing interest in natural fibers as innovative formulations, and techniques are continuously being explored, developed, and applied. This trend indicates that natural fibers are becoming more viable long-term alternatives capable of meeting market demands [5]. The impact of energy absorption by natural fiber composites has been studied in different areas, especially in ballistic science. These include proposals for examining the ballistic impact performance of natural-fiber-reinforced polymer composites designed for multilayer armor systems [6]. A second study aimed to investigate the fundamental characteristics of tucum fiber, focusing on its density, pullout properties, microstructural features, and its application in polymer matrix composites. The composites were subjected to tensile tests, as well as impact tests related to Izod and ballistic conditions [7]. Another relevant study aimed to assess the ballistic performance and energy absorption characteristics of composites reinforced with pineapple leaf fibers, using various conditions evaluated through residual velocity and Izod impact tests [8]. Moreover, finally, a study examined the incorporation of 10, 20, and 30 vol % caranan fiber into epoxy resin, focusing on the properties related to Izod notch toughness and ballistic performance [9]. Additionally, an Asian research group examined the mechanical properties of composites made from banana fiber and ramie fiber reinforced with silicon carbide (SiC) nanoparticles [10]. Figure 1: Helmet components. Furthermore, investigations have been conducted on fabricating a hybrid composite using chopped banyan fiber and sawdust cellulose reinforced with an epoxy polymer matrix through the hand layup. It aims to quantify the impact of the hybrid composite by assessing its mechanical properties and surface morphology [11]. Other areas of research include composites derived from polypropylene and reinforced with hemp fibers, which have been created to enhance the understanding of how this reinforcement affects impact strength and water uptake behavior [12]. Lastly, a natural lignocellulosic fiber was extracted from cow dung waste, and its suitability as a reinforcing material in resin-based polymer composites was assessed [13].