Assessing the effectiveness of a natural cellular material used as safety padding material in motorcycle helmets

Abstract

The efficiency of cork as a material dedicated to energy absorption under impact loading is studied in the present work. The viability of the application of micro-agglomerate cork (MAC) padding on a motorcycle helmet, is studied using finite element simulations of impact tests, considering the specifications of the European Standard ECE-R.22/05. Expanded polystyrene (EPS) is a widely used material, with excellent results in energy-absorption applications. However, after a first impact, the capability of EPS for energy absorption is significantly decreased, due to the almost total absence of elastic springback. However, cork is a material characterized by having both a good energy-absorption capability and high elastic return, due to its viscoelastic behavior, meaning that its capacity to keep absorbing energy is almost unchanged after the first impact. In this work, a three-dimensional numerical model of the helmet-head system is developed, including the outer shell, safety padding and the head, together with its interactions and constitutive models suitable for the analyzed materials. Results show that the developed models can adequately reproduce the behavior of EPS and MAC, in the context of a preliminary analysis. The referred helmet-headform is then submitted to impacts at different points, as specified by the European Standard. The results from helmeted impacts with EPS padding are compared against experimental values. The application of MAC in the protective padding of the helmet is studied and the results, concerning the acceleration of the gravity center of the head, Head Injury Criterion (HIC) values and kinetic energy are presented. Results obtained with EPS and MAC are compared and discussed. Concerning cork, although the maximum acceleration values of the headform and the HIC values were not verified to be within the established limits of the regulatory standard, the results are promising, launching a sound basis for a more thorough work on the application of cork as a new material for advanced applications as an energy-absorption system.

Comparing the mechanical performance of synthetic and natural cellular materials

“…e l s e v i e r . c o m / l o c a t e / m a t d e s for multi-impact applications, such as EPP [7] and agglomerated cork [2,3] as motorcycle helmet liners.…”

“…After one impact, the capacity of this material to keep absorbing energy is almost unchanged. Few researchers recently studying this material also tried to employ it in a great variety of applications, such as road helmets [2,3] and vehicle's passive safety mechanisms [18]. When compared to synthetic cellular materials, cork also appears as a sustainable alternative, once it is fully recyclable and the tree is not harmed as renewing its outer bark every nine years.…”

“…Cellular materials are the material of election also for personal protective devices where the best example can be found on head protection systems such as road-helmets [2][3][4][5]. In fact, under compressive loading, these materials can undergo large strain deformation while maintaining a low stress plateau before reaching densification.…”

Static and dynamic mechanical response of different cork agglomerates

“…Some exceptions are cork (and agglomerated cork) [5][6][7] or EVA (Ethylene vinyl acetate) foam [8] among others, that present the capacity to almost return to its original form after unloading. For EVA, such occurrence can be noticed in many soles shoes and for cork in wine stoppers.…”

New composite liners for energy absorption purposes

“…A recent work studied the possibility of using micro-agglomerated cork as a substitute for expanded polystyrene in motorcycle helmets [12]. This work was based on a helmet already placed in the market and a full replacement of EPS by micro-agglomerated cork was made in the two paddings located between the head and upper top of the helmet as shown in Fig.…”