## 1 Introduction  
The continuous search for efficient and cost-effective materials to deal with crashworthiness applications has been driving many studies over recent decades [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. However, recent self-consciousness and environmental responsibility are promoting the search for so-called green materials featuring good mechanical properties allied to recyclability and sustainability. Presently, a great variety of personal safety gear employs energy-absorbing liners. They are used in sport accessories, protective vests and helmets, among others. The great majority of these liners are usually made of synthetic cellular materials such as expanded polystyrene (EPS) and expanded polypropylene (EPP), sometimes combining different densities within different pads, absorbing impact energy through permanent deformation. Under compressive loading, cellular materials can undergo large strain deformation while maintaining its low stress level almost constant until densification [13]. Due to the major oil-based plastics waste problem, natural alternatives shall be sought. A material such as cork stands out to be employed as energy-absorbing liners in safety gear. Cork is a natural cellular material capable of absorbing great amounts of impact energy [10]. [PERSON] et al. [14] performed quasi-static tests on different cork agglomerates and reported a value of 1.1 GJ/m\({}^{3}\) for a densified AC216 and 634 MJ/m\({}^{3}\) at the end of the stress plateau. The same cork agglomerate absorbed a total of 225 J in 850 J impact tests with 60 mm cubic samples at room temperature [9]. This makes cork an ideal substitute of synthetic cellular materials [3,4]. In particular, agglomerated cork, a by-product obtained from the scraps of wine-stoppers production, has been proving quite promising in terms of crashworthiness, showing nearly isotropic mechanical behavior and capacity to withstand extreme temperatures [15]. In addition, due to its viscoelastic behavior, cork recovers almost entirely after deformation, which is a desired characteristic in multi-impact applications [4,9,14,16] and its properties can be further tailored for specific applications [10,11]. Helmets are one of the most important types of personal safety gear, protecting our heads and especially our brains. An impact to the head can have serious consequences and even be fatal. Thus, head protection and safety helmets are a matter of extreme importance. In fact, road accidents are the main cause of head injuries and motorcyclists and cyclists greatly contribute to this [17, 18, 19]. Presently, helmets are used in many different applications such as military, emergency, and protective services, different types of work, sports, and other recreational activities. Each application demands different technical requirements. However, the main differences are in the shells (materials, geometries, and configurations). Regarding the energy-absorbing liners, there is no substantial difference between the different types of helmets. Helmet standards are also similar, with similar impact test configurations and energies [20, 21, 22]. In real-world accidents, helmets are likely to be subjected to multi-impact loads. Regarding motorcyclists, there is a significant risk of multiple head impacts (for instance, a head impact against a vehicle may be followed by a head impact against the ground) [23,24]. Generally, multiple impacts are expected to become a larger proportion of the total accident population as the latest countermeasures, developed primarily for single mode impacts, take effect [25]. A more recent study concluded that head-on collision, collision with a fixed object, and multiple impacts were the most frequent types of impact in the group of motorcyclist fatalities [26]. To improve helmets quality in terms of safety, comfort, and aesthetics, new materials and designs have been proposed by some manufacturers and researchers. Some of these new concepts have been proposed to enhance the energy absorption properties. [PERSON] et al.