Crash helmet testing and design specifications

Abstract in dissertatio

High-energy impact testing of agglomerated cork at extremely low and high temperatures

Agglomerated cork, made from the scraps of wine stoppers, has been finding a wide set of applications due to its excellent thermal and acoustic insulation properties. The random orientation of grains makes the material nearly isotropic, while its dominant viscoelastic behaviour and nearly zero Poisson's ratio make the material also very interesting in applications where dimensional stability is highly demanded. With proven properties, agglomerated cork has been widely used for manufacturing of architectural facades, in civil construction, aerospace engineering and even home appliances production. For outdoor applications, the performance of cork material under different working temperatures is a vital point to be considered. This paper assesses the capability of five different types of cork agglomerates to withstand 500 J impact energy under different temperature conditions. Keeping 11.2 kg impact mass and velocity of 9.2 m/s, impact tests were performed for a wide range of temperatures starting from sub-zero temperature (−30°C) up to 100°C in order to cover a full span of working circumstances. Results show significant variations of amount of absorbed energy depending on testing temperature, calling the attention of designers and product developers for important aspects to be considered upon the application of this material under extreme weather conditions.publishe

Análise biomecânica de impactos com capacetes: novos materiais e geometrias

Doutoramento em Engenharia MecânicaA cortiça é um material celular natural capaz de suster quantidades consideráveis de energia. Estas características tornam este material ideal para determinadas aplicações como a proteção de impactos. Considerando equipamentos de segurança passiva pessoal, os materiais sintéticos são hoje em dia os mais utilizados, em particular o poliestireno expandido. Este também é capaz de absorver razoáveis quantidades de energia via deformação permanentemente. Por outro lado, a cortiça além de ser um material natural, é capaz de recuperar grande parte da sua forma após deformada, uma característica desejada em aplicações com multi-impacto. Neste trabalho é efetuada uma avaliação da aplicabilidade da cortiça em equipamentos de segurança pessoal, especificamente capacetes. Vários tipos de cortiça aglomerada foram caracterizados experimentalmente. Impactos foram simulados numericamente para avaliar a validade dos modelos constitutivos e as propriedades utilizadas para simular o comportamento da cortiça. Capacetes foram selecionados como caso de estudo, dado as energias de impacto e repetibilidade de impactos a que estes podem ser sujeitos. Para avaliar os capacetes de um ponto de vista biomecânico, um modelo de cabeça humana em elementos finitos foi desenvolvido. Este foi validado de acordo com testes em cadáveres existentes na literatura. Dois modelos de capacete foram modelados. Um modelo de um capacete rodoviário feito de materiais sintéticos, o qual se encontra disponível no mercado e aprovado pelas principais normas de segurança de capacetes, que serve de referência. Este foi validado de acordo com os impactos da norma. Após validado, este foi avaliado com o modelo de cabeça humana em elementos finitos e uma análise ao risco de existência de lesões foi efetuado. Com este mesmo capacete, foi concluído que para incorporar cortiça aglomerada, a espessura teria de ser reduzida. Então um novo modelo de capacete foi desenvolvido, sendo este uma espécie de modelo genérico com espessuras constantes. Um estudo paramétrico foi realizado, variando a espessura do capacete e submetendo o mesmo a duplos impactos. Os resultados destes impactos e da análise com o modelo de cabeça indicaram uma espessura ótima de 40 mm de cortiça aglomerada, com a qual o capacete tem uma melhor resposta a vários impactos do que se feito de poliestireno expandido.Cork is a natural cellular material capable of withstanding considerable amounts of energy. These features make it an ideal material for some applications, such as impact protection. Regarding personal safety gear, synthetic materials, particularly expanded polystyrene, are typically used. These are also able to absorb reasonable amounts of energy by deforming permanently. On the other hand, in addition to cork being a natural material, it recovers almost entirely after deformation, which is a desired characteristic in multi-impact applications. In this work, the applicability of agglomerated cork in personal safety gear, specifically helmets, is analysed. Different types of agglomerated cork were experimentally characterized. These experiments were simulated in order to assess the validity of the constitutive models used to replicate cork's mechanical behaviour. In order to assess the helmets from a biomechanical point of view, a finite element human head model was developed. This head model was validated by simulating the experiments performed on cadavers available in the literature. Two helmet models were developed. One of a motorcycle helmet made of synthetic materials, which is available on the market and certified by the main motorcycle helmets safety standards, being used as reference. This helmet model was validated against the impacts performed by the European standard. After validated, this helmet model was analysed with the human head model, by assessing its head injury risk. With this helmet, it was concluded that a thinner helmet made of agglomerated cork might perform better. Thus, a new helmet model with a generic geometry and a constant thickness was developed. Several versions of it were created by varying the thickness and subjecting them to double impacts. The results from these impacts and the analyses carried out with the finite element head model indicated an optimal thickness of 40 mm, with which the agglomerated cork helmet performed better than the one made of expanded polystyrene

Brain injury mitigation effects of novel helmet technologies in oblique impacts

Cyclists are a rapidly growing group of the world population, particularly after the COVID-19 pandemic which made cycling an attractive form of active mobility for commuters. Yet, cyclists are among the most vulnerable road users. Their severe injury and fatality rate per passenger mile are several folds larger than car occupants and bus passengers. Analysis of accident data shows that impacts to a cyclist’s head occur at an angle in vast majority of real-world head collisions. This produces large rotational head motion. There is significant body of research that shows rotational head motion is the key determinant of brain deformation and subsequent damage to the brain tissue. Hence, novel helmet designs adopt shear-compliant layers within a helmet with the aim of reducing the rotational head acceleration and velocity during an impact, hence reducing risk of brain injury. Cellular materials can be engineered to have interesting mechanical properties such as negative Poisson ratio or anisotropy. Their cellular structure gives rise to a unique combination of properties which are exploited in engineering design: their low density makes them ideal for light-weight design, and their ability to undergo large deformations at relatively low stresses make them ideal for dissipating kinetic energy with near-optimal deceleration. As revealed in this thesis, it also is possible to engineer cellular structures to have high or low shear stiffness with minimal change to their axial stiffness, and vice versa. This has the potential to be very beneficial for cases that require oblique impact management where both axial and shear stiffnesses play a role. However, this domain has seldom been explored, let alone applied to a use case which may result in improved performance that saves lives such as helmets. The main question this thesis aims to address is: Can helmets be improved to reduce the risk of cyclist brain injury in oblique impacts? To answer this question, it was necessary to first assess conventional helmets and emerging technologies aiming to improve helmets in oblique impacts. Hence, 27 bicycle helmets with various technologies were assessed in three different oblique impact conditions. The outcome of studying this proved that helmets may be improved with shear compliant mechanisms between the head and helmet. However, the improvements were marginal and highly dependent on impact site. This is hypothesised to be due to the presence of expanded polystyrene (EPS) foam alongside these shear-compliant mechanisms which hinders their performance. We found that one of the best performing helmets in oblique impacts was one that utilises air and entirely replaces EPS foam yet had some drawbacks such as lack of reusability and shell structure. This encouraged the work that followed which aimed to replace the EPS foam layer in helmets with an air-filled rate-sensitive cellular structure. This work leveraged finite element modelling which employed visco-hyperelastic material models which were validated with axial and oblique impact tests of the bulk material and cellular array samples different speeds. The novelty is that the axial and shear stiffness of the cells could be tailored independently with simple changes to the geometry of the cells. This led to an exciting investigation to determine whether shear-compliant cells outperformed their shear-noncompliant counterparts, which exhibit similar axial stiffness, with respect to brain injury metrics in a helmet. The results showed that, although this may be the case, often the shear-compliant cells dissipated less energy during impact and bottomed-out as a result, leading to adverse effects. Hence, introduction of shear-complaint structures in helmets should be done with care as the energy is dissipated in shear with such cellular structures during oblique impacts which needs to be properly managed. In future, the performance improvements may be implemented for different impact speeds utilising the viscoelastic nature of the cells and inflation of the cells to change their shape.Open Acces

Characterization of cork and cork agglomerates under compressive loads by means of energy absorption diagrams

Cork and cork agglomerates could be suitable replacements for petroleum-based polymeric foams due to their similar internal structure of cells and grains. Additionally, cork products have a renewable origin and are recyclable. Despite these notable properties, few studies have analysed the mechanical properties, especially the specific properties, of these materials under compressive loads. Moreover, although efficiency, ideality, and energy-normalized stress diagrams are commonly used for polymeric foams and 3D-printed lattice structures, these types of diagrams are not yet applied to cork products. It must be highlighted that efficiency diagrams are plotted only against nonspecific properties so, this article proposes additionally the use of nonspecific properties to compare materials not only in terms of properties per unit volume instead but also in terms of properties per unit mass that is more suitable for certain applications in which the weight is crucial. The materials studied herein include three different white cork agglomerates, a brown cork agglomerate, a black cork agglomerate, natural cork, and expanded polystyrene foam, which are subjected to quasi-static compressive loads

Developing novel materials to enhance motorcyclist safety

The number of motorcyclists in Wales has reached record highs and, while accounting for only 0.7% of the vehicles in Wales, they accounted for ~35% of the injuries categorised as killed or seriously injured. Most studies in the literature have shown that the use of motorcycle helmets reduces the probability of brain injury and death, with strong support for their use from international bodies such as the world health organisation. This work aimed to improve motorcyclist head protection by augmenting the single impact performance of existing helmets with multi-impact mitigation. The following objectives supported this aim: An approach to improve elastomeric Fused Filament Fabrication (FFF) manufacturing quality was developed, and an equivalent porosity to injection moulding components was demonstrated. A novel accessible approach, using a uniaxial test machine to characterise elastomers dynamically, was developed. A novel computational method to generate elastomeric rate-dependent energy absorption diagrams was also developed. Additionally, the ability to scale these diagrams between different base elastomers was demonstrated. After selecting a preliminary configuration from an energy absorption diagram, a subsequent simplified simulation of a motorcycle helmet impact enabled efficient optimisation. This approach was successfully used to predict the response of a more complex helmet assembly. A similar agreement between simulation and experimental work was observed for this approach, as was observed when simulating a fully modelled helmet assembly. A prototype helmet, containing an elastomeric cellular structure, was shown to repeatedly pass the requirements of UNECE 22.05 while demonstrating a consistent co-efficient of restitution equivalent to that of an expanded polystyrene (EPS) helmet, even as shell failure occurred. The prototype helmet met the requirements of UNECE 22.05 at three of the four investigated locations. Additionally, it exceeded EPS' performance at one location with a liner thickness of 70% that of EPS