Brain injury mitigation effects of novel helmet technologies in oblique impacts

Abstract

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.