A Novel Anisotropic Composite Foam Concept for Improved Head Protection in Oblique Impacts

Oblique impact is the most common accident situation that occupants in traffic accidents or athletes in professional sports experience. During oblique impact, the human head is subjected to a combination of linear and rotational (angular) accelerations. Rotational (angular) acceleration and also velocity of the head are known to be responsible for traumatic brain injuries and must be minimised.

Cyclists are amongst the most vulnerable road users in traffic and helmets are the only means offering them head protection. Conventional helmets are proven to be effective at mitigation of head linear accelerations. However, they lack dedicated mechanisms to specifically aim at rotational acceleration mitigation, which is also still not a requirement in helmet test standards. Within the KU Leuven Bicycle Helmet Group (newly called IMPACT), it was proposed around the year 2008 that by using (transversely) anisotropic foam with the direction of anisotropy perpendicular to the head surface, the rotational acceleration (deceleration) transferred to the head can be reduced. Early studies demonstrated that a highly anisotropic polyethersulfone foam (PES) with large shape anisotropy ratio could outperform conventional isotropic expanded polystyrene foam (EPS) as helmet liner to reduce both linear and rotational accelerations of the head. However, the better performance of PES foam in comparison to EPS foam could not be solely attributed to anisotropy. This was due to simultaneously different density and solid material properties of PES foam in comparison to EPS. Moreover, processing the highly anisotropic PES foam into complex geometries e.g. of bicycle helmets proved to be challenging and hindered its practical realization as alternative material for helmet liners on large industrial scale.

In this thesis, composite foam with column/matrix configuration is presented as a novel concept for head protection in applications such as protective helmets or headliners in interiors of crashworthy vehicles. This concept creates anisotropy in foam at the ‘’macro level’’ and also enabled to finally perform a clear ‘’proof of principle study’’ to show that the anisotropy of the foam can lead to mitigation of rotational acceleration and velocity transferred to the head in an oblique impact. Moreover, through experimental and numerical parametric studies, it is shown that in the composite foam concept, the level of anisotropy and hence mitigation of rotational acceleration and velocity can be tailored by changing parameters such as the diameter of the foam columns in the structure and the compliance of the matrix foam. The experimental parametric study was carried out by performing biaxial shear-compression and oblique impact experiments on different configurations of composite foams. The numerical parametric study was carried out by simulating oblique impact of a simplified head model, with finite elements. Results have shown reductions in rotational acceleration up to 44% as compared to standard EPS 80 foam by using composite foam. Simulations matched the experimental results and also showed that the shape of the columns of high density foam is less critical.

Another focus of this thesis has been the further development and presentation of test set-ups which can be utilised for preliminary and final assessment of foam materials for helmets in oblique loading, namely a biaxial shear-compression tester and an oblique impact set-up. The biaxial shear-compression test set-up was utilized to study the effect of foam anisotropy on the energy absorption capacity of foams under different loading angles. Moreover, the correlation between combined shear-compression properties of the foams and their behaviour in oblique impact tests was investigated and confirmed. Additionally, in this thesis, further development of the KU Leuven oblique impact set-up was undertaken. For this, a critical comparison between KU Leuven and KTH (Stockholm) oblique impact set-ups was made. Furthermore, a thorough analysis on the instrumentation of the KU Leuven set-up was carried out by designing an apparatus which allowed calibration of its Angular Rate Sensor (ARS) in its wide working range. Moreover, the KU Leuven set-up was simplified by designing and incorporating a solid angled anvil instead of a rotating impact surface. The oblique impact set-up can be utilised for testing foams as flat foam samples as well as for helmet testing.