Polymeric foams are extensively used in applications such as packaging, sports goods and sandwich structures. Since in-service loading conditions are often multi-axial, characterisation of foams under multi-axial loading is essential. In this article, quasi-static combined shear-compression behaviour of isotropic expanded polystyrene foam and anisotropic polyethersulfone foam was studied. For this, a testing apparatus which can apply combined compression and transverse shear loads was developed. The results revealed that the shear and compression energy absorption, yield stress and stiffness of foams are dependent on deformation angle. The total energy absorption of the anisotropic polyethersulfone foam is shown to be direction dependent in contrast to isotropic expanded polystyrene. Furthermore, for similar relative density, polyethersulfone foam absorbs more energy than expanded polystyrene foam, regardless of deformation angle. This study highlights the importance of correct positioning of foam cells in anisotropic foams with respect to loading direction to maximise energy absorption capability ... Read more

Although current standard bicycle helmets protect cyclists against linear acceleration, they still lack sufficient protection against rotational acceleration during oblique impact events. Rotational acceleration is correlated with serious traumatic brain injuries such as acute subdural haematoma and thus should be minimized. This study proposes using highly anisotropic polyethersulfone foam for bicycle helmet liners in order to limit the rotational acceleration. Helmet prototypes, made of polyethersulfone foam with cell anisotropy direction perpendicular to the head, have been produced and compared to a standard commercial helmet. Standard helmets consist of expanded polystyrene foam. Oblique impact tests were performed to measure both linear and rotational accelerations and impact pulse duration. Results demonstrate that the peak rotational acceleration of the polyethersulfone prototype helmet showed a decrease of around 40% compared to the reference expanded polystyrene helmet. Moreover, the peak linear acceleration showed an average decrease of about 37%. Upon impact, the polyethersulfone helmet showed improved head injury protection when analysed based on global biomechanical head injury criteria such as HIC15 and HICrot as well as generalized acceleration model for brain injury threshold, brain injury criterion and head impact power, with a predicted sixfold decrease in likelihood of concussion. ... Read more

Polymer foams are extensively used in lightweight structures, often as energy absorbers. Since most of the loading modes in real life are complex, characterization of core materials under multi-axial loading is of great importance. In this work, a new testing apparatus for measuring combined shear-compression loading of sandwich core materials has been developed. The behavior of expanded polystyrene (EPS) foams has been studied in quasi-static mode. Three different densities of EPS foam have been tested under different angles of displacement (the resultant of shear and compression displacement), particularly 0° 15°, 45°, and 60°.The results reveal that the parameters such as energy absorption in (shear and compression) up to densification, yield stress and elastic modulus of the EPS foam have a strong dependency on the angle of displacement as well as the density of the foams. ... Read more

yclists during bicycle traffic accidents, are prone to oblique impact which leads to rotational accelerations. Rotational acceleration is known to cause significant brain injuries, and should be minimized. Foam materials inside bicycle helmets undergo a combination of shear and compression loads during oblique impact. Therefore, developing an apparatus and a test method which can apply a combination of shear and compression loads to the foam at the same time is of great importance. This testing method has a broad application which is not only limited to the foams for bicycle helmet applications but has general relevance to sandwich core materials in structural composites. In this paper, the shear-compression behavior of different types of foams under different angles, particularly 15 ͦ, 45 ͦ and 60 ͦ is investigated and compared to the standard EPS foam used in bicycle helmets. Shear stresses in anisotropic PES foams under different angles in the combined shear-compression test were lower than for standard EPS, which is favourable because they lead to high rotational acceleration. The peak rotational and translational accelerations of the PES prototype helmets were measured by a rotational impact test set-up and showed a dramatic decrease of around 40% compared to the reference EPS helmet, however at increased pulse duration. ... Read more

lthough current standard bicycle helmets provide protection against so-called linear acceleration, they cannot deliver sufficient protection against rotational acceleration during oblique impact events. These events can cause severe head injuries and the rotational accelerations need to be minimized. We pursue this goal by developing anisotropic foams for bicycle helmets and define test methods to characterize foams under oblique loading. (1) (PDF) DEVELOPMENT OF ANISOTROPIC FOAMS AND CHARACTERIZATION METHODS FOR BICYCLE HELMETS. Available from: https://www.researchgate.net/publication/320110420_DEVELOPMENT_OF_ANISOTROPIC_FOAMS_AND_CHARACTERIZATION_METHODS_FOR_BICYCLE_HELMETS [accessed Apr 28 2020]. ... Read more