Combined Shear-Compression Test to Characterize Foams under Oblique Loading for Bicycle Helmets

Abstract

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.