Among sportspeople and military vets, traumatic brain injury (TBI) is one of the major causes of permanent disability and death. Injury statistics show that the majority of TBIs, of which concussion is a subtype, are associated with oblique impacts, which subject the brain to a combination of linear and rotational kinetic energy forces and cause shearing of the delicate brain tissue.
To improve their effectiveness, helmets worn by military personnel and sportspeople must employ a liner material that limits both. This is where researchers from the University of Wisconsin-Madison come in. Determined to prevent – or lessen the effect of – TBIs caused by knocks to the body and head, they’ve developed a new lightweight foam material for use as a helmet liner.
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For the current study, Thevamaran built upon his previous research into vertically aligned carbon nanotube (VACNT) foams – carefully arranged layers of carbon cylinders one atom thick – and their exceptional shock-absorbing capabilities. Current helmets attempt to reduce rotational motion by allowing a sliding motion between the wearer’s head and the helmet during impact. However, the researchers say this movement doesn’t dissipate energy in shear and can jam when severely compressed following a blow. Instead, their novel foam doesn’t rely on sliding layers.
Oblique impacts, associated with the majority of TBIs, subject the brain to a combination of linear and rotational shear forces
Maheswaran et al.
VACNT foam sidesteps this shortcoming via its unique deformation mechanism. Under compression, the VACNTs undergo collective sequentially progressive buckling, from increased compliance at low shear strain levels to a stiffening response at high strain levels. The formed compression buckles unfold completely, enabling the VACNT foam to accommodate large shear strains before returning to a near initial state when the load is removed.
The researchers found that at 25% precompression, the foam exhibited almost 30 times higher energy dissipation in shear – up to 50% shear strain – than polyurethane-based elastomeric foams of similar density.
