Mitigating Oblique Impacts by Unraveling of Buckled Carbon Nanotubes in Helmet Liners

Background: Helmet systems most commonly experience oblique blunt impacts which cause simultaneous linear and rotational accelerations. The ability to attenuate both linear and rotational accelerations by absorbing the normal shock while accommodating large shear deformations with energy dissipation is critical to developing superior helmet liners that prevent traumatic brain injury (TBI). Objective: To investigate the quasistatic compression-shear response of vertically aligned carbon nanotube (VACNT) foams—which are known for their exceptional specific energy absorption in compression—and explore their potential of accommodating large shear strains at lower shear stress levels, under large compression-shear loadings. Methodology: We investigate the quasistatic compression shear response of freestanding vertically aligned carbon nanotube foams subjected to varied initial precompressions. We use in situ high speed microscopy to visualize the microscale deformations during shear. Results: Vertically aligned carbon nanotube foams exhibit a nonlinear hysteric shear stress–strain response that varies as a function of initial normal precompression. At a given precompression, initial linear response at very low shear strains leads to a behavior showing increasing compliance leading to a plateau like regime at moderate shear strains and then transitions into a stiffening behavior at high shear strains. The shear stress–strain response softens with the increase in initial precompression demonstrating the vertically aligned carbon nanotube foam’s potential to accommodate large shear strains more effectively at severe compression-shear loads unlike other solids that typically jam. In situ high-speed microscopy reveals the unraveling of carbon nanotubes that collectively buckled during precompression, allowing them to accommodate large shear strains at low shear stress levels. Conclusion: We demonstrate the ability of vertically aligned carbon nanotube to accommodate large shear strains at lower shear stress levels under large compression-shear loadings. We propose a model to predict the compression-shear response at different precompressive strains and use this model to develop a deformation modality diagram that categorizes the dominant deformation mechanisms at different loads along different loading angles.