Shock formation and rate effects in impacted carbon nanotube foams

We investigate rate-effects in the dynamic response of vertically aligned carbon nanotube (VACNT) foams excited by impacts at controlled velocities. They exhibit a complex rate-dependent loading-unloading response at low impact velocities and they support shock formation beyond a critical velocity. The measured critical velocities are ∼10 times lower than in other foams of similar densities-a desirable characteristic in impact protective applications. In-situ high-speed microscopy reveals strain localization and progressive buckling at low velocities and a crush-front propagation during shock compression. We correlate these responses to quantitative measurements of the density gradient and fiber morphology, obtained with spatially resolved X-ray scattering and mass attenuation.