Energy absorption of reticulated foams filled with shear-thickening silica suspensions

Abstract

The need for smarter and adaptive, energy absorption materials especially for human protection applications has fueled the interest in new and alternative energy absorbing composites. In this thesis a 'novel' energy absorbing fluid-composite that utilized a shear thickening fluid is developed. Shear-thickening fluids are a class of field responsive fluids that have the ability to transition from low viscosity to high viscosity under an imposed deformation field. Two different types of silica particles are used to create shear thickening fluids. The first are polydisperse and non spherical, with a median diameter of 1.7 ± 1.4 micrometer, while the second are monodisperse spherical particles of 0.3 ± 0.03 micrometer diameter. The particles are dispersed in ethylene glycol at volume fractions of up to [phi]=47% for the polydisperse sample and up to [phi]=60% for the monodisperse spheres. The behavior of the silica suspensions is studied under steady shear, small and large amplitude oscillatory shear flow and also in transient extensional flow. The viscosity of the polydisperse suspension is found to be much greater than the monodisperse one due to the difference in particles shape. Oscillatory experiments indicate that both the onset and magnitude of the shear thickening depends on the frequency and strain applied and show that rapid time-varying deformations result in maximum energy dissipation.

(cont.) Two different regimes are found through extensional flow measurement: at low extension rates the suspensions respond as a viscous rate-thinning fluid, whereas beyond a critical extension rate, the suspension strain-hardens and ultimately fractures in a solid-like fashion. Polyurethane open cell or 'reticulated' foam with relative density of 0.03 and average cell size of 360 micrometer is chosen to envelop the concentrated silica suspensions. The behavior of this nonlinear fluid-solid composite is studied over a range of filling fractions under quasi-static deformation rates (strain rates between 10⁻² - s⁻¹ ), under dynamic impact loading (with energy densities of e = 10⁵ - 10⁶ J/m³) and under high strain-rate deformations (strain rates up to 800 s⁻¹). Results show that, if the foam is filled with a shear thickening suspension, the composite stiffens even at strain rates of 10⁻² s⁻¹ as the impregnated fluid shear-thickens due to the high local strain rates that develop on cellular length scales. High impact load experiments show two different mechanisms for energy absorption: at lower impact energies viscous dissipation is dominant; whereas, after a critical impact energy is reached, the fluid undergoes a transition from liquid-like to solid-like. High-speed digital video-imaging shows that cracks form and propagate through the sample and the impact energy is absorbed by viscoplastic deformation.

(cont.) The addition of these shear-thickening fluids in polyurethane foam is shown to increase the composite energy absorption capability by 35-fold.