Mechanical and Energy-Absorbing Properties of Shear-Thickening Fluids (STFs)

The absorption of dynamic energy during impact of a material is ubiquitous in industrial, biomedical and military applications. From suspension systems to shock absorbers, the ability to divert or dissipate dynamic energy imposes many challenges in developing these systems. Some typical complex composite fluids, such as shear-thickening fluids (STFs), play a vital role in these challenges. STFs, classified as non-Newtonian fluids, are special fluidic composites of dense suspensions which dramatically change their viscosity when subjected to a change in shear rate or stress. These fluidic composite materials display unusual phase transitions between liquid and “solid” phases, due to recoverable changes in viscosity at a critical rate of shear. In recent years, STFs have stimulated much research interest, in which most studies have been focused on the rheological and energy absorption properties of fabrics soaked with STF under impact. The fundamental knowledge of STFs after their shear-thickening transition is still unarticulated. Without full understanding of the fundamental structure-property relationships of STFs after shear-thickening transition it would not be able to design and optimize a material system with STFs, nor could cost-effective development of STFs as energy-absorbing materials be achieved. The aim of the studies in this thesis is to establish fundamental knowledge in developing STFs as adaptive energy-absorbing materials in practical applications. The studies establish methods and approaches for investigating and characterizing the mechanical as well as the energy-absorbing/dissipation properties of a STF systematically after the shear-thickening transition. The STF adopted in this work was composed of 58 vol.% dispersion of styrene/acrylate particles in ethylene glycol. Microscopic examination was conducted to characterize the size, geometry and distribution of styrene/acrylate particles in the STF, and the rheological behaviours of the STF were measured. Double-cantilever-beam specimens with the STF as adhesive layer were adopted to characterize the mode-I fracture energy of the STF at different crack opening displacement rates, following classic fracture mechanics. High-speed digital video-photography was used to observe the deformation behaviour of the STF. The load-displacement curve as well as the high-speed video recording confirmed that the STF showed a “solid” behaviour at high rates by developing rapid but stable crack extension that corresponded to fracture behaviour. The results indicated that the displacement rate and the STF thickness had a significant effect on the magnitude of the mode-I fracture energy of the STF. The fracture energy increased with an increase in the displacement rate, while a plateau value of about 240 J/m2 was observed at high rates. The measured fracture energy can be used as an effective parameter characterizing the crack resistance or the energy-absorbing capacity of the STF in the solid phase. The lap-shear strength and the braking energy of the STF were quantitatively characterized by a modified single-lap shear test method, performed by using two stainless steel adherends with the STF as adhesive layer. The results indicated that the shear rate had a significant effect on lap shear strength and the braking energy of the STF. Moreover, the studies were conducted to quantitatively characterize the energy absorption capacity of the STF under penetration impact and pull-out fracture at different impact or pull-out speeds. The results confirmed that the penetration rate again had a significant effect on the energy absorbing capacity of the STF. In comparisons with the energy absorbing behaviour of some cellular materials in the literature, the STF outperformed a polyurethane foam in terms of energy absorbing capacity. The ability of a STF to maintain its reversible shear-thickening transition behaviour depends on the integrity and durability of the STF. Cyclic dynamic loading at different magnitudes of durations was applied to the STF in order to quantitatively evaluate its aging behaviour. It was observed that cyclic dynamic loading affected the shear-thickening behaviour of the STF because of deterioration of the styrene/acrylate particles caused by abrasive wear during interaction between them, based on the rheological characterization. This research has delivered some new and original results for solutions to the outstanding problems in developing STFs as energy-absorbing materials. It should bring new opportunities for the development of new and advanced material systems with STFs for practical applications