SynopsisThe shear rheology and shear-induced microstructure of poly͑ethylene glycol͒ ͑PEG͒-based suspensions of acicular precipitated calcium carbonate ͑PCC͒ particles of varying particle aspect ratio ͑nominal L / D ϳ 2, 4, 7͒ are reported. These anisotropic particle suspensions demonstrate both continuous and discontinuous reversible shear thickening with increasing applied shear rate or stress similar to that observed for suspensions of spherical colloidal particles. The critical volume fraction for the onset of discontinuous shear thickening decreases as the average particle aspect ratio is increased. However, the critical stress for shear thickening is found to be independent of particle anisotropy and volume fraction. Rather, it can be predicted based on the minor axis diameter of the particles and is found to agree with values for near hard-sphere suspensions. Small angle neutron scattering during shear flow ͑Rheo-SANS͒ demonstrates that long-axis particle alignment with the flow direction is maintained throughout the range of shear stresses investigated, including the shear thickening regimes for both continuously and discontinuously shear thickening PCC/PEG suspensions. Rheo-SANS and transient rheological experiments indicate that this reversible shear thickening is a consequence of lubrication hydrodynamic interactions and the formation of transient hydroclusters of flow-aligned particles.

The energy absorbed in ballistic fabrics is modeled by assuming yarn pull-out, including yarn uncrimping and translation, as the primary energy absorption mechanism. Using a semi-empirical model of yarn pull-out based on laboratory tests, predictions of fabric ballistic performance are compared to ballistic test results. The study demonstrates that quasi-static pull-out results can be correlated quantitatively with yarn pull-out during ballistic impact.

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Yarn pull-out can be an important energy absorption mechanism during the ballistic impact of woven Kevlar® fabric. This study reports the effects of fabric length, number of yarns pulled, arrangement of yarns, and transverse tension on the force-displacement curves for yam pull-out tests on Kevlar® KM-2 fabric under laboratory conditions. A semi-empirical model is presented for predicting the yam pull-out force and energy as a function of pull-out distance, including both yarn uncrimping and subsequent yam translation. This model is found to replicate the experimental data with a high degree of accuracy, and should prove useful for understanding ballistic experiments and improving computational modeling of fabrics.