Hysteresis and strain hardening in the creep response of a polyaniline ER fluid

The electrorheological creep response of PANI/silicone oil suspensions near the yield point is investigated using parallel plate rheometry. Controlled-stress, thixotropic loop experiments exhibit a pronounced hysteresis, from which we determined the static yield stress (σy(static)σy(static)), as the stress where onset of flow occurs on the upward part of the loop, and a dynamic yield stress (σy(dynamic)σy(dynamic)), defined as the stress at which flow ceases on the downward part of the loop. The magnitude of the hysteresis, as characterized by the area under the loop, increases substantially with applied field strength and particle concentration, but decreases with increase of temperature. Consistent with literature data, the creep compliance shows an evolution from viscoelastic to viscoplastic to viscous flow behavior as the applied stress increases through the yield point. In the viscoplastic regime, the apparent equilibrium compliance, Jeapp, shows a discrete pre-yield transition to higher values, indicating a seemingly-enhanced ductility as the applied stress nears the yield point. Measurement of the static yield stress following these creep experiments suggests that the origin of this transition is a pronounced strain-hardening effect. We conclude that strain-hardening contributes to the hysteresis observed in the thixotropic loop test.

The dependence of equilibrium creep compliance on the applied stress of polyaniline-based electrorheological fluid is displayed here at various temperatures. We propose that the creep compliance obeys the following relation:Jeapp=Je,oappexp(ασd2/kBT), where σ is the applied stress, d is the polyaniline particle size, and α is a constant.Figure optionsDownload as PowerPoint slide