Abstract
Relaxation experiments under simple step-strain shear wereperformed on MRF-132LD using a rheometer with parallel-plategeometry. The applied step strains vary from 0.01 to 100%, coveringboth the pre-yield and post-yield regimes. For small step-strainranges, the stress relaxation modulus G(t,γ) is independent ofstep strain, where magnetorheological (MR) fluids behave as linearviscoelastic solids. For large step-strain ranges, the stressrelaxation modulus decreases gradually with increasing step strain.Moreover, the stress relaxation modulus G(t,γ) was found toobey time-strain factorability. That is, G(t,γ) can berepresented as the product of a linear stress relaxation G(t) and astrain-dependent damping function h(γ). The linear stressrelaxation modulus is represented as a three-parameter solidviscoelastic model, and the damping function h(γ) has asigmoidal form with two parameters. The comparison between theexperimental results and the model-predicted values indicates thatthis model can accurately describe the relaxation behavior of MRfluids under step strains.
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