Nonlinear rheological behavior of magnetorheological fluids: step-strain experiments

Relaxation experiments under simple step-strain shear were performed on MRF-132LD using a rheometer with parallel-plate geometry. The applied step strains vary from 0.01 to 100%, covering both the pre-yield and post-yield regimies. For small step-strain ranges, the stress relaxation modulus G(t, gamma) is independent of step strain, where magnetorheological (MR) fluids behave as linear viscoelastic solids. For large step-strain ranges, the stress relaxation modulus decreases gradually with increasing step strain. Moreover, the stress relaxation modulus G(t, gamma) was found to obey time-strain factorability. That is, G(t, gamma) can be represented as the product of a linear stress relaxation G(t) and a strain-dependent damping function h(gamma). The linear stress relaxation modulus is represented as a three-parameter solid viscoelastic model, and the damping function h(gamma) has a sigmoidal form with two parameters. The comparison between the experimental results and the model-predicted values indicates that this model can accurately describe the relaxation behavior of MR fluids under step strains. (Some figures in this article are in colour only in the electronic version).