3.
Rheological properties of MR fluids under large step strain shear are presented in this paper. The experiments were carried out using a rheometer with parallel-plate geometry. Under the large step strain shear, MR fluids behave as nonlinear viscoelastic properties, where the stress relaxation modulus,
G(
t,
γ), shows a decreasing trend with step strain. The experimental results indicate that
G(
t,
γ) obeys time-strain separability. Thus, a mathematical form based on finite exponential serials is proposed to predict MR behavior. In this model,
G(
t,
γ) is represented as the product of a linear stress relaxation,
G(
t), and the damping function,
h(
γ), i.e.
G(
t,
γ)=
G(
t)
h(
γ).
G(
t) is simply represented as a three-parameter exponential serial and
h(
γ) has a sigmoidal form with two parameters. The parameters are identified by adopting an efficient optimization method proposed by Stango et al. The comparison between the experimental results and the model-predicted values indicates that this mathematical model can accurately predict MR behavior.
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