Quantitative investigation on the nonlinear viscoelasticity of magnetorheological gel under large amplitude oscillatory shear

Magnetorheological gel (MRG) is a kind of magneto-sensitive smart composite whose nonlinear rheological behavior is highly controllable by external magnetic fields. For filling the gap that most publications studied the nonlinear property of MR materials in a qualitative manner, this work adopts a quantitative analysis method, called Fourier-transform rheology (FTR), to investigate the nonlinear viscoelasticity of MRG under large amplitude oscillatory shear (LAOS) tests. The influences of magnetic field, strain amplitude and frequency on the nonlinearity of MRG are thoroughly discussed. Results indicate that MRG, under a relative smaller magnetic field, appears higher degree of nonlinearity in the high-frequency region. The magneto-mechanical coupling mechanism of microstructures in MRG is also proposed to explain the various nonlinear phenomena under different loading conditions. Furthermore, for remedy the limitation that higher-order harmonics in FTR lack of clear physical meanings, a geometrical nonlinear parameter, S factor, is utilized to disclose the intra-cycle strain stiffening characteristic of MRG. The effects of strain amplitude, frequency and magnetic field on S factor are similar with that on the third-order harmonic in FTR. Moreover, the energy dissipation density is calculated based on Lissajous curve (i.e. hysteretic stress-strain loop) to characterize the damping property or capacity to dissipate energy of MRG. It is found that frequency only has a great effect on energy dissipation density of MRG under relatively large strain amplitude.