Investigation on the stress-strain rate curve of MR fluids in a combined mode and its application in sheet flexible-die forming

Abstract

During the sheet flexible-die forming process using a magnetorheological (MR) fluid, the MR fluid is always subjected to a three-dimensional stress state, working in a combination of squeezing mode and valve mode. However, there is a lack of deep understanding of the stress-strain rate curve for MR fluids in the combined mode. In this paper, through extrusion test and finite element simulation, the mechanical properties of an MR fluid with a magnetic particle content of 43% and three kinds of viscous mediums used in viscous pressure forming (VPF) were compared and analyzed for the first time. It shows that the adjustment range of the MR fluid’s mechanical properties under different magnetic fields is much larger than that of a viscous medium with a molecular weight of 700 kg/mol. Besides, by changing the magnetic field, the mechanical property of an MR fluid can cover the mechanical properties of viscous mediums with different molecular weights. It is proved that the stress-strain rate curve of the MR fluid changes from a linear relationship to a power exponent one with the increase of magnetic flux density. Meanwhile, the strength coefficient increases, while the strain rate sensitivity index decreases. The obtained curves were applied to the numerical simulation of sheet flexible-die forming, which can well explain the experimental phenomena through the analysis of velocity, pressure, stress, and strain distribution. The results can provide theoretical guidance in the optimization of processing parameters in sheet flexible-die forming using an MR fluid.