Insights into the viscohyperelastic response of soft magnetorheological elastomers: Competition of macrostructural versus microstructural players

Magnetorheological elastomers (MREs) are multifunctional composites that consist of an elastomeric matrix filled with magnetic particles. These materials respond to an external magnetic field by mechanically deforming and/or changing their magnetorheological properties. Such a multi-physical response has made them extraordinary candidates for a wide variety of applications in soft robotics and bioengineering. However, there are still some gaps of knowledge that prevent the optimal design and application of these MREs. In this regard, the effect of viscoelastic mechanisms remains elusive from a microstructural perspective. To the best of the authors' knowledge, this work provides for the first time a numerical homogenization analysis for various magneto-active microstructures accounting for viscous deformation mechanisms. To this end, we propose an incremental variational formulation that incorporates viscoelasticity via internal variables, which is properly modified to deal with the continuity of Maxwell stresses. The proposed framework is applied to study the magneto-mechanical couplings in extremely soft MREs (stiffness < 10 kPa). Such a soft matrix promotes microstructural rearrangements while transmitting internal forces leading to macrostructural synergistic responses. The constitutive parameters are calibrated with experimental tests. The numerical results are accompanied with original magnetostriction tests considering different sample geometries and confined magneto-mechanical tests, reporting the macroscopic response. The results obtained in this work suggest that the effective magneto-mechanical response of the MRE is the outcome of a competition between macrostructural and local microstructural responses, where viscous mechanisms play a relevant role.

Automated summary

Magnetorheological elastomers (MREs) are composites with elastomeric matrix filled with magnetic particles, capable of deforming and changing properties under external magnetic field. They have a wide range of applications in soft robotics and bioengineering. However, understanding of the viscoelastic mechanisms in MREs is still limited. This study provides a numerical analysis of magneto-active microstructures considering viscous deformation mechanisms, shedding light on the competition between macrostructural and microstructural responses in MREs.