Publication: Effects of soft and hard magnetic particles on the mechanical performance of ultra-soft magnetorheological elastomers
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2022-04-25
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Abstract
Magnetorheological elastomers (MREs) mechanically respond to external magnetic stimuli by changing
their mechanical properties and/or changing their shape. Recent studies have shown the great potential of
MREs when manufactured with an extremely soft matrix and soft-magnetic particles. Under the application
of an external magnetic field, such MREs present significant mechanical stiffening, and when the magnetic
field is off, they show a softer response, being these alternative states fully reversible. Although softmagnetic
particles are suitable for their high magnetic susceptibility, they require the magnetic actuation to
remain constant in order to achieve the magneto-mechanical stiffening. Here, we present an alternative solution
based on hard-magnetic MREs to provide stiffening responses that can be sustained along time without
the need of keeping the external magnetic field on. To this end, we manufacture novel extremely soft hardmagnetic
MREs (stiffness in the order of 1 kPa) and characterise them under magneto-mechanical shear and
confined magnetic expansion deformation modes, providing a comparison framework with the soft-magnetic
counterparts. The extremely soft nature of the matrix allows for easily activating the magneto-mechanical
couplings under external magnetic actuation. In this regard, we provide a novel approach by setting the magnetic
actuation below the fully magnetic saturating field. In addition, free deformation tests provide hints
on the microstructural transmission of torques from the hard-magnetic particles to the viscoelastic carrier
matrix, resulting in macroscopic geometrical effects and complex functional morphological changes.
Keywords: Magnetorheological elastomers (MREs), Magneto-mechanics, Experimental characterisation,
Hard-magnetics MRE, Multifunctional materials, Magnetic properties