DMMCTE - DFEE - Preprints

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  • Publication
    Effects of soft and hard magnetic particles on the mechanical performance of ultra-soft magnetorheological elastomers
    (2022-04-25) Moreno Mateos, Miguel Ángel; López Donaire, María Luisa; Hossain, Mokarram; García González, Daniel; Comunidad de Madrid; European Commission; Ministerio de Ciencia, Innovación y Universidades (España)
    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
  • Publication
    Microstructural modelling of hard-magnetic soft materials: Dipole-dipole interactions versus Zeeman effect
    (2021-06-03) García González, Daniel; Hossain, Mokarram; Comunidad de Madrid; European Commission
    Hard-magnetic soft materials are a class of magneto-active polymers (MAPs) where the fillers are composed of hard-magnetic (magnetised) particles. These materials present complex magneto-mechanical couplings, which require the development of modelling frameworks in understanding their responses at the very beginning of conceptualisation and design. Most of the current constitutive approaches available in the literature for hard-magnetic MAPs do not consider dipole-dipole interactions of the embedded particles. However, such interactions among the magnetised particles generate internal forces within the composite that need to be balanced by mechanical stress from the polymeric matrix networks. This fact may imply an initial stretch of the polymeric network and suggests that such dipole-dipole interactions may be important during the MAP deformation process. To address these crucial points, in this contribution, we propose a novel constitutive model relating microstructural characteristics of hard-magnetic MAPs. The model accounts for polymeric network pre-stretch, dipole-dipole interactions, Zeeman effect as well as viscous mechanisms which are formulated on the finite deformation theory. The results obtained herein highlight the importance of accounting for the dipole-dipole interactions and the polymeric network pre-stretch to understand the complex magneto-mechanically coupled behaviour of hard-magnetic MAPs.