Magnetorheological elastomers (MREs) are ferromagnetic particle impregnated rubbers whose mechanical properties are altered by the application of external magnetic fields. In addition, these composite materials can deform at very large strains due to the presence of the soft polymeric matrix without fracturing. From an unconventional point of view, a remarkable property of these materials is that while they can become unstable by combined magneto-mechanical loading, their response is well controlled in the post-instability regime. This, in turn, allows us to try to operate these materials in this critically stable region. These instabilities can lead to extreme responses such as wrinkles (for haptic applications), actively controlled stiffness (for cell-growth) and acoustic properties with only marginal changes in the externally applied magnetic fields. Unlike the current modeling of hierarchical composites, MREs require the development of finite-strain coupled nonlinear magneto-mechanical models in order to tailor the desired macroscopic instability response at finite strains. As a proof of concept, we study experimentally and theoretically the stability and post-bifurcation of a non-linear magnetoelastic film/substrate block in order to obtain active control of surface roughness. The non-intuitive interplay between magnetic field and elastic deformation owes to material and geometry selection, namely a ferromagnetic particle composite film bonded on a compliant passive foundation. Cooperation of two otherwise independent loading mechanisms–mechanical pre-compression and magnetic field–allows to bring the structure near a marginally stable state and then destabilize it with either magnetic or mechanical fields. We demonstrate for the first time that the critical magnetic field is a decreasing function of pre-compression and vice versa. The experimental results are then probed successfully with full-field finite element simulations at large strains and magnetic fields. The magnetoelastic coupling allows for the reversible on/off control of surface wrinkling under adjustable critical magnetic and mechanical fields. In this view, this study constitutes a first step towards realistic active haptic and morphing devices. Novel auxetic and chiral architected MREs are also proposed as potential candidates for future work.
Kostas Danas holds a tenured position as CNRS Research Assistant Professor (Chargé de Recherche) at the Solid Mechanics Laboratory (LMS) at Ecole Polytechnique. He was born and raised in Kozani, Greece and studied at the Department of Mechanical Engineering at the University of Thessaly, Volos, Greece where he received his Dipl. in Mechanical Engineering (2003) with highest honors (rank 1st). He received his M.Sc (2004) from the University of Pennsylvania and his Ph.D. (2008) from the Ecole Polytechique, France and the University of Pennsylvania, PA, USA. After the end of his graduate studies, he moved to the University of Cambridge, U.K. as a postdoctoral Research Associate. In 2009, he applied for a research faculty position at the Centre National de la Recherche Scientifique (C.N.R.S.) where he was ranked 1st in the section of solid mechanics. He has recently obtained his HDR (Habilitation à diriger des recherches, 2016) from University of Pierre and Marie Curie. His main research interests are in the field of solid mechanics and composite materials with an emphasis on the theoretical and numerical description of constitutive laws for composites. He is currently working on the modeling of microstructured active elastomers and their instabilities as well as on the fracture of metallic porous materials. He has recently been awarded an ERC starting grant (2014) to carry out research on the low energy control of instabilities in magnetorheological polymers. Kostas is the recipient of the Bronze Medal of the Centre National de la Recherche Scientifique (CNRS) for 2017.