Development of a magneto-mechanical bench and experimental characterization for magneto-rheological elastomers

Abstract

Magneto-rheological materials are composites consisting of hard or soft magnetic particles embedded in a cross-linked elastomer with various filling ratios. They belong to the class of smart materials whose stiffness and damping can be controlled by an external stimuli as magnetic field. Due to their great flexibility and potential strain amplitude under a various magnetic field, they can find an interest in the field of mechatronic systems such as, soft-robotic or medical technologies.Magneto-mechanical coupling of flexible magnet needs to be clearly understood for development of applications in which they can be submitted to magnetic and mechanical loads. Some researchers have already worked on a full magneto-mechanical experimental characterization for soft magneto-rheological elastomer in case of large deformation [1-3]. However, due to high deformation range, the distances between magnetic particles increase. Thus, the magneto-rheological effect, which is defined by an increase of the stiffness under various magnetic field, is lowered.In the first part of this work we present the recent development of a magneto-mechanical bench for magneto-rheological elastomer filled with hard or soft ferromagnetic particles (figure 1). Concerning the mechanical part, the setup is composed by a system which consist of a displacement loading with a stress controller. By monitoring the displacement, we are able to work up to 1% of its deformation. This deformation mode allows to avoid non-linearities in case of large deformation range. The magnetic loading consists of a reluctant magnetic circuit which is able to produce homogeneous and large magnetic field (up to 40kA/m) in a variable air gap. Mechanical stress and magnetic loading are oriented toward the same direction.A 3D simulation analysis has been performed with a finite element simulation software (Altair Flux ©) for sizing and optimization of the magnetic loading such as intensity and homogeneity. The goal of this study is to expand as large as possible the magnetic field range to which our material would be submitted. For the numerical study, we have chosen a permanent magnet made of NdFeB (Br = 1.3T) and the magnetic flux is driven through a pure iron core. Various magnetic strength field can be reached by adjusting the air-gap thickness of the device. In case of hard magneto-rheological elastomer, composed by silicon-based elastomer filled with 36% vol of NdFeB particles, we are able to explore 10% of the demagnetization curve.The second part of this work focuses on the experimental characterization of magneto-mechanical coupling based on the setup developed. Thanks to this characterization tool, we are able to perform mechanical loading and unloading cycles with and without magnetic field. We observe, without any magnetic field applied, a hysteretic behavior and a remanent deformation specific to elastomeric material (figure 2) [4]. From this unloading stress state, a magnetic field strength of 40kA/m is applied. As it can be seen (figure 2), a tensile stress around 6.5kPa is induced due to magnetic particles interaction in response of the magnetic field applied. These first results are very promising and confirm the ability of the setup to characterize the magneto-mechanical coupling of hard flexible magnet. Further tests will be performed on composites with different elastomeric matrices with various Young’s moduli, different filling ratios and polarization orientation. **