Due to mass constraints, composite materials are possible candidates to replace metal alloys for electromagnetic shielding applications. The design of standard metallic shielding enclosures often relies on finite-element calculations. But in the case of composite materials, the strong dependence on the shielding properties to the microstructure makes the finite-element approach almost impossible. Indeed meshing the microstructure would imply a huge number of elements, incompatible with usual computational resources. We propose in this paper to develop homogenization tools to define the effective electromagnetic properties of composite materials at microwave frequencies. The ratio between the characteristic size of the microstructure and the wavelength is shown to be a key parameter in the homogenization process. The effective properties can then be used as an input for electromagnetic compatibility standard tools, designed for homogeneous media.Index Terms-Effective medium, heterogeneous materials, homogenization, inclusion problem, Maxwell-Garnett model, shielding effectiveness.
International audienceThe use of composite materials for electromagnetic shielding applications contributes to the effort of structure lightening in aerospace industry. In these materials the strong interaction between the electromagnetic field and the microstructure makes the standard numerical tools difficult to implement. Indeed these methods would involve an excessive number of degrees of freedom to describe details of the microstructure. An efficient way to overcome this problem is the use of homogenization techniques providing the effective properties of heterogeneous materials. These effective properties can then be introduced in standard numerical tools to estimate the behavior of shielding enclosures. A recent paper proposes an extension to microwave frequencies of quasistatic homogenization methods. It introduces a characteristic length for the microstructure in the case of a square array of circular 2-D conductive phases embedded in a dielectric matrix. In this paper, a method to identify this length parameter is proposed for random microstructures
Composite materials are increasingly used to contribute to structure lightening in electromagnetic shielding applications. The interactions between electromagnetic waves and composite materials are highly dependent on their microstructure. This gives rise to challenging modelling issues. Considering details of the microstructure would involve an excessive number of unknowns with standard numerical tools for structural analysis. Homogenisation methods-such as Maxwell-Garnett model-are a possibility to overcome this problem. The equivalent homogeneous medium obtained with such methods can be introduced into numerical tools to model full shielding enclosures. A homogenisation model has been recently proposed to obtain the equivalent homogeneous properties of composite materials subjected to electromagnetic waves. It relies on the introduction of a length parameter into classical non dimensional semi-analytical homogenisation methods-also known as mean field approaches. The model is applicable at microwave frequencies as long as the induced currents in the fibres (or inclusions) of the composite materials remain weak. This paper proposes an extension of the approach to include skin effect in the homogenisation method. This is done by considering Joule losses within the fibres of the composite. This extension significantly broadens the frequency range covered by the model. The results show that the optimization of composite shielding properties relies on a subtle compromise between internal reflections and Joule losses. V
Magneto-rheological elastomers belong to the class of smart materials whose mechanical properties can be controlled by an external magnetic field. These materials can be integrated into mechatronic systems and submitted to multiple loadings such as temperature, mechanical stress and magnetic field. Thus, the present work is dedicated to the development of a magneto-mechanical bench and on first experimental characterizations of hard magneto-rheological elastomers taking multiphysics coupling into account. Regarding the mechanical loading, the experimental setup is able to create a uniaxial tensile stress in case of low strain (< 1%) without friction effect. In regards to the magnetic loading, a magnetic circuit made of a strong permanent magnet has been designed to impose a variable and a homogeneous magnetic field strength up to 41 kA/m. Experimental analysis has been performed on silicone rubber filled with 36%vol. of NdFeB particles. The purpose was first to investigate the evolution of the Young modulus with or without magnetic field. Results obtained from measurements show that the developed test bench is able to depict the mechanical behavior and phenomena linked to rubber-like material.
International audienceHomogenization is a mean field approach for the determination of the effective properties of heterogeneous materials. It can provide the average fields per phase but also some information about the field distribution such as second order moments. The use of second order moments of fields can notably improve the estimates of the macroscopic behavior in the nonlinear case. This has been studied mainly in the case of uncoupled behavior. We propose to define second order moments in the case of coupled elasto-magneto-electric behavior using homogenization tools. The results are compared to the field fluctuations obtained from a finite element model
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