Since the beginning of the fellowship, the research work has advanced on three fronts simultaneously: The theoretical study of metamaterials properties, large scale simulations of metamaterials and experimental realisations.
Theoretical study of metamaterials properties
Inspired by early work on metamaterials during my previous postdoc, in 2015 at Imperial College London, I studied the effect of introducing local resonators to wave problems for elastic surfaces (elastic plates first and later to thick elastic substrate such as the earth surface). The metamaterial prototype was obtained by fixing several closely spaced slender beams (resonators) perpendicular to an elastic substrate (Figure 1a and b). From 2016 the research focused mainly on surface Rayleigh waves as this case was the most favorable for the development of seismic metamaterials and it was not yet been considered in literature because very challenging. The metamaterial’s dispersion curves showed unique characteristics such as subwavelength bandgaps and ultra fast/slow modes perfectly suited for wave control. This part of the work was carried out using a theoretical approach based on eigenvalue problems and homogenization theory. This theoretical study of the metamaterial physics has made possible to understand and reveal new phenomena such as the conversion of Rayleigh into shear waves or the lensing phenomena for elastic and seismic waves (Figure 1d).
Large scale simulations and metamaterials optimization
I have tested several new metamaterial designs in real applications featuring full-scale complexities. I have used SPECFEM3D, a state-of-the-art, open source, spectral element based code for elastodynamics problems. The High Performance Computing (HPC) CIMENT in Grenoble (France) and ARCHER in UK have been heavily used to compute the simulations. Starting from the basic resonant metamaterial studied in the theoretical part, the designs have been refined by exploring the parameter space through thousands of simulations. The aim was both improving the performances of the metamaterial (e.g. increase the effective bandwidth) and tailoring it to the specific seismic or laboratory scale (various examples in Figure 1).
Experiments
Together with the theoretical and the numerical part, two experiments have been carried out during the 24 months. In both cases the results have been published (or are being published) in journal articles. For the seismic metamaterial I have co-organized the large-scale experiment on a forest using hundreds of seismometers and an active source of vibrations to validate the concept of resonant seismic metamaterials (Figure 2). The experiment required careful planning and a field trip that lasted two weeks in a forest located near Bordeaux (France). The second experiment was at a much smaller scale, in the ultrasound regime and was carried in the optic laboratory of the University of Nottingham. The aim of this second experiment was to validate the metamaterial design to convert surface wave into shear waves and explore possible use in mechanical engineering to reduce vibrations (Figure 1d).
Finally, the results produced in these 24 months have been also used to obtain funding and implement a large scale experiment on seismic metamaterials (
https://metaforet.osug.fr(öffnet in neuem Fenster)) and more recently, to prepare a research proposal to fund my research group. I have obtained the funding and I will start my group from April 2018 in ETH Zurich.
