Characterisation of the elastic properties of porous foams

International audienceAn experimental method for measuring the elastic constants of poroelastic foams as a function of frequency is presented. The method is based on the measurement of phase velocities of guided acoustic waves in a slab of the material. Standing waves are generated in the material and the phase velocities are evaluated using the spatial Fourier Transform of the displacement profile of the upper surface. The displacement is measured with the help of a Laser Doppler vibrometer along a line corresponding to the direction of propagation of plane surface waves. The spatial Fourier Transform provides the wave numbers and the phase velocities are obtained from the relationship between wave number and frequency. The phase velocity of several guided modes could be measured in highly porous foams saturated by air. The modes were also studied theoretically and from the theoretical and experimental results, it was possible to determine the frequency behavior of the real part of the shear modulus and in a frequency range higher than the traditional methods. Experimental results concerning guided waves in isotropic porous materials tend to suggest that information about the anisotropy of the elastic matrix can also be obtained