Numerical and experimental study of the influence of multiple scattering on surface waves dispersion curves
Résumé
The diagnosis of structures and the evaluation of their residual life is an important issue for civil engineering. Surface waves offer an effective mean for retrieving in a non invasive manner mechanical intrinsic properties of material as a function of depth. In most cases which deal with cover-concrete non destructive evaluation, the assumption of homogeneous layers can be debatable when the aggregates represent approximately 50% in weight and have sizes comparable to the expected depth resolution. Indeed, multiple scattering has an impact on the surface wave effective phase velocity dispersion curve when the incident wavelengths are similar to the radius of inclusions. Multiple scattering theories, such as ISA model, describes the propagation of coherent bulk waves propagating in infinite heterogeneous medium. ISA is known to be theoretically available for a dilute medium and weak mechanical contrast between the inclusions and the matrix. In a In order to validate the assumption that Rayleigh wave velocities can be computed with the effective compression and shear waves velocities, an experimental and numerical approach have been developed. We study the elastic wave propagation in 300mm x 150mm x 80mm heterogeneous blocks made of mass casting polyurethane resin containing 12% of randomly distributed circular aluminum inclusions having a radius of 2.5mm. The ultrasonic measurements are performed with a laser interferometer. The source frequency content lies in the range of 30-300 kHz. According to surface wave properties, the investigation depth in our reduced scale models corresponds to cover-concrete thickness (typically 3-5 cm). The experimental dispersion curves are compared with that of numerical simulations computed by a code based on the Spectral Element Method. The effective phase velocities are compared with the predictions obtained with the ISA model.