Advanced semi-analytical modeling for ultrasonic inspection of concrete structure

Overview & Synopsis

The subject of this thesis falls within the framework of ultrasonic Non-Destructive Testing (NDT) of concrete. Concrete is a highly heterogeneous material: the presence of heterogeneities (aggregates, porosities, micro-cracks, etc.) leads to dispersion of sound velocity and attenuation of ultrasonic waves, which affects the testing of concrete structures. Therefore, it is important to accurately model wave attenuation due to multiple scattering from the various scatterers present in concrete. Fast simulations can be achieved through simplified analytical models. Some properties of concrete have so far been poorly taken into in these models, even if they have a significant influence on inspection observables: for example, the 3D character of the medium, the scattering from porosities, the spatial distribution of aggregates in the medium, their high concentration and their granulometry... This thesis aims at developing and validating an analytical model that incorporates these properties.A new 2D analytical modeling approach is proposed here, followed by a 3D approach. Both 2D and 3D models predict the attenuation and velocity of ultrasonic waves scattered by elastic inclusions (circular or spherical) in an elastic medium. The models can handle a distribution of inclusions of various sizes (granulometry) or multiple distributions of inclusions of different materials (presence of pores alongside aggregates). The major advance of the proposed analytical modeling is the integration in the models of a correlation function between scatterers, realistically describing the random spatial distribution of inclusions in the medium. Prediction errors reported in the literature for high scatterers' concentrations have been corrected thanks to our approach. The analytical results in both 2D and 3D show a very good agreement with numerical simulations based on the finite elements method. Experimental validation of the 3D analytical model on a manufactured concrete sample also proved to be very satisfactory.