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Imagerie ultrasonore multi-éléments de solides anisotropes et multiplement diffusants par analyse de la matrice de réflexion

Abstract : This manuscript describes non-destructive testing of complex media by multi-element ultrasonic methods. Materials of interest are titanium alloys, whose complex microstructure triggers multiple scattering in the backscattered ultrasonic signals. Those alloys can also have a grain flow which makes their microstructure anisotropic. Measured signals are reflection matrices in the canonical basis, also known as inter-element response matrices. To analyze the multiple scattering regime, a simple numerical model is developed based on the Born expansion, to compute the contribution of the reflection matrices associated with each scattering order (single, double, triple...) to the total scattering matrix and analyze their correlations. It allows the computation of reflection matrices of a medium composed of randomly distributed isotropic fluid scatterers. A strong correlation is observed between single and multiple scattering matrices, mainly due to recurrent scattering. The latter is a component of multiple scattering for which the first and last scatterers are located in the same resolution cell. It shares some common properties with single scattering including the "memory effect", which results in a long-range correlation along the matrix anti-diagonals. The proposed simulation also enables the study of the single scattering proportion estimators defined in the literature. We show that they rather estimate the confocal scattering weight, which contains both single and recurrent scattering. Then, a new tool to separate confocal scattering is proposed, which requires the projection of the reflection matrices in focused bases in order to improve the selection of local information. Confocal scattering proportion estimators are experimentally calculated for titanium alloys samples, and show a significant contribution of multiple scattering in the backscattered signals at inspection frequencies. Finally, a method to determine the anisotropy direction of materials is proposed based on reflection matrix acquired with a two-dimensional transducer array. The method is validated with the simulation adapted to segments of aligned scatterers, both in the single and multiple scattering regimes, and applied to the characterization of titanium alloys grain flow.
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Submitted on : Tuesday, February 15, 2022 - 10:11:09 AM
Last modification on : Sunday, June 26, 2022 - 2:20:54 AM
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  • HAL Id : tel-03574386, version 1

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Cécile Brutt. Imagerie ultrasonore multi-éléments de solides anisotropes et multiplement diffusants par analyse de la matrice de réflexion. Acoustique [physics.class-ph]. Université Paris sciences et lettres, 2021. Français. ⟨NNT : 2021UPSLS084⟩. ⟨tel-03574386⟩

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