Inversion of circumferential elastic waves for characterization of concrete pipes

Circumferential elastic waves are used effectively for the non-destructive testing of cylindrical structures. Nevertheless, its potential for testing concrete pipes made out of non-homogeneous media has received only a limited attention. Characterization of such structures poses greater challenges due to the inherent uncertainty in measurements. This paper presents the inversion of elastic waves in concrete pipes that propagate in the circumferential direction for calculating its unknown elastic and geometric properties. As a demonstration, guided wave experiments are conducted on a segment of reinforced concrete pipe (RCP) and its wall thickness, shear, and compression wave velocities of the medium are calculated through parametric inversion using analytical solutions as the forward model. The elastic wave records are measured from the pipe and dispersion properties of circumferential modes are extracted using the cylindrical wavefield transformation. Then the dispersion curves are inverted as a linear inverse problem using the Singular Value Decomposition (SVD) approach. The proposed algorithm is able to invert incomplete datasets from the first two modal branches. The algorithm's reliability is tested extensively for convergence and stability using synthetic and experimental datasets. Finally, the inverted parameters are validated against independent measurements conducted using the impact echo (IE) technique.