Start Date

2016 12:00 AM

Description

Angle-beam ultrasonic inspection methods are frequently based on shear waves since they are readily generated using conventional longitudinal transducers and a wedge. Full wavefield imaging can be employed to measure such waves on the surface of a plate-like specimen after they have scattered from a feature or defect of interest, and multiple shear wave arrivals, or “skips,” are readily visible due to beam spread [1]. Because of the complexity of the total wavefield, which also includes Rayleigh and longitudinal waves, it is challenging to experimentally quantify scattering from the primary skip of interest. Considered here is a methodology to isolate shear wave scattering from a through-hole caused by a specific shear wave skip that is incident upon the hole.

First, the residual wavefield is obtained by performing wavefield baseline subtraction before and after the introduction of a scatterer [2]. Next, a 3-D Fourier transform is performed to convert the residual signals in the time and space domain (t-x-y) to the frequency-wavenumber domain (ω-kx-ky). Then, a 3-D frequency-wavenumber filter is built to extract most of the shear wave energy based upon specified frequency and phase velocity ranges. Finally, the wavefield is reconstructed by performing an inverse 3-D Fourier transform. A comparison of the wavefields before and after filtering shows that only shear waves are left; however, this filtered wavefield still contains scattered energy from multiple skips incident upon the hole. The geometry of the test setup employed here is such that the second shear skip incident upon the hole has the largest energy. The time-dependent trajectory in space of the shear wave scattered by this second skip is calculated by a ray tracing model, which enables its propagation path to be tracked in the measured wavefield. A space-time filter is constructed and implemented to extract this specific scattered wave. After space-time filtering of the residual shear waves after baseline subtraction, scattering can be readily quantified as a function of propagation direction. Results are shown for several through-holes of different diameters in an aluminum plate.

Language

en

File Format

application/pdf

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Jan 1st, 12:00 AM

Windowing of Full Wavefield Data in Multiple Domains to Characterize Angle-Beam Shear Wave Scattering

Angle-beam ultrasonic inspection methods are frequently based on shear waves since they are readily generated using conventional longitudinal transducers and a wedge. Full wavefield imaging can be employed to measure such waves on the surface of a plate-like specimen after they have scattered from a feature or defect of interest, and multiple shear wave arrivals, or “skips,” are readily visible due to beam spread [1]. Because of the complexity of the total wavefield, which also includes Rayleigh and longitudinal waves, it is challenging to experimentally quantify scattering from the primary skip of interest. Considered here is a methodology to isolate shear wave scattering from a through-hole caused by a specific shear wave skip that is incident upon the hole.

First, the residual wavefield is obtained by performing wavefield baseline subtraction before and after the introduction of a scatterer [2]. Next, a 3-D Fourier transform is performed to convert the residual signals in the time and space domain (t-x-y) to the frequency-wavenumber domain (ω-kx-ky). Then, a 3-D frequency-wavenumber filter is built to extract most of the shear wave energy based upon specified frequency and phase velocity ranges. Finally, the wavefield is reconstructed by performing an inverse 3-D Fourier transform. A comparison of the wavefields before and after filtering shows that only shear waves are left; however, this filtered wavefield still contains scattered energy from multiple skips incident upon the hole. The geometry of the test setup employed here is such that the second shear skip incident upon the hole has the largest energy. The time-dependent trajectory in space of the shear wave scattered by this second skip is calculated by a ray tracing model, which enables its propagation path to be tracked in the measured wavefield. A space-time filter is constructed and implemented to extract this specific scattered wave. After space-time filtering of the residual shear waves after baseline subtraction, scattering can be readily quantified as a function of propagation direction. Results are shown for several through-holes of different diameters in an aluminum plate.