Ultrasound imaging of immersed plates using high-order Lamb modes at their low attenuation frequency bands

Abstract

This paper focuses on the use of a Lamb wave-based methodology for ultrasound imaging of immersed plate structures. In these cases Lamb waves can be strongly attenuated due to leaky waves and viscous losses in the liquid, but there are low attenuation frequency bands that may be used for NDT applications. Experimental measurements were conducted to validate the existence of these low attenuation frequency bands, which were also theoretically predicted for some propagation modes, between the frequencies-thickness products of 0.5 MHz mm and 9.0 MHz mm. Using a 5 MHz linear-array and phased-array techniques, A1 and S1 modes are used to obtain images of an immersed aluminum plate with artificial defects. The signals are post-processed in order to select the desired propagation mode and to obtain an image with dynamic focusing in reception. While the A1 mode is strongly attenuated, the S1 mode, at 3.4 MHz mm, can be used to detect and localize defects in the immersed plate.

Selective Generation of Lamb Wave Modes in a Finite-Width Plate by Angle-Beam Excitation Method

“…In particular, the wave mode should be appropriately selected to have optimum performance suitable for the application purpose. For instance, when examining fluid-coupled structures, some applications choose the extensional wave modes to avoid the degradation of the long-distance propagation ability by energy leakage [ 3 , 4 ]. But conversely, the flexural wave modes are preferred to utilize the high energy leakage [ 5 ] in other applications.…”

Multi-modal leaky Lamb waves in two parallel and immersed plates: Theoretical considerations, simulations, and measurements

“…This paper shows that 2D-FFT is indeed very efficient for measuring attenuation in one immersed plate, but purposeless for a second parallel plate. Time-frequency techniques [short-time Fourier transform (STFT) and wavelet techniques] have been used to visualize the propagation of Lamb wave for a plate in air (Niethammer et al, 2000;Xu and Hu, 2017) and less frequently in water (Takiy et al, 2017). However, even if these time-frequency techniques can separate each frequency component of the acoustic signal, they are inefficient in the case of Lamb modes propagating together at the same frequency and temporally overlapping.…”

“…Temporal contributions of each frequency component need to be clearly distinct for each mode: this is easily done when Lamb modes can propagate far enough to separate their echoes, but this condition is hard to fulfil for leaky Lamb waves that are strongly attenuated. One reference has been found where the attenuation is measured with a time frequency technique in one immersed plate (Takiy et al, 2017), but so far there is no evidence of such measurement on a second immersed plate. Moreover, the use of STFT have not yet been validated for leaky attenuation measurement.…”

Identification of Tiny Surface Cracks in a Rugged Weld by Signal Gradient Algorithm Using the ACFM Technique

“…Additionally, the testing result depends on the experience of the operator. Ultrasonic testing (UT) is usually used for the detection of internal defects [7]. Eddy current testing (ET) is easily confused by the lift-off variations, making it hard to identify a tiny crack in a rugged weld [8].…”