Abstract
Less attention has been given to the inspection using the first longitudinal guided wave mode due to its attenuative and dispersive properties at commonly used ultrasonic guided waves (UGWs) operating frequency region (20–100 kHz). However, the first longitudinal guided wave mode has higher flaw sensitivity due to having a shorter wave length and having higher number of non-axisymmetric wave modes at a given frequency. This enhances the capabilities of advanced UGW techniques which require higher number of non-axisymmetric modes. This study has been performed to investigate the potential of mode purity and flaw sensitivity of the first longitudinal guided wave mode compared with other axisymmetric modes in the UGW operating frequency region. Numerical and experimental investigation have been conducted to investigate pure excitation and flaw sensitivity of the first longitudinal guided wave mode. It has been validated that the first longitudinal guided wave mode can be used in the UGW inspection effectively in isolation by adopting transducers with out-of-plane vibration. This reduces the cost and the weight of the UGW inspection tooling. The flaw sensitivity of the first longitudinal guided wave mode has been investigated by aid of an empirically validated UGW focusing technique. Under the studied conditions in this paper, the first longitudinal guided wave mode has $\sim5$ times higher flaw sensitivity compared with the second longitudinal guided wave mode and $\sim 2.5$ times higher than the first torsional guided wave mode. This enhances the capability of UGW flaw detection and sizing.
Highlights
R ESEARCH on ultrasonic guided waves (UGWs) inspection has expanded over recent decades including the use of low frequency ultrasound to screen large specimens e.g. pipes
Compression transducers were manufactured by Smart Material Corp [32] using “1-3 random fiber composite” piezo-ceramic elements for the dimension (13mm × 3mm × 1mm, width, height and depth respectively) which can be fit in the backing block of the current commercial Thickness-Shear Transducers (TSTs)
These transducers can be used with the existing UGW inspection tooling
Summary
R ESEARCH on UGW inspection has expanded over recent decades including the use of low frequency ultrasound to screen large specimens e.g. pipes. They studied the attenuation effect with different fluids in the pipe and presented the attenuation curves based on the numerical calculations They observed that the attenuation is considerably low in the frequency region of 20-100kHz. Dispersion of the first two longitudinal modes, L(0,1) and L(0,2) wave modes were studied empirically in a cylindrical structure filled with liquid [7]. There are research performed to study and counteract for dispersion [4] and attenuation [8] of the L(0,1) wave mode Based on this knowledge, it is be possible to obtain a sufficient test range with the L(0,1) wave mode for UGW inspection.
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