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 ∼5 times higher flaw sensitivity compared with the second longitudinal guided wave mode and ∼2.5 times higher than the first torsional guided wave mode. This enhances the capability of UGW flaw detection and sizing. Index Terms-Pipeline inspection, ultrasonic guided waves, compression transducers, first longitudinal guided wave mode, ultrasonic guided wave focusing. I. INTRODUCTION R ESEARCH on UGW inspection has expanded over recent decades including the use of low frequency ultrasound to screen large specimens e.g. pipes. Pipelines are Manuscript
This paper presents a Split-Spectrum Signal Processing (SSP) with applications to Long Range Ultrasonic Testing (LRUT). The problem of coherent noise due to Dispersive Wave Modes (DWM) in the context of ultrasonic scattering is addressed and a novel solution by utilizing the SSP technique is proposed for reduction of the effects of DWM in the received signal. The proposed technique investigates the sensitivity of SSP performance to the filter bank parameter values such as processing/filter bandwidth, and filter overlap. Therefore, as a result the optimum values are introduced that improve the signal to noise ratio (SNR) significantly. The proposed method has been compared with conventional approaches for synthesized signals for a 6 inch pipe by applying the different recombination SSP techniques. The Polarity Thresholding (PT) and PT with Minimization (PTM) methods were found to give the best result and substantially improve the SNR performance by an average of 10 dB.
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