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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.

Inspection of Cylindrical Structures Using the First Longitudinal Guided Wave Mode in Isolation for Higher Flaw Sensitivity