Ultrasonic Examination – Longitudinal Wave UT

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Longitudinal wave UT is used primarily to determine the remaining thickness of corroded shell components, including the depth of pits. It can also be used to detect internal flaws that have developed during service and that have a reflecting surface essentially parallel to the surface of the workpiece (i.e., perpendicular to the longitudinal ultrasonic wave), such as hydrogen blisters. Surfaces that are perpendicular to the longitudinal ultrasonic wave will usually reflect echos directly back towards the transducer with sufficient amplitude to be detected by the transducer.

Longitudinal wave UT is not an especially useful technique for detecting cracks that have developed during service. Cracks that have a significant effect on the integrity and reliability of a pressure vessel propagate in a direction that is essentially perpendicular to the surface of the component. The reflecting surfaces of these cracks will be essentially parallel to the longitudinal ultrasonic waves, and, therefore, they will not normally reflect echos towards the transducer with sufficient amplitude to be reliably detected.

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