Longitudinal vs Torsional mode in GWT

By: Publisher Team | Feb 18, 2020 15:56 PM

Have you ever asked the question of what is the differences and benefits between Longitudinal and Torsional mode in Guided Wave Inspection? Dr David Alleyne, Dr Thomas Vogt and Prof Peter Cawley wrote a paper on this very subject "The choice of torsional or longitudinal excitation in guided wave pipe inspection"

It makes good reading and a must if you are considering any investment in GWT.

Key Benefits of using ONLY the Torsional mode:

  • It is relatively easy to predict its sensitivity to defects, as it presents a simple mode shape.
  • Its sensitivity to defects can be easily understood
  • It is more sensitive to axial defects than other modes.
  • Its Not affected by presence of liquid (either inside or outside the pipe)
  • Velocity is constant across frequency, can be used across wide frequency band
  • Longitudinal mode obtains poor signal to noise ratio, leading to false calls or missed defects.
  • From an inspection point of view, the use of Longitudinal mode has little value and can significantly increase equipment cost and weight as well as the time for testing, interpretation and reporting of the data.

If you would like more information, then please contact info@guided-ultrasonics.com

Abstract: Long-range screening of pipework using guided waves is now in routine industrial use. The T(0,1) mode is generally preferable to the L(0,2) mode as the transduction system required is simpler and lighter. The L(0,2) mode may be useful in a small number of cases but tests reported here demonstrate that it is unlikely to be useful in liquid-filled pipes. It also tends to give poorer signal-to-noise ratio, and a double mode conversion phenomenon can give extra echoes that could be incorrectly called as defects. In principle it is possible to test each length of pipe using both modes, though at a considerable penalty in equipment complexity, weight and cost. It is argued that if increased confidence is required, a better strategy generally is to test each section of pipe from both directions using the torsional mode.

Reference: Alleyne, D N, Vogt, T, & Cawley, P. 'The choice of torsional or longitudinal excitation in guided wave pipe inspection', Insight - Non-Destructive Testing and Condition Monitoring, Volume 51, Number 7, 1 July 2009, pp. 373-377(5)

Source: https://www.linkedin.com/pulse/longitudinal-vs-torsional-mode-gwt-jackie-berry/