Long Range Guided Wave Pipe Inspection – the Advantages of Focusing

The use of synthetic focusing of torsional guided waves for detecting and sizing defects in pipes using an array of piezoelectric shear transducers clamped around the pipe circumference has been investigated. The performance of the focused system is tested and compared to an unfocused system using data from finite element simulations on pipes of different diameters with circumferential cracks; the finite element results are checked in experiments with a commercial guided wave inspection system on an 8 inch pipe with circumferential slots. The amplitude of the reflection from smaller features is increased by around 16dB with the focused system and the circumferential location of non-symmetric reflecting features can be identified; it is also possible to estimate the circumferential extent of the feature if it is larger than around 1.4 wavelengths of the interrogating signal, in this case, the shear horizontal wavelength. The robustness of the focused system to possible setup errors, such as coupling or phase errors is then thoroughly investigated by corrupting finite element data. The system is shown to be very robust against realistic levels of setup errors. The results therefore show that there is a substantial advantage in using focused long range guided wave inspection systems.