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Acceptance levels are not included in this standard, but can be applied depending on the scope of testing.
The requirements are applicable to both manual and mechanised testing.
5.3.2.3 Ultrasonic guided waves
Guided waves are low frequency ultrasonic waves that propagate along the length of a structure, guided by
and confined in the geometric boundaries of the structure. Several modes of propagation can be used.
Commercial equipment is available, working either by using piezoelectric elements or magnetostrictive
sensors, and this has led to a strong increasing use on site especially for corrosion detection in pipes and
piping.
Transmitted Wave
Reflected Wave
Defect
Pipe
Guided Wave Probe
Ultrasonic Wave propagates through tube wall
Figure 5.18 Ultrasonic guided waves propagation principle
Guided waves are sensitive to changes in cross section (increase or decrease) of the waveguide mainly by
reflection. Other interaction modes are possible, such as mode conversion and diffraction, but they are not
widely used in field applications. Guided waves are a screening technique and are generally not appropriate
to characterise accurately the indications detected. The technique can classify the indications from their
amplitude and when calibration can be carried out, the loss of cross section can be assessed. They are
generally not appropriate to determine the residual thickness except in special cases. The position where
the indication is detected can be assessed at ± 50 cm.
An ISO standard prepared by IIW Sub Commission VC will be issued soon. Several Projects (MOSCO in France,
OPCOM in Europe, Buried Pipe Integrity initiative in US) aiming to assess performances of guided waves
inspection are being undertaken.
Figure 5.19 Testing loop built to
assess performance of guided waves
(Reproduced courtesy: Institut de Soudure)
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