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The main characteristics of TECA signals are:
•
Almost flat liftoff signal
•
Crack-like indications approximately 90° relative to the liftoff signal
•
All crack-like indications feature the same phase shift
•
Signal amplitude linked to the defect depth
Figure 4 Liftoff and phase shift
Obviously, the vertical amplitude of a given indication is impacted by lift off. As shown above,
the vertical component of the signal can be reduced by a factor 2 or 3 when there is significant
lift off. However, since the coil design allows for monitoring the lift off, it can be measured in
order to adjust depth sizing accordingly. Ultimately, the technique relies on multiple depth
curves that are calibrated with various lift off values. The result is equivalent to a 3D “depth
plane” allowing to correlate crack depth, vertical amplitude and lift off (horizontal component),
as illustrated in the next figure.
Figure 5 A 3D “depth plane” is integrated into the software to correlate
crack depth, vertical amplitude and liftoff
