In thickness measurement only one echo counts, the back-wall echo. In flaw detection all the others count: those returning from something that should not be inside the material.
- ✓ A-Scan to interpret the echo, B-Scan to see its profile
- ✓ DAC and DGS curves to evaluate the amplitude against a reference
- ✓ TCG to compensate for attenuation with depth
- ✓ Shear wave angle probes to reach the weld bead
An internal flaw — a crack, a lack of fusion, an inclusion — constitutes an acoustic discontinuity: the pulse that meets it is partly reflected and returns to the transducer before the back-wall echo. The instrument’s task is to make that reflection visible; the operator’s task is to establish what it means.
This second part is the reason why flaw detection is not an automatic measurement. The amplitude of an echo depends on the size of the discontinuity, but also on its orientation, on the depth at which it lies and on the attenuation of the material passed through. Evaluating it requires a reference and a qualification.
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Reference curves serve precisely this purpose: built on blocks with artificial reflectors of known size, they make it possible to compare the real echo with the one expected at the same depth. Without such a reference, the amplitude on the screen is a meaningless number.
The in-depth topics of this section
| Topic | What it answers |
|---|---|
| A-Scan display | What the waveform shows and how to read it |
| B-Scan display | x |
| DAC and DGS curves | How the amplitude of an indication is evaluated |
| TCG and depth compensation | Making echoes from different depths comparable |
| AWS toolkit and indication rating | The calculation required by the American code for welds |
| Angle probes and shear waves | Perché un cordone si ispeziona con il fascio inclinato |
Instrument, probe and reference
A weld inspection requires three coordinated elements: the instrument, which must have the evaluation toolkit required by the applicable code; the angle probe with its wedge, chosen according to the thickness and geometry of the joint; the reference block on which to build the curve.
The Dakota NDT flaw detectors of the FX family include the sizing toolkit with DAC, DGS, TCG and AWS, and also work in thickness measurement mode. The full range is in the Flaw detectors section.
Frequently asked questions
They are two different functions. Some high-end thickness gauges have diagnostic features, but the inspection of a weld bead requires a flaw detector with angle probes and reference curves.
Because the amplitude of an echo also depends on the depth at which the discontinuity lies and on the attenuation of the material. The curve, built on reflectors of known size, provides the term of comparison.
Yes. The interpretation of indications requires an operator qualified according to the standards applicable to the sector and to the type of component inspected.
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Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
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