A flaw in a weld bead sits inside the material and does not reduce the thickness. Finding it calls for a flaw detector with angle beam probes, and a qualified operator able to interpret the indication.
- ✓ Shear wave probes with 45°, 60° and 70° wedges
- ✓ 16-point DAC curve to assess the amplitude of the indication
- ✓ A-Scan and B-Scan to locate and document
- ✓ Dual flaw + thickness mode on the same instrument
- ✓ References EN ISO 17640 and EN ISO 11666
- ✓ Personnel qualified to EN ISO 9712
Cracks, slag inclusions, lack of fusion and porosity have one thing in common: they sit inside the joint and leave no trace on the surface. A thickness gauge, which sends the beam straight down and measures how much material lies under the probe, does not see them: the material is all there, it simply is not sound.
Ultrasonic weld inspection works differently. A shear wave probe mounted on an angled wedge sends the beam in obliquely: the wave enters the material at an angle, crosses the bead and reflects off the discontinuities it meets sideways. By moving the probe back and forth relative to the bead the whole thickness of the joint is explored.
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Finding an echo, though, is not enough: it has to be decided whether that indication is acceptable. This is where the DAC curve comes in, built on known reflectors at increasing distances, which compensates for attenuation and allows amplitudes recorded at different depths to be compared. It is the step where the operator’s qualification weighs as much as the instrument.
What the inspection requires
| Item | Function |
|---|---|
| Flaw detector | It generates and displays the signal, manages gates and DAC |
| Shear wave probe with wedge | It sends the beam in obliquely: 45°, 60° or 70° according to the thickness |
| Reference block | It is used to calibrate the instrument and build the DAC |
| Couplant | It ensures transmission between wedge and part |
| Written procedure | It defines angles, coverage and acceptance criteria |
| Qualified operator | He interprets the indication: it is the part that cannot be automated |
Choosing the angle
The angle of the wedge determines the path of the beam through the joint, and is chosen according to the thickness and the geometry of the bead.
- Larger angles give longer paths and are used on thin sections
- Smaller angles penetrate deeper and are needed on heavy sections
- The procedure often prescribes several angles on the same joint
- The V-path, that is the zig-zag route between the two surfaces, has to be worked out to know where you are looking
Angles and coverage are not improvised: they are set by the written procedure according to the applicable standard. See also the transducers for flaw detectors.
Recommended instruments
| Instrument | Why in this application |
|---|---|
| Dakota FX70-DL / FX71-DL | Portable with A-Scan and B-Scan: inspection in the field and in the workshop |
| Dakota FX81-DL | Benchtop with a large display, for laboratory and acceptance testing |
| Shear wave probes and wedges | Quick change and standard, angles 45°, 60° and 70° |
| Reference blocks | Calibration of the instrument and construction of the DAC curve |
Frequently asked questions
Because it sends the beam straight down and measures how much material lies under the probe. A flaw inside the bead does not reduce the thickness: the material is all there, but it is not sound. An angle beam probe is needed, one that crosses the joint obliquely and catches the discontinuity sideways.
To make indications recorded at different depths comparable. The amplitude of an echo falls with the distance the beam travels, so the same flaw would give different signals depending on where it is. The DAC, built on known reflectors at increasing distances, compensates for the attenuation and allows the acceptance criteria to be applied.
Yes, when the inspection counts as acceptance testing. Ultrasonic weld inspection is a regulated NDT activity: EN ISO 9712 governs the qualification and certification of personnel, and assessing the indication is the part of the work no instrument can replace.
Back to Mechanical Engineering and Steelwork. For the choice of detector and probes, write to us giving thicknesses, material and the applicable acceptance standard.
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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