Examination method
Ultrasonic testing (UT)
High-frequency sound is injected into the material and the echoes returning from internal reflectors are timed and measured, giving both the depth and the size of buried flaws.
How it works
A piezoelectric crystal in the probe converts an electrical pulse into a mechanical vibration at frequencies far above hearing. Couplant between probe and part excludes the air film that would otherwise reflect nearly all the energy. The sound beam travels into the material at a known velocity and reflects wherever it meets a change in acoustic impedance.
Reflections return to the same crystal, which converts them back into electrical signals displayed against time. Because sound velocity in the material is known, elapsed time converts directly to distance, so the instrument reports how deep a reflector lies. Echo amplitude, compared against a calibrated reference, gives an estimate of reflector size.
Weld examination uses angle beams introduced through a wedge, so the sound travels diagonally and bounces off the far surface to sweep the whole weld cross-section. Since planar flaws reflect strongly only when the beam strikes them near perpendicular, the operator scans from both sides and at several angles to catch reflectors of every orientation.
Strengths
- Detects internal flaws throughout the thickness, including deep in heavy sections
- Superior to radiography for planar flaws such as cracks and incomplete fusion
- Gives flaw depth as well as presence, which radiography cannot provide from a single exposure
- No radiation hazard, so work can continue around the inspector without area control
Limitations
- Highly operator-dependent, since interpretation happens live rather than on a permanent record
- Coarse-grained, austenitic, and cast structures scatter sound and can make examination impractical
- Requires couplant, a reasonably smooth surface, and access for the probe to scan properly
- Calibration blocks and reference standards must match the material and geometry being examined
Key variables
- Probe frequency
- Higher frequency resolves smaller flaws but attenuates faster and penetrates less, so it trades sensitivity against reach.
- Beam angle
- Chosen so the sound strikes expected flaw orientations near perpendicular, since a badly angled beam returns almost nothing.
- Calibration and reference level
- Sets the sensitivity against which all indications are judged, and is verified against a block matching the material and thickness.
- Couplant
- Excludes air between probe and part; without it almost all the energy reflects at the surface and never enters the material.
- Scanning pattern
- Coverage, overlap, and the number of directions scanned decide whether the whole weld volume was actually examined.
What an inspector watches for
- Verify calibration on the correct reference block before and after the examination, since drift invalidates everything measured in between
- Confirm the scan covered the full weld volume from the required directions rather than a single pass down one side
- Watch for geometric echoes from the root, cap, and counterbore that can be mistaken for flaws
- Check surface condition and couplant, because poor contact reduces sensitivity without any obvious warning on screen