Volumetric & Advanced NDT · UT
Ultrasonic Testing
Sound waves that measure what light will never see.
Ultrasonic testing (UT) uses high-frequency sound pulses — typically 1–10 MHz — transmitted into a material to detect internal flaws and measure thickness. Echoes return from the far wall and from any internal discontinuity; the time each echo takes to arrive locates the reflector with millimetre precision, from one side of the component only.
The explainer
Watch how it works.
Schematic · loops automatically
1Couple the probe
Couplant gel bridges probe and surface — without it, the air gap reflects the sound before it enters the steel.
2Pulse travels into the material
The piezoelectric element fires a megahertz pulse; the wavefront travels through the thickness at a known velocity.
3The backwall answers
The far surface reflects the pulse; its arrival time on the A-scan measures the full wall thickness.
4A flaw answers early
Moving over a defect, part of the beam reflects sooner — a new peak appears before the backwall echo, locating the flaw’s depth.
5Locate and size
From arrival time and amplitude — against calibrated reference reflectors — the technician positions and sizes the indication for assessment.
The physics
At the heart of every UT probe is a piezoelectric element: a crystal that converts electrical pulses into mechanical vibration and back again. Driven at megahertz frequencies it launches a beam of ultrasound into the material through a thin film of couplant — the gel that bridges the acoustic gap between probe and steel, without which almost all the energy would reflect straight back at the surface.
Sound crossing a boundary between materials of different acoustic impedance reflects — and a flaw is exactly that boundary. A crack, a lamination or a pore filled with air reflects strongly; the pulse returns to the probe and appears on the instrument’s A-scan as a peak whose position measures the sound’s travel time. Since the velocity of sound in the material is known, time converts directly to distance: the readout is a ruler into the metal.
For weld inspection the beam is refracted into the material at an angle using a wedge (commonly 45°, 60° or 70°), sweeping the weld volume from adjacent parent metal, since the cap profile prevents direct coupling. The physics — Snell’s law refraction, beam spread, mode conversion — is the same used at higher sophistication by phased array and TOFD, which is why UT is the foundation discipline of volumetric NDT.
Where it earns its keep
Typical applications
- Weld examination on structural steel to EN 1090 and pressure plant
- Thickness surveys on vessels, tanks and pipework — live plant, one-side access
- Lamination checks before welding and drilling
- Statutory pressure examination follow-up measurements
Standards & schemes
| Designation | Covers |
|---|---|
| EN ISO 17640 | Ultrasonic testing of welds — techniques, testing levels |
| EN ISO 11666 | Acceptance levels for ultrasonic testing of welds |
| EN ISO 16810 | Ultrasonic testing — general principles |
| EN ISO 16809 | Ultrasonic thickness measurement |
Metlab UT technicians hold ISO 9712 method certification to Level 2/3, with weld examinations reported under the INAB-accredited ISO/IEC 17025 laboratory scope (Reg. No. 398T).
What is the difference between UT and radiography for welds?
UT detects planar defects (cracks, lack of fusion) more reliably, measures depth directly, needs access to one side only and has no radiation controls; RT images volumetric defects (porosity, inclusions) superbly and leaves a permanent visual record. Many specifications accept either; the defect types expected usually decide.
How accurate is ultrasonic thickness measurement?
With calibrated equipment and competent technique, typically to about ±0.1 mm on clean steel — comfortably sufficient for corrosion monitoring and remaining-life calculations. Rough or corroded back walls widen the scatter, which is why corrosion surveys report distributions, not single points.
Can UT test through paint?
Thin, well-bonded paint is generally fine for thickness gauging with appropriate technique; heavy, flaking or multi-layer coatings degrade accuracy and are removed locally. For weld defect examination, surface condition requirements are stricter.
Testing · Inspection · Certification
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