Ultrasonic Testing (UT) is an NDT method that sends high-frequency sound into a part to judge its soundness, thickness or other physical properties. Its main strength: UT finds discontinuities deep inside a material while needing access from only one side. It is used on welds, forgings, castings, plate, tubing, and even plastics and ceramics.
How it works: pulses and echoes
Human hearing stops at about 20,000 Hz. A UT flaw detector usually works at around 5 MHz, far above that limit. The sound comes from a piezoelectric transducer: a crystal that vibrates when a voltage is applied, and produces a voltage when returning sound hits it.
The instrument sends a short pulse into the part. Some of the sound reflects from the back wall, some from any discontinuity in its path. Each reflection shows as an echo on the A-scan: the horizontal axis gives distance or depth, the vertical axis gives amplitude. An echo is read at its leading edge, where it starts to rise from the baseline.
Air carries ultrasound very poorly, so a couplant (oil, grease, gel or water) is always needed between the probe and the surface.
Equipment
- Pulse-echo flaw detector with an A-scan display and range, delay, gain (dB), gate and DAC controls.
- Straight-beam (0°) probe for plate, forgings and thickness gauging; dual probe for the near-surface zone.
- Angle beam probe (e.g. 45°, 60°, 70°) for weld inspection.
- Reference and calibration blocks: flat-bottomed hole blocks, IIW (V1) and miniature (V2).
- Couplant suited to the surface condition.
Basic UT inspection steps
- Know what you are looking for: flaw type, orientation and size, and the part geometry.
- Choose the technique, probe and frequency. Use the lowest frequency that still finds the smallest required flaw.
- Calibrate the screen range; for an angle beam probe, find the exit point and check the angle on the IIW block.
- Set sensitivity on a reference block.
- Apply couplant as thinly as possible and scan.
- On plate, scan first with a straight-beam probe for laminations, then with the angle beam probe.
- Size and locate indications, e.g. with the 6 dB drop method, and evaluate them against the acceptance criteria.
- Verify calibration before and after the examination.
Examples of flaws UT finds
In welds, UT finds cracks, porosity, slag inclusions, lack of fusion and incomplete penetration. Each has its own screen signature. A crack usually gives a multi-peaked echo that moves along the time base as the probe moves. A small pore gives one sharp line. Slag shows as a cluster of side-by-side signals of varying height. Laminations in plate are clearest with a 0° straight-beam probe.
Advantages and limitations
| Advantages | Limitations |
|---|---|
| Finds internal flaws with access from one side | Needs couplant and a reasonably smooth surface |
| Shows flaw depth and location | Dead zone just below the surface |
| Works on metals and non-metals | Coarse-grained materials cause noise; some castings are hard to test |
| Finds internal flaws better than PT or MT | Signal interpretation takes training and experience |
Learn UT from scratch
In the Ultrasonic Testing (UT) Level II course you watch the A-scan update live as the probe moves, practise on a flaw detector simulator, and prepare for the Level II exam with a workbook and question bank. Start with free Lesson 1: Uses of UT & certification.
Fact source: UT Level II workbook, pp. 8–24, 32–53, 61–69, 77–85.
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