26 Ultrasonic Testing (UT)
Ultrasonic testing (UT) is non-destructive testing that uses sound waves.
- Expensive
- Lots of training required
- More than one type of UT array
Ultrasonic examination is a true volumetric test method because it can identify the location of discontinuities within a material. UT is very sensitive and is capable of locating very fine surface and subsurface cracks as well as other internal discontinuities and defects.
High‑frequency vibrations or waves are used to locate and measure defects in both ferrous and nonferrous materials. A high-frequency sound beam is directed into a part on a predictable path. The sound beam is reflected back when it encounters an interruption in the continuity of a material. The reflected beam is detected and analyzed to define the presence and location of the discontinuity.
Electronic Components
Electronic components required for UT include:
- An electronic signal generator to provide bursts of alternating voltage
- A sending transducer (crystal) to emit a beam of ultrasonic waves when the AC voltage is applied
- A receiving transducer to convert the sound waves to AC voltage (the receiving transducer and the sending transducer may be combined)
- An electronic device to amplify and demodulate or otherwise change the signal from the receiving transducer
- An electronic timer to control the operation
- A CRT display to characterize or record the output from the test piece. The CRT display uses A‑scan presentation.
Waves
A longitudinal wave (straight beam) is a compression wave that represents wave motion in which the particle oscillation is the same direction as wave propagation.
A shear wave (angle beam) is a transverse wave that represents wave motion in which the particle oscillation is perpendicular to the direction of wave propagation.
UT of Root Passes
UT of the root pass is carried out from both sides of the weld, whenever possible, using a suitable angled probe and prescribed scanning patterns. UT of the root pass detects incomplete penetration or incomplete fusion. Scanning lines are marked at half skip distance back from the original root face on either side of the weld. A guide is then placed so that when the heel of the selected angle probe is butted against the guide, the probe index is on the scanning line.
Other UT
Ultrasonic examination of the fusion face and the weld body requires examining the entire weld volume. The probe is positioned to produce full skip distance to the nearest edge of the weld reinforcement. The probe index is located at a distance from the weld centerline equal to full skip distance plus one-half the full weld reinforcement width. The base metal is marked with two lines, parallel to the weld centerline, on both sides of the weld. The lines are at half skip and full skip distances and mark the boundaries of the scanning pattern.