What is Ultrasonic Testing (UT)?

Ultrasonic testing (UT) is a non-destructive method that sends ultrasonic waves through a material to find internal flaws or measure thickness. Inspectors use it mostly on welds and for thickness measurement.

How it works

Ultrasound is sound above 20 kHz. A transducer sends a short pulse into the object, and the instrument records how strong the returning or transmitted signal is and when it arrives. Typical pulses have centre frequencies of about 0.1 to 15 MHz. Metal testing commonly uses 2 to 10 MHz. Concrete scatters high frequencies, so tests on concrete use about 50 to 500 kHz.

Two arrangements are common. In pulse-echo testing one transducer sends and receives, and the arrival time of an echo shows a back wall or an internal flaw. In through-transmission testing a second transducer receives on the opposite surface, and a flaw weakens the received signal.

A couplant such as gel, oil or water is normally needed between the transducer and the surface.

Use on concrete

The ultrasonic pulse velocity (UPV) test puts transducers on opposite sides of the concrete. Velocity is the path width divided by the pulse transit time. The transducers run at about 40 to 50 kHz. A low velocity can point to cracks, honeycombing or poor patches. The test is indicative only. Destructive testing still gives the structural properties.

ASTM C597, Standard Test Method for Pulse Velocity Through Concrete, is the standard for this test.