Ultrasonic testing (UT) uses the propagation of ultrasonic waves through the material to find internal and surface discontinuities in parts and welds. In addition UT can be used to measure the thickness or length of a part.

Ultrasonic testing can be used on metals and some polymers and composites. It has the advantage over radiography in that it is completely safe and can be performed without evacuating the area. UT uses a piezo electric probe to generate and to receive ultrasonic signals. A water based ultrasonic couplant is used on the material to transfer the signal between the probe and the part.

Ultrasonic testing uses multiple types of waves. One common wave used is the longitudinal wave, or straight beam. This type of beam is used mainly for sending a signal into the material perpendicularly to the part's surface. It is often used for scanning plates for lamination, corrosion mapping, measuring the length of parts, checking bolts for cracks, scanning for bond in weld overlay, some types of weld inspection, and for thickness measurements. Thickness measurements of steel plate and mapping of corrosion wastage is often called audio gauging in the marine industry.

Another common type of wave used is the shear wave, or angle beam. This type of beam creates a slower wave than a longitudinal signal. The wave is sent into the part at an angle and is often used for weld inspection, checking the depth and extent of cracks, checking for manufacturing defects in material stock, and for checking shafts for cracks and other defects.

Other types of ultrasonic tests include phased array, time of flight, and EMATS.

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