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Ultrasonic Testing (UT)

Principles of Ultrasonic Testing

Ultrasonic waves are introduced into a material where they travel in a straight line and at a constant speed until they encounter a surface. At surface interfaces some of the wave energy is reflected and some is transmitted. The amount of reflected or transmitted energy can be detected and provides information about the size of the reflector. The travel time of the sound can be measured and this provides information on the distance that the sound has traveled.


Ultrasonic testing is a very versatile inspection method, and inspections can be accomplished in a number of different ways. Ultrasonic inspection techniques are commonly divided into three primary classifications.

Type of testing technique
  • Pulse-echo and Through Transmission (Relates to whether reflected or transmitted energy is used)
  • Normal Beam and Angle Beam (Relates to the angle that the sound energy enters the test article)
  • Contact and Immersion (Relates to the method of coupling the transducer to the test article)



  • Sensitive to both surface and subsurface discontinuities.
  • Depth of penetration for flaw detection or measurement is superior to other methods.
  • Only single-sided access is needed when pulse-echo technique is used.
  • High accuracy in determining reflector position and estimating size and shape.
  • Minimal part preparation required.
  • Electronic equipment provides instantaneous results.
  • Detailed images can be produced with automated systems.
  • Has other uses such as thickness measurements, in addition to flaw detection.