Part 2 Of Understanding Ultrasonic Testing

Travis Jolliff

Ultrasonic testing comprises a range of non-destructive testing techniques that send ultrasonic waves through an object or material. These high frequency sound waves are transmitted into materials to characterize the material or for flaw detecting. One common application for this test method is ultrasonic thickness measurement, which is used to ascertain the thickness of an object such as when assessing metal corrosion.

This 4 part article provides information that will help the reader better understand Ultrasonic Testing. Key elements include:

  • How does ultrasonic testing work
  • Why is ultrasonic testing used
  • Pulse-echo and Through-transmission ultrasonic testing
  • Contact and Immersion Ultrasonic Testing
  • What Materials Can Be Tested Using Ultrasonic testing
  • Applications for ultrasonic testing
  • Advantages of ultrasonic testing
  • Limitations of ultrasonic testing

Part 1 addressed How does ultrasonic testing work and Why is ultrasonic testing used. Part 2 addresses Pulse-echo and Through-transmission ultrasonic testing and Contact and Immersion Ultrasonic Testing. Part 3 will address What Materials Can Be Tested Using Ultrasonic testing and Applications for ultrasonic testing. Part 4 will address Advantages of ultrasonic testing and Limitations of ultrasonic testing.

Pulse-echo and Through-transmission ultrasonic testing

Ultrasonic testing can be performed using two basic methods – pulse-echo and through-transmission.

With pulse echo testing, the same transducer emits and receives the sound wave energy. This method uses echo signals at an interface, such as the back of the object or an imperfection, to reflect the waves back to the probe. Results are shown as a line plot, with an amplitude on the y-axis representing the reflection’s intensity and distance or time on the x-axis, showing the depth of the signal through the material.

Through-transmission testing uses an emitter to send the ultrasound waves from one surface and a separate receiver to receive the sound energy that has reached the opposite side of the object. Imperfections in the material reduce the amount of sound that is received, allowing the location of flaws to be detected.

Contact and Immersion Ultrasonic Testing

Ultrasonic testing can also be split into two main types: contact or immersion testing.

Contact ultrasonic testing is typically used for on-site inspections accessibility or portability. Contact ultrasonic inspection can be performed where only one side of a test specimen is reachable, or where the parts to be tested are large, irregular in shape or difficult to transport.

Immersion ultrasonic testing is a laboratory-based or factory-based non-destructive test that is best suited to curved components, complex geometries and for ultrasonic technique development. In this method, the component or material is submerged in a water, which acts as a couplant in place of the gels used for contact ultrasound. Immersion UT generally uses pulse-echo method, and robotic probe trajectories can be used to inspect complex surfaces which would be hard to cover with contact probes. Immersion UT can be used for a wide range of wall thickness and material types, making it a suitable testing method for a variety of applications and industries.

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