Ultrasonic Straight Beam

Like all other ultrasonic flaw detection techniques, straight beam testing utilizes the basic principle that sound energy traveling through a medium will continue to propagate until it either disperses or reflects off a boundary of another material, such as the air surrounding a far wall or the gap created by a crack or similar discontinuity. In this type of test, the operator couples the transducer to the test piece and identifies the echo returning from the far wall, aka “the backwall signal”. After identifying the backwall signal location on a good part, the operator then looks for any additional echoes that appear ahead of the said signal or watch for the signal to shift when inspecting a test piece. An acoustically significant echo that precedes the backwall signal may imply the presence of a laminar crack or void. Through further analysis, the depth, size, and shape of the structure producing the reflection can be determined.

Normal/Angle Beam – Normal beam testing uses a sound beam that is introduced at 90 degrees to the surface – for thickness testing, while angle beam utilizes a beam that is introduced into the specimen at some angle other than 90 degrees. The choice between the two is made based on the orientation of the discontinuities that are of interest to the inspector, as the inspector should always try to introduce the sound beam at an angle as close to perpendicular the discontinuities primary axis so that the maximum amount of sound reflection occurs.

Applications for Ultrasonic Straight Beam testing:

  • Flaw detection such as parallel cracks, inclusion, porosity, laminations, etc
  • Erosion/Corrosion thickness gauging and mapping
  • Assessment of bond integrity

Advantages of Ultrasonic Straight Beam Testing:

  • Detects surface and subsurface defects
  • Depth of penetration vs. other test methods is superior
  • Only single sided access is required with a pulse-echo technique
  • High accuracy regarding estimating discontinuity size and shape
  • Minimal specimen preparation is required
  • Instantaneous results produced by using electronic equipment
  • It can be automated and when automated, detailed images can be produced

Limitations of Ultrasonic Straight Beam Testing:

  • Surface must be accessible
  • Requires couplant to promote sound transfer
  • Surface roughness, certain castings, or exceptionally thin materials are difficult to inspect
  • Linear defect oriented parallel to the sound beam go undetected
Ultrasonic Straight Beam

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