Home »  NDT Basic Knowledge » Ultrasonic Flaw Detection
 Basic Knowledge of Ultrasonic Testing (UT)
Invisible and most often covered inhomogenities in almost all objects can be detected and localized nondestructively with ultrasound. The ultrasonic inspection is very important in the case of safety relevant components. Beside flaw detection also measurements are possible, e.g. the wall thickness measurement.

 Main Applications
  • Mobile inspection of welded joints
  • Flaw detection and quality check of castings
  • Automatic testing of mass products with simple geometries like bars, billets, profiles, plates and tubes made of steel, non iron metals and plastic materials.
  • Wall thickness measurement on tubes, pipelines, vessels and chemical plants. This is reasonable where the objects are not accessible from both sides, and where a mechanical slide gauge cannot be used.
Ultrasound is defined as sound with a frequency above the human hearing threshold, i.e. above 20,000 Hertz (= 20 kHz). The main frequency range for materials testing lies between 0.5 and 10 MHz and thus considerably above the hearing threshold. In case of special applications also frequency above 10 MHz and below 0.5 MHz are possible.

When an incident wave hits an interface between two different media, one part is reflected and another part is transmitted into the second medium. The reflection and transmission factor depends on the difference of the medium properties (e.g. in sound velocity and density). At an interface between steel and air almost 100% of the ultrasonic wave is reflected.

Inhomogenities in objects are pores, cracks, shrinkage cavities, slag inclusions etc. Here the ultrasound is well reflected and runs back to the probe under good circumstances.
In order to transmit the ultrasound from the probe into the object, most often a liquid couplant is used, e.g. water, oil or gel.
For automatic testing the whole object and the probe are often completely immersed in water.

If the sound velocity of a material is known, the depth of a defect can be calculated precisely from the ultrasonic time of flight. The sound velocity is a material constant and has for example a value of 330 m/s in air (at 0°C; 344 m/s at 20 °C) and 5920 m/s in steel.
A wall thickness can be measured, if the ultrasonic time of flight between front surface and backwall of an object is determined.
A resolution down to the micrometer range can be achieved.
Wall thickness measurement devices are small, offer a simple operation and often display only the wall thickness value.

A table of acoustic specifications of various materials, such as longitudinal and transverse sound velocity, density and acoustic impedance can be found here.
If inhomogenities like a delamination in a rolled plate is large compared to the sound beam diameter, the dimensions of the defect can be determined with a scanning motion. However, the exact determination of smaller flaw sizes (smaller than sound beam diameter) is not possible with this method. In these cases the reflectivity of such a defect is compared with the echo amplitude of an artificial test reflector (flat bottom hole, cylindrical reflector). Also here, it is necessary that the ultrasound hits the inhomogenity and the test reflector perpendicularly.

The size of the smallest detectable flaws depends on the ultrasonic wave length. With higher sound frequencies, e.g. 20 MHz the wave length in steel is 0.3 mm. In case of the often used frequency of 2 MHz the wave length is 3 mm. As a rule of thumb, defects in the order of a wavelength can be detected.

Often angle beam probes are used, for example for weld testing. The appropriate insonification angle is selected according to the defect types and orientations. Also here it is important that the ultrasound hits the pore or lack of fusion almost perpendicularly.

 Advantages of Ultrasonic Testing
  • Detection of inhomogenities at the surface and in the volume
  • All materials with a good sound conductivity can be tested (sometimes even up to 10 m material thickness can be tested, e.g. forged turbine shafts)
  • The ultrasonic testing can be automated (by evaluation the acoustic signals which were converted into electric signals)
  • Fast test method (in the order of 1000 measurements per second are carried out)
  • Radiation protection must not be provided
  • Reliable detection of planar inhomogenities (e.g. cracks, delaminations in plates, lacks of fusion in welds, etc. )

 Further Information

Knowledge floater "Ultrasonic Measurement and Testing"

You can find further extensive information from the following knowledge floater video "Ultrasonic Measurement and Testing":

Note: This video frame links to Youtube. Please ensure that no internal browser setting prevents external loading or click here for direct access to our NDTChannel on Youtube.
Alternative: The Knowledge Floater can be downloaded as zipped Powerpoint ® file here (size 47 MB approx.). The zip-file also contains the necessary powerpoint viewer. Unpack the archive into a separate directory and start the presentation by opening the file "play.bat".

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