Ultrasonic testing

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The ultrasonic testing is an acoustic method for detection of material defects by means of ultrasound . It belongs to the non-destructive testing methods . This means that components can also be checked when they are installed, e.g. B. the supporting elements of an aircraft. The procedure is sometimes called sonography , as in medicine .

principle

Ultrasonic testing is based on the fact that sound waves propagate at different speeds in different media. They are partially reflected at interfaces of different wave impedance , another part spreads further - often with a changed direction. As the difference in wave impedance increases, the reflected portion also increases. This difference is particularly great at every air-metal boundary.

Like all test methods, the ultrasound control is standardized and carried out according to guidelines. A coupling agent (e.g. paste ( solution ), gel, water or oil) is applied to the surface of the workpiece . The surface to be tested is scanned by means of a test head which emits and receives ultrasound from 0.02 to 50 MHz . This can be done manually, mechanically or automatically (within the production lines). With the latter, the test piece is often immersed in a suitable liquid (immersion technique) or wetted in a defined manner in order to transmit the sound signal.

Changes in the acoustic properties at interfaces (e.g. a blowhole , an inclusion, a crack or other separation in the structure ) inside the part to be tested reflect the sound pulse and send it to the transducer in the test head, which is both Both sender and receiver functions back. The time elapsed between sending and receiving allows the route to be calculated ( pulse-echo method ). Using the measured time difference, a signal image is generated and made visible on a monitor. Using this image, the position can be determined and the size of the defect ( called discontinuity in technical terms ) can be estimated by comparing it with a replacement reflector (flat bottom hole (circular disc reflector), groove, cross hole). In general, discontinuities with a size of approx. 0.6 mm can be detected, with special processes even up to 10 −7  mm. In the case of automatic test systems, the information is stored, compared to the test piece and documented in various ways immediately or later.

Calibrated ultrasonic wall thickness gauges show the wall thickness directly as a numerical value.

application

Ultrasonic testing is a suitable test method for sound-conductive materials (this includes most metals) to find internal and external defects, e.g. B. for welds , forgings, castings, semi-finished products or pipes . Installed railroad tracks are routinely checked by test trains. Finding external (surface) defects is particularly important for parts where the other, mostly internal, surface is not accessible.

In automobile production, ultrasonic testing using the pulse-echo method is widely used to monitor the quality of resistance weld points . The quality of the welding point is derived from the shape of the echo sequence in the A-scan.

Principle of the A-pictures of resistance weld points

A special application is the measurement of layer thicknesses , in which the differences in the running time of the sound are used to determine the material thickness of coatings such as paint layers .

Areas of application

Typical areas of application for ultrasonic testing include:

  • Weld inspection
  • Wall thickness measurement
  • Sheet metal testing
  • Forging inspection
  • Purity test
  • Binding test of white metal bearings

Procedure

Two basic procedures are used:

  • The reflected sound method (also pulse-echo method)

The smallest discontinuity that can be displayed is greater than half the wavelength of the sound, which is dependent on the speed of sound of the material and the frequency of the probe. At a frequency of 4 MHz you get around 0.7 mm in steel. The frequency level cannot be increased indefinitely because the grain boundaries of the structure lead to scattering and absorption and thus the depth of penetration is reduced. The coarser the crystal formation in z. B. is cast steel, the longer the wave frequency must be selected. In the case of cast iron and austenite , the testability is usually very limited.

The largest material paths that are usually echoed using the pulse-echo method are 5 m, which corresponds to a sound path of 10 m. Layer thicknesses can be measured from around 10 µm - even on polymeric materials. The test pulses reach 100 MHz here.

This procedure involves two probes connected to the same ultrasound device. One probe sends the pulse, the other receives the pulse on the opposite side of the test piece. This method is very special and is mostly used for checking for duplications . In addition, zones very close to the surface can also be checked, as this is not possible with the reflection sound method due to a "dead zone". The “dead zone” arises because a probe cannot send and receive at the same time.

The squirter technique can serve as an example of the combination of diving technique testing and sound transmission. The two probes are not directly on the test piece, but a few centimeters apart and are only coupled by water. This technology is used e.g. B. at Airbus in the large-area CFRP / GFRP testing of tail unit components.

Special test procedure

There are around fifty different so-called special test procedures, with some new ones being added every year. A special test is understood to mean the use of the sound effect in materials outside of the “classic” processes. Important branches of the process are phased array, TOFD , guided waves, LORUS and others. With phased array probes (also called group emitters), for example, the transmission characteristics can be specifically influenced by the independent control of the individual emitters. In this way, oblique insonification or sound focusing can be achieved.

Sometimes the methods only differ in the processing of the signals. For example, the ultrasonic testing of aircraft engine disks.

With this technology, no more defects are sought. The multi- zone test is a procedure according to which the engine components are scanned in different depth zones in the range of 0.16 to 0.4 mm test sensitivity. This process takes place in a water tank with the disk rotating. The image recorded digitally during the test is the so-called C-image . On the connected computer you can see an image with different colors, each color represents a certain ultrasonic echo height. Overall, the homogeneity of the pane is determined. A power plant disk the shape and size of a parasol stand has about 50 to 60 test zones and testing takes up to 20 hours.

Ultrasonic testing standards

German Institute for Standardization (DIN)
  • DIN EN ISO 16810, Non-Destructive Testing - Ultrasonic Testing
  • DIN EN 1330-4, Non-Destructive Testing - Terminology - Part 4: Terms used in ultrasonic testing
  • DIN EN 1712, non-destructive testing of welded joints - ultrasonic testing of welded joints - permissibility limits (replaced by DIN EN ISO 11666)
  • DIN EN 1713, non- destructive testing of welded joints - ultrasonic testing - characterization of indications in welds (replaced by DIN EN ISO 23279)
  • DIN EN 1714, non- destructive testing of welded joints - ultrasonic testing of welded joints (replaced by DIN EN ISO 17640)
  • DIN EN 10160, ultrasonic testing of flat products made of steel with a thickness greater than or equal to 6 mm (reflection method )
  • DIN EN 10228-3, Non-destructive testing of steel forgings - Part 3: Ultrasonic testing of ferritic or martensitic steel forgings
  • DIN EN 10228-4, Non-destructive testing of steel forgings - Part 4: Ultrasonic testing of forgings made of austenitic and austenitic-ferritic stainless steel
  • DIN EN 10308, Non-Destructive Testing - Ultrasonic testing of steel bars
  • DIN EN 12223, Non-Destructive Testing - Ultrasonic Testing - Description of the calibration block No. 1
  • DIN EN 12668, Non-Destructive Testing - Characterization and verification of ultrasonic testing equipment
  • DIN EN 12680, foundry - ultrasonic testing
  • DIN EN 14127, non- destructive testing - thickness measurement with ultrasound
  • DIN EN 15617, non- destructive testing of welded joints - time-of- flight diffraction technique (TOFD) - permissibility limits (replaced by DIN EN ISO 15626)
  • DIN EN ISO 7963, Non-Destructive Testing - Ultrasonic Testing - Description of the calibration block No. 2
  • DIN EN ISO 10863, Welded joints - Application of the time-of- flight diffraction technique (TOFD) for testing welded joints

Other institutes

  • SEP 1923, edition: 12/1990, ultrasonic testing of forgings with higher requirements, especially for components in turbine and generator systems
  • ÖNORM M 3002, edition: July 1st, 2004, ultrasonic testing of formed products (forged, rolled) with higher requirements made of steel and other metallic alloys - implementation, scope of testing, quality classes
  • CCH 70-3 UT 70-3, functional specification for the acceptance of steel castings for hydraulic machines
  • AD-Merkblatt HP 5/3, edition: 01/2002
  • AD-Merkblatt HP 5/3 Annex 1, edition: 01/2002
  • AD2000-Merkblatt HP 5/3, edition: 02/2007
  • AD2000-Merkblatt HP 5/3 Annex 1, edition: 02/2007

In order for the tests carried out to be recognized, the testers should be qualified / certified according to the following standards: (EN) ISO 9712; USA: ASNT - SNT-TC-1A / ASME NDE (ANDE) Personnel Certification

See also

literature

  • Herbert Krautkrämer, Josef Krautkrämer, and Otto Rüdiger : A supersonic device for non-destructive testing of materials. Steel iron, 1949.
  • Josef Krautkrämer: Ultrasonic testing in the service of accident prevention. 1957.
  • Josef Krautkrämer: Defect size determination with ultrasound. In: Report of the Materials Committee of the Association of German Ironworkers. No. 1211. Steel iron, 1959.
  • Josef Krautkrämer, Herbert Krautkrämer: Materials testing with ultrasound. 5th edition. Springer, Berlin 1986, ISBN 3-540-15754-9 .
  • Josef Krautkrämer, Herbert Krautkrämer: Ultrasonic Testing of Materials. 4th edition. Springer, Berlin 1990, ISBN 978-3-540-51231-8 .

Web links

Individual evidence

  1. ↑ H. Polrolniczak: Ultrasonic testing as a means of quality assurance in resistance spot welding . Krautkrämer - special print No. SD 297
  2. ^ A b W. Roye: Ultrasonic testing of spot welds in automobile construction . Krautkrämer - Reprint No. SD 298
  3. ↑ tec-science: Ultrasonic testing. In: tec-science. July 13, 2018, accessed on October 25, 2019 (German).