Microwave testing

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The microwave test (microwave defectoscopy) is an electromagnetic method to find material defects using harmless microwaves . It belongs to the non-destructive testing methods . It can also be used to test components when they are installed, e.g. B. built-in, invisible seals in plastic valves.

principle

B-image of a foam-GRP sandwich at 100 GHz. The indication at x = 120 mm comes from moisture in the foam, approx. 20 mm deep under the DUT surface. (Becker, Keil, Becker Photonik GmbH: Annual Conference DGZfP 2017, Article Mi3C2)
GRP pipe wall. C-picture. In the middle: indication of an error at a depth of 60 mm, 24 GHz

Microwaves are electromagnetic waves with frequencies between 300 MHz and 300 GHz or wavelengths between 1 m and 1 mm. A sub-range of these are the millimeter waves with frequencies between 30 GHz and 300 GHz or wavelengths between 10 mm and 1 mm. Microwaves propagate at different speeds in different dielectrics - these are non-conductive materials - and are reflected at the interfaces between them due to their different wave resistances , while another part spreads further. As the difference in wave resistance increases, so does the reflected portion.

To find material defects, a test head that emits and receives microwaves is moved in a touching manner or at a small distance above the surface of the workpiece to be tested. This can be done manually, mechanically or automatically.

Changes in the dielectric properties at interfaces (e.g. a blowhole , a foreign material inclusion or a crack) inside the part to be tested reflect the transmitted microwave and send it to the test head, which functions as both a transmitter and a receiver , back.

The electronic data evaluation then leads to the presentation of the test results, e.g. B. as a B-image (sectional view) or as a C-image (top view).

Procedure

In addition to the reflection method, the transmission method is also possible, in which separate transmitting and receiving antennas are used. The back of the test object must be accessible for the radiographic method. The method does not provide any information about the depth of the defects.

In addition to testing with constant frequency ( CW ), frequency tuning ( FMCW ) is also used. The latter has advantages in determining the depth of defects.

NIDIT radiographic image of a rotor blade trailing edge with artificially distributed adhesive

A test head placed on the component surface provides information about the material distribution under the contact point. When moving over the component surface, a lot of such individual information is recorded point by point and then processed into an overall image. Imaging procedures work faster. Direct imaging processes for microwave testing work either purely electronically or with a microwave area detector consisting of a microwave-absorbing film and a thermal camera (NIDIT process).

Applications

The microwave test is a suitable test method for dielectric materials. These include plastics, glass fiber reinforced plastics (GRP), plastic foams, wood, wood-plastic composites (WPC) and most types of ceramics. Internal and external defects can be found, e.g. B. for semi-finished products or pipes .

FSC measuring device for the non-destructive measurement of paint thickness on CFRP, here on an aerobatic aircraft

Special applications of microwave testing are non-destructive

Microwave testing is used in many industries:

  • Aerospace, e.g. B. Paint thickness measurement on CFRP
  • Automotive industry, e.g. B. NDT of organic sheets and GRP leaf springs
  • Construction industry, e.g. B. Radar Applications
  • Energy supply, e.g. B. Testing of rotor blades of wind turbines
  • Security, e.g. B. Body scanners for use in airports

Because the need for testing in general and especially for dielectric materials has increased in recent years and because microwave technology has now found its way into consumer products and has thus become cheaper, non-destructive testing is also increasingly being carried out with microwaves. Due to the increasing importance of the microwave and terahertz method of the German Society for Non-Destructive Testing (DGZfP) in 2011 and the American Society for Non-Destructive Testing (ASNT) in 2014 the Microwave Testing Committee . Standardization work on microwave testing is in the initial phase.

literature

  • Joseph T. Case, Shant Kenderian: MWNDT - An Inspection Method. In: Materials Evaluation. March 2017, pp. 339–346. (This article contains many references on microwave testing)
  • Reza Zoughi: Microwave Non-Destructive Testing and Evaluation. Kluwer Academic Publishers, Dordrecht 2000, ISBN 0-412-62500-8 .

Web links

Individual evidence

  1. ↑ a b c d Overview of microwave testing Application note from FIT-M. Retrieved August 14, 2017.
  2. ↑ a b Security through technology information from ROHDE & SCHWARZ GmbH & Co. KG. Retrieved August 14, 2017.
  3. ↑ Video on paint thickness measurement on CFRP application of the FIT-M. Retrieved August 14, 2017.
  4. ↑ Microwave testing of GRP leaf springs Conference contribution. Retrieved from the ndt.net portal on August 14, 2017.
  5. ^ Christiane Maierhofer: Radar applications in construction. In: ZfP newspaper. No. 72, December 2000, pp. 43-50, accessed from the ndt.net portal on August 14
  6. ↑ FA MTHz of the DGZfP . Retrieved August 14, 2017.