New ways of nondestructive inspection are offered by the resonance testing method based on known principles but innovative evaluation technique allowing automated processing.

The Principles of Operation

A sonic probe transmits oscillations into the test object. If the frequency of the oscillations is varied step-by-step, the test object always heavily vibrates when one of its selfresonances are met. These resonances depend on the dimensions, the vibration mode, the density and the elastic constants of the object.

The resonances of the test object are picked up by a receiver probe and evaluated in the system instrument. The system not only defines elastic constants, density, hardness and dimensions but also detects and classifies inhomogenities like e.g. cracks, cavities, porosities, chips, a missing tooth on gearwheels. These inhomogenities can not only be detected but also defined in dimension and location. Inhomogenities, depending on location, size and mode of the vibration, cause a shift or a more or less significant split of some - but not all - resonance frequencies. This allows to derive data about the location and size of an inhomogenity and separate disturbing influences by e.g. changes of density or dimensions of the test object.

The Applications

The resonance testing method is not only especially suited for automated testing of mass-produced parts made out of metal, powder metal, ceramic, stone and composite but also for the inspection of critical components in operation. In spite of conventional testing methods, the resonance testing method is not limited by location, orientation or shape of inhomogenities. For example, when testing the rolls of roller bearings differences in length of l0,um, corresponding to 0.1 % can be measured as well as cracks with dimensions of 0.3 mm x 1 gm x 0.8,um. The simpler the shape of the test objects, the easier the testing technique's layout. The computer controls the total testing cycle. The parts can be inserted manually or with special handling mechanics. A couplant between probe and test object is not necessary.

The Future

The largest automated system on record is in operation for about one year at a large car manufacturing plant and has tested more than 20,000,000 ceramic parts for automobiles. Very short testing cycles, 100 % volume inspection capability and independence from defect shape, orientation and location are the special merits of the new method and lead to new testing applications: in automotive industry for example, spark plug ceramic or airbag container, or in medical field for ceramic implants - to name just a few.

In addition, the resonance test method can significantly improve the inspection of many parts currently being tested with conventional methods.

For more information contact: Krautkraemer GmbH

| UT online| MTQ Dortmund report |

Rolf Diederichs 01. Dec 1996, info@ndt.net