The specimen under test has sound waves of a frequency well above the audible range introduced into it. A flaw in the material results in the distortion of the sound waves which are transmitted and received in a probe containing a single crystal. The apparatus is most effective when the flow is perpendicular to the plane of transmission of sound waves. Close-grained materials such as the result due to forging, rolling, etc., show the most satisfactory result.

Forgings of up to 30' thick can be examined by this method against a maximum of about 10 by X and gamma rays.



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RADIOGRAPHY

X Radiography

In this test rays of electrons from a high voltage source are passed through the specimen and record the intensity of distribution of photographic material in the form of a shadow picture. Defects in the specimen can be seen by inspection of the picture. The required voltage depends on the type and thickness of material and varies from 10KV to 5000KV. A portable 10KV set can be used to inspect steel from about ?" thick to 1½" thick depending on the technique used. The exposure time is approximately two minutes.

Gamma Radiography

In this case, the rays from radio-active substance are used in a similar manner to the above. The substances produced from industrial use are cobalt, tantalum and iridium. In general gamma rays are used when the thickness of metal is great or where castings are in any way complicated. The advantages over X rays are that parts of varying thickness may be shown in one radiograph and that the technique is much simpler. The disadvantages are that the time of exposure is greater and that the results produced are generally inferior to those by X-ray. Great care must be taken with the radioactive source, all nonessential personnel cleared from the test area.

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