Published: Jan 1931
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I desire to describe a method which may be considered as supplemental to the X-ray method, and in some instances an extension of it. The development of this method was carried out at the Naval Research Laboratory in Washington, under the direction of Mr. Robert Mehl, who invited me to come and work with him in the summer of 1929. Figure 1 illustrates the principle of the X-ray testing method with which everyone is familiar. An X-ray tube is connected with a high-voltage current supplied through a transformer and control devices. X-rays are generated in the tube. The object to be tested is placed in the path of those rays. A defect, inhomogeneity or flaw in the object permits X-rays to pass through it more intensely than through the sound, dense metal adjacent to the flaw. If a photographic film is placed behind the object being tested, a shadow of the flaw is registered on the film. With this X-ray method it is possible to test steel to a thickness of about 31/2 in., and a correspondingly thicker section of a metal of less density, for instance, aluminum. With thin sections and where it is possible to bring the sections to the X-ray equipment, excellent results are obtained. It is sometimes desirable, however, to test thicker sections for soundness, especially in the development of steel castings. If it were possible to use waves of shorter wave length than X-rays, (“wave length” in X-rays means the same thing as “wave length” in radio broadcasting) thicker sections could be penetrated in a reasonable length of time, and the photographic registration would be likewise effective. The idea of using a natural rare metal, radium, was conceived by Mr. Mehl, and support for these tests was given by the Naval Research Laboratory. It was possible with this method, to carry on to thicker sections of metal the work done by the X-ray, with certain limiting features and certain advantageous features as compared to the X-ray process.
Doan, G. E.
Associate Professor of Physical Metallurgy, Lehigh University, Bethlehem, Pa.