Published: Jan 1931
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Except for the hydrostatic test of welded pipes or other pressure vessels, a non-destructive test which could be considered satisfactory for commercial use has not been available until recently. A means of testing the welds without damaging the completed structure has long been sought for, and of late favorable attention has been given the stethoscopic test and the X-ray examination. A great number of pressure vessels and the thousands of pipe joints both in power lines and in cross-country transmission lines offer ample evidence of the ability to obtain safe welds through the use of a suitable procedure control. Under the procedure controls which have been employed, the examination has consisted almost entirely of tension and bend tests of material of the same thickness, and welded under the same conditions as those employed in the structure. Each operator who is to work on a given job is required to make test plates which will meet definite specifications. This is analogous to testing by sampling. However, on account of the attitude of various regulatory bodies, and because of the increased confidence which may be given those not familiar with welding, more extensive application of welding may be made when satisfactory non-destructive tests of the welds are employed. Non-destructive examination is useful for welds not made under procedure control. For these reasons the industry is fortunate in having the stethoscope and X-ray methods available. The stethoscope was first used for testing welds by A. B. Kinzel and this application has since been investigated and developed further by Mr. Kinzel and others of the Union Carbide and Carbon Research Laboratories. The same principle is involved as is used by a blacksmith when he strikes a piece of metal to find if it rings true, or when staybolts are tested by tapping them with a hammer. The stethoscope is the type commonly used by physicians (Fig. 1). The coneshaped end piece is preferred, and over this end piece is placed a soft rubber cap. When listening through the stethoscope, the operator presses the end of the cone with the soft rubber covering firmly against the metal containing the weld which is to be tested. This prevents undue damping such as might occur if the hard cone were in immediate contact with the metal, and because of intimate contact between the soft rubber and the metal, outside noises do not interfere. The weld is tapped with a hammer, as shown in Fig. 2, at intervals of about one inch along its entire length. The size of the hammer and the force of the blow will depend on the thickness of the metal being examined. The pipe, pressure vessel or structural steel member being tested will have a natural vibration, and there will be also forced vibration, both of which are set up by the blow. The sound due to the natural vibration should first be determined, and then the important things to be listened for are the first sounds obtained upon striking.
Dawson, J. R.
Metallurgical Engineer, Union Carbide and Carbon Research Laboratories, Inc., Long Island City, N. Y.