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The need for a satisfactory non-destructive test for welds has been keenly felt ever since the introduction of welding, and the lack of such a test has required that our growing faith in welded structures be founded upon faith in the skill and integrity of well-trained men using trustworthy welding materials and equipment, and upon careful supervision and inspection, with periodic destructive tests of samples. The aim of the author has been to develop a non-destructive test method which would be commercially feasible for use in the average shop, by shop men, and sufficiently versatile that the same equipment might be used for a variety of products. The success thus far attained is limited to butt welds in steel, and in general to the detection of faults in such welds; but under the most favorable conditions an approximate measure of the strength of the joint is possible. The author's attention has not been entirely confined to magnetic methods, but after experimenting also with some suggested thermal, electrical, and aural methods, it was concluded that for butt welds, at least, the magnetic methods were most sensitive and practicable. The foundation of magnetic testing of welds is the fact that faults in welds increase their reluctance, and at flux densities from about 13,000 to 15,000 gausses-the differences between good and poor welds, of similar type and size, are very easily detected. Therefore magnetic weld testing equipment consists, essentially, of some means for producing a magnetic flux through the steel practically perpendicular to the welded seam, and some means of detecting the regions of abnormally high reluctance. The means of detection may be broadly classified as “magnetographs” (the iron-powder method), and instruments measuring magnetic potential-drop or leakage flux.
Watts, T. R.
Research Engineer, Westinghouse Electric and Manufacturing Co., East Pittsburgh, Pa.