โ Scribed by R.H.O.
No coin nor oath required. For personal study only.
Pneumatic Fatigue Testing Machine.--A pneumatically operated fatigue testing machine; whose operating principle is very similar to that of a slide trombone, was described by its developer, F. B. Quinlan of the Schenectady Works Laboratory of the General Electric Company, at the recent Buffalo meeting of the American Society for Testing Materials.
Originally developed to test gas turbine buckets, the device has proved so efficient and adaptable that it will undoubtedly have wide peacetime applications, Mr. Quinlan said.
The operating mechanism of the new instrument is extremely simple, consisting of a tuned air column in which the tuning is accomplished by decreasing the length of the air path, much as a trombone player changes the tone of his instrument by moving the slide.
The part to be tested is placed in such a position between the two air paths or tubes that the air from the open ends strikes on the upper end of the sample piece, setting it to vibrating. Since the piece vibrates at the same frequency as the note in the testing trombone, and air is continually fed through the tubes, the air impulses will cause continuous vibration of the test piece. Adjusting the tube length so that the air in them vibrates at the same frequency as that of the test sample makes it possible to produce very large and stable amplitudes of vibrations with comparatively small amounts of air.
The sample vibrates, of course, until it fatigues to the extent that its vibrations are out of phase with those of the tuned air and a break occurs.
The pneumatic fatigue tester has produced stresses as high as IOO,OOO pounds per square inch, Mr. Quinlan said in his talk, with no more air being used than that supplied by the .average vacuum cleaner. The tester has no parts to wear out, as nothing moves put the part to be broken. Three different models have been built, the one described, one that has proved especially efficient for torsion testing, and one for strip testing. All of them operate on the same principle, but differ in size and arrangements for holding the piece to be tested. With the exception of the strip fatigue machine, all parts may be tested at temperatures from below zero to 17ooยฐF.
Current research with the pneumatic tester is providing preliminary data which indicate that by using a recording frequency meter, the chart will show what appears to be a point of incipient failure. These data, of course, are extremely valuable to the designer and engineer. Brittle materials show this point of incipient failure to be very close to that of complete fracture, the author pointed out in conclusion, but in ductile materials it appears long before visible fracture becomes apparent. R. H. O.
๐ SIMILAR VOLUMES