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This paper describes a large scale fatigue testing machine of the vibratory type which is quite different in principle of operation from the resonance type machines. It operates on the principle of the dynamic vibration absorber and uses to advantage the condition that at some frequency between the two resonance frequencies of the system the disturbing force is equal to the force developed in the damper spring. In the machine described the disturbing force consists of an alternating couple developed by two sets of rotating eccentric disks on parallel shafts rotating in the same direction, 180 deg out of phase with each other, and the absorber consists of a weighted test specimen subjected to bending loads. At the operating speed of the machine, the impressed alternating couple is balanced by the moment developed in the specimen. The specimen is so supported that the bending moment is uniform over the length of the 40-n. span. The main frame of the machine is designed for a capacity of 400,000 in-lb which may be increased from zero to a maximum in 50 steps through adjustment of multiple eccentric disks. The mean moment during the cycle is zero so that the machine is now limited to applying completely reversed stresses only. Results of fatigue tests on riveted and welded joints obtained with the machine are generally in agreement with the work reported by other investigators. The machine has proved sufficiently rugged and adaptable to permit additional investigations into the fatigue properties of various large scale joints in glass-reinforced laminated plastics.
Cordiano, H. V.
Head, Naval Material Laboratory, New York Shipyard, Brooklyn, N. Y.