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A combined experimental and analytical investigation of the low-cycle fatigue strength of pressurized tubes is described. The tubes were thin-walled, closed-ended, and fabricated of heat-treated type 403 stainless steel. Repeated pressurization of both notched and unnotched tubes over a range from zero to relatively high maximum pressure resulted in the desired low-cycle fatigue failures. Fatigue strengths were also determined for notched and unnotched, flat, axially loaded specimens of the same material subjected to correspondingly high maximum load levels. Comparisons of these results, on a maximum stress basis, indicated that the unnotched tube fatigue strength was significantly less than the axially loaded specimen fatigue strength and comparable to the fatigue strength of the axially loaded, notched specimen. Two possible theoretical approaches to the low-cycle, pressurized-component fatigue problem are described. The first approach neglects the cyclic aspects of the loading and considers only pressurization that increases monotonically to failure. The alternative procedure, specifically devised for high-cycle strengths, applies to fatigue strength under multiaxial stress conditions. Predictions based on these theories are compared with the test results, and are shown to be unsatisfactory. Areas of required analytical and experimental investigation are therefore suggested.
Bell Aircraft Corp., Buffalo, N. Y.
Space Technology Laboratories, Los Angeles, Calif.