Effect of magnitude of vibratory load superimposed on mean tensile load on mechanism of and time to fracture of specimens and correlation to engine blade

Ferguson, Robert R.

Tensile fatigue tests were run on specimens of seven turbine blade alloys at a test temperature of 1500 degrees F and a mean stress of 22,000 pounds per square inch with superimposed alternating stresses of 0, plus or minus, +/-5000, plus or minus, +/-10,000, and plus or minus, +/-15,000 pounds per square inch. The same three types of fracture occurring in turbine blades - stress rupture, stress rupture followed by fatigue, and fatigue - were obtained in the specimens. The type of fracture obtained was found to be a function of the material and the magnitude of the alternating stress. With increasing alternating stress, the mechanism of failure changed from stress rupture to stress rupture followed by fatigue and then to fatigue. Six of the seven alloys showed the same mechanism of failure for specimens tested at an alternating stress of plus or minus, +/-5000 pounds per square inch as for blades tested in a J33-9 jet engine.

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