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Low-cycle fatigue tests were made on low-alloy cast steel in three conditions, as well as a few tests on wrought specimens. Specimens 1 in. in diameter with a ground surface were tested by reversed, push-pull, strain-controlled mode at room temperature. True stress-true strain, tension tests were also made on the materials.
The strain-hardening exponent for a single hysteresis loop agrees closely with that at certain portions of the tensile curve. Whether these materials will cyclic-strain harden or soften at a specified strain range could also be determined from the tension test data. Using these observations, and the work of Martin, Manson and Morrow and their colleagues, a formula was synthesized which predicted fatigue lives of all durations from tensile data and the fatigue limit of the material. When the fatigue limit was adjusted for the specimen conditions the cyclic lives were predicted with useful accuracy. For example, micro-shrink in one casting was found to reduce the cyclic life sharply without much loss in tensile fracture ductility. Use of the notched specimen fatigue limit in the formula gave a reasonably accurate prediction of cyclic life for this casting. The formula is also used to calculate cyclic life under conditions of variable stress and to calculate the reduction of fatigue limit due to cyclic pre-stressing at stresses higher than the fatigue limit.
low-cycle fatigue, cast steels, prediction, formula, tension, true stress-true strain, cracks, hysteresis energy, micro-shrink
Manager, Materials and Processes Laboratory, General Electric Co., Schenectady, N.Y.