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Cumulative fatigue behavior of a wrought cobalt-base superalloy, Haynes 188, was investigated at 538°C under various single-step sequences of axial and torsional loading conditions. Initially, fully-reversed, axial and torsional fatigue tests were conducted under strain control at 538°C on thin-walled tubular specimens to establish baseline fatigue life relationships. Subsequently, four sequences (axial/axial, torsional/torsional, axial/torsional, and torsional/axial) of two load-level fatigue tests were conducted to characterize both the load-order (high/low) and load-type sequencing effects. For the two load-level tests, summations of life fractions and the remaining fatigue lives at the second load level were computed by the Miner's linear damage rule (LDR) and a nonlinear damage curve approach (DCA). In general, for all four cases predictions by LDR were unconservative. Predictions by the DCA were within a factor of two of the experimentally observed fatigue lives for a majority of the cumulative axial and torsional fatigue tests.
axial fatigue, cumulative fatigue, cyclic hardening, damage curve approach, life prediction, linear damage rule, load-type sequencing, torsional fatigue
Senior research engineer, Ohio Aerospace Institute, NASA Glenn Research Center, Cleveland, OH
Materials research engineer, U.S. Army Research Laboratory, NASA Glenn Research Center, Cleveland, OH