Cumulative axial and torsional fatigue: An investigation of load-type sequencing effects

Cumulative fatigue behavior of a wrought cobalt-base superalloy, Haynes 188, was investigated at 538 degreesC 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 degreesC 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.