Characterization of Strain-Controlled Low-Cycle fatigue and fracture behavior of P91 steel at elevated temperatures

The strain-controlled low-cycle fatigue behavior of P91 steel is investigated under various strain amplitudes ranging from 0.20% to 1.0% at 25, 538, and 566 degrees C. The fatigue life decreases with an increase in the test temperature; however, a clear reduction is observed at low strain amplitudes, resulting in the degradation of the fatigue limits. The amount of cyclic softening increases with an increase in the strain amplitude and is significantly higher at elevated temperatures. Based on experimental results, Basquin-Coffin-Mason models are established to express the relationship between the strain amplitude-fatigue life curves and the cyclic stress-strain. The transition fatigue life increased from 6677 reversals at 25 degrees C to approximately 12,000 reversals at the elevated temperatures of 538 and 566 degrees C. The more pronounced effect of temperature at the very low strain amplitude condition is attributed to the occurrence of multiple oxidation-enhanced crack initiations after long-term exposure to elevated temperatures.