Effect of Relative Mean Strain in High-Temperature Low-Cycle Fatigue of a Stainless Steel

The effect of the relative mean strain in the low-cycle fatigue of a stainless steel (AISI 304) in air at 650°C and at a constant strain rate (4 × 10⁻³s⁻¹) is studied. Two modes of strain measurement (axial and diametral) were used separately in nonzero mean strain experiments with controlled axial strain in real time. The effect of the relative mean strain on the fatigue life has been found to be negligible in the range of lives 200 ⩽ N ⩽ 10,000 cycles when the mean strain does not exceed 1.5 times the total range. This effect, however, becomes important when the mean strain is several times the total strain range. In addition, for N < 200 cycles at failure, the effect of even a small value of the mean strain appears to be significant. The extension of a method previously developed is presented for establishing the high-temperature fatigue behavior under isothermal conditions with positive mean strain. On the basis of the results of a short-term tensile test, the technique gives predictions in good agreement with experimental data.