Fatigue crack growth rates and thresholds in steels under oxidising conditions

Abstract

The effect of R (minimum load/maximum load) on fatigue crack growth in tests on CrMoV steels at 550°C was investigated under oxidising conditions. Most results were described adequately by linear elastic fracture mechanics using just the tensile loading to calculate stress intensity amplitude ΔK. Crack growth rates in air and under vacuum were decreased by loading at R = 0 compared with tests at R = −1 or R > 0 especially near the threshold ΔK 0 · R = −1 tests produced upper bound crack growth rates for any conditions. Crack growth in air was faster than under vacuum except at low amplitudes, so that for tests in air at 10 −2 Hz and R = 0 the threshold amplitude was higher than under vacuum. Higher frequencies ( e.g. 1 Hz), an inert environment and compressive or raised mean tensile stresses all lowered ΔK 0. These effects are interpreted in terms of the cyclic crack tip displacement being reduced by shear lips at the crack surface, and oxide filling. Limited creep ratcheting per cycle in these creep-ductile steels was found not to increase crack growth rates.