Multiaxial fatigue behaviour of cyclic hardening and softening steels under variable elastic-plastic strain amplitudes

To investigate the cumulative multiaxial fatigue behaviour of cyclically softening and hardening steels 30 Cr Ni Mo 8 (similar to AISI 4340) and X 10 Cr Ni Ti 18 9 (AISI 321) strain controlled tests with variable amplitudes under uniaxial loading and under combined in and out of phase axial and shear strains were carried out. While the results of variable amplitude uniaxial tests as well as the results of constant amplitude multiaxial tests can be predicted within a reasonable accuracy, the results for multiaxial variable amplitude loading differ from the prediction being significantly on the nonsconservative side. The inconsistancies observed underline the importance of the appropriate evaluation of the multiaxial strain state for variable amplitude loading. The state of art justifies the use of high safety margins for the fatigue life evaluation in the pressure vessel technology. (LBF)