Isothermal, thermal–mechanical and complex thermal–mechanical fatigue tests on AISI 316 L steel—a critical evaluation

Results are presented and compared with strain-controlled ‘isothermal fatigue’ (IF), strain-controlled ‘thermal–mechanical fatigue’ (TMF) and ‘complex thermal–mechanical fatigue’ (CTMF) tests. The IF and TMF tests were carried out with a common single-specimen testing system while the CTMF tests were performed with a two-specimen testing system which allows the interaction between a ‘hot’ and a ‘cold’ location of a component, e.g. the outer and the inner side of a cooled turbine blade, to be simulated. The test samples were made from the austenitic steel AISI 316 L. The complex interaction between both specimens in a CTMF test results in cyclic creep to negative total strains, which does not occur in strain-controlled IF and TMF tests. The cyclic deformation behaviour under CTMF conditions is not only determined by the hardening or softening of the material investigated, but even to a larger extent by the interaction of the specimens within the CTMF system. However, the microstructural changes under CTMF conditions can be correlated with the respective observations made under IF and TMF loadings.