Realization of complex thermal-mechanical fatigue by a two-specimen testing system

Many components are exposed to high thermal and mechanical loadings. For example, the blades of gas turbines are subjected to thermally and mechanically induced strains and stresses at varying temperatures. The former arise from inhomogeneous temperature fields, which are due to start-stop cycles, resulting in thermal fatigue. The latter arise from centrifugal forces, which arise from the rotation of the turbine, resulting in mechanical low cycle fatigue and creep during service. The combination of thermally induced loading and mechanically induced loading can neither be investigated in a conventional (strain controlled) thermal-mechanical fatigue (TMF) test nor in a conventional (stress controlled) creep test. Also the interaction between different volume elements within a component can not be investigated in a single specimen experiment. To simulate such "complex" thermal-mechanical fatigue loading, a two-specimen testing system was build up. At this testing system the thermal-mechanical loading of the specimens, each of them representing a distinct volume element of a component, is generated just by varying the temperature-time history of the two specimens and the coupling conditions between them. Furthermore, it is possible to superimpose an external force, e.g. representing the centrifugal force. The distribution of this force on the two specimens and the resulting deformation behaviour are the result of the interaction of the two specimens. The testing and interpretation methods as well as the results of first experiments with a 12% chromium steel and a 316 type stainless steel are presented.