Optimisation of start and restart procedure in TMF fatigue testing

Thermomechanical fatigue is a technique that is going to be widely used for the characterisation of gas turbine components. As the experimental technique is quite complex, it requires a defined procedure to initialise the experiments. In particular the evaluation of the systematic temperature-mechanical strain shift due to the equipment response and the thermal strain compensation during thermal cycling, the verification of zero stress before starting have to be defined in order to produce TMF tests comparable from different laboratories. This paper describes the experimental activity carried out in order to determine a procedure for the evaluation of thermal strain, for the optimisation of the phase between temperature cycle and mechanical strain and for the selection of the best point at which test should start. The zero stress test performed before TMF test is described and confirms its utility to ensure a correct start of TMF testing. TMF tests have been performed on Nimonic 90 nickel-base superalloy in the temperature range of 400–850 °C and 180° out of phase cycle. Some tests were stopped and restarted in order to determine the influence of such stops on TMF life and the best restart procedure. At the end few tests have been planned for the verification that the adopted procedure is generally correct.