Failure behaviour of EUROFER 97 in the low-cycle fatigue region under multi-step loading

For verification of high temperature creep–fatigue interaction rules for EUROFER 97, different LCF multi-step experiments (2- and 3-step tests) were performed at 450 and 550 °C. The load conditions were varied by changing the total strain amplitude during an experiment. In 2-step tests the load was changed after a given number of cycle from a high level to a low level or vice versa and afterwards the experiments were continued until fracture (high–low, or low–high loading). In 3-step tests, the load was changed in the first and second steps after given numbers of cycles. In the third step, it was switched back to the load applied in the first step and the experiments were continued until fracture (high–low–high, or low–high–low loading). As expected, the results show a non-linear damage accumulation. In 2-step experiments, it was found that beginning the tests with a low total strain amplitude resulted in longer normalised total lifetimes than in the case of high total strain amplitudes at the beginning. In 3-step tests, one can detect a similar behaviour. But here, additionally it is also observed that the number of cycles in the first step influences the lifetime. A comparison of the results with the standard design curve shows that there is more than enough space available to be on the safe side for calculations of constructions.