Reduced softening of EUROFER 97 under thermomechanical and multiaxial fatigue loading and its impact on the design rules

Within the development of the test blanket module (TBM) for use in the ITER and DEMO fusion power plants, interest increasingly focuses on design rules for components built from EUROFER 97. One of the specific characteristics of EUROFER 97 as a ferritic–martensitic steel is its cyclic softening yielding to lower stresses under strain-controlled fatigue loading and, hence, longer lifetimes. Our thermomechanical and multiaxial fatigue tests, however, revealed lifetimes that were remarkably lower than those expected on the basis of isothermal uniaxial fatigue tests. The reduced cyclic softening observed in these experiments is believed to be one of the reasons of the shorter fatigue lifetimes. When applying the design rules derived for EUROFER 97 from isothermal uniaxial data considering the recommendations in the ASME and RCC-MR codes to our thermomechanical and multiaxial fatigue tests for verification purposes, strong loss of their conservatism was found. The lifetimes observed in some of the multiaxial experiments were even lower than the design lifetimes that had been supposed to be sufficiently conservative. To overcome this problem, new design rules will be proposed, which are based among others on the damage and lifetime prediction model developed lately for EUROFER 97.