Reduced activation ferritic martensitic (RAFM) steels, among others EUROFER97 and F82H, are promising candidates as structure materials for first wall components of future fusion power plants.
During operation the structure material will be subjected to complex thermo-mechanical loading and high irradiation doses. Correct modeling of its deterioration under these loading conditions is a precondition of a sufficiently reliable lifetime prediction procedure.
In our approach we started developing a deformation damage model describing the behavior of RAFM steels in the un-irradiated state. The model accounts for many characteristics originating from the unique microstructure of these materials, among others the non-linear hardening behavior under arbitrary loading, complex non-saturating softening under cyclic loading (Fig. 1) and material deterioration under creep-fatigue loading (Fig. 2). To determine the temperature and material dependent model parameters an appropriate fitting strategy was developed and implemented in a computer code - called fitten_RAFM - which allows the parameter determination on the base of material data obtained in standard tensile, creep and low cycle fatigue tests. Using this strategy the model was successfully applied to describe the behavior of EUROFER97 and F82H mod [1, 2]. In addition the model has been implemented in the finite element code ABAQUS as a user material subroutine - called umat_RAFM - allowing best estimation of the mechanical behavior of real structures built from RAFM steels.
Recently the model was modified to take irradiation into consideration by modeling the irradiation induced hardening and its interaction with the deformation and damage behavior. The resulting irradiation hardening model comprises the hardening induced by neutron irradiation (Fig. 3) as well as its alteration due to inelastic deformation (Fig. 4) and its recovery at high temperatures. All these phenomena are observed in post irradiation examinations on RAFM steels. However the applicability of the irradiation hardening model developed is not restricted to RAFM steels and can be extended to other materials showing at least qualitatively similar behavior. The coupling of the irradiation hardening model with the model describing the deformation and damage behavior of RAFM steels in the un-irradiated state provides a powerful tool for the prediction of the constitutive behavior of RAFM steels during and after neutron irradiation under low cycle fatigue conditions.
In ongoing activities the model will be applied to describe the behavior of EUROFER97 and F82H mod as observed in post irradiation examinations and thus to verify furthermore its prediction ability. In addition the resent coupling with the irradiation hardening model will be added to the implementation in the finite element code ABAQUS.