Micro-mechanical FE-simulations of the plastic behaviour of steels undergoing martensitic transformation

Sammanfattning: This thesis is concerned with the mechanical behaviour of steels undergoing martensitic transformation. More specifically, the aim has been to develop a method to numerically stimulate the mean strain (or stress) response associated with a prescribed temperature and mean stress (or strain) variation.Based on the simplification that the jump of deformation gradient associated with the propagating martensitic interfaces for each martensitic variant can be treated as a welldefined metallurgical parameter, a constitutive description of a grain has been developed, in which evolution equations for the microscopic plastic strain and volumic fractions of the different variants are specified. Thus, also the kinetics of the transformation can be extracted from the simulation results.A trilinear isoparametric finite element based on the adopted constitution has been implemented in the FE code TRINITAS. The updating of the internal variables in the integration points has been accomplished with an elastic predictor - plastic corrector procedure, in which the different couplings between the inelastic processes are considered.By modelling a representative cell of the material as a cubical stacking of dubical grains, where each grain is represented by one finite element, numerical creep test simulations of a Fe-25Ni-0.66C steel have been performed. The results obtained are found to be in qualitative agreement with experimental results for both the plastic behaviour and the transformation kinetics.