  A microstructure-based model that accounts for the effect of grain size has
been developed to study the effect of grain size on the fatigue life of Inconel
718 alloys. The fatigue behavior of two alloys with different grain size was
determined by means of uniaxial cyclic deformation tests under fully-reversed
deformation ($R_\varepsilon$ = -1) at 400$^\circ$C in the low cycle fatigue
regime. The model was based in the determination of the fatigue indicator
parameter (based on the local crystallographic strain energy dissipated per
cycle) by means of computational homogenization of a representative volume
element of the microstructure. The mechanical response of the single crystal
within the polycrystal was modelled through a phenomenological crystal
plasticity model which was modified to account for the effect of grain size on
the monotonic and cyclic hardening/softening mechanisms. The
microstructure-based crack initiation model parameters were calibrated from the
experimental tests of the material with fine grain size. The results of the
fatigue simulations were in good agreement with the experimental results in
terms of the cyclic stress-strain curves and of the number of cycles for
fatigue crack initiation. The model did not show any grain size effect on the
fatigue life for the largest cyclic strain ranges while the predicted fatigue
life predicted was considerably longer in the case of the microstructure with
fine grain size for the lowest strain ranges, in quantitative agreement with
experimental data. These differences were attributed to changes in the
deformation modes between homogeneous plastic deformation at large cyclic
strain ranges and localized plasticity in a few grains at low cyclic strain
ranges.
