  A microstructure-based model is presented to predict the fatigue life of
polycrystalline metallic alloys which present a bilinear Coffin-Manson
relationship. The model is based in the determination of the maximum value of a
fatigue indicator parameter obtained from the plastic energy dissipated by
cycle in the microstructure. The fatigue indicator parameter was obtained by
means of the computational homogenization of a representative volume element of
the microstructure using a crystal-plasticity finite element model. The
microstructure-based model was applied to predict the low cyclic fatigue
behavior of IN718 at 400$^\circ$C which exhibits a bilinear Coffin-Manson
relationship under the assumption that this behavior is triggered by a
transition from highly localized plasticity at low cyclic strain amplitudes to
more homogeneous deformation at high cyclic strain amplitudes. The model
predictions were in very good agreement with the experimental results for a
wide range of cyclic strain amplitudes and two strain ratios ($R_\epsilon$ = 0
and -1) and corroborated the initial hypothesis. Moreover, they provided a
micromechanical explanation for the influence of the strain ratio on the
fatigue life at low cyclic strain amplitudes.
