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This work deals with the effect of high mean stress on the high cycle
fatigue behavior of a cast aluminum alloy AlSi7Cu0.5Mg0.3. Uniaxial fatigue
tests have been carried out at different load ratios, ranging from R=-1 (push-pull)
to R=0.95. In addition, in-situ meso-scale observations, using a optical
microscope have also been performed during the tests. The aim of these
observations is to identify the different crack initiation mechanisms that occur at
different mean stress values. These mechanisms are directly related to the material
microstructure and the evolutions of plastic deformation and material damage as a
function of the imposed loading condition. Specimen fracture surfaces were
consequently examined using a scanning electronic microscope (SEM). Two
different fatigue crack mechanisms are clearly identified. These observations will
be used to establish a fatigue criterion capable of taking into account the change
in mechanism and therefore be applicable to the complete range of mean stress
values.