Two phase metals can suffer from non-uniform load sharing between the phases or stress heterogeneity on the micro-scale, elastic/plastic anisotropy and other mechanical interactions on the scale of grain size during cyclic loading. These effects strongly influence the fatigue damage and crack initiation behaviour of the two phase metals. In this paper, the fatigue damage and crack initiation behaviours of two austenitic-ferritic duplex stainless steels, UNS S32750 and UNS S32906, have been studied by both experimental investigations such as low cycle fatigue tests, SEM and TEM study, and X-ray and neutron diffraction and simulation using multi-scale material modelling. It has been found that the material damage and crack initiation in these two duplex stainless steels during cyclic loading occur mainly in the ferritic phase that is the weakest phase if the deformation hardening is considered.
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