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The evolution of the fatigue resistance of austenitic-ferritic duplex stainlesssteels modified by high nitrogen additions or by heat treatments (475°C) does not followthat of tensile properties, in particular at high applied strains or stresses. The cyclicdamage mechanisms in such alloys are discussed from the point of view of interactionsbetween short cracks and microstructural barriers. A generalised model of barriers takinginto account the short crack nucleation site (which can be different in duplex steelsaccording to the applied stress/strain amplitude), the physical properties of both phasesand their respective responses to the cyclic loading is applied to interpret the experimentaldata of low cycle fatigue tests carried out on a medium nitrogen commercial SAF 2507duplex steel. Compared with the first generation DSS (low nitrogen), the nitrogen additionof approximately 0.25% induces a quasi single phase fatigue behaviour of the materialwithout a significant increase of the fatigue life at applied plastic strain amplitude. Theradical reduction of the fatigue life which follows the structural hardening of the ferrite