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This paper deals with a study of the fatigue crack propagationin the near-threshold domain and in the mid-rate range on two austeniticstainless steels type 304L and 316L widely used in nuclear industry. Theaim is to reassess the propagation laws and the threshold levels to answerrecent industrial safety problems related to damage tolerance. Thepropagation curves established at room temperature, 150°C and 300°C atR=0.1 and 0.7, are discussed in comparison to literature. The investigationof crack closure has shown a substantial contribution of this phenomenonin the near-threshold area as well at R=0.1 as at R=0.7. Propagationmechanisms are illustrated and documented by mean of SEM observations.