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Curves providing a failure strain value depending on a local triaxiality factor have been obtained experimentally by a number of investigators and are usually applied in the structural analysis. In this paper, the experimental and theoretical bases of this practice are reviewed. Results obtained in a campaign performed on cruciform specimens at the Joint Research Center of Ispra are also considered. The possible dependence of the failure strain on the whole history of the local triaxiality and on parameters other than triaxiality is explored. Finally, experimental difficulties in checking the available failure theories are shortly discussed.