1 The RED criterion for fatigue life assessment of metals under non-proportional loading Camilla Ronchei a, *, Andrea Carpinteri b , Daniela Scorza c , Andrea Zanichelli b , Sabrina Vantadori b a Department of Civil Engineering, University of Calabria, via Pietro Bucci, 87036 Arcavacata di Rende (CS), Italy b Department of Engineering & Architecture, University of Parma, Parco Area delle Scienze 181/A, 43124 Parma, Italy c Department of Engineering, University of Naples Parthenope, Centro Direzionale Isola C4, 80143 Napoli, Italy *Corresponding author: camilla.ronchei@unical.it ABSTRACT In the present paper, a novel criterion for fatigue life assessment of engineering components under multiaxial low-cycle fatigue regime is proposed. The criterion, named Refined Equivalent Deformation (RED) criterion, allows to take into account, when the load is non-proportional, the fatigue strength decrease experimentally observed for those metals sensitive to non-proportionality. The criterion is applied to two different metals (304 stainless steel and 6061 aluminum alloy) characterised by different non-proportional sensitivities. The accuracy of such a criterion is compared with those of other criteria available in the literature.