An assessment of the true stress-true strain relationship has been done by means of tensile and small ball punch tests on austenitic and tempered martensitic steel at room temperature. A finite element model was developed and validated to calculate the force-deflection curve obtained from the ball punch experiment. The effects of the specimen thickness and material properties on the overall shape of the ball punch test curve are discussed. The constitutive behavior assigned to the specimen for the calculations was determined from the tensile test but we showed that assumptions have to be done to extend it to large strains as those arising during the punch tests. Using an inverse methodology, it was possible to show that a linear strain- hardening stage takes place at large strains. The potential for evaluating the evolution of the strain-hardening capacity after irradiation is outlined. (C) 2004 Elsevier B.V. All rights reserved.