The primary mission of the international fusion material irradiation facility (IFMIF) is to generate a material database to be used for the design of various components, for the licensing and for the assessment of the safe operation of a demonstration fusion reactor. IFMIF is an accelerator-based high-energy neutron source whose irradiation volume is quite limited (0.5 litre for the high fluence volume). This requires the use of small specimens to measure the irradiation-induced changes in the physical and mechanical properties of materials. In this paper, we have developed finite element models to better analyse the results obtained with two different small specimen test techniques applied to the tempered martensitic steel F82H- mod. First, one model has been used to reconstruct the load- deflection curves of small ball punch tests (SPTs), which are usually used to extract standard tensile parameters. It has been shown that a reasonable assessment of the overall plastic flow can be done with SPTs. Second, we have investigated the stress field sensitivity at a crack tip to the constitutive behaviour, for a crack modelled in plane strain, small-scale yielding and fracture mode I conditions. Based upon a local criterion for cleavage, which appears to be the basis to account for the size and geometry effects on fracture toughness, we have shown that the details of the constitutive properties play a key role in modelling the irradiation-induced fracture toughness changes. Consequently, we suggest that much more attention has to be paid and efforts made to investigate the post- yield behaviour of the irradiated specimens and, in order to reach this goal, we recommend the use of not only tensile specimens but also of compression ones in the IFMIF irradiation matrices.