Dependence of the critical current in ex situ multi- and mono-filamentary MgB2/Fe wires on axial tension and compression

The transport critical current I-c of single- and four-core MgB2/Fe wires under tensile and compressive axial strain 8 was measured with a U-shaped spring set up. In all wires, 1, increases linearly and reversibly with applied tensile strain, up to a sample-dependent reversible strain limit epsilon(irr). The same reversible and linear I-c(epsilon) relation is found when applying compressive strain. This shows how the I-c(epsilon) increase with tension can be understood as a gradual release of the thermal pre-compression strain induced in the MgB2 filaments of the composite. The value of the reversible strain limit epsilon(irr) depends mainly on the degree of pre-compression, but also on the shape of the composite wire. It is highest in a square wire and lowest in the flattest tape. Tensile strain levels above epsilon(irr) cause an irreversible degradation of 1, This I-c(epsilon) degradation, due to filament cracking, is significantly steeper in heat-treated wires than in as-deformed ones. The different I-c(epsilon) regimes are analysed and the mechanical behaviour of the MgB2/Fe composite is compared with that of typical Nb3Sn wires and Bi,Pb(2223) tapes.