Critical currents of MgB2 wires prepared in situ and ex situ subjected to axial stress

Single-core MgB2 composite wires prepared by in situ and ex situ PIT processes were subjected to axial tension and pressure. Their transport critical currents Ic were measured as a function of axial strain ε with a U-shaped spring set-up at liquid helium temperature and external magnetic fields ranging from 3.5 to 6 T. For all wires, Ic changes linearly and reversibly with applied strain, up to a sample-dependent reversible strain limit εirr. The value of εirr depends mainly on the degree of pre-compression, on the mechanical properties of the sheath and also on the structure of the MgB2 core. In contrast to ex situ wires, the aspect ratio of in situ wires does not affect the value of εirr. On the other hand, in situ wires are typically more sensitive to axial strain, i.e. they display a stronger reversible Ic variation and a steeper irreversible degradation, which is attributed mainly to the higher porosity of in situ cores