I. Introduction
Nb3Sn is recognized as one of the important practical superconducting materials for a high field magnet over l0T. However, it is well known that the strong sensitivity of its superconducting properties to stress/strain still remains as a serious problem [1]. In addition, the critical current density of the practical Nb3Sn wires is deteriorated due to the residual strain by the thermal contraction difference between Nb3Sn filaments and the materials composing the wire, such as bronze matrix, Cu, Nb and so on. Therefore, can be enhanced if one may realize a strain-free state for Nb3Sn filaments. In fact, Ochiai et al. have already reported that the loaded and unloaded tensile strain treatment increases and for the Nb3Sn wires [2]. From this point of view, the react and wind technique is a convenient and efficient method to control the residual strain. Generally, it has been considered that the bending strain above 0.5% induced from pulleys and a coil bobbin during the winding process gives rise to a serious damage for the Nb3Sn wires [3]. Recently, we found that for high strength Nb3Sn wires, the repeated bending loads applied as coil-winding enhance not only , but also upper critical field and critical temperature [4], [5]. We call the repeated bending strain at room temperature “prebending strain”. It was confirmed that increases for three different type of the bronze route Nb3Sn practical wires, and the reinforced Nb3Sn wire with Cu-24wt%Nb in outer part of the wire, CuNb/Nb3Sn, shows larger enhancement of than the ordinary Nb3Sn wire [4]. Comparison of the axial tensile strain dependence of for high strength Nb3Sn wires with and without prebending strain treatment indicates a reduction of the axial residual strain, and also the enhancement of the maximum value at the peak for the axial strain dependence of [6]. In other words, the critical current at the axial-strain-free state, , is also increased by the prebending treatment for the CuNb reinforced Nb3Sn wires. This is one possible reason why the enhancement of CuNb/Nb3Sn wire by the prebending treatment is larger than those of the other wires. It also suggested that the radial/tangential residual strain is different from zero even for the axial strain free state. Therefore, of the composite Nb3Sn wires can be enhanced drastically, if we optimize the strain states three-dimensionally. For this purpose, we have to understand the three dimensional residual strain state and its effect on the superconducting properties.