Critical current behavior in bending and tensile stressed Nb3Sn composites at temperatures below 4.2 K

Critical current measurements were obtained on bronze-diffused reacted Nb3Sn multifilamentary conductors as a function of mechanical stress loads and applied transverse magnet fields up to 13 T for temperatures in the range between 4.2 K and 1.9 K. To simulate the winding handling of reacted Nb3Sn wires for superconducting magnets, operated in normal or superfluid helium the effects of room temperature bend strain on current carrying capacities of monolithic and flat cable conductors were investigated by controlled winding processes using mandrels with various diameters. The results show a field-dependent degradation in critical current with increasing bend strain for both conductor types. Only a very small precompression effect (< 0.5%) on critical current is observed. At higher strain, beginning at 0.4%, the critical current decreased rapidly, reaching at 13 T by more than 50% at a bending strain of 0.9% calculated on overall conductor cross section. Both optical and scanning electron microscopy were used to examine the samples before and after testing. Additional data on the mechanical properties (tensile stress-strain diagrams) of Nb3Sn samples are presented, along with some preliminary critical current versus uniaxial tensile stress measurements at 2.2 K and 4.2 K, respectively. First results indicate that the increase in critical current at 2.2 K is relatively larger (about 25%) than that at 4.2 K. This fact is interesting for Nb3Sn magnets cooled with superfluid helium to obtain a broad strain tolerance.