Pre-Bend Effect of Nb3Sn Composite with Different Structures

Masayoshi Wake( )
Pre-bend effect of Nb3Sn composite conductor has possibility in the enhancement of@current carrying capability. Nb3Sn is formed at high temperature. Due to the thermal contraction difference among composite materials, internal stress in the conductor is developed during the cool down. Such internal stress deforms the lattice of Nb3Sn and reduces the current carrying capacity. By giving bending to the conductor, it is possible to relax the internal stress of the composite. Finite element method is a good tool to look into the mechanism of the pre-bend effect.
The conductors are often reinforced by Cu-Nb to stand for electro-magnetic force. The location of Cu-Nb has influence to the pre-bend effect. Two ways of reinforcement are compared. Circumference reinforcement has Cu-Nb pipe@in the circumference. Core reinforcement has Cu-Nb core in the center. Some ANSYS are shown here. Plots are Von Mieses stress. Both models are of diameter 1mm and copper ratio 0.6.

The simulation result is summarized in the left. Core reinforcement shows larger effect of prebending. However, the degradation itself caused by the thermal contraction is less in the circumference reinforcement. Probably, Ic characteristic is better in the circumference reinforcement as a result. The critical current density Jc may be given by:

    circumference reinforcement     core reinforcement stage conditions
without prebend with prebend without prebend with prebend
T=925 K, no bend
T=500 K, no bend
T=300 K, no bend
The internal stress in the composite is already accumulated by the thermal contraction difference at room temperature.
T=300 K, horizontal bend
Bending is given by the 2.0 N/mm force gradient. The bending radius is 6 cm.
T=300 K, bend back
T=300 K, adjust bend
T=300 K, bending force removed
T=300 K, vertical bend
T=300 K, bend back
adjust bend
Bending force removed
The internal stress in the pre-bent composite is much less than that of composite with no-prebend.
T=50 K
T=4.2 K
Internal stress in the composite is built up again by the cooling.
Electro-magnetic force pulling of 20 N.
20N in total is given to the composite diameter of 1.0 mm. The force is given so that the strain in the composite become even.
Electro-magnetic force pulling of 40 N.
Electro-magnetic force pulling of 60 N.
Electro-magnetic force pulling of 80 N.
Electro-magnetic force pulling of 100 N.
Electro-magnetic force pulling of 120 N.
Electro-magnetic force pulling of 150 N.
Electro-magnetic force pulling of 200 N
Electro-magnetic force pulling of 300 N
Note that the color scales are different for the pictures above 300N.
Electro-magnetic force pulling of 400 N.
Electro-magnetic force pulling of 500 N.
Electro-magnetic force pulling of 600 N.