Investigation of Frictional Force Applied to Strands Surrounded by Other Strands and Tribological Analysis of Contact Surface in CIC Conductor

At the SULTAN test facility, the assessment of the performance of toroidal field coils has been progressing. Unpredictable strand buckling was observed during the destructive investigation of the conductor after significant degradation of the current sharing temperature. The buckling direction was perpendicular to the Lorentz force (LF), and the mechanism causing this was due to the thermal shrinkage occurring because of the difference in the thermal contraction between the strand material and the stainless steel conduit. Our previous research was based on a two-dimensional string model and demonstrated that the observed 2 mm strand bending could have led to strand buckling if the total amount of slide at the contact crossover was assumed to be 80 μm. To verify this assumption, we fabricated a device for the measurement of the friction force (shear force) between strands under a constriction force comparable to the LF of several hundred of kilonewtons per meter. The results for a Cr-coated 0.89 mm diameter strand surrounded by bare Cu strands indicated that the thermal contraction force would be sufficient to overcome the static friction force when the contraction force is reduced to a tenth of the maximum LF. The mechanism of the slide motion could be divided into two processes: separation of the inner wall of the conduit and the gradual separation from other strands due to a gradual reduction of the LF. A tribological analysis revealed that the real contact area increases by 10% when the shear force is applied; this result would help us evaluate the contact resistance between strands of conductors.