Mechanical Properties of Roebel Coated Conductor Cable A

Mechanical Properties of Roebel Coated Conductor Cable A. Kario 1, S. Otten 1, 2, C. M. Bayer 1, M. Vojenciak 1, 3, A. Kling 1, B. Ringsdorf 1, B. Runtsch 1, W. Goldacker 1 1 Karlsruhe Institute of Technology, Institute for Technical Physics 2 University of Twente, Chair of Energy, Materials and Systems 3 Institute of Electrical Engineering, Slovak Academy of Science

Motivation forces field current G. Kirby et al. , Eu. CARDII Meeting • Bending properties • Tensile stress • Transverse stress • Need of impregnation

Outline Bending properties Transversal stress Tensile stress Impregnation

Bending radii > 10 mm do not degrade the Ic • Cable geometry: 12 mm cable, 10 strands, TL: 126 mm • Measurements on single strands show degradation in the bent section by < 1% • Small degradation, < 6. 5 % of total cable Ic • Cable degradation was not caused by bending r = 50 mm r = 10 mm To be

Roebel cable is fragile to bending-torsion Cable 10 or 15 mm diameter • 5% or 10% degradation at 20° winding angle • No reversibility in a case of 10 and 15 mm thickness

Outline Bending properties Tensile stress Transversal Impregnation stress

Modelling of the stress in meander structure C. Barth et al. , Supercond. Sci. Technol. 25 (2012) 025007 (9 pp) • 4 mm Roebel cable • Superpower tape: 100 µm thick, Yong modulus: 120 GPa • Applied tensile load 1 k. N • Finite Element Method, Comsol Multiphysics

Roebel strand geometry: outer corner and 10 mm inner radius • High von Mises stress – concentrated in small areas, favour the growth of cracks • Low von Mises stress – distributed over large area, cable can withstand high tensile load C. Barth et al. , Supercond. Sci. Technol. 25 (2012) 025007 (9 pp)

Outline Bending properties Tensile stress Transversal Impregnation stress

Different compressive stress values 10 – 40 MPa D. Uglietti et al. , Supercond. Sci. Technol. 26 (2013) 074002 (5 pp) • J. Fleiter et al. , Supercond. Sci. Technol. 26 (2013) 065014 (5 pp) • GCS cable: 15 strands, TL: 300 mm • Effective stress: 111 MPa (36% surface) • KIT cable: 10 strands, TL: 126 mm • Effective stress: 167 MPa (24% surface) degradation of the individual strands Ic by 30% at a transverse stress of 10 MPa.

Transversal stress test with additional copper strands stainless steel Roebel strands • Contact area of stainless steel: 4 mm x 85 mm Copper strands

5% Ic degradation by 68. 8 MPa effective stress • Average transverse stress (cable surface) • Effective transverse stress To be

Defects on the Roebel-bridge Magneto-optical Imaging 2 mm 60 K, 20 m. T 2 mm 61 K, 30 m. T 2 mm • Ic measurements after deassembly of the Roebel cable 60 K, 25 m. T To be

Outline Bending properties Tensile stress Transversal Impregnation stress

Epoxy choice • Commercially available resins with fillers • Thermal expansion • Thermal conductivity • Chemical compatibility with REBCO Low Tc cable example Fred M. Asner, High Field Superconducting Magnets CLARENDON PRESS OXFORD 1999 • Resin working temperature • Resin viscosity Stress concentrations

Thermal expansion similar to REBCO for more then 50% filled epoxy's Thermal expansion (%) -1, 60 -1, 40 * -1, 20 -1, 00 * -0, 80 -0, 60 * -0, 40 * -0, 20 0, 00 xy d lle i f n U o ep ) ) ) ) e % % % ap t 0 0 8 % 56 % 0 7 6 5 6 8 ( (4 )3 ( 00 CO a( a( 06 5 T T c c 6 ( ( B i i ( l l H N e r Si Si CN l(O RE ina hit ve +C + l i m p A. e u a S Al hit np Gr p o a rb Gr Ca N. Bagrets, ITEP, KIT 8 4 - ) % • Thermal expansion < 0. 7 % preferred To be *published C. Barth, Ph. D thesis (2013)

Equal thermal conductivity at 4. 2 K for silica and alumina filled epoxy's λ (W/ m*K) 1, 4 * 300 K 1, 2 * 1 0, 09 0, 06 0, 05 0, 6 0, 04 * 0, 2 0, 01 % (6 0 - 0(5 Al um in a ite ra ph 70 60 0 a G ) ) % ) (6 0 lic Si lic a (5 80 Si 0(6 er lv Si % % ) % (5 )3 H (O Al N C + G ra ph ite 6 -8 (4 T T N C p. + U ar bo n % ) % -8 (4 d ille nf ) xy ep o % C Al um in a( 60 -7 0 -6 0 (5 0 0 (6 ite ph ) ) % % ica (5 0 ica -8 0 Sil Gr a Sil ve r( 60 (5 6 3 ) % ) ) % % Al (O T CN + te hi ap H) (4 -8 -8 (4 T CN + p. Gr ) % ) y ox ep le d fil on ) 0 0 Un * 0, 03 0, 4 rb * 0, 07 0, 8 Ca * 4. 2 K 0, 08 S. Drotziger, ITEP, KIT • At 4. 2 K, all epoxies have very low thermal conductivity • Effect of fillers is small compared to 77 -300 K To be published * C. Barth, Ph. D thesis (2013)

Impregnation of dummy Roebel cables • Wet-winding • Vacuum impregnation with fused silica and glass fibre To be

Vacuum impregnation with Araldite and 50% fused silica • Dummy between two stainless steel tapes • Araldite CY 5538/HY 5571 with 50 wt% fused silica Epoxy filled central hole • T = 80 °C, P = 0. 3 k. Pa • Thickness < 1 mm → high current density 100 µm stainless steel tape • Dummy with one SP REBCO strand • Ic measurement at 77 K • No Ic degradation To be Ic [A] n Before impregnation 171. 7 28. 1 After impregnation 2 x 170. 2 170. 9 26. 8 28. 5

Transverse pressure tests (U. Twente) Samples to be tested • Reference cable • Impregnated cable (CY 5538/HY 5571 + 50 wt% silica) Cryogenic press (UTwente) • T = 4. 2 K • Imax = 50 k. A • Bmax = 11 T (perpendicular) • Fmax = 260 k. N • U-shaped samples W. Van de Camp, master thesis, University of Twente, 2012

Summary Bending • Easy direction down to 10 mm radius: TL: 126 mm, 10 strands cable • Depends on: TL, no. of tapes, SC tape Tensile stress • Optimised structure: outer corner, inner radius 10 mm (fixed) Transverse stress • Impregnation support needed Bending properties ? ? Tensile ? stress ? ? ? Transversal Impregnation stress Impregnation • Vacuum impregnation with Araldite 50% fused silica proposed