Mechanical Properties of Roebel Coated

Conductor Cable

A. Kario1, S. Otten1,2, C. M. Bayer1, M. Vojenciak1,3, A. Kling1, B. Ringsdorf1, B. Runtsch1, W. Goldacker1

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

• Bending properties

• Tensile stress

• Transverse stress

• Need of impregnation

current

fieldforces

G. Kirby et al., EuCARDII Meeting

Outline

Bending properties

ImpregnationTransversal stress

Tensile stress

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 bendingr = 50 mm

r = 10 mm

10 20 30 40 50500

600

700

800

900

1000

REBCO - outside

REBCO - inside

Ic (A)

radius (mm)

10

12

14

16

18

20n-value

To be published

Roebel cable is fragile to bending-torsion

0 10 20 30 40 50 60 700.5

0.6

0.7

0.8

0.9

1.0

15 mm, sc outside

Winding angle [°]

Ic norm [A]

10 mm, sc outside

10 mm, sc inside

15 mm, sc inside

10 %

5 %

LN2, SF

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

15 mm, sc outside

Winding angle [°]

Ic norm [A]

10 mm, sc outside

10 mm, sc inside

LN2, SF

• No reversibility in a case of 10 and 15 mm

thickness

• 5% or 10% degradation at 20° winding angle

Cable

10 or 15 mm diameter

Outline

Bending properties

ImpregnationTransversal stress

Tensile stress

Modelling of the stress in meander structure

• 4 mm Roebel cable

• Superpower tape: 100 µm thick, Yong modulus: 120 GPa

• Applied tensile load 1 kN

• Finite Element Method, Comsol Multiphysics

C. Barth et al., Supercond. Sci. Technol. 25 (2012) 025007 (9pp)

Roebel strand geometry: outer corner and 10 mminner 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 (9pp)

Outline

Bending properties

ImpregnationTransversal stress

Tensile stress

• degradation of the

individual strands Ic

by 30% at a transverse

stress of 10 MPa.• KIT cable: 10 strands, TL: 126 mm

• Effective stress: 167 MPa (24% surface)

• GCS cable: 15 strands, TL: 300 mm

• Effective stress: 111 MPa (36% surface)

Different compressive stress values 10 – 40 MPa

J. Fleiter et al., Supercond. Sci. Technol. 26 (2013) 065014 (5pp)

D. Uglietti et al., Supercond. Sci. Technol. 26 (2013) 074002 (5pp)

Transversal stress test with additional copper strands

• Contact area of stainless steel: 4 mm x 85 mm

stainless steel

Roebel strands

Copper strands

0 10 20 30 40 500.5

0.6

0.7

0.8

0.9

1.0

Ic normalised n-value

pressure (MPa)

5 % Ic degradation =

27.7 MPa

2

4

6

8

10

12

5% Ic degradation by 68.8 MPa effective stress

• Average transverse stress (cable surface)

• Effective transverse stress

0 20 40 60 80 100 1200.5

0.6

0.7

0.8

0.9

1.0

Ic normalised

pressure (MPa)

n-value

5 % Ic degradation =

68.8 MPa

2

4

6

8

10

12

To be published

Defects on the Roebel-bridge

1 2 3 4 5 60

10

20

30

40

50

60

70

straightstraightbridge bridge

strand 3 strand 4 strand 5 strand 6 strand 7 strand 8 strand 9

Ic (A)

position on Roebel strand

bridgestraight

60 K, 20 mT

61 K, 30 mT

2 mm

2 mm

60 K, 25 mT

2 mm

Magneto-optical Imaging

• Ic measurements after deassembly of the

Roebel cable To be published

Outline

Bending properties

ImpregnationTransversal

stress

Tensilestress

Epoxy choice

• Commercially available resins with

fillers

• Thermal expansion

• Thermal conductivity

• Chemical compatibility with REBCO

• Resin working temperature

• Resin viscosity

Stress concentrations

Low Tc cable example

Fred M. Asner, High Field Superconducting Magnets, CLARENDON PRESS OXFORD 1999

Thermal expansion similar to REBCO for more then 50% filled epoxy's

• Thermal expansion < 0.7 % preferred

* C. Barth,PhD thesis (2013)

Unfilled epoxy

Carbon p. + C

NT (4-8 %

)

Graphite + C

NT (4-8 %

)

Al(OH)3 (5

6 %)

Silver (6

0-80 %)

Silica (5

0 %)

Silica (6

0 %)

Graphite (5

0-60 %)

Alumina (60-70 %

)

REBCO tape

-1.60

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

N. Bagrets, ITEP, KIT

*

*

*

Thermal expansion (%)

*

To be published

Equal thermal conductivity at 4.2 K for silica and alumina filled epoxy's

• At 4.2 K, all epoxies have very low thermal conductivity

• Effect of fillers is small compared to 77-300 K

* C. Barth, PhD thesis (2013)

Unfilled epoxy

Carbon p. +

CNT (4-8 %

)

Graphite

+ CNT (4-8 %

)

Al(OH)3 (5

6 %)

Silve

r (60-80 %

)

Silica

(50 %

)

Silica

(60 %

)

Graphite

(50-60 %

)

Alumina (60-70 %

)

REBCO tape

0

0.2

0.4

0.6

0.8

1

1.2

1.4

S. Drotziger, ITEP, KIT

*

**

λ (W/ m*K) λ (W/ m*K)

300 K

Unfille

d ep

oxy

Carbo

n p.

+ C

NT (4-8

%)

Graph

ite +

CNT (4

-8 %

)

Al(OH)3

(56

%)

Silver

(60-

80 %

)

Silica

(50

%)

Silica

(60

%)

Graph

ite (5

0-60

%)

Alumina

(60-

70 %

)0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

4.2 K

*

* *

To be published

Impregnation of dummy Roebel cables

• Wet-winding

• Vacuum impregnation with fused silica

• Vacuum impregnation with fused silica and glass fibre

To be published

Vacuum impregnation with Araldite and 50% fused silica

• Dummy between two stainless steel tapes

• Araldite CY5538/HY5571 with 50 wt% fused

silica

• T = 80 °C, P = 0.3 kPa

• Thickness < 1 mm → high current density

100 µm stainless steel tape

Epoxy filledcentral hole

  Ic [A] n

Before impregnation 171.7 28.1

After impregnation 170.2 26.8After impregnation 2x 170.9 28.5

• Dummy with one SP REBCO strand

• Ic measurement at 77 K

• No Ic degradation

To be published

Transverse pressure tests (U. Twente)

W. Van de Camp, master thesis, University of Twente, 2012

Samples to be tested

• Reference cable

• Impregnated cable

(CY5538/HY5571 + 50 wt% silica)

Cryogenic press (UTwente)

• T = 4.2 K

• Imax = 50 kA

• Bmax = 11 T (perpendicular)

• Fmax = 260 kN

• U-shaped samples

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

Impregnation

• Vacuum impregnation with Araldite 50% fused silica proposed

Summary

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Bending properties

ImpregnationTransversal

stress

Tensilestress