mechanical properties of roebel coated conductor cable

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