1. Ignacio Aviles Santillana
16 T magnet development program with a focus on the yoke laminations. Structural limits of ARMCO©
Ignacio Aviles Santillana

2. Outline Introduction and motivation Design approaches
Results obtained on ARMCO© at room temperature
Rationale and motivation of the cryogenic testing
Results obtained on ARMCO© at cryogenic temperature (4.2 K)
Discussion and implementation of the results
Ignacio Aviles

3. Introduction and motivation
16 T magnet development for FCC
3 different types
They all need an ‘iron’ yoke
Mechanical characterization at cryogenic temperature of currently used materials: ARMCO©
One forth of the block – coils mechanical structure
One forth of the cosine - theta mechanical structure
One forth of the common - coil magnet cross section
Ignacio Aviles

4. Design approaches: strength of material
MATERIAL RESISTANCE
DRIVING FORCE
STRESS (σ)
𝑆∙𝜎<𝑅 𝑚
𝑆∙𝜎<𝑅 𝑒𝐿
Security factor
Tensile strength
Lower yield strength
Ignacio Aviles

5. Design approaches: fracture mechanics
DRIVING FORCE
MATERIAL RESISTANCE
STRESS
TOUGHNESS
FLAW SIZE & GEOMETRY
𝑆∙𝑌∙𝜎∙ 𝜋∙𝑎 <𝐾 𝐼𝐶
Safety factor
Geometrical factor
Flaw size
Fracture toughness
Ignacio Aviles

6. Test campaign ARMCO © 2 families of materials were tested:
ARMCO © as received (after hot rolling). Grade 1
ARMCO © annealed. Grade 1
Uniaxial tensile RT & 4.2 K  CERN (thanks to M. Crouvizier)
Fatigue 4.2 K  TIPC (CN)
Fracture 4.2 K  KIT (GE)
Ignacio Aviles

7. Results: Tensile tests
At RT, we are mostly interested in the σm as it has a major impact in the fabrication costs. However, ReL > 180 MPa is required as the coils are loaded at room temperature.
Material
ReL [MPa]
σm [MPa]
A [%]
Emod [GPa]
ARMCO as received
229 ± 1
293 ± 2
42 ± 1
196 ± 3
ARMCO annealed (Rolling direction)
237 ± 3
304 ± 2
41 ± 2
199 ± 3
ARMCO annealed (Transverse direction)
251 ± 1
301 ± 1
44 ± 2
201 ± 3
Sample geometry according to ISO Thickness: 4 mm
Ignacio Aviles

8. Mechanical properties at cryogenic temperature
In order to asses the static, dynamic and toughness properties at temperature close to operation.
Electromagnetic forces induced during powering / unpowering the coils.
Maximum principal stress in the yoke at 1.9 K for 0 A (left) and nominal current (right)
Ignacio Aviles

9. Results: Cryogenic tensile tests
Material
σm [MPa]
A [%]
ARMCO as received
1043 ± 4
0,4 ± 0,1
ARMCO annealed (Rolling direction)
972 ± 8
0,2 ± 0,1
ARMCO annealed (Transverse direction)
975 ± 6
0,3 ± 0,1
Width in the calibrated section reduced from 12.5 mm to 8 mm to guarantee breakdown outside the heads.
They all broke in the elastic region (brittle)
Annealing slightly decreased 4.2 K
Ignacio Aviles

10. Sample designation for fatigue and toughness @ 4.2 K
Material
Specimen IDs
Test
ARMCO®
Ar-1, Ar-2, Ar-3
Fatigue
annealed
An T-1, An T-2, An T-3,
An L-1, An L-2, An L-3
MQ T-1, MQ T-2, MQ T-3,
MQ L-1, MQ L-2, MQ L-3
Ar-CT1, Ar-CT2
Fracture toughness
An TL-CT1, An TL-CT2
An LT-CT1, An LT-CT2
ARMCO© as received was tested in the rolling direction.
ARMCO© annealed was tested in both rolling and transverse directions.
MQXF cycles were only implemented for ARMCO© annealed both in rolling and transverse directions.
Ignacio Aviles

11. Cryogenic fatigue testing
In order to rule out a premature failure during the whole lifespan of the components.
Cycles tailored for 11T dipole and MQXF cases.
Number of cycles: (EDMS )
Maximum principal stress at 1.9 K [MPa]
11 T
MQXF
Zero current
~ 0
213
Nominal current
142
232
Ignacio Aviles

12. Cryogenic fatigue testing
Ignacio Aviles

13. Results: Cryogenic fatigue testing
Specimen
Temp.
[K]
Fatigue Parameters
Survival Cycles
σmax
[MPa]
R ratio
Frequency
[Hz]
Ar-1, 2 & 3
4.2
 180
0.1 7
>400,000
An T-1, 2 & 3
An L-1, 2 & 3
MQ T-1, 2 & 3
500
MQ L-1, 2 & 3
All the samples which were tested survived the designed load cycles for 400 kcycles
Ignacio Aviles

14. Fracture toughness results @ 4.2 K
In order to implement a fracture toughness based design
Compact tensions specimens (5.8 mm thickness)
‘K’ tests for brittle materials
Specimen ID
Material
Fracture toughness (KIC); [MPam]
Fracture toughness uncertainty; [MPam]
Ar -CT1
ARMCO as received
27.98
0.22
Ar – CT2
26.91
An TL-CT1
ARMCO annealed (short side)
24.44
0.16
An TL-CT2
25.71
0.52
An LT-CT1
ARMCO annealed (long side)
25.37
0.21
28.17
0.53
Ignacio Aviles

15. Discussion of the results
Rolling direction
Defects due to lamination: Planar 2D defects or
surface defects due to a problem with the lamination equipment
Ignacio Aviles

16. Discussion of the results
𝜎 𝑚𝑎𝑥 < 𝐾 𝐼𝐶 𝑆 𝑌 𝜋𝑎
𝐾 𝐼 < 𝐾 𝐼𝐶
Reported in literature:
𝐾 𝐼𝐶 = 30 MPam
2a = 1 mm
S = 1.5
Y = 1.8
𝜎 𝑚𝑎𝑥 =280 𝑀𝑃𝑎
Ø = p/2 1
Ignacio Aviles

17. Discussion of the results
Surface semi elliptic crack
More critical than embedded lamination crack.
It is used as benchmark NDT defect (fabricated via EDM)
𝐾 𝐼𝐶 = 27 MPam
a = 0.8 mm (control in class 6 of EN 4050 – 4)
S = 1.5 (from ITER DDD)
𝐾 𝐼 < 𝐾 𝐼𝐶
Ignacio Aviles

18. Discussion of the results
Direct application of ASTM fracture toughness to structures is conservative
Ignacio Aviles

19. Discussion of the results
NDT
Under the required conditions, it is technically possible to control via UT in class 6 (ASTM 4050 – 4), what would allow to detect defects of 0.8 mm.
It is typically done for high added value products (e.g. austenitic stainless steel ~ 10 €/Kg for this range of thickness).
To be discussed with the supplier the additional cost of the control.
State – of – the – art steel plants offer in – line NDT (e.g. surface visual inspection).
Respecting best practices and standards in force, the maximum UT speed is ~ 2 hours / m2 to be able to detect defects of 0.8 mm.
It requires a suitable surface state (typically free of oxide).
In addition, a surface inspection (visual, Eddie currents, penetrant testing) could be also put in place in order to detect surface cracks.
Alternatively, a statistical NDT program could be performed for some ‘as fine blanked’ products at the surface and cross sections
Ignacio Aviles

20. Conclusions
Yoke lamination with well-defined yield strength at warm and cold are required for a reliable and cost-efficient design.
The static, dynamic and toughness properties at cryogenic temperature (4.2 K) have been assessed for ARMCO ©.
All samples which were tested survived the designed fatigue load cycles (security factor of 20 in the number of cycles).
When a fracture mechanics’ approach to design is applied, the σmax = 280 MPa value reported in literature seems extremely conservative. An approximation for the case of a semi - elliptic surface crack points out that a higher σmax could be achieved without jeopardizing the structural integrity of the magnets.
It is however strongly recommended to perform a FEA with the particular geometry of the components in order to calculate more accurately the stress intensity factor.
With a suitable NDT program, 100% of the volume can be controlled. Defects of 0.8 mm can be detected, but would most likely increase the production costs.
An NDT aimed to detect surface defects (the most critical) integrated in the production combined with an statistical control of fine blanked products could be implemented.
Ignacio Aviles

22. Additional slides
Ignacio Aviles

23. Discussion of the results
S = 1.5 (from [1])
2a > 0.8 mm (control in class 6 of EN 4050 – 4)
Embedded elliptical crack
𝜎 𝑚𝑎𝑥 < 𝐾 𝐼𝐶 𝑆 𝑌 𝜋𝑎
𝐾 𝐼 < 𝐾 𝐼𝐶
Ignacio Aviles