Ignacio Aviles Santillana 16 T magnet development program with a focus on the yoke laminations. Structural limits of ARMCO© Ignacio Aviles Santillana 09.05.2017

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

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

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

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

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

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 6892. Thickness: 4 mm Ignacio Aviles

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

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 σm @ 4.2 K Ignacio Aviles

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

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: 20000 (EDMS 1171853) Maximum principal stress at 1.9 K [MPa] 11 T MQXF Zero current ~ 0 213 Nominal current 142 232 Ignacio Aviles

Cryogenic fatigue testing Ignacio Aviles

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

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

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

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

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

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

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

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

Additional slides Ignacio Aviles

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