Samuli Heikkinen CLIC Workshop 16-18 October 2007 a short summary of the doctoral thesis on the CLIC fatigue study

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Samuli Heikkinen CLIC Workshop 16-18 October 2007

a short summary of the doctoral thesison the CLIC fatigue study

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• Introduction to CLIC fatigue problem• Undertaken fatigue tests and the most important

results:• Ultrasound (CERN, S. Heikkinen)

• Laser (CERN, S. Calatroni, H. Neupert)

• RF (SLAC, S. Tantawi, V. Dolgashev)

• Conclusions and suggestions for future work

Contents

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Introduction to CLIC fatigue problem

H

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20 μs

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Virginmetal

Cyclic loading

Cyclic slip PSB’sIntrusionsExtrusions

Stage I crackpropagation

Stage II crackpropagation

Dislocationactivation

Final failure

Stress concentrations

The dislocation (defect) density increases Crack initiates Crack propagates

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Steels, Mo, Ti, …Cu, Al, …

Wöhler curve, SN-curve (stress versus number of cycles to failure)

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Material survey

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Ultrasonic fatigue tests (Heikkinen, CERN)

• Mechanical stressing of materials at 24 kHz• Reversed and fully compressive stress ratios• Fast, cheap and reliable tool to collect high cycle fatigue data

(CLIC lifetime in 30 days)

Laser fatigue tests (Neupert, Calatroni, CERN)

• Thermal stressing of materials with pulsed laser at 20 Hz and 200 Hz

• Conditions close to RF, cyclic loading due to pulsed surface heating

• Limited in number of cycles

RF fatigue tests (Tantawi, Dolgashev, SLAC)

• Thermal stressing of materials with high power klystron at 60 Hz• Conditions closest to CLIC parameters (except the achievable

number of cycles)

Fatigue experiments

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Fatigue by ultrasonic experiments I

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Fatigue by ultrasonic experiments II

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Fatigue by laser experiments

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Cu-OFE_1 (ΔT ~ 70ºC) Cu-OFE_2 (ΔT ~ 110ºC)

Fatigued zone

RF breakdown zones

110ºC

70ºC 70ºC

Fatigue by RF experiments

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Wöhler curves of the test results (Stress vs. N)

RF SLAC, ΔT~70ºC

RF SLAC, ΔT~110ºC

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Wöhler curves of the test results (Magn. field vs. N)

RF SLAC, ΔT~70ºC

RF SLAC, ΔT~110ºC

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Conclusions

• Ultrasound: The collected fatigue data classifies the materials.

• The best commercially available candidates have most likely been identified and tested.

• Laser: The order of the materials is the same as for the ultrasound. The stress levels are lower, but so is the failure criteria.

• Radio frequency: The fatigue experiments are launched and ready to produce data to be compared with ultrasound and laser.

• Cold worked Copper Zirconium C15000 is the best candidate if high temperatures (>300°C) during the manufacturing process and operation can be avoided.

• ΔT 60 % higher than cold worked copper C10100.

• GlidCop® Al-15 is my choice if high temperatures are inevitable.

• ΔT 55 % higher than cold worked copper C10100 .

• RF breakdown resistance is an open question, two sets of contradictory experimental data (RF at SLAC and DC at CERN).

• Machinability is worse than for CuZr and Cu. (machining tests under way)

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• More radio frequency fatigue experiments!

• Define the failure criteria of the cavities.

• Is it the drop in Q or increase in breakdown probability, due to the extrusions on the surface?

• More laser fatigue experiments!

• Currently poor statistics.

• Specimens should be pushed further than the Ra 0.02 µm. It would be interesting to see cracks also there.

• More ultrasound fatigue experiments!

• Attention should be put on the surface evolution at high number of cycles. Even when the cracks doesn’t appear.

• In the end a statistical study should be done to define the needed safety factor.

Suggestions for future work

Documents

Samuli Heikkinen CLIC Workshop 16-18 October 2007 a short summary of the doctoral thesis on the CLIC fatigue study