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

Samuli Heikkinen CLIC Workshop October 2007 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

Samuli Heikkinen CLIC Workshop October 2007 Introduction to CLIC fatigue problem H

Samuli Heikkinen CLIC Workshop October 2007

20 μs

Samuli Heikkinen CLIC Workshop October 2007 Virgin metal Cyclic loading Cyclic slipPSB’s Intrusions Extrusions Stage I crack propagation Stage II crack propagation Dislocation activation Final failure Stress concentrations The dislocation (defect) density increasesCrack initiates Crack propagates

Samuli Heikkinen CLIC Workshop October 2007 Steels, Mo, Ti, … Cu, Al, … Wöhler curve, SN-curve (stress versus number of cycles to failure)

Samuli Heikkinen CLIC Workshop October 2007 Material survey

Samuli Heikkinen CLIC Workshop October 2007 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

Samuli Heikkinen CLIC Workshop October 2007 Fatigue by ultrasonic experiments I

Samuli Heikkinen CLIC Workshop October 2007 Fatigue by ultrasonic experiments II

Samuli Heikkinen CLIC Workshop October 2007 Fatigue by laser experiments

Samuli Heikkinen CLIC Workshop October 2007 Cu-OFE_1 (ΔT ~ 70ºC) Cu-OFE_2 (ΔT ~ 110ºC) Fatigued zone RF breakdown zones 110ºC 70ºC Fatigue by RF experiments

Samuli Heikkinen CLIC Workshop October 2007 Wöhler curves of the test results (Stress vs. N) RF SLAC, ΔT~70ºC RF SLAC, ΔT~110ºC

Samuli Heikkinen CLIC Workshop October 2007 Wöhler curves of the test results (Magn. field vs. N) RF SLAC, ΔT~70ºC RF SLAC, ΔT~110ºC

Samuli Heikkinen CLIC Workshop October 2007 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 C GlidCop® Al-15 is my choice if high temperatures are inevitable. ΔT 55 % higher than cold worked copper C 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)

Samuli Heikkinen CLIC Workshop October 2007 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