LIGO Document E2400425-v2

Convolution UHV Leak Failure Rate Estimate

Document #:
LIGO-E2400425-v2
Document type:
E - Engineering documents
Other Versions:
Abstract:
The Cosmic Explorer (CE) and Einstein Telescope (ET) projects are considering the use of corrugated, thin-walled, tubing for their beam tubes as an alternative to thick-walled, smooth, straight piping or ring-stiffened piping (for a number of reasons; see for example https://dcc.ligo.org/LIGO-T2400337) The unstiffened and ring-stiffened tubes require bellows (expansion joints) to allow for expansion during vacuum bakeouts, as well as adjustments for alignment. The corrugated tube alternative does not require these discrete bellows (expansion joints) since they are (in effect) long bellows. However, the corrugated tube option incorporates many more convolutions to the overall assembly than a set of discrete bellows. If reliability against UHV leaks scales with convolutions, then the reliability of the system may be insufficient.

The convolutions are typically formed by either mechanical expansion or hydroforming. In either case, high plastic strains are induced in the forming of the bellows. Regardless of the method employed in forming the initial cylindrical shell (roll formed or spiral welded), axisymmetric convolutions will cross the weld. The heat affected zone (HAZ) of the weld is often the initiation site of microcracks leading to leaks later during the operational phase of the system. (If the tube is spiral welded and also spirally corrugated, at the same helix angle, then the corrugations would not cross the weld.)

Authors:
Notes and Changes:
v2:
1)Noted that the estimation of UHV leak failure rate is approximate, not definitive, and meant to motivate the need for further investigation.
2) Noted that although the LEP bellows are sealed with aluminum gaskets, the reported bellows failures do not involve the gaskets.
3) Added a comparison of the LEP bellows requirements to LIGO bellows requirements to show that LIGO requirements may be more stressing.
4) Added a calculation of cyclic fatigue life for bellows and corrugated tubes.
5) Added a discussion of potential Stress-Corrosion Cracking (SCC) concerns.
6) Added conclusions and recommendations.
Referenced by:

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