LIGO Document G1100720-v1
- The 40 kg mirrors of the Advanced LIGO interferometers will be suspended on four circular cross section silica fibres with a diameter 400 mm. It is known from literature that silica fibres of this geometry can exhibit an ultimate breaking stress in excess of 5Gpa. However any minor damage or pre-existing surface micro flaws significantly reduce the fibre strength. A preliminary study of the fabrication, profiling, and breaking of flame and laser pulled silica fibres has been performed. The majority of thin fibres (diameter 200m) show a strength exceeding 5 GPa, however some fraction always demonstrate a much lower strength. This weak fraction becomes more significant when the fibre becomes larger. For a better understanding of fracture mechanisms we conducted a series of breaking experiments with high speed photography and video imaging. We confirm that our surface preparation procedure (based on a flame or laser polishing of the silica stock) allow us to pull fibers with a flawless surface. However, thick fibers (diameter >230m) break mainly at the fiber ends at a much lower stress. The observed reduction in strength is shown to correlate with heating of the neck and stock area in order to aid in alignment of the fibre prior to strength testing.
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