A study of the fracture mechanisms in pristine silica fibres utilising high speed imaging techniques

The 40 kg test masses of the Advanced LIGO interferometric gravitational wave detector will each be suspended on four fibres fabricated from Heraeus Suprasil synthetic silica glass. The ultimate tensile breaking stress and fracturing process of loaded silica fibres was investigated in order to develop the technology required to suspend the test mass in a robust and safe manner. The majority of the fibres in this study were pulled by hand in a H-2-O-2 flame; for comparison, fibres were also pulled on a CO2 laser machine. Carefully prepared fibres were shown to be pristine, i.e. free of surface cracks. Such fibres exhibited breaking strengths as high as 5 - 6 GPa. To analyse the mechanisms of fracture a high speed photographic setup was developed in addition to the use of a high speed video camera. The pristine fibres break in the zone of maximal stress as expected in the model of flawless fibres. Some fibres break at a lower stress and these fractures occur at the fibre ends. This type of fracture is related to the thermal stress induced by local heating which was used to align the fibre in the test structure. The most likely fracture mechanism is based on a thermo-kinetic approach.