Inert strength of subthreshold and post-threshold Vickers indentations on fused silica optical fibres

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Abstract
The strength of Vickers indentations made in air on fused silica optical fibres has been studied with indentation loads in the range 5mN-20N which spans the threshold region for radial crack formation. Handling and strength-testing techniques are described which permit measurements to be made in this strength range that is intermediate between ‘pristine’ and post-threshold. The results of inert strength measurements (i.e. made in liquid nitrogen) are compared with existing models for the behaviour of subthreshold and post-threshold indentations and reasonable agreement is found between experiment and theory if it is assumed that the true threshold (i.e. for indentations performed in an inert environment) is substantially higher than observed for indents made in air. The experimental results are somewhat stronger than previously published data and this is at least partially attributed to the latter not employing a completely inert test environment.