Long-term mechanical behaviour of optical fibres coated with a u.v.-curable epoxy acrylate

Abstract

Long-term strength tests have been performed on glass fibres drawn from both Suprasil rods and graded index pre-forms in a laser furnace and in a graphite resistance furnace and coated in both cases with a u.v.-curable epoxy acrylate. Results of stress-free ageing tests in humid environments over a period of 18 months show that the fibres weaken at a rate that depends not only upon humidity level but also upon which heating source is used for fibre drawing. The weakening phenomenon has been ascribed to stress-induced reactions between silica and adsorbed water at the glass-polymer interface which occur spontaneously during the tensile test (dynamic fatigue). Static fatigue tests in 90% r.h. show that the fatigure behaviour deviates from that predicted by the theories of Charles and of Charles and Hillig for uncoated glasses; specifically plots of log (stress) versus log (time-to-failure) and stress versus log (time-to-failure) show downward curvatures which become more pronounced as the applied stress is lowered. In the absence of fatigue data for uncoated fibres, however, it is not possible to determine whether such non-linear behaviour is brought about by the presence of the organic coating layer.