Tensile fatigue properties of fibre Bragg grating optical fibre sensors

This paper presents an experimental study into the tensile strength and fatigue properties of uncoated optical glass fibres containing Bragg grating (FBG) sensors. The protective polymer coating of the optical fibres must be removed by chemical or mechanical stripping methods to produce the Bragg gratings in the fibre core. This paper investigates the effects of chemical or mechanical stripping on the sensor integrity, operating life and damage mechanisms of the fibres when they have not been recoated after producing the Bragg gratings. The fatigue properties of the chemically or mechanically stripped fibre sensors were evaluated under cyclic tension–tension loading when externally bonded to carbon fibre-epoxy substrates representative of aircraft composite structures. The tensile strength of the mechanically stripped fibre was lower than the chemically stripped fibre because larger surface flaws were created during removal of the polymer coating. Tension fatigue life (S–N) curves of the chemically and mechanically stripped fibres were measured, and the fatigue life and signal quality of the FBG sensors decreased rapidly with increasing maximum fatigue strain. The signal quality of the FBG sensors degraded with increasing number of load cycles due to the formation of sub-critical cracks within the fibre by stress-corrosion. The fatigue life and sensor performance under cyclic loading was superior for the chemically stripped sensors. The S–N curves exhibit a fatigue endurance limit (at 106 load cycles), below which chemically and mechanically stripped fibres have an infinite fatigue life and there is no degradation to the FBG sensor signal quality.