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Detection of disbonding in bonded joints with a spatially displaced CFBG sensor

Optical fibre based sensors are being increasingly accepted by industry as an alternative to wire based strain gauges and are now available "off-the-shelf" [e.g. 1]. The fibre based sensors are regarded as intrinsically safe, light-weight and can be multiplexed, further reducing the mass of a distributed sensor system. Fibre Bragg grating sensors have been widely used to monitor strain in civil engineering structures [e.g. 2] and research has demonstrated their use for strain sensing and damage monitoring in composite structures [3,4,5]. This paper describes a study of the sensitivity of chirped fibre Bragg grating (CFBG) sensors to disbonds in a generic bonded joint. In previous work, it has been shown unambiguously that an embedded sensor can detect the initiation and growth of a disbond when the sensor is located adjacent to the growing delamination/disbond. However, the spatial sensitivity of the sensor to a disbond in a remote part of the structure is unclear. Consequently, the focus of the current work is to investigate the sensor sensitivity to a disbond when the CFBG sensor is located in an undamaged part of the bonded joint. The complete study comprises virtual testing and experimental validation; this paper covers the theoretical part of the study, using a combination of finite element modelling and optical response modelling.