Abstract
Composite materials are increasingly used in many different applications. However, the use of composite materials introduces different failure modes. Delamination within, and disbonding of, composite structures are types of damage which can lead to failure of the structure, and therefore in-situ monitoring of such damage is of increasing importance. In this thesis, the use of surface-mounted chirped fibre Bragg grating (CFBG) sensors is investigated; these sensors can be installed at any point in the life of a structure to monitor delamination or disbonding. The work began by using a CFBG sensor bonded to the surface of a unidirectional GFRP double cantilever beam (DCB) specimen to show that it is possible to identify the location, and monitor the growth, of a delamination. A dip in the spectra reflected by the sensor indicated the approximate location of the delamination front. Analysis and refinement of the technique allowed the location of the delamination front to be identified to within four millimetres. The same approach was developed for a bonded composite DCB specimen. The reflected spectra showed smaller dips than seen previously and required a signal processing technique to be developed, enabling the location of the disbond to be identified to within about four millimetres. Finally, the surface-mounted CFBG sensors were used to identify the location of a disbond between a CFRP plate reinforcement and a steel beam. This was achieved using thermally induced strains, rather than mechanical loading (as used in the earlier work), and an artificial disbond. In addition, with the aid of an artificially grown cut in the adhesive (simulating disbond growth), it was possible to show that the technique can be used to monitor disbond growth, using thermal loading alone, to within about one millimetre of its actual location; the temperature variation required to monitor the growth is about 20C.