Abstract
This thesis reports investigations into the application of a type of optical sensor to detect and monitor damage in composites structures. The chirped fibre Bragg grating (CFBG) is a type of optical component that is sensitive to both strain and temperature and is commonly found in the telecommunications industry as a filter. Its small diameter and low cost makes it a potentially inexpensive sensor ideal for embedding in composite materials. This work consisted of extending the use of CFBG sensors into three areas: monitoring of disbonding in composite-metal bonded joints; detection of manufacturing defects in composite-composite bonded joints; monitoring of delamination lengths for a specimen subjected to mode II loading. All of these areas were investigated using a combination of experimental testing and theoretical predictions (using finite element analysis and optical prediction software). The first area investigated used a bonded joint composed of one metal adherend (aluminium) and one composite adherend with an embedded CFBG, bonded together at elevated temperature. Thermal strains were generated in the bonded joint on cooling the joint from the adhesive cure temperature to room temperature. The joint was then subject to tension-tension fatigue loading, which caused the joint to progressively disbond. The relaxation of the thermal strain in the composite adherend due to the advancing disbond front caused perturbations in the reflected spectra from the embedded CFBG sensor which allowed the development of the disbond to be tracked to within about 2 mm. In the second area, CFBG sensors were embedded within one of the composite adherends in a composite-composite bonded joint with an included manufacturing defect in the bondline. The first defect used was PTFE and the reflected spectrum from the CFBG sensor showed a clear perturbation at the defect location (the joint was externally loaded). Subsequent tests used more realistic defects (i. e. “air gaps” introduced into the bond) as the defect, and perturbations were observed in the CFBG spectra for both glass fibre reinforced plastic (GFRP-GFRP) joints and carbon fibre reinforced plastic (CFRP-CFRP) bonded joints. The effect of defect size and position was also investigated. The results for detecting the air defects were not as clear as for the PTFE artificial defect; however defects of 5 mm or longer could be detected at any position within the joints. The third area investigated in this work used embedded CFBGs to monitor delamination lengths in specimens subjected to mode II loading. A CFRP End-notch flexure specimen (ENF) was modelled using finite element analysis and predictions were made of the reflected CFBG spectra to establish the effect of sensor position on the reflected spectra. ENF specimens were then manufactured with sensors embedded in the positions which would produce the clearest perturbations (based on the computer modelling). Delaminations were grown in the ENF specimens and the effects on the reflected spectra were compared with the predictions, with excellent agreement between the extension of the delaminations as monitored directly and by the CFBG sensors, but only when low-reflectivity sensors (i. e. 40% reflectivity) were used. Indeed, a very important and supplementary part of the delamination study showed that sensor sensitivity to non-uniform strain fields (i. e. the type of strain distribution generated by damage in composite materials) is increased with decreasing CFBG reflectivity.