Adhesively bonded composite-composite single-lap joints, with cross-ply GFRP adherends, have been cyclically loaded to initiate disbonding at either end of the overlap length. Disbond initiation and growth have been monitored using a combination of in situ photography (the joint is transparent) and a single chirped fibre Bragg grating (CFBG) sensor embedded within one composite adherend (with the low-wavelength end of the sensor adjacent to the cut end) and not in the adhesive bondline. Sensors having the same spectral bandwidth (20 nm), and lengths in the range 15 mm to 60 mm have been tested. The experimental results have been modelled using a combination of finite-element analysis and commercial software for predicting FBG spectra, and the predictions are in very good agreement with the experimental results. In all cases, it has been shown that the position of the disbond front can be located using the CFBG sensors with a precision of about 2 mm.
Matrix cracking damage is a generic type of damage that develops under load in the off-axis plies of laminated composites and is generally the precursor of more serious damage mechanisms, particularly delamination. Hence, it is important to identify and if possible locate this type of damage. Chirped fibre Bragg grating sensors have been embedded in a transparent glass fibre reinforced plastic crossply laminate and changes to the reflected spectra as a consequence of crack development have been studied. An approximately sinusoidal variation of the intensity of the reflected spectrum occurs at the position of the crack, enabling both crack development and crack position to be identified. A simulation of a reflected spectrum, incorporating a stress transfer model to predict the strains and an optical model to predict the reflected spectrum, is in reasonable agreement with the experimental results.
Chirped fibre Bragg gratings (CFBGs) have been used to monitor disbond initiation and disbond growth in composite bonded joints. The CFBGs have been embedded within, but near the surface of, a transparent GFRP composite adherend that has been bonded to a second transparent adherend. The low wavelength end of the CFBGs has been arranged to be adjacent to the end of the first adherend. Disbond initiation is readily detected as a modification to the reflected spectrum of the CFBG, consisting of the low wavelength part of the reflected spectrum being shifted to lower wavelengths; this is due to the unloading of the adherend resulting from the disbond. Disbond growth is detected by the movement of a dip in the reflected spectrum of the CFBG (as a consequence of the load redistribution at the disbond front); this dip moves to higher wavelengths as the disbond propagates. The relationship between the shift of the dip in the reflected spectrum with the position of the disbond front has been determined directly through the use of transparent GFRP joints.
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