Detection of disbond initiation and growth in a bonded joint using a chirped fibre Bragg grating sensor

Palaniappan, J. and Ogin, S. (2007) Detection of disbond initiation and growth in a bonded joint using a chirped fibre Bragg grating sensor. Deformation and Fracture of Composites Conference (DFC9), Sheffield, UK. 10 - 12 Apr 2007.

Abstract

Adhesive bonding of composite structures are considered superior to mechanical fasteners for cost saving, weight reduction, stresses distributed over the entire bond area and not at a hole (as in bolted joints). The serious concern with these bonded joints is disbond initiation and growth in either quasi-static loading or fatigue loading and hence structural health monitoring of this bonded joints is important. Recent researches have proven that health monitoring using optical fibre sensors is the new technological innovation for easy interpretation and cost effective (figure 1).
A novel optical method for monitoring the disbond in composite bonded joints using chirped fibre Bragg grating (CFBG) is presented in this paper. Experiments were performed by embedding CFBG sensor of 15mm length in one of the (02/90/06)s GFRP adherend along outer 0° ply from the bondline. The bonded joint is subjected to a fatigue loading which both initiates disbonds between the adherends and then causes the disbonds to grow. The CFBG reflection spectrum shows a shift to lower wavelength due to disbond initiation. Comparing the reflected spectrum at different fatigue cycling, it is possible to predict the size and locate the debonding front as the perturbation of the reflection spectrum shifted to higher wavelength (figure1). Visual observations of the disbond front in the optically transparent joint measured in-situ by a camera and from the CFBG reflected spectrum are compared, showing that the sensor can determine the disbond front position to within about a millimeter (figure 2).
Figure 1: CFBG Reflected spectra at 11500 (before the disbond), 13000 and 14000 cycles showing (a) the reflection spectrum shift to lower wavelength due to disbond initiation and (b) shift of perturbation to higher wavelength as disbond propagates.
Figure 2: Comparison of CFBG determination of the disbond front position with the photographic determination.

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