Changes in the reflected spectra of embedded chirped fibre Bragg gratings used to monitor disbonding in bonded composite joints

Disbond growth detection in composite–composite single-lap joints using chirped FBG sensors

“…During manufacture of the panels, optical fibres containing the CFBG sensors were embedded near the first 0/90 interface, and therefore approximately 0.5 mm from the adhesive bondline. The adherends containing the sensors were cut so that the low-wavelength end of the sensor was adjacent to the cut end of one adherend (further fabrication details can be found in previous papers [15,16]). For this work, CFBG sensors with a range of sensor lengths have been used (15,30,45 and 60 mm), although all sensors had the same spectral bandwidth, having a full width at the half-maximum of the reflected spectrum of 20 nm (i.e.…”

“…Figure 1 shows a schematic diagram of the single-lap joint and the position of the embedded CFBG sensor. The optical fibre containing the sensor was spliced to the optical arrangement which consisted of a broadband light source, coupler and optical spectrum analyser (details are provided in [14][15][16]). The bonded joints were subjected to fatigue loading using a computer-controlled servohydraulic fatigue machine (Instron 1341) with a peak load of 8 kN, an R-value (R=  min / max ) of 0.1, and a sinusoidal waveform with a frequency of 3 Hz.…”

“…Earlier work using a CFBG sensor with a length of 45 mm [16] produced a reflected spectrum which showed a perturbation at the position of the disbond front that was significantly more symmetrical in shape compared to the perturbation shown in Figure 4 for a 15 mm sensor. To investigate the effect of sensor length on the reflected spectrum in the presence of a disbond, spectra for sensors with the same spectral bandwidth of 20 nm, but different sensor lengths (15 mm, 30 mm, 45 mm and 60 mm) have been predicted.…”

“…In addition to uniform fibre Bragg gratings, chirped fibre Bragg grating (CFBG) sensors have been investigated more recently for damage monitoring in composite materials, bonded joints and sandwich structures (e.g. [12][13][14][15][16][17]), following the initial demonstration by Takeda, Okabe and colleagues [e.g. 18] that such sensors could both detect and locate damage development in composite materials.…”

“…In previous work [15,16] it was shown that a CFBG sensor could be used to monitor disbond initiation from one end of the overlap of a composite-composite bonded joint. In the present paper, the work is extended (a) to show that disbond monitoring at both ends of the overlap is possible with a single embedded sensor, and (b) to determine the effect of using different sensor lengths.…”