The use of a CFBG fibre optical sensor to detect disbond development in composite/composite and metal/composite adhesively bonded joints

This paper describes a novel technique to monitor disbond initiation and propagation in both composite-composite and metal-composite adhesively bonded joints using chirped fibre Bragg grating (CFBG) sensors embedded within fibre-reinforced composite adherends. Characteristic changes in the reflected spectra from the sensors indicate both (a) disbond initiation (due, for example, to fatigue loading) and (b) the current position of the disbond front (to within about 2 mm). For composite-composite bonded joints, the reflected spectra are recorded with the joint under a small load (in practice, this could be the self-weight of the structure). In the case of a metal-composite joint, the difference in the coefficient of thermal expansion between the adherends is sufficient to enable disbond propagation to be monitored when the joint is unloaded. The experimental results have been modelled using a combination of finite-element analysis and commercial software for predicting FBG spectra, with very good agreement between the experimental observations of the reflected spectra and the predicted spectra, for disbonds of different lengths

NDE detection of manufacturing defects in composite bonded joints using CFBG sensors

This paper reports the use of chirped fibre Bragg grating (CFBG) sensors to detect defects introduced during the bonding of composite joints. The single-lap joints were made from transparent GFRP cross-ply composites which had a CFBG sensor embedded within one of the two adherends during the manufacturing stage. Joints with defects simulated using PTFE or introduced real air-gaps have been investigated. The sensors were interrogated with no load on the joints and also under a moderate quasi-static load. For the joints with PTFE “defects”, the perturbations observed in the reflected spectra provided a clear indication of the location of the defects and were in very good agreement with modelling predictions. Changes in the reflected spectra for the air-gap defects also provided an indication of the location of the defects and the spectrum changes were in reasonable agreement with modelling predictions. However, further work is required to establish the limitations of the technique for air-gap defects

The use of an embedded chirped fibre Bragg grating sensor to monitor disbond initiation and growth in adhesively bonded composite/metal single lap joints

Chirped fibre Bragg grating (CFBG) sensors embedded within glass fibre reinforced plastic (GFRP) adherends have been used to monitor disbond initiation and growth in an adhesively bonded GFRP/aluminium alloy single lap joint. The elevated temperature curing of the adhesive used in the manufacture of the joint leads to thermal strains being generated within the GFRP and aluminium adherends. Disbond initiation and growth between the adherends during fatigue cycling causes a relaxation of the residual thermal strains within the composite adherend and perturbations (peaks or dips) in the reflection spectra from the CFBG sensor. These perturbations can be used, when the joint is unloaded, to monitor both the initiation of a disbond in the joint and to monitor the growth of the disbond with fatigue cycling

Reflected spectra prediction for chirped fibre Bragg gratings used for disbond detection in composite/composite bonded joints

In this paper it is shown that a chirped fibre Bragg grating sensor embedded within a composite adherend can be used to monitor disbond initiation and propagation in an adhesively bonded single lapjoint. Characteristic changes in the reflected spectra from the sensor indicate both disbond initiation and the current position of the disbond front to within about 2 mm (a distance which depends on adherend material and sensor position in relation to the adhesive bondline). When the sensor extends the full overlap length, disbond initiation from either end of the overlap can be monitored. The results have been modelled using a combination of finite-element analysis and commercial software for predicting FBG spectra; the predicted spectra are in very good agreement with experiment. The CFBG sensor technique could provide the basis for monitoring a wide range of bonded joints and structures where one adherend is a composite material

Detection of defects in as manufactured GFRP-GFRP and CFRP-CFRP composite bonded joints using chirped fibre Bragg grating sensors

Chirped fibre Bragg grating (CFBG) sensors have been embedded in composite coupons which have been used to form one half of single lap bonded joints. The bonded joints have been made with deliberately included defects, consisting of either a PTFE insert or an air gap, and the sensors have been used to detect the presence and location of the defects. The experimental results, and the modelling, show that defects in both GFRP-GFRP joints and CFRP-CFRP joints can be detected by the embedded CFBG sensors, though it is easier to detect defects in the lower stiffness (GFRP) joints. <br/

The use of chirped fibre Bragg grating sensors to monitor disbond growth

Composite-composite or composite-metal bonded joints, are used in various industries and applications. The difficulty with any bonded construction with regard to structural integrity monitoring is, of course, that the bonded structure cannot be disassembled easily. Various NDE (non-destructive evaluation) techniques have been suggested to monitor bonded joints based on ultrasonic, acoustic, thermal and backface strain measurements [e.g. 1-3]. Among NDE optical techniques suggested, the use of uniform fibre Bragg grating (FBG) sensors has been the most widely favoured, with a number of demonstrations available in the literature for monitoring bonded joints, repairs and structures [e.g. 4-6]. In addition, chirped fibre Bragg grating (CFBG) sensors have been investigated more recently for damage monitoring in composite materials, bonded joints and sandwich structures (e.g. [7-11])

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

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 15–60 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

Systematics of the Pacific monkey-faced bats (Chiroptera : Pteropodidae), with a new species of Pteraloplex and a new Fijian genus

Copyright © 2005 The Natural History MuseumThe fruit-bat genus Pteralopex comprises the monkey-faced bats, a group of six endangered species found only in old-growth forests on certain islands in the south-west Pacific (the Solomon Islands and Fiji). The taxonomy of the genus is reviewed in detail and updated accordingly. Two ‘cryptic’ biological species are shown to occur in sympatry on both Bougainville and Choiseul in the northern Solomon Islands (corresponding to Pteralopex anceps Andersen, 1909 and a previously undescribed species) and each is accordingly described and reviewed. A new genus (Mirimiri) is erected for the Fijian monkey-faced bat (formerly Pteralopex acrodonta), which differs greatly both morphologically and genetically from species of Pteralopex in the Solomon Islands. Ecomorphological differences between sympatric Pteralopex species are briefly reviewed, including potential differences in functional morphology and feeding ecology. Geographic patterns of occurrence and future survey priorities for monkey-faced bats are also discussed.Kristofer M. Helge