embedded optical fibre bragg gratings for aerospace monitoring.pdf

7/30/2019 EMBEDDED OPTICAL FIBRE BRAGG GRATINGS FOR AEROSPACE MONITORING.pdf

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EMBEDDED OPTICAL FIBRE BRAGG GRATINGS FOR AEROSPACE MONITORING

M. L.Dockney., I. J. Read', P.D. Foote' and R. P. Tatam'

In-fibre Bragg gratings have been embedded into an aerospace c a r b n fibre composite material for the purposeof quantitative dc strain measurement. Using a three-point bend test, the theoretical strain applied to a &nfibre test beamwas compared to strain measurements made by five surface mounted strain gauges and twentyembedded fibre gratings. 'Ihe gratings were written around 8 3 0 m into two types of comm ercial fibre and bothunjacketed and recoated grating sensorswere investigatedIntroductionThe past nine years has seen the emergence of the field of smart s tructures and sma rt materials. One of the keyareas of development is the fabrication of composite materials with integral sensory capability for aerospacestructures. A typical requirement W i g to measure strain near the surface of a structural panels in the rangefo.5%ove r a temperature range of -50C to 160C. Optical fibre sensors systems are now seen as one of themost promising candidate technologies [l]. The small cross-section of fibre sensors, ypically 125 microns,allows them to be embedded within a composite material with minimal disruption to the sumu nding host (for asurvey of the effects on composite strength of including optical fibres in carbon fibre composite material seeI21). Also the breaking strain of optical fibres is at leas t that of carbon fibres at around 1% . n particular, in-fibre Bragg grating sensors I31 have attracted much attention due to their inherent spectral encoding of themeasurand signal. This enables fibre grating sensors o perform absolute wavelength measulements and allowsthem to be readily wavelength multiplexed to form quasi-distributed sensor networks.Carbon fibre test beamThree point bend tests were performed on a carbon fibre composite test beam which had dimensions of 403"x 23.5". 'Ihe beam was m anufactured from Ciba-Geigy 914 "300 re-preg with a stack sequence ( 4 5 . 4 5 ,0,0,90,0,-45,45,0,90,0).he sequence was repeated with the 4 5 lies on the outer surfaces giving 22 plieswith a total thickness of 3.4W.I". Four optical fibres were embedded into the composite beam between theadjacent 0.0plies at a depth of 0.5" below the beam surface. The fibres were embedded running parallel tothe ply direction (along the length of the beam) to minimise disturbances to the surrounding host material andprevent the formation of resin eyes which could weaken the bonding between the optical fibre and compositematerial [e.g. Ref. 11.Each optical fibre contained an array of five Bragg gratings, each 2" ong, which were written using awavelength tuneable, frequency doubled Nd:YAG pumped dye laser system [4 ] and a m i m r interferometer. 'Ihegratings were written around 83Onm. and had a nominal 4nm Bragg wavelength separation for the purpose ofwavelength multiplexing. The gratings in each array were separated by 5 0 .0 .1 m m . the centre grating beingpositioned mid-way along the length of the test beam with the location known to an accuracy of 5 S m m . ?heposition of the fibre gratings within the carbon fibre test beam is illustrated in figure 1. Two types of standardtelecommunications fib re were investigated: FibrecoreSM800 nd Spectran FS SMC-A0780B.

M. L. Dockney and R. P. Tatam are at the Optical Sensors Group, Centre for Photonics and OpticalEngineering, School of Mechanical Engineering, Cranfield University,UK.+1. J. Read and P.D. Foote are with the Sowerby Research Centre of British Aerospace plc, Bristol. UK

0 1997The Institution of Electrical Engineers.Printed and publishedby he IEE. Savoy Place. LondonWC2R OBL,'1


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One fibre of each type had a Desoto acrylate coating applied, using a Vytran RC-100 ecoater, to the smallregions where the normal fibre jacketing had been removed for grating fabrication, since it has already beenshown that the presence of a coating can affect the bonding between the sensor and host material [6].E veelecaica l, resistive foil strain gauges of2" ength were bonded to the surface of the test beam, at the samelocations along the length of the beam as he f ibre gratings.For the threepoint bend test the beam supports wereseparated by 384flmm and the load was applied mid-way between the two supports using a micrometer screwgauge. The beamwas deflected in lm m fo.05pm steps with a total deflection range of m m m . To induce strainof the opposite sign the composite beam was simply inverted. Using this configuration the theoretical surfacestrain applied to the centre of the test beam was in the range S O 0 strain.Grating interrogation systemTo measure the change in Bragg wavelength of the fibre gratings and monitor the resistance of the surfacemounted s t r a i n gauges the experimental arrangement shown in figure 2was used. Fibre grating arrays wereintermgated by a system based upon a broadband superluminescentdiode (Superlum Ltd)and a piezo scanned,in-line Fabry-Ptrot tuneable filter. Bragg wavelengths are determined using a novel signal subtraction processwhich after appropriate filtering can yield data of a similar quality to the more conventional scan and dithertechnique[7], but has the advantage of permitting a data fate over two orders of magnitude greater than the scanand dither technique. For this work fibre grating arrays were interrogated with an update rate of 125Hz.Scandata was recorded using a storage oscilloscope before beiig transferred to a personal computer for signalprocessing. Using t h i s system Bragg wavelength changes could be measured to an accuracy of approximately6pants per million which for the gratings used in this work corresponds to a strain accuracy of


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however, had a strainldeflection response 4% less on average than the uncoated gratings (excluding SM800uncoated, grating 1) and up to 8.1% less than the theoretical value. The lower strain response indicates that theapplied coating acted to reduce the strain ransferred to the fibre gratings.Of comeh bove discussion assumes that the gratings are well bonded to the surrounding host material andthat the fibre gratings remains fixed between the same ply layers throughout the cure process. However, it ispossible that some movement could occur during this period. In t h i s experiment an uncertainty of 1 ply layer,would lead to an error of 12.5% in the calculated fibre grating strain values.Also displayed in table 1 is the residual strain imparted on the embedded optical fibres as a result of the cure!cycle. The train was calculated from the change in the Bragg wavelength of the fibre gratings measured priorto embedding and after curing. It can be seen that most of the fibre gratings within an array have a similarresidual strain, but there are wo notable exceptions. Grating 1 of the uncoated SMSOO fibre has a much lowerresidual strain than the other gratingswithinthe array. This is interesting because its strain/de!flection responseis greater than the theoretical value by 12.4%. a much greater difference than fo r the other uncoated gratings asnoted above. This type of result would be expected if the fibre was poorly bonded to the surrounding compositeand drifted from its original position during the cure process. However, the second anomalous residual strainvalue, grating 5 of the m a t e d SM800 fibre. does not appear to give a notably spurious straiddeflectionresult.

ConclusionWithin the applied surface strain range of EM00 t ra in themajority of embedded fibre gra tings remained wellbonded to the surrounding carbon fibre composite. The results obtained from the uncoated fibre gratings weregenerally in good agnxment wth the strain gauge data and theoretical values. The strain sensitivity of al l butone of the uncoated gratings beiig within 4.5% of the theoretical value. The acrylate re-coating materialappears to reduce the strain transferred to a fibre grating sensorby approximately 4%.AcknowledgementsMLD acknowledges a UK Engineering and Physical Sciences Research Council CASE studentship inassociation with British Aerospace Ltd.References1. Measures, R.M., Fibre optic sensing for composite smart structures, CompositesEngineering, 3, 1993, pp715-750.2. Jensen, D.W., Pascual. J. and August. J.A., Perform anceof graphite bismaleimide laminates withembedded optical fibres, Smart Ma t. and Struct., 1, 1992, p24.3. Morey, W.W., Meltz,G., and Glenn, W.H., Fiber Optic Bragg grating sensors, SPIEVol. 1169,Fiber Optic a n d h s e r sensors VII, 1989, pp98-107.4. Dockney, M.L., James S.W., and Tatam R.P., Fibre Bragg gratings fabricated using a wavelength

tuneable source and a phase mask based interferometer., Meas . Sci. Technol. 7 , 1 9 9 6 , 4 4 5 4 8 .5. Sirkis, J.S., and Dasgupta, A., Optimal coatings for intelligent structure fiber optic sensors,SPIE Vol. 1370,Fiber Optic Smart Structures and skin s I I I , San Jose, 1990, pp129-140.6, Waite. S..Tatam,R.P. nd Jackson, D.A., The se of optical fibre for damage and strain detection incomposite materials., J. Com p. Technol. and Res. Com posites,19, 1 9 8 8 , 4 3 5 4 2 .


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7. Kersey, A.D., Berkoff, T.A., and Morey,W.W.,Multiplexed fiber Bragg grating strain sensorsystem with a fiber Fabry-Pdrot wavelength filter', Opr. Lett., 18, 199 3, pp1370-1372.

S rainldFibre I Grating I Residualstrainmeasured Theory

FibrecoreSM800uncoated

FibrecoreSM800m a t e d

specmFS SMC-A0780BuncoatedspecmFS SMC-A0780Bm a t e d

13551579134212051263

65.3100.8136.3100.865.3

66.2W. 1100.9fo.2138.8M.2100.8M.16 4 . W 166.1M.1101.8M.l140.5M.2101.4kO.l64.4M. 165.6M.1101.7M. 2138.4M.21 0 0 . m . 164.5M.6 5 . M . 1' 101.m.2137.2kO.2100.5kO.2j 63.520.1

ain/mm)Fibregrating

73.4M.31 0 3 . M . 3134.8M.399.8M.262.5M.362.4M.396.2W.2129.4M.298.9M.260.7kO.265.3M. 2104.7kO.2141.4M.3101.8fo.366.6M.26 1 W . 3102.5M.2125.3M.395.3M.26 9 . M . 2

Table 1. Theoretical strain response compared to that measured by electrical strain gauges and n-fibreB r a s gratings. Also, esidual strain measured by fibre gratings after embedding in carbonfibre composite.


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1::::-U:-1::::::::::::::- ::::::::::::=7

gm=

specaan-s r v lmbSM800- -: ::::=:::::::::::::: ::: : :5

Figure 1 Diagram showing the location of the fibre gratings within the carbon fibre composite est beam.

ReferenceSuperlum In-line tumble I3SLD Fabry-Pht fdter

beam

Personal U0 Boardcomputer Strain gaugebridge

Figure 2 Experimental arrangement for grating interrogation system based upon an n-line tuneableFabry-Wrot filter and a broad band source.


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4000n-9 3000E?.t:2000i 1000.II oUQ5-1000un2-2000cr-4000

-25 -20 -15 -10 -5 0 5 10 15 20 25Beam deflection (mm)

(a)

4000n-9 30002000E!U

L-2 1000Ee ov

5-1000m#-2000cc2-3000-4000

CA

-25 -20 -15 -10 -5 0 5 10 15 20 25Beam deflection (mm)

(b>Figure 3 Theoretical strain,electrical sUain gauge data, and fibre grating data obtained from (a) grating3 of uncoated SM800 ibre and (b) grating3 of recoatedSM805 ibre.

embedded optical fibre bragg gratings for aerospace monitoring.pdf

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