THERMAL ISOLATION TECHNIQUES FOR CURE MONITORING USING FBG OPTICAL SENSORS

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1 THERMAL OLATION TECHNIQUES FOR CURE MONITORING USING FBG OPTICAL SENSORS E.K.G. Boateng, P.J. Schubel, N.A. Waio Polyme Composites Goup Division of Mateials, Mechanics and Stuctues Faculty of Engineeing The Univesity of Nottingham Univesity Pak Nottingham NG7 2RD, UK SUMMARY This pape epots on expeimental themal isolation techniques fo Fibe Bagg Gating (FBG) sensos used fo the cue monitoing of themoset composites. A numeical model fo the FBG is intoduced. This study is pimaily focused on pepeg mateials used in the wind enegy industy. Keywods: FBG sensos, cue monitoing, esidual stesses and pocessing, esistance netwok. INTRODUCTION Cue monitoing can impove the efficiency of pocessing as it can identify citical points in the cue eaction such as the equied degee of coss linking fo temination of the cue cycle. Theefoe, the poduction time fo the poduct can be educed and optimised without dange of poducing incoectly cued mateial. Fibe optic stain sensos have mainly been applied to composite mateials fo stuctual health monitoing in the aeospace and wind enegy industy. The physical size of an FBG optical fibe is extemely small (26µm) compaed with taditional stain measuing systems, which enables it to be embedded into composite stuctues fo detemining the stain distibutions with minimal affect on the mechanical popeties of the host mateial (Figue 1a). The FBG senso is fomed of thee component mateials; a coe, glass cladding and potective coating (Figue 1b). The mechanical popeties of the cladding and coe ae the same, although the eflective index of the coe is slightly highe in ode to satisfy Snell s law. The potective coating is made fom an acylate o polyimide mateial and povides shielding against moistue absoption and damage. In the case of embedded optical fibes, it is geneally accepted that polyimide coating povides a bette adhesion to the host composite mateial than the standad telecommunication acylate coating [1] and a highe intefacial stength [2]. The FBG senso is a peiodic modulation in the coe of an optical fibe. The cente wavelength of the light eflected fom a Bagg gating is dependent on the efactive index of the coe and the peiodicity of the Bagg gating. FBG sensos ae sensitive to tempeatue change which affects both the efactive index and the peiodicity of the gating.

2 (a) (b) Figue 1 (a) SEM image of a sectional view of a polyimide coated FBG senso in an E- glass composite. (b) Single model fibe. Hence, it is essential to devise a method fo eliminating the affect of tempeatue. The elationship between the efactive Bagg wavelength shift ( λ B ) coesponding to the change of stain and tempeatue at the gating egion (ε g ) can be expessed as λ B /λ B =K ε g+ β T (1) Whee β = (α CTE -α n ) = Tempeatue sensitive facto α CTE = coefficients of themal expansion of the optical fibe α n = themo-optic coefficient K = stain-optic coefficient. Themosetting esins geneally shink duing cue in the ode of 1 to 8% [3], depending on the esin chemisty, which intoduces intenal stain into the laminate. It is the monitoing of this stain which povides infomation on the degee of cue of the esin and povides an oppotunity to feedback intenal cue stain data into the FE design stage of the component. The challenge in utilising FBG sensos fo the pupose of cue stain monitoing is to accuately isolate themal fluctuations fom the tue stain induced by chemical esin shinkage and themal contaction. Undestanding pocessing conditions and the intenal cuing stain of themoset composite stuctues is an impotant step in the undestanding pocessing and mateial development and stuctual optimisation. The eseach epoted hee aims to investigate expeimental methods of isolating themal influences on FBG s when embedded within a cuing themoset composite laminate. Published eseach on the use of fibe optic sensos fo cue monitoing of composites is limited and tends to focus on tansmission spectoscopy, evanescent wave spectoscopy and efactive index monitoing [4]. The use of fibe optics fo tempeatue monitoing of cuing laminates has been epoted with some success using high biefingence fibes [5], whee the tansvese stain developed duing the cue pocess of a glass fibe/epoxy esin was detected. Howeve, these optic sensos ae not commecially available.

3 EXPERIMENTAL PROCEDURE The sensos used in each expeiment consisted of FBGs with a gating length of 25mm witten onto a polyimide coated Single Mode fibe. The wavelength shift was ecoded using an Insensys thee channel OEM-13 fibe senso inteogato unit and tempeatue was ecoded using NI hadwae and Labview softwae at an acquisition fequency of 1 Hz. All heating and cuing was done on a flat aluminium tool using a silicon mat heate and PID contolle. Themal eo of ±.1ºC was ecoded fo the pocess descibed. Compaison of diffeent isolated capillay tubes Thee diffeent capillay tubes wee studied as a stain isolation media based on thei co-efficient of themal expansion, conductivity and elative diamete to the fibe optic (Table 1). FBG sensos wee encased in the capillay tubes and then sealed at both ends using a apid cuing epoxy. The isolated FBG s wee then placed on top of the heating tool with a efeence FBG senso (unpotected) and a K type themocouple. A light felt based insulating cove was then placed on top to help etain the heat. The fibes wee then heated at vaying heating ates of.5, 1 and 2ºC/min (fom 25 ºC to 1 ºC). Table 1: Geomety of the isolation mateial Isolation mateial Inne Diamete (mm) Thickness (mm ) Length (mm ) Stainless steel 16 gauge Stainless steel 25 gauge Glass capillay tube Figue 2 coss-sectional view of an isolation mateial. The glass capillay tubing showed minimal sensitivity to changes in heating ate elative to the efeence senso (Figue 3). A sinusoidal fluctuation was obseved fo all sample ates of between -3µε & -5µε afte the fist 15mins. At heating ates of 2 ºC/min o above, neithe stainless steel capillay tubes tested would be suitable as a stain isolation system due to the esidual stain obseved. Based on the low themal lag and matched coefficient of themal expansion obseved in these tests, the glass capillay tubing was used to isolate the stain effects fom the FBG when embedded within a cuing pepeg laminate.

4 Stain (µε) Tempeatue ( o C) Stainless steel 16 gauge Stainless steel 25 gauge Glass capillay.3 dia Tempeatue Figue 3 Diffeences in stain between each FBG senso isolation mateial and efeence senso at a heating ate of 2ºC/min Isolation of themal effects fom chemical shinkage Unidiectional plaques wee moulded fom two plies of heavyweight UD16gsm wind enegy gade pepeg. Two paallel FBGs (one isolated fom cuing stain by a glass tube) wee moulded in the cente of each plaque, and two plaques wee made; one whee the FBG sensos wee aligned ( o ) with the einfocement axis and the second, whee the FBG was pependicula (9 o ) to the einfocement axis. The esulting laminate was consolidated using a conventional vacuum bag pocess at 95% vacuum pessue, with a cue schedule as pe manufactue s guidelines NUMERICAL MODELLING (a) (b) Figue 4: (a) Coss-sectional view of FBGs layes in isolation mateial (b) Simplified 1D themal esistances netwok

5 The heat flow is simplified to an axysimmetic, 1-D themal esistances netwok as seen in Figue 4(b) and the heat loss fom each cylindical laye is shown below: The heat tansfe fom oute suface of isolation mateial to mid section of isolation mateial can be expessed as: ( T1 T2 ) q & 1 2 = = M 2C2T & (2) 2 R2 2D Out Out ln ln Out mid R RConv PR R 2 = ( ) + Cond ( ) = + (3) 2πhPR L 2πk L Heat tansfe fom mid-section of isolation mateial to mid-section of ai egion: ( T2 T3 ) q & 2 3 = = M 3C3T & (4) 3 R3 R R + R + R 3= Cond ( ) Conv( Ai ) Cond ( Ai ) mid 2D in Out Ai ln ln ln in in mid Ai R 3 = + + (5) 2πk L 2πh Ai L 2πk Ai L The equations fo the heat tansfe fom oute suface of isolation mateial to mid section of isolation mateial follow the same fom as (4) and (5). The isolation mateial has good bonded chaacteistics to the potective coating so we can assume equilibium condition between the layes: ε = ε = ε = ε Coe L F ε = = + α T 2 (6) L A E The equations fo ε,ε and ε Coe follow the same fom as (6) F + F+ F+ FCoe= A E A Eα T4 + A Eα T SS SS F = ASS ESS + A E + A E A E + A E α SS T2 The equations fo F, F and F Coe follow the same fom as (7) ( ) ( 5 ) ( )( ) (7) Whee the symbols epesent: Isolation Mateial Ai Ai Potective Coating Glass cladding Coe Coe In Inne suface Out Oute suface mid Mean Radius Out D Oute diamete Out

6 L α Cond ( ) Length of gating Themal expansion R Conduction esistances R Convection esistances Conv( Ai ) h Convection coefficient (hee between potective coating and glass cladding) k Conduction coefficient E Young s modulus F Foce ε Stain A Simulink model was ceated allowing equations 1-6 to be solved at diffeent tempeatues (Figue 5) and the esults compaed with the expeimental isolation mateial. Figue 5: computational logic diagams fo Simulink model RESULTS & DCUSSION Cuing of embedded FBG sensos in the einfocement fibe diection ( ). Both FBG sensos (isolated in a glass capillay tube and non-isolated) embedded paallel to the pepeg einfoced fibe diection follows the tempeatue pofile (Figue 6). Duing the amp up stage thee was a diffeence in stain of less than 1µε between the two FBG sensos embedded in the laminate. Fo the sensos pependicula to the pepeg einfocement diection (9 ), the stain eadings follow the same geneal tend as the tempeatue pofile (Figue 6b); as seen in the diection. Duing the amp up stage thee was a diffeence in stain of 25µε between the isolated and non-isolated FBG sensos embedded in the laminate, indicating an initial themal expansion of the esin and fibes.

7 The stain effects within the pepeg wee detemined by the supeposition of the isolated and non-isolated FBG fo each oientation within the pepeg laminate. (Figue 7). Themal expansion of the esin and fibes was detected in both the º and 9 oientations; illustated by an initial positive stain eading. The esults show that the amount of themal expansion stain was geate in the 9 oientation compaed to the oientation. Chemical esin shinkage was obseved between 25mins and 55mins into the cue cycle in the oientation to the einfocement. Due to the difficulty faced in sepaating themal expansion effects of the fibe fom the esin shinkage, it is unclea whethe esin shinkage stats 3 mins o 42 mins into the cue cycle in the 9 oientation to the einfocement - the fibe diection of the pepeg potentially has an effect on the detection of cue shinkage. Figue 7 shows that a significantly lage amount of esin shinkage is detected in the 9 oientation whee the themal expansion effects ae lage. The isolated sensos have eliably detected the esidual compessive stain induced afte the specimen has cooled. Simila esults wee epoted by Cossby et al. [6] by using tansmission nea-ir optical fibe senso. Stain (µε) 15 1 (a) Tempeatue ( o C) Stain (µε) 15 1 (b) Tempeatue ( o C) -1 degee FBG Glass capillay.3 dia Themocouple -1 9 degeesfbg Glass capillay.3 dia Themocouple Figue 6 Compaison between two FBG sensos (non-isolated and isolated in a Ø.3mm glass capillay tube) embedded into uni-diectional E-glass pepeg (a) Paallel to the einfoced diection. (b) Pependicula to the einfoced diection Stain (µε) Tempeatue ( o C) -1 FBG stain without tempeatue ( degee) FBG stain without tempeatue (9 degee) Themocouple Figue 7 Compaison of the chemical esin shinkage detected in the einfoced diection (9 degee) and paallel to fibe diection ( degee) with the effects of tempeatue emoved.

8 Stain (µε) Glass capillay.3 dia. Simulink model Stain Themocouple Figue 8: Compaison between the nomalised expeimental isolation mateial and the Simulink model The numeical model shows good ageement with the nomalised expeimental data. Figue 8 has the potential to be used in moe complex einfocement and tempeatue Figue 6 fields. CONCLUSIONS Tempeatue ( o C) This pape has evaluated seveal expeimental methods of tempeatue isolation techniques fo cue monitoing. It was found that encasing the FBG sensos into a.3mm diamete glass capillay tube has the least effect on the FBG tempeatue detection ove that of the othe systems tialled. The esults show successful isolation of the themal and tue stain effects at a constant tempeatue egion of the cuing cycles. Futhe wok is needed to undestand the ecoded stains in the unstable cooling stage of the cue cycle. The esults showed good coelation between the Simulink model and the expeimental fibes encased in the glass capillay tube. Chemical esin shinkage was detected in both the and 9 oientations to the einfocement duing cuing. A two-fold incease in detectable stain was obseved when compaing the 9 oientation to the oientation. This is a function of the estictions ceated by the uni-diectional einfocement. ACKNOWLEDGEMENTS The authos would like to thank the A.I.R.P.O.W.E.R consotium. This poject is cofunded by the Technology Stategy Boad's Collaboative Reseach and Development pogamme, following an open competition. The Technology Stategy Boad is an executive body established by the Govenment to dive innovation. It pomotes and invests in eseach, development and the exploitation

9 of science, technology and new ideas fo the benefit of business - inceasing sustainable economic gowth in the UK and impoving quality of life. REFERENCES 1. Udd E. Fibe optic smat stuctues. John Wiley & Sons; Levin K. Intefacial stength of optical fibes. In: ICCM-12, Pais; July 5 9, Schubel, P.J., et al., Chaacteisation of Themoset Laminates fo Cosmetic Automotive Applications. Suface Chaacteisation 26, 37(1): p Jones, M., Fibe optic senso development fo eal-time in-situ epoxy cue monitoing. Jounal of Composite Mateials, 1997: p E Chehua, A.A.S., C-C Ye, S W James, I K Patidge Stain development in cuing epoxy esin and glass fibe/epoxy composites monitoed by fibe Bagg gating sensos in biefingent optical fibe. Smat Mateial and Stuctues, : p Guemes JA, Menéndez JM. Response of Bagg gating fibe-optic sensos when embedded in composite laminates. Compos Sci. Technol. 22; 62(7): p