Study of the mechanical behavior of the optical fiber by a mark-tracking method

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1 EPJ Web of Conferences 6, (2010) DOI: /epjconf/ Owned by the authors, publshed by EDP Scences, 2010 Study of the mechancal behavor of the optcal fber by a mark-trackng method V. Chean, E. Robn, R. El Abd and J-C. Sangleboeuf LARMAUR Equpe Verres ERL CNRS 6274, PPT - Bâtment 10B - Campus de Beauleu, Rennes Cedex, France Abstract. The mark-trackng method was used n the unaxal tensle test to determne the elastc propertes of optcal fbers. The mark-trackng method s based on the followup of two markers on the specmen wth the help of an mage processng technque. It allows us to determne the true stran wth respect to the small strans assumpton (e 1%) or the fnte strans (e>1%) wthout any mpact of the rgd sold movement nether pulley fber sldng on the measured stran. Optcal fbers used n ths study are commercal Verrllon sngle mode slca fbers, 125 µm n dameter wth a two layers 62.5 µm thck epoxy-acrylate polymer coatng. Both as-receved optcal fber and strpped fber were subjected to the unaxal tensle test and the cantlever beam bendng test. The strpped fber Young s modulus results under both tests were found to be very smlar. Thus, the mark-trackng method s adaptable to the tensle test of optcal fbers and the elastc behavor of both as-receved optcal fber and strpped fber s found to be lnear. Ther Young s modulus are 22GPa and 83GPa, respectvely. These results revealed that those coatngs are playng a mechancal role n the fber elongaton. 1. Introducton Besdes sgnal transmsson for telecommuncatons, fbers are used n an ncreasng number of devces and the avalablty of slca fbers enlarges the feld of the possble applcatons such as optcal fber sensors, remote chemcal analyss, thermal measurements and thermal magng, reflectometry, optcal nstrumentaton and also laser power delvery [1-2]. Most medcal applcatons of fbers, wth ther specfc requrements, are related to ths group. Moreover, optcal fbers sensors are wdely used n smart materals [3-5], for example n order to montor cvl engneerng structures to assess ther durablty. In the framework of ths study, we use composte materal embedded optcal fbers n order to mprove the use n servce of Hgh Technology Brdge. Therefore, elastc propertes have been a major consderaton n the applcaton of optcal fbers. In order to characterze the elastc propertes of optcal fber, both as-receved fber and strpped fber (strpped polymer coatng fber) were subjected to mechancal tests. In ths work, cantlever beam bendng test and unaxal tensle test were performed to determne the Young s modulus of optcal fber. Optcal fbers used n ths study are commercal Verrllon sngle mode slca fbers whch are 125 µm n dameter wth a two layers 62.5 µm thck epoxy-acrylate polymer coatng. Ths type of coatng s the most wdely used n standard optcal fber. Ths s an Open Access artcle dstrbuted under the terms of the Creatve Commons Attrbuton-Noncommercal Lcense 3.0, whch permts unrestrcted use, dstrbuton, and reproducton n any noncommercal medum, provded the orgnal work s properly cted. Artcle dsponble sur le ste ou

2 EPJ Web of Conferences The Young s modulus of strpped fber s determned by a cantlever beam test. Then, to valdate ths result some unaxal tensle tests were performed. The unaxal tensle test was usually used to characterze the mechancal behavor of optcal fber by many studes [6-7]. In ths paper, we also used ths knd of test by usng an optcal method (mark-trackng method) to measure fber elongaton nstead of usng the crosshead machne dsplacement. 2. Expermental Tests 2.1. Cantlever beam bendng test A cantlever s a beam supported only on one end as shown on fgure 1b. Ths test, thanks to strength of materals theory, leads to determne the Young s modulus of slca fber (strpped polymer coated fber) [8]. The gauge length of the specmen s 244 mm and strpped fber densty ρ s 2.15g/cm 3 (measured by a helum pycnometry). a) b) y M W N x Strpped fber F N L Fg. 1: Cantlever beam test; a) schema of cantlever beam b) cantlever beam test of strpped fber The geometry of the bended fber and the coordnate system that wll be used s on fgure 1a. Deflecton along the beam s gven by: 2 Wx 2 2 y = ( x 4Lx + 6L ) (1) 24EI π d I = d W = ρ. g. π 4 2 (2) (3) Where: E and I are respectvely the Young s modulus and the moment of nerta of the strpped fber W s the self weght load (N/mm) d s the dameter of the strpped fber and s 125µm measured by optcal mcroscopy and also SEM (Scannng Electronc Mcroscope). g s equal to 9.81 m/s p.2

3 14th Internatonal Conference on Expermental Mechancs Wth deflecton measured at the free end of the beam, the Young s modulus of a strpped polymer coatng fber s calculated as: 4 ql E = f I 8 N (4) Where f B s elastc deflecton n (mm) at pont N. The dameter of the core (wave gude) doped by Germanum s 8±1µm and s postoned on the neutral axs of the beam so that the Young s modulus obtaned s the one of slca. The deflecton measured at the end of the beam f N s 114mm correspondng to a Young s modulus of strpped polymer coatng fber E = 83 GPa, whch s dfferent from the one of slca fber E= 73GPa found n the lterature [6-7]. Therefore, to confrm ths value, a unaxal tensle test wth the attachment of mark trackng method was performed as follow Unaxal tensle test wth mark-trackng method Mark-Trackng method The mark-trackng method s the optcal technque allowed for local stran measurement [9]. For ths study, two markers are deposted, gently and carefully n order to avod the ncdental surface cracks, on the surface of specmens wth permanent marker pen. Determnaton of stran was done under the prncple of measurement mentoned above Automatc measurement devces Algorthm of the mage processng allows us to calculate the barycenter coordnates of the two markers applyng on the sample. The coordnates of the marks are gven by the followng equatons [10]: xg = yg = x ( I I ) s ( I I ) s y ( I I ) s ( I I ) s (5) Where I s the grey level of the pxel whose coordnates are (, ) x y and I s s the threshold value by whch the pxels of markers are dstngushed. As ths method s operated only n reference ( O, X, Y ), out of plan deformaton was controlled carefully by usng a mrror nclned 45 comparng to the axs of Camera CCD. Once the marks are located, the lengths between the two markers l( t ) and the stran ε ( t) by the tme (every second) are calculated followng the prncple of deformaton measurement as ths method p.3

4 EPJ Web of Conferences Prncple of deformaton measurement In fgure 2, the lne segment M 1 M 2 consdered as segment of two markers at ntal state at tme t=0 n the reference ( O, X, Y ) changes nto a segment M3 M 4 at tme t once the load s appled (collected every second). Intal state (tme t=0) Deformed state (tme t) Y Y Y 2 M 2 Y 4 M 4 Y 1 M 1 Y 3 M 3 O X 1 X 2 X O X 3 X 4 X Fg. 2: Deformaton of lne segment So the deformed length l(t) and the orgnal length L are gven by: l( t) = ( X X ) + ( Y Y ) (6) and L = ( X X ) + ( Y Y ) (7) Stran calculatons were performed wth respect to the small strans assumpton (stran 1%) or the fnte strans (stran>1%). The engneerng normal stran or engneerng extensonal stran e of the fber axally loaded s expressed as a change n the length L per unt of the orgnal length L of the lne element or fber. The normal stran s postve f the materal fbers are stretched. Thus, we have: l l( t) L l( t) e = = = 1 = λ( t) 1 (8) L L L Where λ s a stretch rato or extenson rato and t s defned as the rato between the fnal length l(t) and the ntal fbers length L. Typcal normal stran, e, s greater than 1%, thus we use the logarthm stran or true stran n ths study. The logarthm stran consdered as an ncremental stran s gven by: δl δε = L (9) p.4

5 14th Internatonal Conference on Expermental Mechancs and the logarthmc stran s obtaned by ntegratng ths ncremental stran: l ( t ) δε = L δl L l( t) ε ( t) = lg = lg ( ) L ( λ t ) (10) (11) The parameters whch are to be determned are the coordnates of M 1, M 2 (Intal state) and M 3, M 4 (deformed state). Then we can calculate the axal strans of the fber, whch can be small ( 1%) or large (>1%) Tensle Tests The montor was used to control the fber deformaton durng the test (spot dffracton or markers mages). Recordng of markers postons s done wth a CCD vdeo camera sensor and a data acquston card. Then all data nformaton was stored n a computer for stran calculatons, wth correspondng tensle forces data for stress calculatons. a) b) Tensle Machne Pulley Specmen mage presented by mrror Fber specmen Gauge length Real specmen mage Camera CCD Mrror Fg. 3: a) Data acquston system devces b) Image analyss Optcal fbers were wnd round the pulleys to perform the tensle test as shown on fgure 3. The gauge length was 0.2 m and the tests were performed at room temperature (19±1 C, 40~50% RH). The cross-head speeds were 8 mm/mn and 4mm/mn for polymer coated fber and strpped polymer coatng fber respectvely. In order to avod fbers slppng and damages on the contact area double sded tap was appled on the lteral surface of each pulley on whch tree wndngs of fber specmen were carred out. For the strpped polymer coatng fber specmen, strpped length was 2 cm ~ 3 cm n the mddle part of the specmen (polymer coated fber). The tensle strength of the optcal fber σ s calculated by: p.5

6 EPJ Web of Conferences σ = F A (12) Where: F s the measured force A s the cross-sectonal area of the optcal fber for polymer coated fber testng or crosssectonal area of glass fber for strpped polymer coatng fber. 3. Expermental results and dscusson The man purposes of these tensle tests are to fnd out Young s modulus of polymer coated fber (PCF) and strpped fber (SF), and also ts lnearty. Durng the tensle tests, none of any out of plan deformaton were notced as the stran curves of the real specmen and of the mrror-presented specmen were supermposed. As a result, the marktrackng method was avalable. Table1 and table2 revealed that Young s modulus of as-receved fber and strpped fber were found to be 22GPa and 83GPa wth an elastc lnear behavor n term of deformaton 5~ 6% and 3% respectvely as shown on fgure 4. Stress (MPa) Stran SF1 SF2 SF3 SF4 SF5 PCF1 PCF2 PCF3 PCF4 Fg. 4: Stress-stran curve of polymer coated fber (PCF) and strpped fber (SF) Ther mechancal propertes were reported and gven by table1and table2. Table 1: Mechancal property of as-receved fber N Dameter 10-3 mm Falure stran (%) Young s modulus (GPa) Average Standard devaton p.6

7 14th Internatonal Conference on Expermental Mechancs Table 2: Mechancal property of strpped fber N Dameter 10-3 mm Falure stran (%) Young s modulus(gpa) Average Standard devaton 1.01 In prevous studes, Young s modulus of slca fbers were found to be around 73GPa, whch s absolutely dfferent from the results mentoned n our study. Moreover, the stress-stran relaton of optcal fbers was frst examned by Mallnder and Proctor [11] and later by Glaesemann et al. [12]. They found that the relaton between stress and stran of an optcal fber s nonlnear. Then, the nonlnear behavor was used n the strength measurement of optcal fber by many studes [13-15]. Hence, there are two aspects to be consdered and to be dscussed. The frst aspect refers to stran calculaton. We would compared the logarthm stran n our case, wth the small assumpton stran, e, and also wth the crosshead dsplacement-characterzed stran. As the strans results vared from 3% to 6%, the strans calculated by the small assumpton, e, and the logarthm ones are very smlar. Once usng the crosshead dsplacement-characterzed strans, fgure 5 shown that the mechancal behavor of strpped fber was nonlnear, but the Young s modulus value s about only 40GPa whch was two tmes smaller than the one measured by usng the mark-trackng method. Stres (MPa) y = 83375x y = 40268x Mark-trackng Crosshead Stran Fg. 5: Stress-stran curves of strpped fber specmen n 4 The second aspect refers to the Young s modulus aspect. We found that Young s modulus of the strpped fber s 83GPa whch seems to be a hgh value compared to the one of fused slca fber p.7

8 EPJ Web of Conferences usually about 73GPa whose behavor was nonlnear. Optcal fbers used for ths study are commercal Verrllon sngle mode slca fbers whch may be dfferent from the other commercal fbers. Thus, ths hgh value of the Young s modulus and the lnear relaton between stress and stran mght be caused by the concentraton of fluorde n ths optcal fber claddng. Consequently, we wll do some addtonal tests to observe the lnearty of the behavor on other standard commercal optcal fbers whose Young s modulus s 73GPa. On the part of as-receved fbers, the fbers were consdered as a composte materal whose Young s modulus could be calculated by respectng the mxture model of Vogt. Ths Young s modulus was fnally confrmed by expermental tests n ths study. 4. Concluson and perspectve The cantlever beam bendng test s an effectve test allowng a rapd measurement of the Young s modulus of the fber. The tensle test provdes not only the Young s modulus, but also ts lnearty presented by the stress-stran curves. Wth the attachment of the mark-trackng method n the tensle test, we proved that the Young s modulus of the as-receved fber and the strpped one are dfferently about 22GPa and 83GPa wth an elastc lnear behavor n term of deformaton 5~ 6% and 3% respectvely. The Young s modulus of the strpped fber s the same usng cantlever beam bendng test or unaxal tensle test. These results pont out those coatngs playng a mechancal role n the fber elongaton. Thus, the mark-trackng method s fnally the potental and avalable technque for optcal fber mechancal propertes measurements. Accordng to the authors, other knd of commercal fbers wll be tested wth the same expermental protocol test n order to study the lnear or nonlnear behavor n the stress-stran relaton. References 1. I.B. Kwon, M.Y. Cho, H. Moon, Sensors and Actuators A 112, 8 (2004) 2. B. Lee, Optcal Fber Technology 9, 23 (2003) 3. M.V. Gandh, B.S. Thompson, Smart Materals And Structure, CHAPMAN & HALL, 309 (1992) 4. C.S. Shn, C.C. Chang, Internaton Journal of Fatgue 28, 7 (2006) 5. M. Majumder, T.K. Gangopadhyay, A.K. Chakraborty, Sensors and Actuators A 147, 15 (2008) 6. B. Delobelle, F. Thebaud, D. Chappelle, D. Perreux, V. Placet, R. Ferrère, Comptes Rendus des JNC 16-Toulouse, 10 (2009) 7. C.P. Chen, T.H. Chang, Materals Chemstry and Physcs 77, 7 (2002) 8. J.L. Fanchon, Gude de MECANIQUE scence et technologes ndustrelles, Edtons Natan, 480 (1996) 9. F. Brémand, J.-C. Dupré, Lagarde, Proc. Photomécanque 95 Etude du comportement des matéraux et des structures, 7 (1995) 10. R. Rotnat, R. Té b,v. Valle, J.-C. Dupré, Stran No.3 Vol.37, 10 (2001) 11. E.P. Mallnder, B.A. Proctor, Physs and chemstry of glass 5, 13 (1964) 12. G.S. Glaesemann, S. T. Gulat, J. D. Helfnstne, Tech.Dg.-11 th opt. Fber Comm. conf., TUG5, (1988) 13. W. Grffoen, J. Am. Ceram. Soc., 75, 5 (1992) 14. M. John Matthewson, Proceedng of a conference held 8-9 Septemberboston, Massahusettes, 28 (1994) 15. Prabhat K. Gupta, Charles R. Kurkjan, Journal of Non-Crystallne Solds 351, 5 (2005) p.8