Two Sorts of Fiber Optic Sensor Monitoring the Cure ...

High Technology Let ters , Vol . 6 , No . 2 , April .2000 　

Two Sorts of Fiber Optic Sensor Monitoring the Cure Process of Composite Laminate Li Chensha(李辰砂), Z hang Boming , Wang Dianfu , Du Shanyi ( Center fo r Composite M aterials , Harbin Insti tute of Technology , Harbin 150001 , P . R. China) Abstract T w o new sorts of fiber optic sensor are designed .Firstly , the variation of ref ractive index of resin surrounding t he embedded fiber optic w as mean to observed by measuring t he speckle spatial spect rum at the end-f ace of the fiber optic .Secondly , i t is proposed to measuring the change of t hickness of the laminate by using fiber optic microbend attenuat ion .T he experiment results measured by the fiber optic sensors are given .T he f irst so rt of sensor w as found to be able to detect t he viscosi ty process of t he resin matrix during cure process includeing the minimum point of viscosi ty , t he gelation point and the end point of cure process ;the seco nd sort of senso r could detect t he course of change of the thickness of composite laminate pressed by operation pressure . It is benef it for us to optimize the operation technology and t o est ablish the intelligent monitoring system about curing process of composites for t he reason t hat the viscosi ty process and the change course of the thickness of laminate can be monito red . Key words :Composites , Cure , Fiber optic senso r , Speckle , Microbend

ing system of composite is show n in figure 1 . 0 　Introduction Advanced composite materials are being incorporated in increasing amounts in combat and tactial vehicle systems , launch vehicles , space platforms , composite w ings and other primary and secondary structures in aircraft , the automotive industry , and civil engineering applications .parts and structures constructed from continuous fiber reinforced thermosetting resin composites are m anufactured by arranging the uncured fiber-resin mixture (prepreg) into the desired shape and then curing the m aterial inside the autoclave .T he curing process is accom plished by exposing the m aterial to elevated tem peratures and pressures for a predetermined length of time . T he elevated temperatures applied during the curing provide the heat required for initiating and m aintaining the chemical reactions in the resin ;T he applied pressure cause the force needed to compress the laminate and squeeze excess resin out of the m aterialo , to consolidate the lamination [ 1] structure, and to compress voids . T he cur-

Fig . 1 　T he curing system of composite

T he cure cycle has a significant effect on the quality of the finished part .Some m ajor considerations in selecting the proper cure for a given composite material include the uniform temperature distribution and the lim ited highest temperature during the curing process , the minized voids and residual st resses of the finished product[ 1] .some other specifications about the operation approach are as follow s : Starting curing process , the autoclave temperature is elevated at a predeterminated rate . In the early stage , the viscosity of resin goes dow n to the nadir w ith tem perature rising .T he viscosity will go up after the nadir as

S upport ed by the Commissi on of National Def ence Science and Indust ry of China. Received N ov. 18 , 1999 .

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High T echnology Let ters , Vol . 6 , No . 2 , April .2000

the starting of chem ical reactions w hich cause the crosslinking changes in the m olecular structure .the viscosity w ill significantly rise nearby the gelatinization point at w hich the m aterial becomes so viscous that resin cannot flow under the applied pressure . During cure it m ust be ensured that all the plies are fully com pacted before the gelatinization .T o assure proper volum e ratio of resin and fiber as well as the uniform distribution of the inner pressure , it is demanded to add pressure nearby the nadir of viscosity for the reason that the resin has a highly flow behaviour to facilitate the compaction .To prevent the rapid drop , followed by a rapid rise in viscosity so that there is plenty of time for the laminate to be completely com pacted , it is needed to keep the viscosity nearly constant at the low est level by maintaining the autoclave temperature tem porarily constant (that is called intermediate tem perature constant stage)from the start of the compression until compaction is completed .Once compaction is completed , the temperature inside the autoclave should be elevated again to the designed highest value w hich is named curing temperature to accelerate the chemical reactions and raise the viscosity in the resin m atrix in order to avoid the resin being excessively lost .When the chemical reactions have been on the point of end , the cure operation should be stopped so that the cure time is shorten and the production cost is saved . Because the processes of curing inside the autoclave can not be directly observed , the variables such as the viscosity and the cure deg ree of resin matrix or the thickness of the laminate at every time etc are difficult to be guessed .T herefore , it is difficult for us to adjust the autoclave temperature and the pressure during the cure process . T he traditional technologies based on empirical approach are undesirable in that they can not ensure the qualities of the products and the efficiency of production . T he operation condition can be effectively — 62 —

cont rolled if every variables of the cure system are monitored by sensors fixed in the cure system .T he general gauge of m onitoring the change process of viscosity of resin m atrix is dielectric sensor[ 2] , but this sort of sensor is undesirable for its high cost and inferior capability w ith the monitored material . T he developed thickness gauges have shortingcomings of structural com plication and more disturbing to normal cure process[ 1] . 1 　The Principles of Two Sort of Fiber Optic Optical fiber sensors become the preference of sensors used in smart technologies and structures for their superior sensing perform ance and compatibility w ith matrix[ 3] . In this paper, we describe the development of a realtime monitoring technique based on two sorts of special m anufactured optical fiber sensor which are called the speckle spatial spectrum (SSS)fiber optic sensor and the microbend attenuation fiber optic sensor .By m easuring the speckle spatial spectrum at the end of the fiber optic, the refractive index of the resin w hich reflects the viscosity variation of the resin during cure process is obtained .On the other hand , the compression process of the composite laminate caused by the applied pressure is reflected by measuring the microbend attenuation of the light transmitted through the fiber optic em bed the com posite laminate .these methods can indicate the w hole curing process including the nadir of the viscosity , the gelation point , the end point of the cure process and can indicate the process of thickness changing w ith time after the pressure is applied .If these key points are determinated , the operation processes of autoclave temperature and pressure w ill be rightly executed . Moreover , our work lay a foundation for applying the fiber optic sensors in intelligent online system w hich can real time monitor and control the curing process of composites , then the aims of ensuring the qualities of products and elevating the efficiency of the production will

High Technology Let ters , Vol . 6 , No . 2 , April .2000

be realized . After the end of cure , the fiber optic maintains the performance of optical waveguide w hich can be used in sensing vibration , stress and fatigue etc during service of composites . 1.1 　FO Sensor Measuring Resin' s Ref ractive Index As resin cure , the variations of viscosity , density and components of the resin m atrix w ill change the ref ractive index of the resin m atrix .T he index of refraction of m ajority therm osetting resin system s increases in the range of 0 . 01 ～ 0 . 04 f rom initial uncured resin to final cured products .Several authors have speculated that there is close correlation betw een the gain in refractive index and the cure [ 4 -6] deg ree of the resin . T he ref ractive index of a given m aterial depends on the types of m olecular bonds it contains and its density .If a m aterial consists of several different types of m olecular bonds , as is often the case of polymeric materials , the ref ractive index depends on the weighted average of the molar refractivity of each bond type .In terms of Lorentz-Lorenz formula , the molar refractivity of one kind of molecular bond is w ritten as : n 2 -1 R =(N m / N) 2 (1) n +2 W here N is the num ber of m olecules per unit volume , N m is Avogadro' s num ber , n is the refractive index .Let N m / N =D , n can be described as : D +2R (2) D -R T he molar refractivity R depends on the w eighted average of each bond type for the m aterials w hich consist of several different types of m olecular bond .Af ter the starting of cure reaction , the types of molecular bond and their ratio in all types of molecular bond contained in the resin are changing as the cure deg ree increases , it certainly m ake the refractive index of resin matrix changes .T he density D also influences the ref ractive index , n =

as show n in eq. (2).M any polymers can exist in either amorphous form or crystalline form , the gelation point at w hich the resin transforms from amorphousness to crystalline during cure process can be observed as an abrupt change of the refractive index of the resin for the reason that the density changes obviously near the gelation .With regard to the polymer fluids , the change of density also indicates the f ree-volume expanding or lessening among the molecular bonds , w hich can reflect low ering or increasing of viscosity of the materials .T herefore , the viscosity of resin can also be reflected by the refractive index . T he FO speckle spatial spectrum (SSS) sensor is based on the special design of the optical fiber .T he fiber core is made of the glass w hose refractive index (n 1)is a little higher than that of the resin matix being monitored (n p).A fiber optic w ith a short segment coat wiped off is embedded between any tw o bordered plies of the uncured composite (prepreg).Under the application of a coherent light , a single multim ode fiber becomes a interferometer w hich has many arms , all coexisting as propagation m odes . W hen the output from the fiber optic is projected upon a screen , a circular pattern consisting of a very large num ber of “speckles” of varying intensities is observed . The distribution of these speckles depends on the angle of incidence , numerical aperture of the fiber and the perturbation of the fiber . Assuming that nonleaky modes are equally excited , the speckle spatial pow er spectrum [ 7] can be described as : GIP(v)= A[ GI(ρ)] ∞

ρ GI(ρ )

∫ ρ -v dρ

≡2

v

2

2

(3)

Where A[ ] denote an Abel t ransform , GI(ρ ) can be expressed w ith the so-called Chinese hat function : GI(ρ )=chat(ρ / ρ0) 2 2 　　 ≡ cos-1 ρ / ρ0 -ρ /ρ 1 -(ρ / ρ0) , 0 π 　　　　ρ≤ ρ0 (4) — 63 —


W here υ is spatial frequency , ρ is integration variable , ρ0 = (sin(2θm ))/ λ, θ m = π/ 2 - sin-1(n 1 2 - n 22), λ is the w aveleng th of light . As a segment of the embed fiber optic is w ipped off the coat , the naked core contacts the surrounding resin directly .the varying refractive index of the resin m atrix during cure process changes the numerical aperturc of the fiber optic , and further changes the speckle spatial pow er spectrums at the output end of the fiber optic, thus the curing process can be m onitored. In this paper , the core ref ractive index n 1 w as chosen to be 1 . 60 or 1 . 62 , the normalized speckle spatial spect rums w ere calculated w ith the clad refractive index n 2 = 1. 54 , 1 . 56 and 1 . 58 , which are the variation range of the refractive index of resin m atrix during cure process . The output results show that the norm alized speckle spatial spectrums should change obviously with variation of the refractive index of resin in cure process .T he variation of the normalized speckle spatial spectrum s w ill not be obviously if the core refractive index n 1 w as chosen to be 1 . 65 or 1. 67 which are excessively higher than that of the resin being monitored . T he results of theoretic analysis have been verified by the experim ent carried out by the co-worker of substituting the resin by the m atching liquid w ith similar refractive index , and the relationship betw een cutoff frequency of speckle spatial spectrums and the ref ractive index is fitted nearly linear as follow s[ 8] : n 2 =-0 . 0700v c +1 . 6210 (5) T herefore , the absulate value of the ref ractive index of resin m atrix can be calculated by processing the output data from the fiber ptic. 1.2 　 FO Microbend Sensor to Measure Thickness Change of the L aminate In optical fiber , the intensity of the transmitted light is lost in response to the microbends w hich are some minor bends dis— 64 —

tributed on the fiber without modifying the [ 9 , 10] macro geom etry of the fiber .T his loss occurs most f requently w hen the highest-order mode in the fiber core is coupled to the first cladding (radiation)m ode , then is rapidly attenuated .T his coupling can be induced by environmental effects , such as temperature , pressure , impact , or acoustic waves w hich creat structural strain .M echanical coupling of structural changes toward an embedded fiber optic modifies the micro geometry of the fiber optic and allow s light to be coupled between the modes , thus modulates the light intensity transmitting through the fiber optic . T he fibre bundle contained in the com posite laminate has a stationary spatial periodicity w hich can cause microbend of the fiber optic and lead the attenuation of the light transmitted through the fiber optic .T his kind of intrinsic m icrobend assembly may be employed in smart structures for its much compact internal structure[ 9 , 10] . T he considerated sensors include tw o aspects :the fiber m icrobending sensitivity , represented by ΔT / ΔX , and the m echanical design of the device , associated with ΔX/ ΔF . ΔT ΔT ΔX = (6) ΔF ΔX ΔF where T is the fiber transmission , F is the applied force , and X is the deformation of fiber normal to its axis .T he w ays of elevating the Sensitivity of m icrobending , or elevating the item of ΔT / ΔX , are decreaseing the value of num erical aperture , decreasing the number of m icrobend points and increaseing the disturbing periodicity[ 9] . One example of the influences on the total sensitivity of the microbend sensor is given by the length of the fiber optic .It has been found that the microbend sensitivity of the sensor ΔT q ∝l (7) ΔX W here l is the length of the microbend fiber , and 0 ≤q ≤1 .A fiber w ith a perfectly absorbent jacket w ould have q =1 and a sensitivity that depends linearly upon the sensing length[ 9] .


2 　The Experiment of Monitoring Cure Process of Composites T he speckle spect rum fiber optic sensing system s is show n in fig . 2 .The diameter of optical fiber is chosen to be 50 ～ 100 μm to ensure enough modes , the refractive index of the core is 1 . 62 .A segment of the fiber (about 20mm)w hose coat is wipped off acts as sensor .T he fiber optic including the sensor part is embed between any two bordered plies near the middle plies with tw o ends outside of the composite laminate acting as the ports by w hich the light transmits through the fiber optic .

T he fiber optic with asegment naked core is embed between two bordered plies near the middle of the composite laminate with tw o ends outside of the laminate .T he light transmits through the optical fiber from one end to another end .T he bare segment of the optical fiber surrounded by the resin matrix just likes black painted clad so as to have q ≈1 , w hich means highly microbending sensitivity tow ards monitoring the com pression of laminate caused by external pressure .

Fig . 3 　 Int rinsic fiber optic microbend sensor

F ig . 2 　System of FO Speckle Spect rum(SSS)collection experiment

T he microbend fiber optic sensing system is show n in fig . 3. It has been found that the m icrobend sensitivity of telecom munication optical fibers is low for the reason that the antidam ping quality of general fiber optics is emphasized in order to transmit signals without losting .Contrary , it is demanded that the microbend fiber optic applied in monitoring the compression process of the laminate should has the perform ance of m aking the transmitted light attenuated in a certain ex tent .In our experiment , specially m anufactured PCS glass fiber optic is em ployed .In order to ensure the definite sensitivity , the selected core radius is 100μm and clad radius is 150μm .The coat of a short segment optical fiber(about 20m m)is also w ipped off to strengthen the sensitivity .

T he polymer coats of tw o sorts of fiber optic have high capabilities with the materials and are easy wiped off by chemistry m ethod . Single m ode He-Ne laser with high coherence is used as light source , a CCD cam era is used to gather speckle distribution and a silicon photodiode is used to detect microbend attenuation .The datum gathered at any time are automatically imported to the computer and immediately processed by codes , thus the refractive indexs of the resin and the attenuations of the transmitted light can be derived in real time .T he composite laminate containing the fiber optic sensor are cured on a hotpress . T he SSS signal recorded during curing of pure bism aleimide (BM I ) resin and carbon fiber reinforced/ BM I resin mat rix prepreg are show n in fig . 4 and fig . 5 respectly .T emperature-time profiles of the cure cycles are also show n.Actually the temperature is fluctuated about 5 ℃ due to temperature controlled system . — 65 —

High T echnology Let ters , Vol . 6 , No . 2 , April .2000 　　

the cure process more distinctly , every data point in fig . 5 are replaced by the mean value of this point and a few neighboring points . T hen one order derivative is calculated , as show n in fig .6 .W hen one order derivative reaches the maximum value , resin has gelatinized .Cure process is indicated complete when one order derivative keep fluctuating near zero in the post period of cure process .

F ig . 4 　SSS signal monito ring t he mould curing w ith pure BM I-Ⅰ resin

T he results of the experiments are discussed as follow .As the reactions have not started in the early stage of cure process , the viscosity of resin goes dow n to the m inimum point with temperature rising , thus the refractive index of the resin goes dow n to the m inim um point a .After then the refractive index rising w ith the rising of the viscosity due to the starting of reactions of cross-link in the resin . T he gel point b can be determinated in the curve because the refractive index should rise sharply near the gelation point .After the gel point , the refractive index rem ains in rising because the types of m olecular bonds continuously change with the developing of cure reactions .T he cure rate declines up to 0 w hen the process of cure reaction attains the termination , thus the varing rate of refractive index declines and the curves of refractive index drive to being constant near the end point of cure reaction c , then the end time of cure reaction can also be determinated .Fig .4 is the m onitored results of pure BM I resin and fig . 5 is that of carbon fiber reinforced/ BM I resin m atrix composite , but they are m uch coincident .This situation makes out that the fiber optic sensors have the right capabilities w ith the composites in our experiment that can exactly measure the param eters of resin m atrix w ithout being influenced by the reinfoced fiber bundles . In order to present the variation w ithin — 66 —

Fig . 5 　SSS signal monit oring t he mould curing w ith BM I-Ⅰ/ T 300 prepreg

Fig . 6 　T he processed results of monit ored data of BM I-Ⅰ/ T 300 prepreg

In order to verify the relability of our experiment , the experiment results of SSS are compared with the results m onitored by dielectric sensor w hose principle is based on the fact that the ionic conductivity of resin m atrix changes w ith the viscosity of resin as the process of cure reaction and can also m onitor the minimum point of viscosity of resin , the gela[ 2] tion point and the end tim e of cure process . T he curve derived from the monitored results of carbon fiber reinforced/ bismaleimide(BM I)


resin matrix composite by dielect ric sensor is show n in fig .7 , which includes the m inimum point of viscosity , the gelation point and the end point of cure reaction .We can conclude by comparing fig . 6 w ith fig . 7 that the trends of two curves of the same kind of com posite monitored by different sorts of sensor are almost same .T he heating-up rate in fig . 7 is less than that in fig . 6 , thus the cure process show n in fig . 7 logs behind the cure process show n in fig . 6 , Certainly the m inimum viscosity time , gelation time and cure end time in fig . 7 log behind the corresponding points in fig . 6.

F ig . 7 　Ex periment results of dielectric monito ring cure process of BM I-Ⅰ/ T 300 prepreg

T he m icrobend fiber optic sensor can m onitor the changing process of thickness of composite laminate caused by external pressure w hen the resin matrix is at the fluidity stage . In terms of the model about resin flowing through laminates presented by Gutowski[ 11] , T he uncured composite w ith viscous resin m atrix can be modeled as a porous , nonlinear elastic media that is filled with a viscous liquid . W hen pressure is applied , it m ust be shared by both the resin and the fiber structure . In terms of the experiment results of microbend of fig . 8 .Initially , as the viscosity is lowered and the fluidity of resin is enhanced , the force of the plies acting on the fiber optic is w eaken.T hus the m icrobend of fiber optic should be w eaken , the pow er attenuation of

transmitted lights also be faded .The loss in resin caused by pressure gradient w ill allow the composite to com pact , thereby compressing the fiber netw ork , elastic deformation occurs in the fiber bundles and part of the applied load is carried by the fibers .As a result of this , microbend of the fiber optic can be reinforced by the elastic deformation of the fiber network , thus the pow er attenuation is enhanced .The finial fiber volume fraction of the com posite is determined by the deformation characteristics of the fiber strcture .W hen the cm posite laminate is com pacted , the microbend attenuation attains m aximum without varying .T hus the time at which com paction is achieved should be correctly judged from the curve of m icrobend attenuation . It is found by multi experim ents that the measured results w ill be distinct if the fiber optic is intersected with the fiber boundles of the adjacent plies .T he reason of this situation is that the fiber optic across the fiber bundles of the plies can be strongly supported by the fiber bundles , and thus the microbend is easily formed.One of the best measured results is show n in fig .8 , the applied pressure is 0.1M Pa ;the layup of the plies is 0/ 0/ 0/ 90/ 90/ FO/ 90/ 90/ 0/ 0/ 0 ;fiber optic is em bed betw een the tw o middle plies and is perpendiculared w ith the fiber bundles of the tw o adjacented plies .Gutow ski has found by compuation and experiment that under the external pressure , the deform ation of fiber bundles of composite is enhanced slow ly in the beginning [ 11] but subsequently enhanced fastly . T his regular parten w hould lead the phenomenon that the microbend attenuation enhances relatively slow ly after initial decreasing process , then enhances fastly , as show n in fig .8 .W hen the composite laminate is nearly com pacted , the varing rate of microbend attenuation declines to 0 because of the ending of deform ation of fiber bundles .T hus the points at which the compression of lam inate is started and completed can be determinated from fig . 8 .T he sm all peak value of the curve in the early stage is — 67 —


caused by heat expansion of the plies .the fluctuations occured in anaphase are caused by the fluctuation of tem perature control system .

F ig . 8 　Ex periment results of micobend sensor for BMI-I/ T 300 prepreg

3 　Conclusion Smart materials and structures containing fiber optic sensors are rapidly developed and highly regarded in the fields of aircraft , astronayigation etc since these years .According to the latest tendency of sm art technologies and structures , w e have system atically explored the technology of monitoring the parameters by using multimode fiber optic , and found that multimode fiber optic sensors can be applied in monitoring cure process of composite .The apparatus w hich are employed in our experimenrts are desirable in sensitivity and realibility , sim ple in structure and econom ic in cost . In this paper , two specially manufactured types of fiber optic sensor are presented .T he viscosity process including the nadir of viscosity , the gelation point and the end time of cure process can be m onitored by speckle spatial spectrum (SSS)fiber optic .T he compression process caused by external pressure , incuding starting point and ending point of compression of laminate can be monitored by microbend attenuation fiber optic sensor . The monitored results w ere found to be accurately repetitive by several exeriments .T herefore , w e can determinate the time at w hich intermediate constant temperature stage begins and the time at — 68 —

which operation pressure is applied according to the early declining process of viscosity of resin mat rix ;T he applied pressure can be adjusted according to the change rate of thickness of laminate and the tem perature can be secondary elevated on time according to the monitored compaction point of laminate so as to avoid the resin being excessively lost .M oreover, w hen the end point of cure process is monitored , the cure operation can be terminated on tim e so that the cure cycle can be saved . If these key operation steps are effectively controled , the quality of products whould be ensured and the production costs w hould be saved , as be expatiated in front . Moreover , our w ork lay a foundation for applying the fiber optic sensors in intelligent online system w hich can real time m onitor and feedback control the curing process of composites[ 12] .Every variables of the composite can be gathered by fiber optic sensors every several seconds and processed by com puter , then the corresponding operating orders established according to the tendencies of variables are feed back to the curing system by the intelligent online monitoring system .T hus the operation conditions at every intervals can be autom atically adjusted to make the curing operation proceeding along the optimum course . In this paper , the tw o sorts of fiber optic sensor are experim entized apartly , but they can be set ted in the same cure system and monitor the different variables simulately . F urthermore , the laser light emited from the light source can be split into tw o beams and be rip into two fiber optics separately without employing tw o lasers .It is also found that the measured results have nothing to do w ith the transmitted light intensities .therefore it is unnecessary to set reference light to com pensate the light fluctuation of the transmitted light . On the other hand , the monitoring system employed in our experiment has the design features as follows :A general laser diode is employed as light source .the parts of gathering and processing signals can be integrated based


on chip microcom puter .T he fiber optic links at the inlet port and the outlet port of the composite laminate can be realized by connectionpeg and pinboard .Thus the total reability of the monitoring will be greatly elevated and the investment will be saved . After the end of cure , the fiber optic m aintains the performance of optical w aveguide w hich can be used in sensing vibration , stress and fatigue etc during service of compos[ 13] ites .T herefore , fiber optic sensors have m ultiuses of monitoring the parameters of the composites from producing stage to service stage that other sensors can not possess . References [ 1] Ciriscioli P R , Springer G S .SmartAutoclave Cure of Composites, L ancaster , PA :T echnomic Publishing Co , Inc .1990 [ 2] Ciriscioli P R , Springer G S .S AMPE Journal , 1989 , 25 :35 [ 3] Eric Udd .Overview of f iber optic smart structure f or aerospace application .In :Fiber Optic Smart Structures And Skins I , E .Udd , ed . SPIE .1988 , 986 , 2 [ 4] Milkovich S M , et .al .In Situ Sensors fo r Intelligent P rocess Cont rol fo r Fabrication of Polymer-Matrix Composite M aterials , Sm art Sensing .In :Processing , and Inst rumentation I I , James S .Si rkis , ed .SPIE .1994 , 2191 , 349

[ 5] M ay R G , et .al .Combined fiber optic strain sensor and composite cure monitor fo r smart st ruct ure applications , Smart Sensing .In :Processing , and Inst rumentation II , James S . Sirkis , ed .SP IE .1994 , 2191 , 46 [ 6] Kai-Yuen Lam , Af romowit z M A .Cure Sensor Appl .Opt . , 1995 , 34 :5635 [ 7] Raw son E G , Goodman J W , Norton R E .J . Opt .Soc .Am . , 1980 , 70 : 968 [ 8] Z hang N M .High Technology L etters , 1998 , 8 (10):35 [ 9] Tim Clark , Herb Smit h .Microbend fiber optic sensors (Eric Udd ed).John Wiley & Sons, Inc . , 1994 , 319 [ 10] Suopajarvi P , et al .Opt ical Engi neeri ng , 1995 , 34(9), 2587 [ 11] Gutow ski T G , Mo rigaki T , Cai A .Journal of Composite Materials , 1987 , 21 : 172 [ 12] Ciriscioli P R, George S .Journal of Composi te Materials , 1991 , 25(10): 1542 [ 13] Spillman W B , Jr .Et .al .Fiber optic vibrat ion senso rs for structural cont rol applications .In : P roc .Damping' 89 Conference , 1989 , Vol . Ⅲ , ICA1 -ICA21 Li Chensha , bo rn in 1969 , g raduated f rom Xi′ an Jiaoto ng University in 1991 .He received his Ph . D .deg ree from Harbin Institute of T echnology in 1999 .Now he w orks as a post-doctor in T singhua Universi ty .His main research interests is smart material sy stem and st ructure .

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