Development of Fiber Optic Ingress/Egress Methods. For Smart Composite Structures

Transcription

1 Development of Fiber Optic Ingress/Egress Methods For Smart Composite Structures H K Kang, J W Park, C Y Ryu, C S Hong and C G Kim Department of Aerospace Engineering Korea Advanced Institute of Science and Technology Kusong-dong, Yusong-gu, Taejon , Korea To whom correspondence should be addressed. Tel : , Fax : , cshong@kaist.ac.kr

2 Abstract The fiber optic ingress/egress is one of the important issues for the application of fiber optic sensors to real structures because the optical fiber is so fragile at the ingress/egress point. Little attention has been paid to methods of practically accessing embedded optical fibers in the literature of sensor activities. In this paper, we proposed the novel out-of-plane ingress/egress method to improve the limit of the existing inplane ingress/egress method. For the safety of the embedded optical fiber, we developed the component to protect the optical fiber at the ingress/egress point and the preservation method of an optical fiber against the composite curing environment of high temperature and pressure. And we also developed an optical connection fixture to integrate the connection part and a composite structure. Some tests were performed to confirm the safety and survivability of the embedded optical fiber using the devised ingress/egress method. 2

3 1. Introduction External effects, such as excessive load above design limit, impact and fatigue, can damage aerospace composite structures. The defects due to damage may degrade the structural performance and integrity of composite structures. For assuring the structural integrity, it is desirable to monitor the health of composite structures in real-time. Fiber optic sensors have many advantages in that they are easy to be embedded in composites, they are very sensitive, and they can give a lot of information from one probe. As rapid progress of fiber optic sensors has been made, the potentiality of wide applications has been reported to measure various physical parameters from strain to magnetic field [1] using them. Many researches for the structural health monitoring have been performed using fiber optic sensors [2-7]. It is essential to have the durable sensor-system for the real-time health monitoring of structures during both the fabrication and in-service. Most researches on sensor development just focus on the development of sensing schemes. Little attention, however, has been paid to methods of practically accessing the embedded optical fiber in the literature of sensor activities. For the field applications of optical fiber sensors, the practical researches, such as sensor installation, mechanical properties of sensor, and ingress/egress to connect with an exterior system, etc, should be studied. Spillman et al [8] dealt with general issues on the fiber optic ingress/egress methods in detail. In this literature, however, they only reported the in-plane ingress/egress methods and suggested the concept of the out-of-plane method. In some papers, only the concepts of fiber optic ingress/egress were introduced. Little work has been carried out to consider actual problems of ingress/egress in fiber optic sensor 3

4 systems. Previous researches have generally been carried out using only single composite panels or beam elements. For the wide application of optical fiber sensor system it is important to efficiently access the embedded optical fiber at ingress/egress point of the fiber from the host structures. It is of importance how to configure the individual structural elements to maintain the sensing capability of the smart structure when certain elements are to be replaced due to wear or damage. These problems must be solved for applying the fiber optic sensor to a smart structure. The laboratory level demonstrations of the combination of a composite structure and fiber optic sensor technology have shown that a short length of fiber is embedded into the composite and later connected to longer fiber using any of many splicing techniques. These schemes are not robust enough for field use because the spliced parts may be easily damaged by harsh environment [8]. In this paper, we examined the previously reported ingress/egress methods and their problems. The novel ingress/egress method is demonstrated to overcome the limitations of the conventional ingress/egress method. Smart composite structural elements with embedded fiber optic sensors were fabricated using the devised ingress/egress method in this study. And the safety and survivability of the embedded optical fiber were confirmed by tests. 4

5 2. Survey of previous ingress/egress methods and their problems The fiber optic ingress/egress method is one of the important issues for the construction of smart structures with embedded fiber optic sensors. Optical fibers are very fragile when they are extracted outer and connected with the exterior system. One of the advantages of optical fiber is to easily embed into the composite structure. In general, it is a simple process to manufacture a composite part embedding an optical fiber. In machining and connecting work after the cure, however, there would be several problems. Firstly, the optical fiber is vulnerable and fragile at the edge of a composite part where the entry and exit points are located. Secondly, the existence of embedded fibers causes difficulties to machine the composite laminate for the designed dimension. Thirdly, it is difficult to connect an embedded optical fiber with an adjacent part or an exterior system. The in-plane ingress/egress method is simple to use in laboratories. In this method, the ingress/egress point of an optical fiber is located at the edge of a composite laminate. In this case, a major problem exists at the ingress/egress point of the optical fiber in the edges of the composite laminate. At this point, the optical fiber is vulnerable and fragile in fabricating process of a composite part so that breakage and damage of an optical fiber frequently occurs during separation of the composite laminate from the curing tool and handling of the laminate. During the heating period in curing process, the viscosity of composite matrix resin decreases and the resin, which is not absorbed to bleeders and peel-plies, flows over the optical fiber out of a laminate area. And during the cooling period, the resin becomes hard and makes the optical fiber brittle. Moreover, the excessive resin bonds the optical fiber to curing tools such as caul plates. Therefore, it is 5

6 very difficult to safely separate and handle the laminate without breakage and damage of the optical fiber since the accretion of the resin causes the fragility of the optical fiber. Several techniques were suggested to overcome this problem. Green et al [9] used embedded thermoplastic sleeves and locally laminated restricted flow film adhesives for resin flow control at the laminate edges. Hong et al [10] used several types of tubes for the reinforcement of the optical fiber at the ingress/egress point as shown in Figure 1. These methods protected the fiber from both resin flow and local stress concentration during curing, and relieved strain at ingress/egress points during handling in subsequent laboratory testing. However, these methods would be impractical for realistic in-service application since size adjusting and machining work needed extreme precaution as mentioned before. The splicing region where the embedded fiber was connected to a longer exterior fiber was not robust. And it is found that, even with close fitting shrunkon tubing, the low viscosity resin could easily flowed into the gap between the sleeve and the fiber and made the fiber brittle after cure. The method using edge connector [9], which contained a precision ceramic ferrule, was developed to remove the fiber hardening at the ingress/egress point and to connect easily. In the method, two additional aligning studs should be embedded with the ferrule to align and set the embedded ferrule and a standard fiber optic connector to be connected. A precision drilled tooling block was necessary to prevent the resin flow contamination of the polished ferrule end. This method is not also appropriate for real production environment since the edges of the most of composite parts are trimmed and shaped after cure. Therefore, connection methods that have ingress/egress points away from the laminate edge should be devised for robust fiber connection. One of them is the out-of-plane ingress/egress method as shown in Figure 2. Green et al [11] demonstrated the surface mounted connectors as an out-of-plane 6

7 ingress/egress method. In the method, an AMP Fiber Optic Backplane Connector was used which consists of daughter card built-in ferrule and mother board that were used in electronic equipments. By using this method, the edge trimming and size adjusting problems were solved, but the bonding of an optical fiber to the ferrule and polishing of the ferrule end surface were complicated and time consuming jobs. We performed the preliminary work for the out-of-plane ingress/egress method as shown in Figure 3. In the work, we made an optical cable pigtail using an optical cable with an optical connector. The fiber optic sensor was spliced to the end of the optical cable and embedded in the composite laminate. Throughout this study, we have come to understand two problems that must be improved. First, the optical fiber (cable) near the ingress/egress point was structurally so weak that the optical fiber was often broken at this point. Therefore, this point must be reinforced. Second, the exposed optical fiber to the outside of a composite laminate must be safely preserved not to be damaged in the curing environment of high temperature and high pressure. 7

8 3. Fabrication of smart composite element using out-of-plane ingress/egress method 3.1. Lay-up of composite laminate with embedded optical cable The optical cable was connected with an EFPI (extrinsic Fabry-Perot interferometer) sensor at its end and it was embedded into a unidirectional composite laminate. The stacking sequence of the laminate was chosen as [0 5 /{0}/0 15 ] T where {} marks the location of an embedded sensor. The two employed optical cables were the FC/APC type of optical jumper cords (Korea Electric Terminal Co., Ltd.) which have diameters of 0.9 mm and 2.4 mm with polymer jacket, respectively. And they have the average insertion loss less than 0.2dB. Laminates were made of CU-125 NS graphite/epoxy prepreg (HFG Co.) and its material properties are as follows: E1 = GPa, E2 = E3 = 9. 6GPa, G12 = G13= G23= 4. 8GPa, ν 12 =ν13= 0. 31, ν = We stacked composite prepregs up to the layer to embed the EFPI. After placing the EFPI on the laid up prepregs, the remaining plies were stacked with extracting the optical cable up through holes and slits made for the ingress/egress of the optical cable as shown in Figure 4 (a). Because the optical cable is thicker compared with a composite prepreg, the embedded optical cable can degrade properties of host composite material. Therefore, the length of the embedded optical cable must be minimized. As shown in Figures 2 and 4 (a), most part of the embedded optical fiber was the bare fiber with EFPI and the optical cable was embedded from the proximity of an ingress/egress point to reinforce the weak part of fiber and not to degrade properties of the composite material. Hence, for the efficient application of optical cable, the 8

9 lengths of bare fiber and optical cable to be embedded must be exactly designed before fabrication of smart composite structures. Figure 4 (b) shows the lay-up of composite laminate embedding an optical cable with a fiber optic sensor. 3.2 Protection of an optical cable at the ingress/egress point An optical cable must be properly protected and reinforced since the optical cable at the ingress/egress point is structurally very weak during both curing process and inservice. In previous researches, however, there are few works about this subject. We developed the fiber protector at the ingress/egress point as shown in Figure 5. This component could effectively protect and reinforce the optical cable at the ingress/egress point. It was made of aluminum for the weight saving and composed of two parts, and assembled by bolt for easy application to composite structure manufacturing. This component was installed on a composite laminate before curing then bonded to the composite part by adhesive film and excessive resin in the curing process. This component could be designed in several shapes, as shown in Figure 6, according to the purpose and shape of an applied structure. The type A and type B in Figure 6 can be applied to the flat surface of stuructures, while the attachment of type C can be applied to the curved surface of a filament wound vessel and a radome, etc. These components can be made of various materials such as light metals and composites, etc, which are light enough for weight saving of smart structures. 9

10 3.3 Preservation of an optical cable during fabrication process Figure 7 illustrates the lay-up of the composite laminate, peel-plies, bleeders, Teflon films, and caul plates before the cure. The caul plates were used to uniformly pressurize to the composite laminate and to avoid the optical cable from sinking into the laminate. The application of caul plates is demonstrated in Figure 7 (b). The optical cable exposed to the exterior of the laminate must be protected against the curing environment of high temperature and high pressure. Otherwise the optical cable may be compressed or damaged by this environment. Hence, we preserved the optical cable by wrapping it with breather fabric and covering it with metal elements as shown in Figure 8. We cured these composite laminates in an autoclave. 3.4 Development of optical connection fixture Figure 9 shows the assemblies of constructed smart composite elements with the external cable. The type of optical connectors was FC/APC and the same type of optical adapter was used for the connection between connectors. We developed optical connection fixtures to fasten the connecting element on the smart structure element, as shown in Figure 9, which was made of aluminum for weight saving. These fixtures can be designed in various shapes according to the purpose and the connecting direction of optical fibers. They were attached by FM-73M adhesive film (CYNAMID) and the optical adapter was built-in on the laminate with installed connecting element. The optical cable of 2.4 mm diameter was utilized in the horizontal connection and 0.9 mm diameter cable was used in the vertical connection as shown in Figure 9. These 10

11 commercial standard optical cable and connector made it easy to connect the embedded optical fiber with the outer data transferring fiber. In addition to this advantage, as it is compatible with another optical system or a sturucture part that contain the same type of the connector, it is possible to expand the structure by connecting adjacent parts or systems. Moreover the use of commercial optical component may give the convenience of the replacement and repair of the old or damaged component. The price reduction of commercial optical component make this method powerful and economical. If the optical connection part is not fixed on a structure, the movement and vibration of the structure can cause damage of optical components, transfer loss of light signals, and noise at the connection part. Therefore, it is very important to robustly fix the optical connection parts on the structure. These fixtures may be applied to not only embedding case but also surface attaching case of optical fiber sensors. 11

12 4. Bending test of constructed smart composite structural element 4.1. Test apparatus and method The ingress/egress method of the optical fiber must be stable to be applied to real structures. We performed tests of composite elements by adopting out-of-plane ingress/egress method to confirm the safety and survivability of the embedded optical fiber. Bending tests were performed and the schematic of tests is shown in Figure 10. The configuration of the utilized EFPI is shown in Figure 11. Electrical strain gage (ESG) was surface-mounted below the embedded EFPI sensors. The stacking sequence of the laminate was chosen as [0 5 /{0}/0 15 ] T. Hence the EFPI was embedded between the 5 th and the 6 th plies and ESG was attached on the surface which is 10 plies away from the mid-plane. Shapes of composite structural elements and positions of the embedded EFPI and surface-mounted ESG are illustrated in Figure 12. Gage lengths of EFPI s are about 5.4 mm for the horizontally connected element and about 5.0 mm for the vertically connected element. In order to detect the tensile deformation by EFPI and ESG, one end of the composite element was clamped and the opposite end where the optical connection part was located was subjected to upward transverse loading and unloading as shown in Figure Strain calculation of fiber optic EFPI sensor 12

13 For the interferometric fiber optic sensor (FOS) such as EFPI, Michelson interferometer, the relation between the phase of FOS and deformation can be written as follows: 4π φ = ( n L+ L n) (1) λ 0 where φ means the cumulative phase caused by the deformation in EFPI. L is the gage length of the embedded EFPI. n is the refractive index of EFPI in the gage length, λ 0 is the wavelength of the laser diode in vacuum state, 1305 nm. In case of EFPI, there is no change of the refractive index in the gage length since the light medium of EFPI is air ( n 1, n = 0 ). Therefore, Equation (1) can be rewritten as Equation (2). φ 4πn L = λ 0 (2) If N is the number of half waves of the interferometric fringe of EFPI, φ can be written as follows: φ = Nπ (3) From Equation (2) and Equation (3), the signal of EFPI can be transformed into the strain of the deformed structure by using Equation (4). 7 N ε = (4) L 13

14 4.3. Results and Discussions The objective of this test was to confirm the enhancement of the safety and survivability of the optical fiber in the construction of the smart composite structure made by the novel out-of-plane ingress/egress method. Results of bending test are shown in Figures 13 and 14. Raw signals of embedded EFPI s and surface-mounted strain gages are shown in Figures 13 (a) and 14 (a) and strains calculated from signals of both sensors are shown in Figures 13 (b) and 14 (b). Signals of EFPI s were successfully transferred to the signal acquisition unit as shown in Figures 13 (a) and 14 (a). Hence, it is readily apparent that the novel ingress/egress method of the optical fiber can be effectively applied to the real composite structures. If the damage of the optical fiber occurred or the optical connection part had some problem in curing process, the loss of the light signal would take place so that the acquisition of any signal might be difficult. In the out-of-plane ingress/egress method of this study, the survivability of the employed optical fiber was enhanced since we reinforced the structurally weak part of the ingress/egress point and preserved the optical cable and connector exposed to the exterior of the laminate against the curing environment. In addition, the robust integration of the optical connection part and the structural element has given the structural safety of this part and the stability of signals that had little loss and noise of the light signal. Several splicing techniques have been used to join two optical fibers, but this splicing region was mechanically very weak and frequently broken. In the developed ingress/egress method, the standard optical cable and connector were used to connect two optical fibers so that the connection part could be strong and maintain the safety. 14

15 For the bending deformation the ratio of the strain from EFPI sensor to that from ESG should be theoretically 0.5. That is because the total number of prepreg plies was 20 and the EFPI was embedded between the 5 th and the 6 th plies while ESG was attached below the 10 th ply from the mid-plane. Ratios of both measured strains, however, are about 0.42, which shows that there is a certain amount of the deviation from the linear variation of calculated strain values with thickness location of FOS as shown in Figures 13 (b) and 14 (b). This means that FOS has moved to the mid-plane during the curing process of the laminate. The possible cause of this movement of FOS is that the FOS which had the diameter of about 2 prepreg plies thickness easily moved toward the mid-plane due to the pressure during cure because the viscosity of matrix resin had decreased [10]. The strain ratio of 0.42 illustrates that the embedded FOS moved toward mid-plane by about 0.8 ply thickness. This may cause an error when we measure the strain induced by bending. Therefore, we should consider the movement of the embedding location of FOS through the thickness in thin laminated composites subjected to the out-of-plane loading for the precise measurement of strain. The developed ingress/egress method in this study will be able to be applied to not only composites but also other materials that can embed the fiber optic sensor. 15

16 5. Conclusions The development of fiber optic ingress/egress method is very important for the real application of fiber optic sensors to smart composite structures. We developed the alternative ingress/egress method that utilized the out-of-plane extraction of an optical fiber. Using the conventional in-plane ingress/egress method, there exist critical problems in the fragility of optical fiber, edge trimming and size adjusting work of composite laminate, and connection with an outer system for the real application of the optical fiber. In our preliminary studies about out-of-plane ingress/egress method, we found several problems: first, the optical fiber (cable) near the ingress/egress point is structurally so weak that the optical fiber was often broken at this point. Therefore, this point must be reinforced; second, the exposed optical fiber to the outside of a composite laminate must be safely preserved in order not to be damaged in the curing environment of high temperature and high pressure. So we developed the fiber protector to reinforce the optical fiber at the ingress/egress point and the preservation method of the fiber against the curing environment. And optical connection fixtures were developed to integrate as single structure element, which fixed the connection part on the composite panel. The use of commercial optical elements gave us the easiness of connection, replacement and repair and improved the mechanical property and safety of the connection part. We confirmed by tests that the safety and survivability of the embedded optical fiber were enhanced by the ingress/egress method devised in this study. 16

17 Acknowledgments The authors would like to thank the Ministry of Science and Technology, Korea, for the financial support by a grant from the Critical Technology 21 project. 17

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