Intrinsic Microbenci Fiber Optic Sensors

Transcription

1 Intrinsic Microbenci Fiber Optic Sensors Huang, Rui1 *; Yuan, Shenfang' ; Jiang, Yun2; Tao, Baoqi' 'The Aeronautical Key Lab for Smart Materials & Structures Nanjing Univ. ofaeronautics & Astronautics, Nanjing , PRC 2Nanjing Fiberglass Research & Design Institute ABSTRACT In this paper, special optical fibers are embedded into 3-D braided carbon/epoxy composites to constitute intrinsic microbend fiber optic sensors for internal strain measurement. Samples with embedded optical fibers are braided through four-step method and processed through Resin Transfer Molding (RTM) process. Special measures are taken to prevent damage of the sensors. The microbend optic fiber sensor works well in a three-point bending experiment. The experiments show that the output intensity of the microbend sensor is proportional to the internal strain of the sample while the load increases. Similar results can be achieved as the load decreases. The results show that the embedded microbend fiber optic sensor is fit for internal strain measurement in 3-D braided composites. Keywords: microbend, fiber optic sensor, braided composites 1. INTRODUCTION As one kind of intensity modulation sensors, microbend fiber optic sensors utilize intensity loss resulting from optical fiber bending to measure change of environmental parameters, e.g. strain, pressure and impact. Having advantages of simplicity, cost-effectiveness, the microbend sensors have been used widely in various composites for mechanical parameter measurement, structural health monitoring and process monitoring [ 1-3]. Braided composites have advantages over conventional laminated composites[4-6]. Using 3-D braiding process, a fully integrated structure can be directly fabricated into various shapes[7], which leaves out procedures of fiber cloth tailoring and laying, and leads to a lower production cost. It also eliminates the problem of delamination of laminated composites. Through braiding process, optical fibers can be embedded into braided composites and form many microbend structures. Thus intrinsic microbend fiber optic sensors are constituted. The braiding process is flexible and many parameters (braiding angle, number of axial yarns, radial yarns) are adjustable. With the help of that, performance of the microbend sensors can be optimized. * Correspondence: Tel: ; Fax: Fiber Optic Components, Subsystems, and Systems for Telecommunications, Suning Tang, Xiaomin Ren, Editors, Proceedings of SPIE Vol (2001) 2001 SPIE X/01/$15.00

2 In this paper, optica' fibers are embedded into 3-D braided carbon/epon composite testpieces to constitute intrinsic microbend fiber optic sensors. Internal strain of the testpieces is measured in three-point bending experiments. 2. MECHANISM OF MICROBEND SENSORS 131 Microbend results in intensity loss of transmission light in optical fibers. This is because the highest order transmission mode couples with radiation mode and attenuates quickly. The profile can be expressed as follow: no' 11 where A = 2n0 2 (r) = fl02[1 24rJ1 is relative index difference, n0 and nd' the refractive index of core and cladding, a, radius of fiber, a =oo for step-index fibers, a 2 for graded index fibers. Spatial wavelength of disturbance A can be described as A r where /1 is the fl' /3 propagation constant. The sensitivity of microbend fiber optic sensors is decided by the following formula: = --- AT LTAX (2) z\f AXAF AT AX where is decided by fiber features represents the design of the sensor. T is the AX AF transmission light energy, F the disturbance, X the axial deformation of fiber. 3. FABRICATION OF TESTPIECES 3.1. BRAIDING METHODS OF TESTPIECES There are two methods to braid optical fibers into braided structures. One is that the optical fibers are treated as axial yarns. Therefore they are oriented straight and parallel to the axis. The other is that the optical fibers are braided together with reinforced fibers, which makes optical fibers curving. Because the braiding of optic fibers is not easy and the optical fibers may endure some residual stress, which may affect operation of sensors, the first method is adopted in this paper. A 3-D braided girder form testpiece with braided optical fibers is shown in Figure 1. Considering the communication fiber is not much sensitive to microbend, special fibers (monomode, cut-off wavelength 1 550nrn) are used and embedded into 3-D carbonlepoxy braided composite testpieces. Reinforced fibers used are carbon fibers since they are used widely in aerospace. The testpiece is braided through four-step method. Its dimension is 25OmmX (1) Proc. SPIE Vol

3 20mm X 6mm; fiber volume content is 50 percent and yarn number is 28 X 8. Special jigs are designed to protect optical fibers from fracture and other bad effects. For the convenience of molding, the optical fibers are lead out ofthe testpieces from two sides, as shown in figure 1. ::::::::::::::::::::::::::::::::::::::::::::::::::: op)ical fiber Figure 1. Schematic drawing ofa testpiece with braided optical fibers 3.2. MOLDING PROCESS OF TESTPIECE WITH BRAIDED OPTICAL FIBERS Among molding processes of braided composites are vacuum impregnation, mold pressing and resin transfer molding (RTM) [6]. As the fittest molding process for braided composites, RTM itself is also one of the main research projects of Advanced Composite Technology supported by NASA [8]. During RTM process, preform is placed in airtight dies and subjected to a certain temperature and pressure, resin is then infused into the dies and cured. Products with high surface quality, fiber volume ratio and low void ratio can be produced through RTM. As one kind of clamp close molding process, RTM has a high demand on dies, especially airtightness, which makes optical fiber leadout difficult. Therefore, special dies are designed for the leadout and protection ofthe optical fibers. The combined steel dies are used in the experiment, which are composed oftop cover, cavity die and base plane. The cavity die is shown in figure 2. A seal groove is used. Two plugged impressions with pores are designed in the cavity die. At the leadout, sleeved with glass fiber tube, the optical fibers are lead out through the pores to the cavities at the two sides of the cavity die. The above design solves the contradiction of die airtightness and optical fiber leadout. Besides, it can prevent the optical fibers from contact with resin, which makes the optical fibers crispy. bolt hole seal groove plugged impression\ 4 4#4,,,,pore Figure 2. Schematic diagram of the cavity die 150 Proc. SPIE Vol. 4604

4 During manufactllring the test specimen by using the specially designed dies, matrix is Epon 618; release agent is silicone oil; curative is polyamide. A testpiece cured through RTM process is shown in figure 3. Figure 3.A testpiece cured through RTM process 4. EXPERIMENTAL METHODS AND RESULTS OF STRAIN MEASUREMENT Three-point bending experiments are performed using the embedded sensor. A schematic drawing of experimental set-up is shown in Figure 4, where the light source is LD and the light detector is optical fiber multimeter HP8153A. LD optical fiber testpiece HP 8153A Figure 4 Set-up of three-point bending experiment The results are shown in Figure 5 and Figure 6, where x axle is surface strain near loading point (measured with resistance strain gauge) and y axle is output value of light intensity, I, whose unit is i- w. As can be seen in Figure 5, when the testpiece is under a bending load, as the load increases, so does the output light intensity. In the range from 30 microstrains to 1300 rnicrostrains, the output light intensity is nearly linear to the measured surface strain. Because the strain applied on the optical fiber is proportional to the measured surface strain, the output light intensity is also proportional to the strain applied on the optical fiber, i.e. the internal strain of the braided testpiece. Proc. SPIE Vol

5 : E /microstrain Figure 5 Experimental result while load increases Figure 6 shows the result when the load decreases, which is similar to the result when the load increases C /microstrain Figure 6 Experimental result while load decreases 5. CONCLUSIONS In this paper, microbend fiber optic sensors are embedded into braided testpieces for strain measurement. The results show that the microbend sensors are applicable to internal strain measurement for braided composites. The microbend fiber optic sensors have advantages of simple structure, convenient measurement technique and low cost. But its sensitivity needs improvement. In order to get more microbend loss, increase of core diameter, decrease of numerical aperture, change of braiding process and other measures are needed. Both theoretical and experimental work is necessary to the optimum design of the sensors. ACKNOWLEDGMENTS This paper is to the memory of reverent Prof. Baoqi Tao. The work is supported by Natural Science Foundation of China (grant number: ). 152 Proc. SPIE Vol. 4604

6 REFERENCES [1] Udd E Fiber optical sensor overview Fiber Optical Smart Structures ed E Udd (New York: John Wiley & Sons Inc.) pp , 1995 [2] Measures R M Fiber optical strain sensing Fiber Optical Smart Structures ed F Udd (New York: John Wiley & Sons Inc.) pp , 1995 [3] Clark T, Smith H Microbend fiber optic sensors, Fiber Optic Smart Structures, ed E Udd (New York: John Wiley & Sons, Inc.) pp , 1995 [4] Fedro M J and Willden K, Characterization and manufacture of braided composites for large commercial aircraft structures, NASA/CP 3 154, Proc. 2" NASA Advanced Composites Technology Conference, pp , 1991 [5] Kamiya R, Cheesernan B A, Proper P and Chou TW, Some recent advances in the fabrication and design of three-dimensional textile preforms: a review, Composite Science and Technology, 60 pp , 2000 [6] Yang Gui, Ao Daxing, Zhang Zhiyong and Yao Xuefeng, Fabrication, Process and Industrial Practice of Braided Structures, (Beijing: Science Press) pp 14-20, 1999 [7] El-Sherif M A and Ko F K, Co-braiding of sensitive optical fiber sensor in 3-D composite fiber network, Proc. SPIE , 1992 [8] Dow M B. and Dexter H B, Development of stitched, braided and woven composite structures in the ACT program and at Langley Research Center ( ), NASA/TP , 1997 Proc. SPIE Vol