Embeddable Fiber Optic Strain Sensor for Structural Monitoring
Amardeep Kaur1, Sriram Nagarajan2, Sudharshan Anandan2, Lei Yuan1, K. Chandrashekhara2, Steve E. Watkins3, Hai Xiao*1 1 2 Photonics Technology Laboratory, Composites Research Laboratory, 3Applied Optics Laboratory, Missouri University of Science and Technology, Rolla, MO, USA 65409-0040 Nam Phan Naval Air Systems Command Patuxent River, MD 20670-1906
ABSTRACT An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for measurement of strain at high ambient temperatures. The sensor is fabricated using a femto-second (fs) laser. The EFPI sensor is fabricated by micromachining a cavity on the tip of a standard single-mode fiber and is then self-enclosed by fusion splicing another piece of singlemode fiber. The fs-laser based fabrication makes the sensor thermally stable to sustain temperatures as high as 800 ºC. The sensor is relatively insensitive towards the temperature as compared to its response towards the applied strain. The sensor can be embedded in Carbon fiber/Bismaleimide (BMI) composite laminates for strain monitoring at high ambient temperatures. Keywords: fs-laser, EFPI, optical fiber, temperature, strain, embeddable, structural monitoring.
1. INTRODUCTION Repair and maintenance of structures in developed countries demand a considerable amount of resources to be used each year. Recent years have seen a new development in the field namely embedded sensors as a smart structures technology. Embedding sensors while constructing new structures or repairing the old ones allows for continual monitoring of structural health thus giving an estimate of remaining utility. This capability provides an opportunity for preventive measures, e.g. performing repairs in time to prevent any major damage. Embedded sensors have been used for cure monitoring [1], fatigue detection [2, 3], strain profiling [4-6], and temperature measurement [7]. Optical fiber based sensors have gained wide interest for structural health monitoring applications due to their compact size, immunity from electromagnetic interference, multiplexing capabilities etc. Different types of optical fiber sensors like Fiber Bragg gratings [7,8], extrinsic Fabry-Perot interferometeric (EFPI) sensors [2,9], intrinsic Fabry-Perot interferometeric (IFPI) sensors [10], long period fiber gratings, and combinations of these sensors [7] have been used for monitoring strain, temperature, cure monitoring, and pressure in the field of structural health monitoring. Among various optical fiber based sensors, EFPI-based sensors are best suited for strain monitoring applications where the ambient temperatures can become very high. Bragg gratings and IFPI sensors have been demonstrated for strain applications over the years [10,11], but they are very sensitive towards the ambient temperature as well. EFPI-based optical fiber sensors, however, are almost insensitive towards the ambient temperature when compared to their sensitivity towards the strain. Smart materials like carbon fiber composites are used in aerospace and civil infrastructure monitoring applications where the sensors are required to monitor strain profiles in high temperature environment. These requirements present an appropriate opportunity for fs-laser micro-machining to fabricate EFPI sensors. Fs-laser fabrication yields thermally stable structures that can withstand higher temperatures as compared to the regular optical fiber sensors. An extrinsic Fabry-Perot interferometric fiber optic sensor for measurement of strain at high ambient temperatures is presented in this paper. The sensor is relatively insensitive towards temperature as compared to its strain sensitivity.
*Author for correspondence – E-mail:
[email protected], Phone: 573-341-6887
Temperature and strain responses of the sensor are demonstrated in air, and after embedding in fiber reinforced carbon fiber composite laminates.
2. FABRICATION AND OPERATIONAL PRINCIPLE The EFPI sensor consists of two reflective surfaces created by fabricating a cavity. In this work, the version of EFPI is formed by the cavity on the tip of a standard single mode fiber and then fusion splicing with another piece of fiber. The cavity is fabricated by etching a 40 µm deep square surface (65 µm x 65 µm) on the tip of a single-mode fiber using a laboratory integrated fs-laser micro-machining system. The cavity length of 40 µm results in a gauge length of 40 µm. The system operates at a center wavelength of 800 nm with the repetition rate and pulse width of 1 kHz and 120 fs, respectively. The laser system provides an output of ~0.8 W which was then reduced to 40 mW by using a combination of wave plates and polarizers. An objective lens was used to focus the laser beam on the tip of the fiber. The fiber was mounted on a computer-controlled five-axis translation stage (Aerotech, Inc.) with a resolution of 1 µm. The cavity was then fused with another piece of fiber using an electric arc based fusion splicer. Figure 1(a) shows the schematic diagram of the EFPI sensor; dcavity is the cavity length or gauge length. Input light traveling in the core of the fiber reflects back from the two walls of the cavity and the difference (induced by the cavity length change) between these two reflections in turn causes a change in the wavelength of the original input signal. Figure 1(b) shows a confocal microscopic image of the EFPI sensor after the cavity was fusion spliced with another piece of single-mode fiber.
dcavity
Cladding Core
(a)
Cavity
Fusion Joint
(b) Figure 1.(a) Structural diagram of the EFPI. (b) Confocal microscopic image of the cavity after fusion splicing it with another piece of fiber.
The fs-laser ablated surfaces have low reflectivity making contribution from multiple reflections negligible. Thus using plane wave approximation for a two-wave interferometer, the reflected signal intensity IR can be expressed as [12]:
−−−− I R = I 21 + I 22 + 2√I 1 I 2 Cos(ϕ 1 − ϕ 2 )
(1)
where I1, I2 are the two reflections (as shown in Figure 1(a)) at the cavity walls and ϕ1, ϕ2 are the respective phase lags.
The cavity length, dcavity is given in the equation below where λ is the central wavelength and FSR is the free spectral range.
d cavity =
1 λ2 2 FSR
(2)
It can be observed, from the above equation, that the wavelength shift is not dependent upon ambient temperature change. Also, the coefficient of thermal expansion (CTE) of silica (0.55 x 10-6/ºC) makes the single mode silica fiber an ideal candidate to be used for high temperature applications. The wavelength shift induced during the temperature change is actually due to thermal strain. At the same time by differentiating equation (2), we observe that the wavelength shift is linearly dependent upon the strain (Δd/dcavity). With the application of axial strain, the cavity length dcavity will change which will in turn induce a change in the central wavelength resulting in a wavelength shift. These inherent qualities make the cavity-based EFPI sensor an ideal candidate for strain monitoring at high temperatures. For embedding the sensor, twelve-layer unidirectional laminates were fabricated (12 in. x 1 in.) , using IM7/AR4550 prepreg, by out-of-autoclave process. A sensor was embedded between the central layers. The prepreg layup was cured at 190.56 ºC for two hours. The embedded sensor was used to perform cure monitoring. Once cured, the sample was used to observe the temperature response using the same set-up as described before. The temperature was raised from room temperature in steps of 25 ºC to reach a temperature of 225 ºC. The glass transition temperature (271 ºC) of the BMI resin used to bind the sample limited the upper limit of the temperature tested. Table 1 lists the sample size of the laminates used and some properties of the BMI resin. Table 1. Properties of the BMI resin.
Sample size used for embedding the sensor Glass Transition Temperature Cure Temperature Tensile Modulus Major Poisson Ratio (ν12)
12 in. x 1 in. 271 ºC 190.55 ºC 20.6 Mpsi 0.29
3. EXPERIMENTAL SET-UP A 100 nm broadband source (B&W TEK INC.) was used as input, a 3 dB coupler was used to send the signal to the sensor and recieve the reflected signal back which was then recorded using an optical spectrum analyzer (OSA). The sensor was placed inside a box furnace (Lindberg/Blue M). The temperature of the furnace was raised from room temperature to 800 ºC in steps of 50 ºC and the resultant wavelength shift in the spectrum was recorded. Figure 2 shows the set-up used. Broadband Source 1 X 2 Coupler Optical Spectrum Analyzer
Sensor Head
Box Furnace
Computer for Data Processing Figure 2. Equipment set-up for temperature testing and the sensor response monitoring.
Figure 3 presents the schematics of the equipment set-up used for applying strain to the non-embedded EFPI sensor. The sensor was fixed to a translation stage (Newport) at one end and to a magnetic block at the other. A certain amount of pre-strain was applied in order to make sure that the sensor was stretched and not loose. For the embedded sensor, an INSTRON 5584 tensile test machine was used to apply tensile load on the embedded sample. In both the cases, an axial strain was induced along optical fiber/sensor axis resulting in a wavelength shift in the reflection spectra. These spectra were then recorded using an optical spectrum analyzer. The recorded data was then processed to find the corresponding wavelength shift for each applied strain step. Figure 4 shows the placement of composite sample with embedded sensor between the grips of the test equipment. The optical fiber can be seen coming out of the right side of the sample; the shrinking tube used to prevent the fiber from breaking at the egress point can also be seen. The figure shows a longitudinal split as a result of the substrate failure at which point the sensor also broke. The split results due to a nonperfect alignment of the laminate sheets. Broadband Source 1 X 2 Coupler
Sensor Head
Optical Spectrum Analyzer
Strain Application
Computer for Data Processing
Figure 3. Equipment set-up for strain application and sensor response monitoring (non-embedded sensor).
Composite laminate sample with embedded sensor
Optical fiber input/output
Figure 4. INSTRON machine used for strain testing of the embedded sensor.
4. RESULTS AND DISCUSSION In this section, the experimental results for temperature, and strain response of the sensor are presented and discussed. The EFPI sensors that produced these results were very similar in terms of cavity length, fringe visibility, and background noise. Fringe visibility of the sensors for which the results are presented here ranged from 12 to 20 dB with a background loss of 6 to 9 dB. Figure 5 shows the wavelength shift resulting from the increasing ambient temperature as observed by using the nonembedded EFPI sensor. The slope of the response was calculated to be 0.585 pm/ºC and CTE of silica was calculated to be 0.715 x 10-6/ºC which is 1.3 times larger than that of the actual CTE. Temperature response of the sensor embedded in carbon composite laminates is presented in Figure 6. The slope and CTE calculated from the sensor response were 1.742 pm/ºC and 1.615 X 10-6 /ºC , respectively. The CTE of the BMI resin calculated in case of the embedded sensor was 1.615 x 10-6/ºC which is smaller than the actual value of the resin that ranges from 24 x 10-6 /ºC to 44 x 10-6 /ºC [13] but about 2.3 times larger than the calculated CTE of silica. For the non-embedded sensor, the calculated strain resulting from the cavity expanion was 0.747 µε/ºC whereas the strain exerted for embedded sensor was calculated to be 2.25 µε/ºC. Larger strain in case of embedded sensor might have resulted from the strain exerted on the sensor by the surrounding composite laminates. The sensor is not very sensitive towards the temperature, but it does respond very well towards the thermal strain of the host structure.
Slope&=&0.585&pm/ºC&
Figure 5. Response of the non-embedded EFPI sensor towards the ambient temperature.
Slope&=&1.742&pm/ºC&
Figure 6. Response of the embedded sensor (in carbon fiber reinforced composites) towards the ambient temperature.
Responses of the free and embedded sensor at room temperature are presented in Figure 7. The non-embedded sensor has higher slope (1.55 pm/µε) as compared to the embedded sensor (0.6 pm/µε). A cycle of tensile test was performed where the applied strain was increased uptil 4000 µε and then decreased to a zero strain state in steps of 500 µε. It can be observed from the plot that the slope of unloading response is slightly higher than the loading; this resulted from a small amount of residual strain. For the non-embedded sensor, the strain was applied until the fusion joint broke loose from the cavity; it was verified under a microscope that the cavity broke at the fusion joint. The breaking point for the EFPI was observed to be at 3800 µε.
Figure 7. Measured strain response of the embedded and the non-embedded sensors at room temperature.
5. CONCLUSIONS In conclusion, an extrinsic Fabry-Perot interferometric fiber optic sensor is presented in this paper for measurement of strain at high ambient temperatures. The fs-laser fabrication results in thermal stability of the sensor thus enabling the sensor to withstand very high temperatures. At the same time, a single fusion joint allows for better structural integrity and ease-of-fabrication over tube-encapsulated designs. The inherent properties of the fused silica and the EFPI cavity structure enable the sensor to be used as an efficient strain sensor. This sensor was tested for its embedded response by embedding it inside carbon composite laminates. High performance carbon composites are used in aerospace applications as they provide good mechanical performance at elevated temperatures. They are used as a high temperature substitute for toughened epoxy resins. The sensor is relatively insensitive towards temperature as compared to its strain sensitivity. Temperature and strain responses are demonstrated for the non-embedded and the embedded sensor. Responses of the sensor towards ambient temperature, and axial strain were presented and discussed. The temperature, and strain responses of the non-embedded sensor, and embedded (in BMI composite laminates) sensor were compared. Though there were expected differences in the slopes of the responses and the CTE values calculated for silica were higher than the actual CTE of the silica, the sensor responds linearly towards the temperature increase and the wavelength shift is minimal at temperatures as high as 850 ºC as compared to its strain sensitivity. The resultant wavelength shift for the embedded sensor over a range of 100 ºC was about 0.071 nm and the wavelength shift for an applied strain of 1000 µε was observed to be about 0.58 nm. The fitting linearity for the temperature, and strain response was 0.995, and 0.999 respectively. An ease of fabrication, small size, embedding capability, low temperature and high strain sensitivity for carbon fiber composite laminates makes this sensor a very good candidate for structure monitoring applications.
ACKNOWLEDGEMENT This research was sponsored by Integrated Systems Solutions, Inc. The authors would like to thank Stratton Composite Solutions for the materials supplied.
REFERENCES [1] Murukeshan, V. M., Chan, P. Y., Ong, L. S., and Seah, L. K., “Cure monitoring of smart composites using Fiber Bragg Grating based embedded sensors”, Sensors and Actuators A: Physical, Vol. 79, 153-161 (2000). [2] Etches, J., and Fernando, G., “Evaluation of Embedded Optical Fiber Sensors in Composites: EFPI Sensor Response to Fatigue Loading,” Polymer Composites, Vol. 31, 284-291, (2010). [3] Zetterlind, V., Watkins, S., and Spoltman, M., “Fatigue Testing of a Composite Propeller Blade using Fiber Optic Strain Sensor,” Sensors Journal IEEE, Vol. 3 (4), 393-399, (2003). [4] Jin, X., Chung, J., and Venkat, V., “Simultaneous Measurement of Two Strain Components in Composite Structures Using Embedded Fiber Sensor,” Journal of Composite Materials, Vol. 33 (15), 1376-1389 (1999). [5] Choquet, P., Juneau, F., and Dadoun, F., “New Generation of Fiber-Optic Sensors for Dam Monitoring” Proceedings of the 1999 International Conference on Dam Safety and Monitoring, pp. 19–22, (1999). [6] Bhatia, V., Sen, M., Murphy, K., Claus, R., Jones, M., Grace, J., Tran, T., Greene, and J., “Optical Fiber Extrinsic Fabry-Perot Interferometric Strain Sensor for Multiple Strain State Measurements,” SPIE, Vol. 2444, 115-126 (1995). [7] Rao, Y., Yuan, S., Zeng, X., Lian, D., Zhu, Y., Wang, Y., Huang, S., Liu, T., Fernando, G., Zhang, L., Bennion, I., “Simultaneous Strain and Temperature Measurement of Advanced 3-D Braided Composite Materials using an Improved EFPI/FBG system,” Optics and Lasers in Engineering, Vol. 38, 557-566, (2002). [8] Luyckx, G., Voet, E., Lammens, N., and Degrieck, J., “Strain Measurements of Composite Laminates with Embedded Fibre Bragg Gratings: Criticism and Opportunities for Research,” Sensors, Vol. 11, 384-408, (2011). [9] Huang, Y., Wei, T., Zhou, Z., Zhang, Y., Chen, G., and Xiao, H., “An Extrinsic Fabry–Perot Interferometer-based Large Strain Sensor with High Resolution,” Measurement Science and Technology, Vol. 21 (10), 10538 (8pp), (2010). [10] Huang, Z., Zhu, Y., Chen, X., and Wang, A., "Intrinsic Fabry–Perot fiber Sensor for Temperature and Strain Measurements", IEEE Photonics Technology Letters, Vol. 17(11) , 2403 -2405 (2005).
[11] Majumder, M., Gangopadhyay, T., Chakraborty, A., Dasgupta, K., and Bhattacharya, D., “Fiber Bragg Gratings in Structural Health Monitoring—Present Status and Applications,” Sensors and Actuators A: Physical, Vol. 147(1), 150-164 (2008). [12] Gangopadhyay, T., “Prospects for Fibre Bragg Gratings and Fabry-Perot Interferometers in fibre-optic vibration sensing,” Sensors and Actuators A: Physical, Vol. 113, 20-38, (2004). [13] Nelson, J., Hine, A., Segdwick, P., Lowe, R., Rexeisen, E., King, R., and Patz, N., “ “Development of Nano-Silica Bismalimide (BMI) Matrix Resins for Prepeg Tooling Composites: Fortified Tooling Prepeg BMI,” SAMPE Technical Conference, May 21-24, Baltimore, MD, 1-13 (2013).
