- Email: [email protected]
Strain and damage monitoring of CFRP in impact loading using a fiber Bragg grating sensor system
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 1353–1361 www.elsevier.com/locate/compscitech
Strain and damage monitoring of CFRP in impact loading using a fiber Bragg grating sensor system Hiroshi Tsuda *, Jung-Ryul Lee Structural Health Monitoring Group, Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, Tsukuba 305-8568, Japan Received 9 May 2006; received in revised form 18 September 2006; accepted 22 September 2006 Available online 9 November 2006
Abstract A fiber-optic system featuring strain measurement and ultrasonic detection was constructed with fiber Bragg gratings based on wavelength–light intensity conversion technique. This fiber Bragg grating sensing system consists of a broadband light source, a broadband optical filter for strain measurement and a narrowband tunable filter for ultrasonic detection. The system was applied to strain measurement in impact loading to carbon fiber-reinforced plastics and the subsequent impact damage detection. Experimental results demonstrated that fiber Bragg grating sensors could measure strain with higher resolution compared with conventional strain gauges. Furthermore, ultrasonic inspection in which ultrasonic sensitivity was maximized by controlling the transmissive wavelength of the tunable filter could detect a 6.3 · 9 mm2 impact damage. 2006 Elsevier Ltd. All rights reserved. Keywords: Fiber Bragg gratings; D. Non-destructive testing; D. Ultrasonics
1. Introduction Carbon fiber-reinforced plastics (CFRP) have been widely applied to automobile and aerospace applications because of their high specific strength and rigidity. However, CFRP have a grave disadvantage that the compressive strength declines substantially after being subjected to impact damage. Hence, detection of impact loading and damage is very important to secure the reliability of CFRP structures. Impact loading can be detected by strain measurement and the induced damage can be inspected by ultrasonic technique. Conventionally, strain has been measured with resistive strain gauges and ultrasound has been detected with piezoelectric sensors. Smart structures in which structural conditions are monitored with built-in sensors have drawn attention in these days [1–4]. In smart structures including detection of both *
Corresponding author. Tel.: +81 29 861 92 84; fax: +81 29 861 58 82. E-mail address: [email protected] (H. Tsuda).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.09.017
impact loading and damage using conventional technique, strain gauges and piezoelectric sensors would be installed. However, these electric sensors have serious drawbacks that they suffer from electromagnetic interference and cannot be used in explosive environment. Moreover, they need individual cables. Therefore, the monitoring system applied to large-scale structures where multi-point sensing is performed would inevitably include complex cable network. In order to overcome these technical difficulties, fiber Bragg gratings (FBGs) featuring multifunction, multiplexibility as well as immunity to electromagnetic interference have been expected to be promising sensors for structural health monitoring [5]. FBG reflects a narrowband light whose central wavelength is called the Bragg wavelength when broadband light is launched. FBG can work as strain sensor since the Bragg wavelength varies with strain FBG is subjected to. In the past studies [6–9], the Bragg wavelength has been evaluated with the optical spectrum analyzer (OSA). Application of FBG strain sensor has been limited to static strain
1354
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
measurement because the sampling rate of OSA is a few hertz at a maximum. The objective of the present study is to develop a simple FBG sensing system which can measure strain and detect ultrasound for structural health monitoring. We constructed an FBG sensing system including a broadband light source and two optical filters for sensing strain and ultrasound. A wavelength–light intensity conversion technique was utilized to detect the shift in the Bragg wavelength at fast speed. The FBG sensing system was applied to monitor strain in CFRP during impact loading and to detect the induced damage. 2. Wavelength–light intensity conversion technique Consider the optical system shown in Fig. 1. Broadband light is transmitted to an FBG via an optical circulator and the light reflected from the FBG is sent to an optical filter. We assume that the optical filter has characteristics whose transmissivity declines with wavelength in a wider wavelength range compared with the reflective bandwidth of the FBG. The Bragg wavelength shifts to longer wavelength when the FBG is elongated. Then the intensity of light transmitted through the filter decreases because the transmissivity at longer wavelength is smaller. Conversely, the intensity of transmissive light increases when the FBG is compressed. Thus, shift in the Bragg wavelength induced
by strain can be converted into light intensity with optical filters. Light intensity can be measured with photodetectors. The response frequency of photodetectors is usually over 10 MHz. Hence, strain can be measured at fast speed by measuring the intensity of light demodulated with optical filters. Details of ultrasonic detection using wavelength–light intensity conversion technique have been described in a previous paper [10]. An outline of detection principle is given here. Propagation of ultrasound in materials causes strain change in the microstrain range over several tens of kHz. Shift in the Bragg wavelength caused by the subtle strain change ranges in sub-pico meters. In order to detect such a subtle Bragg wavelength change, the optical filter must meet stringent optical characteristics whose transmissive bandwidth is comparable with the reflective range of the FBG sensor as shown in Fig. 1. The intensity of light transmitted through the filter is represented by the area where the reflectivity of the sensor overlaps with the transmissivity of the filter. Thus, the small variation in the Bragg wavelength of FBG can be detected as the change in the intensity of light transmitted through the filter. 3. Experimental procedure As mentioned in the previous section, it is necessary to use broadband and narrowband optical filters to measure
Fig. 1. Principle of wavelength–light intensity conversion technique using optical system consisting of a broadband light source and an optical filter.
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
strain and ultrasound, respectively. In the present study, an optical system which can measure two quantities of strain and ultrasound was constructed by changing the optical path of light reflected from the FBG using an optical switch as shown in Fig. 2. Broadband light is transmitted to an FBG sensor and the light reflected from the FBG is sent to the optical switch. The optical path switches to port A where broadband filter is connected when strain is measured. Light transmitted through and reflected from the broadband filter goes to photodetectors where the intensity of light is converted into voltage signal. The switch is set to port B where narrowband tunable filter is connected when ultrasound is detected. Then, the intensity of light transmitted through the narrowband filter is converted into voltage signal. This optical measurement system requires a very stable light source and optical filters without any drift from the external disturbance such as change in temperature. Amplified spontaneous emission (ASE) light source whose fluctuation in optical power is less than ±0.5% was employed. The transmissive wavelength of both optical filters used in the present study fluctuates a few picometers per 1 C. Experiments were performed under constant temperature conditions in order to eliminate the influence of temperature variation on transmissive wavelength of optical filters. Optical characteristics of the broadband filter for strain measurement are shown in Fig. 3. The broadband optical filter is made of dielectric multilayer. The transmissivity and reflectivity monotonically decreases and increases with wavelength, respectively, in a range from 1548 to 1553 nm. Transmissivity of narrowband filter for ultrasonic detection is shown in Fig. 4 along with the reflective curve of FBG sensor. The narrowband optical filter is a tunable optical filter whose transmissive wavelength can be controlled by computer. FBG employed in the present study was provided by Hitachi Cable Co. Ltd. The Bragg wave-
1355
Fig. 3. Optical characteristics of the optical filter for strain measurement.
length at strain free was 1550.1 nm and the grating length was 1.5 mm. The diameters of the cladding and outer polyimide coating were 40 lm and 52 lm, respectively. Although this fiber has a smaller diameter, the FBG characteristics are the same as those of conventional FBGs [11]. The full widths at half-maximum (FWHM) of the FBG and optical filter are 0.57 nm and 0.38 nm, respectively. The monitored specimen was a 300 · 300 · 1 mm3 CFRP (T800H/3631) whose stacking sequence was [0/90]2s. FBG was attached on the surface of the specimen 50 mm away from the center of CFRP plate. A resistive strain gauge was attached next the FBG sensor in order to measure strain in impact loading for reference. The specimen was bilaterally supported by a steel clamp so that the specimen was suspended in impact loading. The impact loading at an energy of 2.7 J was applied at the center of the specimen using a semi-sphere impactor. A 6.3 · 9 mm2 elliptically-shaped damage was observed by C-scope scan although only small dent could be seen in damaged area from naked eye obser-
300 broadband light source shear wave transmitter 50
FBG sensor
A
broadband optical filter for strain measurement
50 optical switch
9
300
transmitted light
reflected light
B
6.3 narrowband tunable filter for ultrasonic detection
strain gauge weight drop impact damage
pulse generator
[0/90]2s CFRP
steel clamper
strain amp
photodetector
recorder
sampling rate = 100kS/s in strain measurement = 100MS/s in ultrasound detection /mm
Fig. 2. Experimental setup for strain measurement in impact loading and ultrasonic inspection.
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
Normalized reflectivity of FBG
1
1
0.5
0.5
tunable filter FWHM = 0.38nm
1.5mm long FBG FWHW = 0.57nm
0
Normalized transmissivity of tuanble filter
1356
0 -1
0
1
-1
0
1
Relative wavelength, nm Fig. 4. Optical characteristics of the FBG sensor and the tunable filter.
vation. Strain measured from the FBG sensor and strain gauge was recorded at a sampling rate of 100 kS/s. A shear wave ultrasonic transmitter whose nominal frequency was 250 kHz was attached on the specimen surface in alignment with the optical fiber axis 100 mm away from the FBG sensor. An impulse signal was input to the transmitter to generate ultrasound. Response to ultrasound was recorded at a sampling rate of 100 MS/s before and after impact damage. 4. Experimental results 4.1. Calibration of FBG strain measuring system In strain measurement through wavelength–light intensity conversion, strain is evaluated from change in intensity of light demodulated by optical filter. The formulation of the following two relations is a prerequisite for quantitative strain evaluation. 1. Shift in the Bragg wavelength with strain applied to the FBG. 2. Variation in intensity of light transmitted through and reflected from the optical filter with the Bragg wavelength of FBG. These relations were obtained from the preliminary experiment in which an FBG attached on a thin stainless steel plate was strained by bending and the strain was evaluated with a resistive strain gauge attached next to the FBG. The relation between the Bragg wavelength of small diameter FBG and strain is shown in Fig. 5. The Bragg wavelength kB varies in proportion to strain in a strain range of ±0.1% and the relation is given by
Fig. 5. Relation between the Bragg wavelength of FBG sensor and strain applied to FBG.
kB ðnmÞ ¼ 1550:1 þ 12:4 e ð%Þ
ð1Þ
An imposed strain of 1% leads to a 12.4 nm shift in the Bragg wavelength of the FBG employed in the present study. The optical filter for strain measurement has characteristics which vary monotonically in a wavelength range from 1548 nm to 1553 nm. In the combination of the FBG and the optical filter, intensity of light transmitted through and reflected from the optical filter changes monotonically with strain in a range from 0.17% to 0.23% since the Bragg wavelength at strain free was 1550.1 nm Variation in two photodetector outputs which correspond to intensities of light transmitted through and reflected from the optical filter with the Bragg wavelength is shown in Fig. 6. As expected from the filter characteristics, intensity of light transmitted through and reflected from the optical filter decreases and increases with the Bragg wavelength, respectively. This figure shows the light
2.0
1357 2.0
transmitted light, case 1 transmitted light, case 2 1.5
1.5
1.0
1.0 reflected light, case 1 reflected light, case 2
0.5
0.0 1547
0.5
1548
1549
1550
1551
1552
PD output for light reflected from the filter, Vreflect
PD output for light transmitted through the filter, Vtrans
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
0.0 1553
Bragg wavelength λB, nm Fig. 6. Change in intensity of light transmitted through and reflected from the optical filter with strain applied to FBG.
intensities in two cases where fiber-optic link was changed. In the present experimental system, fiber-optic link was made by mechanical splice using the FC optical connectors [12]. Connector loss in mechanical splice varies whenever connection is changed. This is because alignment of optical fibers in connectors varies at every connection. Connector loss in case 2 was smaller than that in case 1 because intensity of light in case 2 was higher than that in case 1. Fiber-optic system with mechanical splices inevitably gives rise to variation in connector loss. Therefore, the Bragg wavelength cannot be determined directly from the intensity of light transmitted through and reflected from the filter. A parameter, R which corresponds to the ratio of the difference to the sum of the two photodetector outputs is introduced in order to eliminate the influence of connector loss on the relation between the Bragg wavelength and light intensity [13]. R¼
V reflect V trans V reflect þ V trans
ð2Þ
where Vtrans and Vreflect denote photodetector outputs for light transmitted through and reflected from the filter, respectively. Relation between the parameter R and the Bragg wavelength is shown in Fig. 7. The relation shows one-to-one relation between the Bragg wavelength and the parameter R in any optical connections. Fig. 8 shows the relation between the parameter R and strain which was obtained by converting the Bragg wavelength in Fig. 7 into strain using Eq. (1). The relation between the parameter R and strain was approximated by the 5th order polynomial given by Eq. (3). The correlation coefficient between the approximate polynomial and the experimental results is over 0.999, so the polynomial gives good approximation.
e ð%Þ ¼ a0 þ a1 R þ a2 R2 þ a3 R3 þ a4 R4 þ a5 R5 ð3Þ where a0 ¼ 21:46E 3 a1 ¼ 175:14E 3 a2 ¼ 7:7474E 3 a3 ¼ 141:74E 3 a4 ¼ 321:63E 3 a5 ¼ 446:12E 3 Strain can be evaluated from Eq. (3) by substituting the parameter R which is experimentally determined from photodetector outputs for intensity of light transmitted through and reflected from the optical filter. 4.2. Strain measurement in impact loading Strain measured with the FBG sensor and the strain gauge during impact loading to the CFRP specimen is shown in Fig. 9. Strain behavior obtained from both sensors shows a good agreement. The maximum tensile strains evaluated from FBG sensor and strain gauge were 204le and 208le and the maximum compressive strains were 99le and 97le, respectively. The experimental results demonstrated that FBG could measure strain correctly through wavelength–light intensity conversion technique. Note noise level in two sensor signals. The background noise in strain gauge is greater than that in FBG sensor. Spikes in strain gauge signal are observed at 0.014 s and 0.14 s as arrowed in Fig. 9. Moreover, sudden relatively large fluctuation appeared at 0.09 s only in strain gauge signal. These considerable noises in strain gauge signal would result from electromagnetic interference. By taking the
1358
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
Ratio of the difference to the sum of the two photodetector outputs, R
0.8
0.6
case 1 case 2
0.4
0.2
0.0
-0.2
-0.4
-0.6 1547
1548
1549
1550
1551
1552
1553
Bragg wavelength λB, nm Fig. 7. Relationship between the Bragg wavelength of FBG and the parameter R.
0.2 case 1 case 2
Strain ε, %
0.1
fitted curve
0.0
-0.1
-0.2 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Ratio of the difference to the sum of the two photodetector outputs, R Fig. 8. Relationship between the parameter R and strain.
definition for the minimum detectable signal as the rootmean-square value of signal recorded before impact loading, the minimum detectable strain for FBG and strain gauge are estimated to be 1.2 le and 8.1 le, respectively. The frequency domain representation of two sensor signals is shown in Fig. 10. While both sensor signals have a similar distribution up to 1 kHz, strain gauge signal exhibits relatively high intensity at 1.5 kHz and over 9 kHz. Here we assume that the frequency components over 1 kHz are noise because the normalized intensities in FBG sensor signal are always less than 102. Signalto-noise ratios of FBG and strain gauge signals are evaluated to be 40 dB and 8 dB, respectively. Signal-to-noise ratio can be improved by low-pass filtering, whereas an
abrupt strain change over the cutoff frequency of low-pass filter cannot be measured. It can be confirmed from the experimental results that FBG sensors can measure strain more precisely in wider frequency range compared to conventional strain gauges. 4.3. Impact damage detection by ultrasonic inspection In ultrasonic detection using the present sensing system, the difference between the central wavelength of the filter and the Bragg wavelength of the FBG influenced significantly on ultrasonic sensitivity. Influence of the wavelength difference on ultrasonic sensitivity was investigated by shifting the transmissive wavelength of the tunable filter
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
Fig. 9. Strain measured with FBG sensor and strain gauge in impact loading.
Fig. 10. Frequency domain representation of FBG sensor and strain gauge signals.
in order to determine the optimum wavelength difference for ultrasonic detection. Fig. 11 shows the variation of ultrasonic sensitivity with the wavelength difference along with the reflective curve of FBG sensor. Signal-to-noise ratios of ultrasonic response signal are plotted as circles in the figure. Signal-to-noise ratio exceeds 40 dB when the filter wavelength is 1.4 nm shorter than the Bragg wavelength. Signal-to-noise ratio gradually increases as the wavelength difference decreases and reaches a peak at a wavelength difference of 0.42 nm. Then, signal-to-noise ratio considerably declines as the wavelength difference diminishes. Fig. 12 shows positional relationship between the filter transmissive curve and the FBG reflective curve at the maximum ultrasonic sensitivity. In combination of the filter and FBG sensor employed in the present study, ultrasonic sensitivity was maximized when the intersection of both curves was located at 0.6 in the transmissivity or reflectivity. From these results, the tunable filter was set to 0.42 nm shorter wavelength than the Bragg wavelength of FBG in ultrasonic inspection.
1359
Fig. 11. Variation of ultrasonic sensitivity with difference between the central wavelength of tunable filter and the Bragg wavelength of FBG sensor along with the reflective curve of FBG.
Fig. 12. Positional relationship between the filter transmissive curve and the FBG reflective curve at the maximum ultrasonic sensitivity.
FBG sensor response to ultrasound excited by impulse signal before and after impact damage is shown in Fig. 13. The normalized response signal is depicted because intensity of response signal depended on the state of contact between the transmitter and the specimen. Compared with response signal before impact damage, response signal after impact damage exhibits earlier initial response by 0.2 ls and longer response period. Frequency domain representation of the response signal before and after impact damage is shown in Fig. 14. The maximum frequency components in response before and after damage are 60 kHz and 90 kHz, respectively. The intensity of frequency components in response after damage declines sharply over 200 kHz compared with response before impact damage. There are many matrix cracks and delamination in damage area. Ultrasound propagating through the damage is scattered because matrix crack and delamination result in mechanical discontinuity. Ultrasonic scattering increases with frequency of ultrasound. For that reason, frequency components more than
1360
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
Fig. 13. Response to ultrasound excited by spike signal before and after impact damage.
Fig. 15. Dispersion curve for [0/90]2s.
Fig. 16. Dispersion curve for 0 monolayer.
Fig. 14. Frequency domain representation of ultrasonic response before and after impact damage.
200 kHz in response signal after impact damage must have abated. Consider the reason why response after impact damage shows earlier initial response by 0.2 ls. S0 lamb wave must have dominated ultrasound propagated in the specimen because ultrasound was generated with a shear wave transducer which caused in-plane displacement [10]. S0 wave is propagated through [0/90]2s before impact damage. After specimen was impacted, S0 wave is propagated in 0 and 90 layers separately within the damaged area where delamination exists. S0 wave propagating in 0 layer travels faster than that in 90 layer because ultrasound travels faster in material with higher Young’s modulus. Dispersion curves for [0/90]2s and 0 monolayer are shown in Figs. 15 and 16, respectively. Response after impact damage has the maximum frequency component at 60 kHz. The velocities of S0 wave in [0/90]2s and 0 monolayer at 60 kHz are 7,300 m/s and 9,740 m/s, respectively. Thus, S0 wave propagated in 0 layers within damaged area travels faster than that in intact area. Since S0 wave was propagated in 6.3 mm long damaged area after impact damage,
the expected time difference in S0 wave arrival before and after impact damage, Dt is given by Dt ¼
6:3E 3 6:3E 3 ¼ 0:22 ls 7300 9740
ð4Þ
The expected time difference agrees well with the experimental results. Mode conversion between S0 and A0 can occur at the edge of damaged area. Consider the case that S0 wave is converted into A0 wave when entering damage area and the A0 wave is converted into S0 wave when getting out of damage area. As shown in Fig. 16, velocity of A0 wave in 0 monolayer is 740 m/s which is much smaller than that of S0 wave. The time where A0 wave passes through 6.3 mm long damage area is estimated to be 8.5 ls while the time for S0 wave is only 0.6 ls. Influence of mode conversion on initial response behavior can be negligible because the time difference between two cases is 7.9 ls (see Fig. 13). The longer period of response signal after impact damage would result from ultrasonic scattering in damaged area. Matrix cracking and delamination give rise to mechanical discontinuity in damaged area. Ultrasound
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
propagated in materials with mechanical discontinuity is subjected to scattering which broadens the period of response signals. 5. Conclusions An FBG sensing system including strain measurement and ultrasonic detection for structural health monitoring was constructed. This system consists of a broadband light source, a broadband optical filter for strain measurement and a narrowband tunable filter for ultrasonic detection. The system was applied to strain measurement in impact loading to CFRP and impact damage detection by ultrasonic method. Experimental results proved that FBG sensors could measure strain with higher signal-to-noise ratio in wider frequency range compared with conventional strain gauges. FBG sensors would be highly reliable in long-term strain measurement because of its self-referencing and immunity to electromagnetic interference. Furthermore, ultrasonic inspection in which ultrasonic sensitivity was maximized using the tunable filter could detect a 6.3 · 9 mm2 impact damage. These experimental results have confirmed that the FBG sensing system can be useful tools to monitor strain and damage in composite materials. Acknowledgements This project was conducted as a part of the project, ‘‘Civil Aviation Fundamental Technology Program–Advanced Materials & Process Development for Next-Generation Aircraft Structures’’ under contract from RIMCOF, founded by the Ministry of Economy, Trade and Industry (METI) of Japan. The authors also thank Shunji Eguchi for technical assistance.
1361
References [1] Lin M, Chang F. The manufacture of composite structures with a built-in network of piezoceramics. Compos Sci Technol 2002;62(7–8): 919–39. [2] Ihn J, Chang F. Detection and monitoring of hidden fatigue crack growth using a built-in piezoelectric sensor/acturator network: II. Validation using riveted joints and repair patches. Smart Mater Struct 2004;13(3):621–30. [3] Chang F, editor. Structural health monitoring. Lancaster: DEStech Publications; 2005. [4] Wang B, Takatsubo J, Akimune Y, Tsuda H. Development of a remote impact damage identification system. Struct Control Health Monit 2005;12(3–4):301–14. [5] Othonos A. Fiber Bragg grating. Rev Sci Instrum 1997;68(12): 4309–41. [6] Kuang K, Kenny R, Whelan M, Cantwell W, Chalker P. Embedded fibre Bragg grating sensors in advanced composite materials. Compos Sci Technol 2001;61(10):1379–87. [7] Singhvi A, Mirmiran A. Creep and durability of environmentally conditioned FRP-RC beams using fiber optic sensors. J Reinf Plast Compos 2002;21(4):351–73. [8] Lin Y, Pan C, Kuo Y, Chang K, Chern J. Online monitoring of highway bridge construction using fiber Bragg grating sensors. Smart Mater Struct 2005;14(5):1075–82. [9] Ling H, Lau K, Cheng L, Chow K. Embedded fiber Bragg grating sensors for non-uniform strain sensing in composite structures. Meas Sci Technol 2005;16(12):2415–24. [10] Tsuda H. Ultrasound and damage detection in CFRP using fiber Bragg grating sensors. Compos Sci Technol 2006;66(5): 676–83. [11] S. Kojima, K. Satori, K. Fukuchi, A. Hongo, N. Takeda, Development of multipoint strain measurement systems using small-diameter fiber Bragg gratings. In: Smart structures and materials 2002: proceedings of SPIE, Bellingham, Washington, USA: SPIE, 2002. p. 202–9. [12] Derickson D. Fiber optic test and measurement. Upper Saddle River: Prentice-Hall; 1998. p. 621–38. [13] Melle S, Liu K, Measures M. Practical fiber-optic Bragg grating strain gauge system. Appl Opt 1993;32(19):3601–9.
Strain and damage monitoring of CFRP in impact loading using a fiber Bragg grating sensor system Hiroshi Tsuda *, Jung-Ryul Lee Structural Health Monitoring Group, Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, Tsukuba 305-8568, Japan Received 9 May 2006; received in revised form 18 September 2006; accepted 22 September 2006 Available online 9 November 2006
Abstract A fiber-optic system featuring strain measurement and ultrasonic detection was constructed with fiber Bragg gratings based on wavelength–light intensity conversion technique. This fiber Bragg grating sensing system consists of a broadband light source, a broadband optical filter for strain measurement and a narrowband tunable filter for ultrasonic detection. The system was applied to strain measurement in impact loading to carbon fiber-reinforced plastics and the subsequent impact damage detection. Experimental results demonstrated that fiber Bragg grating sensors could measure strain with higher resolution compared with conventional strain gauges. Furthermore, ultrasonic inspection in which ultrasonic sensitivity was maximized by controlling the transmissive wavelength of the tunable filter could detect a 6.3 · 9 mm2 impact damage. 2006 Elsevier Ltd. All rights reserved. Keywords: Fiber Bragg gratings; D. Non-destructive testing; D. Ultrasonics
1. Introduction Carbon fiber-reinforced plastics (CFRP) have been widely applied to automobile and aerospace applications because of their high specific strength and rigidity. However, CFRP have a grave disadvantage that the compressive strength declines substantially after being subjected to impact damage. Hence, detection of impact loading and damage is very important to secure the reliability of CFRP structures. Impact loading can be detected by strain measurement and the induced damage can be inspected by ultrasonic technique. Conventionally, strain has been measured with resistive strain gauges and ultrasound has been detected with piezoelectric sensors. Smart structures in which structural conditions are monitored with built-in sensors have drawn attention in these days [1–4]. In smart structures including detection of both *
Corresponding author. Tel.: +81 29 861 92 84; fax: +81 29 861 58 82. E-mail address: [email protected] (H. Tsuda).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.09.017
impact loading and damage using conventional technique, strain gauges and piezoelectric sensors would be installed. However, these electric sensors have serious drawbacks that they suffer from electromagnetic interference and cannot be used in explosive environment. Moreover, they need individual cables. Therefore, the monitoring system applied to large-scale structures where multi-point sensing is performed would inevitably include complex cable network. In order to overcome these technical difficulties, fiber Bragg gratings (FBGs) featuring multifunction, multiplexibility as well as immunity to electromagnetic interference have been expected to be promising sensors for structural health monitoring [5]. FBG reflects a narrowband light whose central wavelength is called the Bragg wavelength when broadband light is launched. FBG can work as strain sensor since the Bragg wavelength varies with strain FBG is subjected to. In the past studies [6–9], the Bragg wavelength has been evaluated with the optical spectrum analyzer (OSA). Application of FBG strain sensor has been limited to static strain
1354
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
measurement because the sampling rate of OSA is a few hertz at a maximum. The objective of the present study is to develop a simple FBG sensing system which can measure strain and detect ultrasound for structural health monitoring. We constructed an FBG sensing system including a broadband light source and two optical filters for sensing strain and ultrasound. A wavelength–light intensity conversion technique was utilized to detect the shift in the Bragg wavelength at fast speed. The FBG sensing system was applied to monitor strain in CFRP during impact loading and to detect the induced damage. 2. Wavelength–light intensity conversion technique Consider the optical system shown in Fig. 1. Broadband light is transmitted to an FBG via an optical circulator and the light reflected from the FBG is sent to an optical filter. We assume that the optical filter has characteristics whose transmissivity declines with wavelength in a wider wavelength range compared with the reflective bandwidth of the FBG. The Bragg wavelength shifts to longer wavelength when the FBG is elongated. Then the intensity of light transmitted through the filter decreases because the transmissivity at longer wavelength is smaller. Conversely, the intensity of transmissive light increases when the FBG is compressed. Thus, shift in the Bragg wavelength induced
by strain can be converted into light intensity with optical filters. Light intensity can be measured with photodetectors. The response frequency of photodetectors is usually over 10 MHz. Hence, strain can be measured at fast speed by measuring the intensity of light demodulated with optical filters. Details of ultrasonic detection using wavelength–light intensity conversion technique have been described in a previous paper [10]. An outline of detection principle is given here. Propagation of ultrasound in materials causes strain change in the microstrain range over several tens of kHz. Shift in the Bragg wavelength caused by the subtle strain change ranges in sub-pico meters. In order to detect such a subtle Bragg wavelength change, the optical filter must meet stringent optical characteristics whose transmissive bandwidth is comparable with the reflective range of the FBG sensor as shown in Fig. 1. The intensity of light transmitted through the filter is represented by the area where the reflectivity of the sensor overlaps with the transmissivity of the filter. Thus, the small variation in the Bragg wavelength of FBG can be detected as the change in the intensity of light transmitted through the filter. 3. Experimental procedure As mentioned in the previous section, it is necessary to use broadband and narrowband optical filters to measure
Fig. 1. Principle of wavelength–light intensity conversion technique using optical system consisting of a broadband light source and an optical filter.
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
strain and ultrasound, respectively. In the present study, an optical system which can measure two quantities of strain and ultrasound was constructed by changing the optical path of light reflected from the FBG using an optical switch as shown in Fig. 2. Broadband light is transmitted to an FBG sensor and the light reflected from the FBG is sent to the optical switch. The optical path switches to port A where broadband filter is connected when strain is measured. Light transmitted through and reflected from the broadband filter goes to photodetectors where the intensity of light is converted into voltage signal. The switch is set to port B where narrowband tunable filter is connected when ultrasound is detected. Then, the intensity of light transmitted through the narrowband filter is converted into voltage signal. This optical measurement system requires a very stable light source and optical filters without any drift from the external disturbance such as change in temperature. Amplified spontaneous emission (ASE) light source whose fluctuation in optical power is less than ±0.5% was employed. The transmissive wavelength of both optical filters used in the present study fluctuates a few picometers per 1 C. Experiments were performed under constant temperature conditions in order to eliminate the influence of temperature variation on transmissive wavelength of optical filters. Optical characteristics of the broadband filter for strain measurement are shown in Fig. 3. The broadband optical filter is made of dielectric multilayer. The transmissivity and reflectivity monotonically decreases and increases with wavelength, respectively, in a range from 1548 to 1553 nm. Transmissivity of narrowband filter for ultrasonic detection is shown in Fig. 4 along with the reflective curve of FBG sensor. The narrowband optical filter is a tunable optical filter whose transmissive wavelength can be controlled by computer. FBG employed in the present study was provided by Hitachi Cable Co. Ltd. The Bragg wave-
1355
Fig. 3. Optical characteristics of the optical filter for strain measurement.
length at strain free was 1550.1 nm and the grating length was 1.5 mm. The diameters of the cladding and outer polyimide coating were 40 lm and 52 lm, respectively. Although this fiber has a smaller diameter, the FBG characteristics are the same as those of conventional FBGs [11]. The full widths at half-maximum (FWHM) of the FBG and optical filter are 0.57 nm and 0.38 nm, respectively. The monitored specimen was a 300 · 300 · 1 mm3 CFRP (T800H/3631) whose stacking sequence was [0/90]2s. FBG was attached on the surface of the specimen 50 mm away from the center of CFRP plate. A resistive strain gauge was attached next the FBG sensor in order to measure strain in impact loading for reference. The specimen was bilaterally supported by a steel clamp so that the specimen was suspended in impact loading. The impact loading at an energy of 2.7 J was applied at the center of the specimen using a semi-sphere impactor. A 6.3 · 9 mm2 elliptically-shaped damage was observed by C-scope scan although only small dent could be seen in damaged area from naked eye obser-
300 broadband light source shear wave transmitter 50
FBG sensor
A
broadband optical filter for strain measurement
50 optical switch
9
300
transmitted light
reflected light
B
6.3 narrowband tunable filter for ultrasonic detection
strain gauge weight drop impact damage
pulse generator
[0/90]2s CFRP
steel clamper
strain amp
photodetector
recorder
sampling rate = 100kS/s in strain measurement = 100MS/s in ultrasound detection /mm
Fig. 2. Experimental setup for strain measurement in impact loading and ultrasonic inspection.
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
Normalized reflectivity of FBG
1
1
0.5
0.5
tunable filter FWHM = 0.38nm
1.5mm long FBG FWHW = 0.57nm
0
Normalized transmissivity of tuanble filter
1356
0 -1
0
1
-1
0
1
Relative wavelength, nm Fig. 4. Optical characteristics of the FBG sensor and the tunable filter.
vation. Strain measured from the FBG sensor and strain gauge was recorded at a sampling rate of 100 kS/s. A shear wave ultrasonic transmitter whose nominal frequency was 250 kHz was attached on the specimen surface in alignment with the optical fiber axis 100 mm away from the FBG sensor. An impulse signal was input to the transmitter to generate ultrasound. Response to ultrasound was recorded at a sampling rate of 100 MS/s before and after impact damage. 4. Experimental results 4.1. Calibration of FBG strain measuring system In strain measurement through wavelength–light intensity conversion, strain is evaluated from change in intensity of light demodulated by optical filter. The formulation of the following two relations is a prerequisite for quantitative strain evaluation. 1. Shift in the Bragg wavelength with strain applied to the FBG. 2. Variation in intensity of light transmitted through and reflected from the optical filter with the Bragg wavelength of FBG. These relations were obtained from the preliminary experiment in which an FBG attached on a thin stainless steel plate was strained by bending and the strain was evaluated with a resistive strain gauge attached next to the FBG. The relation between the Bragg wavelength of small diameter FBG and strain is shown in Fig. 5. The Bragg wavelength kB varies in proportion to strain in a strain range of ±0.1% and the relation is given by
Fig. 5. Relation between the Bragg wavelength of FBG sensor and strain applied to FBG.
kB ðnmÞ ¼ 1550:1 þ 12:4 e ð%Þ
ð1Þ
An imposed strain of 1% leads to a 12.4 nm shift in the Bragg wavelength of the FBG employed in the present study. The optical filter for strain measurement has characteristics which vary monotonically in a wavelength range from 1548 nm to 1553 nm. In the combination of the FBG and the optical filter, intensity of light transmitted through and reflected from the optical filter changes monotonically with strain in a range from 0.17% to 0.23% since the Bragg wavelength at strain free was 1550.1 nm Variation in two photodetector outputs which correspond to intensities of light transmitted through and reflected from the optical filter with the Bragg wavelength is shown in Fig. 6. As expected from the filter characteristics, intensity of light transmitted through and reflected from the optical filter decreases and increases with the Bragg wavelength, respectively. This figure shows the light
2.0
1357 2.0
transmitted light, case 1 transmitted light, case 2 1.5
1.5
1.0
1.0 reflected light, case 1 reflected light, case 2
0.5
0.0 1547
0.5
1548
1549
1550
1551
1552
PD output for light reflected from the filter, Vreflect
PD output for light transmitted through the filter, Vtrans
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
0.0 1553
Bragg wavelength λB, nm Fig. 6. Change in intensity of light transmitted through and reflected from the optical filter with strain applied to FBG.
intensities in two cases where fiber-optic link was changed. In the present experimental system, fiber-optic link was made by mechanical splice using the FC optical connectors [12]. Connector loss in mechanical splice varies whenever connection is changed. This is because alignment of optical fibers in connectors varies at every connection. Connector loss in case 2 was smaller than that in case 1 because intensity of light in case 2 was higher than that in case 1. Fiber-optic system with mechanical splices inevitably gives rise to variation in connector loss. Therefore, the Bragg wavelength cannot be determined directly from the intensity of light transmitted through and reflected from the filter. A parameter, R which corresponds to the ratio of the difference to the sum of the two photodetector outputs is introduced in order to eliminate the influence of connector loss on the relation between the Bragg wavelength and light intensity [13]. R¼
V reflect V trans V reflect þ V trans
ð2Þ
where Vtrans and Vreflect denote photodetector outputs for light transmitted through and reflected from the filter, respectively. Relation between the parameter R and the Bragg wavelength is shown in Fig. 7. The relation shows one-to-one relation between the Bragg wavelength and the parameter R in any optical connections. Fig. 8 shows the relation between the parameter R and strain which was obtained by converting the Bragg wavelength in Fig. 7 into strain using Eq. (1). The relation between the parameter R and strain was approximated by the 5th order polynomial given by Eq. (3). The correlation coefficient between the approximate polynomial and the experimental results is over 0.999, so the polynomial gives good approximation.
e ð%Þ ¼ a0 þ a1 R þ a2 R2 þ a3 R3 þ a4 R4 þ a5 R5 ð3Þ where a0 ¼ 21:46E 3 a1 ¼ 175:14E 3 a2 ¼ 7:7474E 3 a3 ¼ 141:74E 3 a4 ¼ 321:63E 3 a5 ¼ 446:12E 3 Strain can be evaluated from Eq. (3) by substituting the parameter R which is experimentally determined from photodetector outputs for intensity of light transmitted through and reflected from the optical filter. 4.2. Strain measurement in impact loading Strain measured with the FBG sensor and the strain gauge during impact loading to the CFRP specimen is shown in Fig. 9. Strain behavior obtained from both sensors shows a good agreement. The maximum tensile strains evaluated from FBG sensor and strain gauge were 204le and 208le and the maximum compressive strains were 99le and 97le, respectively. The experimental results demonstrated that FBG could measure strain correctly through wavelength–light intensity conversion technique. Note noise level in two sensor signals. The background noise in strain gauge is greater than that in FBG sensor. Spikes in strain gauge signal are observed at 0.014 s and 0.14 s as arrowed in Fig. 9. Moreover, sudden relatively large fluctuation appeared at 0.09 s only in strain gauge signal. These considerable noises in strain gauge signal would result from electromagnetic interference. By taking the
1358
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
Ratio of the difference to the sum of the two photodetector outputs, R
0.8
0.6
case 1 case 2
0.4
0.2
0.0
-0.2
-0.4
-0.6 1547
1548
1549
1550
1551
1552
1553
Bragg wavelength λB, nm Fig. 7. Relationship between the Bragg wavelength of FBG and the parameter R.
0.2 case 1 case 2
Strain ε, %
0.1
fitted curve
0.0
-0.1
-0.2 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Ratio of the difference to the sum of the two photodetector outputs, R Fig. 8. Relationship between the parameter R and strain.
definition for the minimum detectable signal as the rootmean-square value of signal recorded before impact loading, the minimum detectable strain for FBG and strain gauge are estimated to be 1.2 le and 8.1 le, respectively. The frequency domain representation of two sensor signals is shown in Fig. 10. While both sensor signals have a similar distribution up to 1 kHz, strain gauge signal exhibits relatively high intensity at 1.5 kHz and over 9 kHz. Here we assume that the frequency components over 1 kHz are noise because the normalized intensities in FBG sensor signal are always less than 102. Signalto-noise ratios of FBG and strain gauge signals are evaluated to be 40 dB and 8 dB, respectively. Signal-to-noise ratio can be improved by low-pass filtering, whereas an
abrupt strain change over the cutoff frequency of low-pass filter cannot be measured. It can be confirmed from the experimental results that FBG sensors can measure strain more precisely in wider frequency range compared to conventional strain gauges. 4.3. Impact damage detection by ultrasonic inspection In ultrasonic detection using the present sensing system, the difference between the central wavelength of the filter and the Bragg wavelength of the FBG influenced significantly on ultrasonic sensitivity. Influence of the wavelength difference on ultrasonic sensitivity was investigated by shifting the transmissive wavelength of the tunable filter
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
Fig. 9. Strain measured with FBG sensor and strain gauge in impact loading.
Fig. 10. Frequency domain representation of FBG sensor and strain gauge signals.
in order to determine the optimum wavelength difference for ultrasonic detection. Fig. 11 shows the variation of ultrasonic sensitivity with the wavelength difference along with the reflective curve of FBG sensor. Signal-to-noise ratios of ultrasonic response signal are plotted as circles in the figure. Signal-to-noise ratio exceeds 40 dB when the filter wavelength is 1.4 nm shorter than the Bragg wavelength. Signal-to-noise ratio gradually increases as the wavelength difference decreases and reaches a peak at a wavelength difference of 0.42 nm. Then, signal-to-noise ratio considerably declines as the wavelength difference diminishes. Fig. 12 shows positional relationship between the filter transmissive curve and the FBG reflective curve at the maximum ultrasonic sensitivity. In combination of the filter and FBG sensor employed in the present study, ultrasonic sensitivity was maximized when the intersection of both curves was located at 0.6 in the transmissivity or reflectivity. From these results, the tunable filter was set to 0.42 nm shorter wavelength than the Bragg wavelength of FBG in ultrasonic inspection.
1359
Fig. 11. Variation of ultrasonic sensitivity with difference between the central wavelength of tunable filter and the Bragg wavelength of FBG sensor along with the reflective curve of FBG.
Fig. 12. Positional relationship between the filter transmissive curve and the FBG reflective curve at the maximum ultrasonic sensitivity.
FBG sensor response to ultrasound excited by impulse signal before and after impact damage is shown in Fig. 13. The normalized response signal is depicted because intensity of response signal depended on the state of contact between the transmitter and the specimen. Compared with response signal before impact damage, response signal after impact damage exhibits earlier initial response by 0.2 ls and longer response period. Frequency domain representation of the response signal before and after impact damage is shown in Fig. 14. The maximum frequency components in response before and after damage are 60 kHz and 90 kHz, respectively. The intensity of frequency components in response after damage declines sharply over 200 kHz compared with response before impact damage. There are many matrix cracks and delamination in damage area. Ultrasound propagating through the damage is scattered because matrix crack and delamination result in mechanical discontinuity. Ultrasonic scattering increases with frequency of ultrasound. For that reason, frequency components more than
1360
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
Fig. 13. Response to ultrasound excited by spike signal before and after impact damage.
Fig. 15. Dispersion curve for [0/90]2s.
Fig. 16. Dispersion curve for 0 monolayer.
Fig. 14. Frequency domain representation of ultrasonic response before and after impact damage.
200 kHz in response signal after impact damage must have abated. Consider the reason why response after impact damage shows earlier initial response by 0.2 ls. S0 lamb wave must have dominated ultrasound propagated in the specimen because ultrasound was generated with a shear wave transducer which caused in-plane displacement [10]. S0 wave is propagated through [0/90]2s before impact damage. After specimen was impacted, S0 wave is propagated in 0 and 90 layers separately within the damaged area where delamination exists. S0 wave propagating in 0 layer travels faster than that in 90 layer because ultrasound travels faster in material with higher Young’s modulus. Dispersion curves for [0/90]2s and 0 monolayer are shown in Figs. 15 and 16, respectively. Response after impact damage has the maximum frequency component at 60 kHz. The velocities of S0 wave in [0/90]2s and 0 monolayer at 60 kHz are 7,300 m/s and 9,740 m/s, respectively. Thus, S0 wave propagated in 0 layers within damaged area travels faster than that in intact area. Since S0 wave was propagated in 6.3 mm long damaged area after impact damage,
the expected time difference in S0 wave arrival before and after impact damage, Dt is given by Dt ¼
6:3E 3 6:3E 3 ¼ 0:22 ls 7300 9740
ð4Þ
The expected time difference agrees well with the experimental results. Mode conversion between S0 and A0 can occur at the edge of damaged area. Consider the case that S0 wave is converted into A0 wave when entering damage area and the A0 wave is converted into S0 wave when getting out of damage area. As shown in Fig. 16, velocity of A0 wave in 0 monolayer is 740 m/s which is much smaller than that of S0 wave. The time where A0 wave passes through 6.3 mm long damage area is estimated to be 8.5 ls while the time for S0 wave is only 0.6 ls. Influence of mode conversion on initial response behavior can be negligible because the time difference between two cases is 7.9 ls (see Fig. 13). The longer period of response signal after impact damage would result from ultrasonic scattering in damaged area. Matrix cracking and delamination give rise to mechanical discontinuity in damaged area. Ultrasound
H. Tsuda, J.-R. Lee / Composites Science and Technology 67 (2007) 1353–1361
propagated in materials with mechanical discontinuity is subjected to scattering which broadens the period of response signals. 5. Conclusions An FBG sensing system including strain measurement and ultrasonic detection for structural health monitoring was constructed. This system consists of a broadband light source, a broadband optical filter for strain measurement and a narrowband tunable filter for ultrasonic detection. The system was applied to strain measurement in impact loading to CFRP and impact damage detection by ultrasonic method. Experimental results proved that FBG sensors could measure strain with higher signal-to-noise ratio in wider frequency range compared with conventional strain gauges. FBG sensors would be highly reliable in long-term strain measurement because of its self-referencing and immunity to electromagnetic interference. Furthermore, ultrasonic inspection in which ultrasonic sensitivity was maximized using the tunable filter could detect a 6.3 · 9 mm2 impact damage. These experimental results have confirmed that the FBG sensing system can be useful tools to monitor strain and damage in composite materials. Acknowledgements This project was conducted as a part of the project, ‘‘Civil Aviation Fundamental Technology Program–Advanced Materials & Process Development for Next-Generation Aircraft Structures’’ under contract from RIMCOF, founded by the Ministry of Economy, Trade and Industry (METI) of Japan. The authors also thank Shunji Eguchi for technical assistance.
1361
References [1] Lin M, Chang F. The manufacture of composite structures with a built-in network of piezoceramics. Compos Sci Technol 2002;62(7–8): 919–39. [2] Ihn J, Chang F. Detection and monitoring of hidden fatigue crack growth using a built-in piezoelectric sensor/acturator network: II. Validation using riveted joints and repair patches. Smart Mater Struct 2004;13(3):621–30. [3] Chang F, editor. Structural health monitoring. Lancaster: DEStech Publications; 2005. [4] Wang B, Takatsubo J, Akimune Y, Tsuda H. Development of a remote impact damage identification system. Struct Control Health Monit 2005;12(3–4):301–14. [5] Othonos A. Fiber Bragg grating. Rev Sci Instrum 1997;68(12): 4309–41. [6] Kuang K, Kenny R, Whelan M, Cantwell W, Chalker P. Embedded fibre Bragg grating sensors in advanced composite materials. Compos Sci Technol 2001;61(10):1379–87. [7] Singhvi A, Mirmiran A. Creep and durability of environmentally conditioned FRP-RC beams using fiber optic sensors. J Reinf Plast Compos 2002;21(4):351–73. [8] Lin Y, Pan C, Kuo Y, Chang K, Chern J. Online monitoring of highway bridge construction using fiber Bragg grating sensors. Smart Mater Struct 2005;14(5):1075–82. [9] Ling H, Lau K, Cheng L, Chow K. Embedded fiber Bragg grating sensors for non-uniform strain sensing in composite structures. Meas Sci Technol 2005;16(12):2415–24. [10] Tsuda H. Ultrasound and damage detection in CFRP using fiber Bragg grating sensors. Compos Sci Technol 2006;66(5): 676–83. [11] S. Kojima, K. Satori, K. Fukuchi, A. Hongo, N. Takeda, Development of multipoint strain measurement systems using small-diameter fiber Bragg gratings. In: Smart structures and materials 2002: proceedings of SPIE, Bellingham, Washington, USA: SPIE, 2002. p. 202–9. [12] Derickson D. Fiber optic test and measurement. Upper Saddle River: Prentice-Hall; 1998. p. 621–38. [13] Melle S, Liu K, Measures M. Practical fiber-optic Bragg grating strain gauge system. Appl Opt 1993;32(19):3601–9.