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Ultrasound and damage detection in CFRP using fiber Bragg grating sensors
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 66 (2006) 676–683 www.elsevier.com/locate/compscitech
Ultrasound and damage detection in CFRP using fiber Bragg grating sensors Hiroshi Tsuda
*
Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 2, Tsukuba 305-8568, Japan Received 10 November 2004; accepted 18 July 2005 Available online 10 October 2005
Abstract Ultrasound detection using fiber Bragg gratings (FBGs) and its application to damage detection were investigated. Two types of FBG ultrasonic sensing system were constructed using different light source: a broadband light source and a tunable laser source. Ultrasonic waves generated with a piezoelectric transducer were propagated through a cross-ply CFRP and were detected by the FBG sensing systems at several ultrasonic source-sensor intervals. Furthermore, the FBG sensing systems were applied to ultrasonic inspection of an impact damaged cross-ply CFRP in order to examine their capability for damage detection. Both FBG sensing systems produced better detection of the damage in the composite than piezoelectric sensors. The FBG ultrasonic sensing system including a tunable laser possessed high sensitivity to ultrasonic wave and proved to be an alternative to conventional piezoelectric sensors in ultrasonic inspection of structural composite materials. 2005 Elsevier Ltd. All rights reserved. Keywords: Fiber Bragg gratings; D. Non-destructive testing; Ultrasonics
1. Introduction Ultrasonic inspection has extensively been conducted in structural health monitoring. Damage in structures affects propagation of ultrasonic wave. Thus, the presence of damage can be identified when the detected ultrasonic signal deviates from the reference signal of undamaged structure. So far ultrasonic inspection of structures has been performed using piezoelectric devices as ultrasonic transmitters and sensors [1]. Piezoelectric sensors, however, have a serious drawback of electromagnetic interference. Fiber-optic sensors that are immune to electromagnetic interference have been expected to be an alternative ultrasonic sensor. Several studies on ultrasonic detection with optical interferometric sensors have been reported [2–7]. Although optical interferometric sensors allow sensitive ultrasonic detection, a phase control system is required to maintain *
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the optimum sensitivity. In the past years, ultrasonic detection using fiber Bragg grating (FBG) sensors have been reported [8–12]. An FBG is a wavelength encoding sensor thereby making the sensor self-referencing, rendering it independent of fluctuating light levels and the system immune to source power and connector losses that plague many other types of fiber-optic sensors. From the advantages mentioned above, FBGs are considered to be promising ultrasonic sensors in structural health monitoring. FBG ultrasonic sensing systems can be classified into two types according to light source employed. One is a system including a broadband light source and an optical filter [8,9]. Ultrasonic wave can be detected through an optical filter processing of the light reflected from FBG sensor. Another is a system with a tunable laser source in which the intensity of light reflected from FBG sensor directly corresponds to ultrasonic response [10–12]. To the authorÕs knowledge, however, capability for ultrasonic detection and damage monitoring of FBG sensing systems is not well investigated.
H. Tsuda / Composites Science and Technology 66 (2006) 676–683 broadband light source
optical circulator
FBG sensor
FBG filter photo detector
a
b
Reflectivity / Transmissivity
In the present study, two types of FBG sensing system with either a broadband light source or a tunable laser source were constructed. Using the systems constructed, ultrasonic wave propagating through a cross-ply CFRP was detected at several ultrasonic source-sensor intervals. Sensitivity to ultrasound of the two systems was evaluated. Then, these systems were applied to ultrasonic inspection of an impact damaged CFRP. Ultrasonic wave was propagated through either damaged area or only intact area and the resultant responses were recorded. The influence of damage on response behaviour was investigated. Furthermore, ultrasonic inspection using a piezoelectric sensor was conducted to compare with damage detectability of the FBG sensing systems.
677
compression
strain free
tension
wavelength
Fig. 1. FBG ultrasonic detection system with a broadband light source and its principle of operation. (a) FBG ultrasonic detection system with a broadband light source, (b) a schematic illustrating the variation in reflectivity of the FBG sensor with applied strain. Solid and dotted curves correspond to the reflectivity of the FBG sensor and the transmissivity of the FBG filter, respectively.
2. Ultrasonic detection using FBGs 2.1. Fundamentals of FBGs [13] An FBG has periodical variation in the refractive index within the core of an optical fiber and acts as a narrowband reflection filter. The central wavelength of light reflected from an FBG is called the Bragg wavelength, kB and is given by
An imposed strain of 1 · 10 would lead to a 1.22 pm shift in the Bragg wavelength of an FBG whose Bragg wavelength at strain free is 1550 nm. The shift in the Bragg wavelength is positive when the FBG expands. Conversely, the Bragg wavelength shifts to negative when the FBG contracts.
the figure are simplified forms than the actual characteristics. Solid and dotted curves in the figure represent the reflectivity of the sensor and the transmissivity of the filter, respectively. The area enclosed by the reflective curve corresponds to distribution of light reflected from the sensor. The light that can be transmitted through the filter is represented by the area enclosed by the transmissive curve of the filter. Hence, the area where the reflectivity of the sensor overlaps with the transmissivity of the filter corresponds to the intensity of light transmitted through the filter. The area is shaded in Fig. 1(b). The Bragg wavelength of the sensor shifts to a longer wavelength kt when the sensor is elongated. Then, the shaded area grows and so the intensity of light detected with the photodetector increases. Conversely, the Bragg wavelength shifts to a shorter wavelength kc and the intensity of light transmitted through the filter decreases when the sensor is compressed. Thus, ultrasonic wave can be detected from an FBG sensor in combination with the broadband light system.
2.2. Ultrasonic detection by a broadband light system
2.3. Ultrasonic detection by a tunable laser system [11,12]
Details about principle of ultrasonic detection by a system including broadband light source were described in the previous paper [9]. A brief explanation is given here using Fig. 1. The system is schematically shown in Fig. 1(a). Broadband light travels to the FBG sensor via an optical circulator and light reflected from the sensor passes another FBG for filtering. Light transmitted though the filter reaches a photodetector where the intensity of light is converted into voltage signal. Here, we assume that the Bragg wavelength of the filter kf stays constant and the FBG sensor in strain free state has a slightly longer Bragg wavelength k0 than kf. Fig. 1(b) shows the transmissivity of the FBG filter and the change in reflectivity of FBG sensor with varying strain applied to the FBG sensor. These characteristic curves shown in
An ultrasonic sensing system including a tunable laser source is shown in Fig. 2(a). The laser emission wavelength is set to kout where reflectivity of the sensor at strain free is reduced by half as shown in Fig. 2(b). The solid curve in the figure represents the reflectivity of the sensor at strain free. Laser light is launched into the FBG sensor via an optical circulator. The light reflected from the sensor is transmitted to the photodetector without filter processing. When the sensor expands, the Bragg wavelength shifts to a longer wavelength. Then, the reflectivity of the sensor at the lasing wavelength rises as shown in Fig. 2(b) and so the intensity of light reflected from the sensor increases. Conversely, intensity of light reflected from the sensor decreases when the sensor contracts. In the system including a tunable laser source, the intensity of light reflected from
kB ¼ 2nK;
ð1Þ
where n and K are the effective refractive index of the fiber core and the grating period, respectively. The relative shift in the Bragg wavelength, DkB for an applied strain along the fiber axis by e under a constant temperature condition is given by DkB ¼ 0:787kB e.
ð2Þ 6
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H. Tsuda / Composites Science and Technology 66 (2006) 676–683
tunable laser
optical circulator
FBG sensor
Reflectivity
a
photo detector
b
Wavelength
Fig. 2. FBG ultrasonic detection system with a tunable laser source and its principle of operation. (a) FBG ultrasonic detection system with a tunable laser light source, (b) a schematic illustrating the variation in reflectivity at the lasing wavelength when the FBG sensor expands.
the FBG sensor directly corresponds to ultrasonic response. 3. Detectability for ultrasound of FBG sensing systems 3.1. Experimental procedure Ultrasonic wave was detected at several ultrasonic source–sensor intervals in order to evaluate ultrasound detectability of FBG sensing systems. Ultrasonic waves were propagated through a 340 · 340 · 1 mm3 CFRP (T800H/3631) whose stacking sequence was [0/90]2s. Ultrasonic waves propagating through thin plates are referred to as the Lamb waves. Lamb waves can be categorized into two modes by the displacement with regard to the plate plane: symmetrical and asymmetrical modes which are, respectively, called S and A modes for short. A previous paper reported that FBG sensors were more sensitive to S mode waves than A mode waves [14]. Thus, a piezoelectric shear wave transmitter (Panametrics, V-150) which generates S mode waves was used as ultrasonic source. The ultrasonic transmitter has a diameter of 25 mm and a central frequency of 250 kHz. Lamb wave propagation is characterized by the product of specimen thickness and the wave frequency. Only fundamental S mode waves called S0 waves can spread over cross-ply CFRP when the product is less than 1 MHz mm [15]. The product is 0.25 MHz mm under the present experimental condition. Hence, only S0 waves can propagate through the specimen. Two types of ultrasonic sensing system were constructed. Experimental setup employed is shown in
Fig. 3. The broadband light system works when optical switches 1 and 2 in the figure are set to port A. Then, the light reflected from FBG sensor passes the FBG filter and the light transmitted through the filter arrives at the photodetector. When both optical switches are set to port B, the tunable laser system works. Then, the light reflected from FBG sensor streams into the photodetector directly. The ultrasonic transmitter was placed in contact with the CFRP plate using a highly viscous gel as a couplant and was driven by a spike signal emitted from a pulser (PAC, C-101HV). FBG sensor was glued with adhesive for strain gauges to the CFRP surface 50 mm away from the edge. S0 waves were detected at a transmitter-sensor interval ranging from 50 to 250 mm every 50 mm. The photodetector output was recorded at a sampling rate of 100 MHz and the average from 512 waveform acquisitions was taken. Wavelength characteristics of FBGs employed and the lasing wavelength are shown in Fig. 4. Both FBGs for sensing and filtering has a grating length of 10 mm, a half-value width of approximately 0.2 nm and their Bragg wavelengths in strain free state are 1550.28 and 1550.17 nm, respectively. In the tunable laser system, the laser emission wavelength was set to be 1550.38 nm where the reflectivity of FBG sensor was nearly 50%. Here, we note the relation between the polarity of response and the deformation of FBG sensor. In the broadband light system, the Bragg wavelength of the sensor is slightly longer than that of the filter as shown in Fig. 4. This is the same Bragg wavelength condition as shown in Fig. 1(b). In the tunable laser system, the position of the lasing wavelength in the reflective curve of the sensor is
H. Tsuda / Composites Science and Technology 66 (2006) 676–683
A optical switch 1 fiber direction
[0/90] 2s CFRP
FBG sensor
optical circulator
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broadband light source
tunable laser B
ultrasonic transmitter
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A FBG filter photo detector
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optical switch 2 sensor - transmitter interval = 50, 100, 150, 200, 250
B
recorder
pulser
/mm
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transmitter-sensor interval = 250mm
1
1.0
0 -1 1
0.8 reflectivity of the FBG sensor Bragg wavelength = 1550.28nm 0.6
transmissivity of the FBG filter Bragg wavelength = 1550.17nm lasing wavelength = 1550.38nm
0.4
Normalized response
Normalized reflectivity and transmissivity
Fig. 3. Experimental setup in the ultrasonic detection test.
0.2
200mm
0 -1 1
150mm
0 -1 1
100mm
0 -1 1
50mm
0 0.0 1549.5
1550
1550.5
-1
1551
0
Wavelength, nm
the same as shown in Fig. 2(b). Consequently, the relation between them accords with the explanation described in Sections 2.2 and 2.3. In either ultrasonic sensing system employed, the response signal increases and decreases when the FBG sensor expands and contracts, respectively. 3.2. Experimental results Figs. 5 and 6 show the evolution of response signals with transmitter-sensor intervals received from the broadband light system and the tunable laser system, respectively. The intensity of response signals was normalized by the maximum value of response signals from respective systems. The abscissa, time was set to be zero when the pulser emitted spike signal to the ultrasonic transmitter. Response from either system demonstrated rising behavior on S0 wave arrival. As mentioned in the end of Section 3.1, the response in either system grows when the FBG sensor expands. Thus, the FBG sensor must have been stretched on the first arrival of S0 wave. Beginning of response delays in proportion to the transmitter-sensor interval. The velocity of S0 wave was mea-
40
60
80
Time,µs
Fig. 5. Response to S0 wave received from the broadband light system.
transmitter-sensor interval = 250mm
1 0 -1 1
Normalized response
Fig. 4. Wavelength characteristics of FBGs and lasing wavelength in the ultrasonic detection test.
20
200mm
0 -1 1
150mm
0 -1 1
100mm
0 -1 1
50mm
0 -1 0
20
40
60
80
Time, µs
Fig. 6. Response to S0 wave received from the tunable laser system.
sured experimentally from the relation between the time of response onset and the transmitter-sensor interval. The measured velocities were 7170 and 7270 m/s from the broadband light system and the tunable laser system,
H. Tsuda / Composites Science and Technology 66 (2006) 676–683
Table 1 Unidirectional YoungÕs moduli, density and PoissionÕs ratio of the CFRP employed 158 8.67 1570 0.38
Another PoissonÕs ratio m21 is given by m21 ¼ EE21 m12 .
respectively. Wave velocity can theoretically be estimated from mechanical properties of material where the wave propagates. Eq. (3) gives the velocity of S0 wave in direction of 0 or 90 of cross-ply composites [16] sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E1 þ E 2 ; ð3Þ V ¼ 2qð1 m12 m21 Þ where E, q and m are YoungÕs modulus, density and PoissonÕs ratio, respectively. The unidirectional YoungÕs moduli of CFRP employed and other properties are listed in Table 1. S0 wave velocity is estimated from Eq. (3) to be 7320 m/s. The velocity estimated theoretically agrees well with experimental results. Influence of the transmitter-sensor interval on the peakto-peak value, Vpp and the signal-to-noise ratio (S/N ratio) given by the following equation was investigated: S=N ratio ¼ 20 log
V pp ; N rms
ð4Þ
where Nrms denotes the root mean square value of signal prior to the ultrasonic response. Figs. 7 and 8 show the change in peak-to-peak value and S/N ratio of response signals from the broadband light system and the tunable laser system, respectively. The value of Vpp should decrease with increasing transmitter-sensor interval while the experimental results showed different behavior. Fluctuation in Vpp would result from the scatter in contact condition between the ultrasonic transmitter and the specimen. The S/ N ratio seems to be independent of the transmitter-sensor interval up to 250 mm in either system and exceeds 40 and 70 dB in the broadband light system and the tunable 80
60
1.0
40
peak-to-peak value, Vpp
S/N ratio, dB
Peak-to-peak value, Vpp
2.0
20
S/N ratio 0.0
0 50
100
150
200
250
Transmitter-sensor interval, mm
Fig. 7. Peak-to-peak value and S/N ratio of response signals from the broadband light system.
Peak-to-peak value, Vpp
Longitudinal YoungÕs modulus, E1 (GPa) Transverse YoungÕs modulus, E2 (GPa) Density, q (kg/m3) PoissonÕs ratio, m12
2.0
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40
peak-to-peak value, Vpp
S/N ratio, dB
680
20
S/N ratio
0.0
0 50
100
150
200
250
Transmitter-sensor interval, mm
Fig. 8. Peak-to-peak value and S/N ratio of response signals from the tunable laser system.
laser system, respectively. In the broadband light system, the sensitivity is limited by a low optical power reflected by the narrowband FBG from a broadband light source. In the tunable laser system, on the other hand, the intensity of light measured with the photodetector amounts to the product of the laser output and the reflectivity of the sensor. Because of the strong light, the tunable laser system permits sensitive ultrasonic detection with a higher S/N ratio. Response signal from the broadband light system, however, has an S/N ratio high enough for practical use. 4. Damage detection using FBG ultrasonic sensing systems 4.1. Experimental procedure Experimental setup employed in the damage detection is shown in Fig. 9. The monitored material was a 290 · 190 · 1 mm3 CFRP (T800H/3631) whose stacking sequence was [0/90]2s. This CFRP plate contains elliptically shaped 65 · 15 mm2 visible damage introduced by a ball dropping at an impact energy of 7.35 J. Fig. 10 shows the damaged area of the CFRP. Splitting and delamination can be seen in the damaged area. The experimental setup except the monitored material was the same employed in the ultrasound detection test shown in Fig. 3. An FBG sensor was attached on the CFRP surface using adhesive for strain gauges. Wavelength characteristics of FBGs employed are shown in Fig. 11. The Bragg wavelengths of the sensor and the filter were 1550.18 and 1550.17 nm, respectively. The laser emission wavelength was set to be 1550.28 nm where the normalized reflectivity of the sensor was approximately 50%. A spike signal emitted from the pulser was input to the ultrasonic transmitter to generate S0 wave. The ultrasonic transmitter was put in two places so that the generated S0 wave could pass the damaged area or only intact area before reaching the FBG sensor. The interval between transmitter and sensor was 100 mm. In addition, ultrasonic inspection using a piezoelectric sensor was also performed for reference. The piezoelectric sensor employed was the
H. Tsuda / Composites Science and Technology 66 (2006) 676–683 A
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broadband light source
optical switch 1 optical circulator
tunable laser B
A
FBG filter
pulser
photo detector optical switch 2 B
ultrasonic transmitter
damaged area
recorder
ultrasonic transmitter 15
65
FBG sensor
100
100
[0/90] 2s CFRP
fiber direction /mm
Fig. 9. Experimental setup for ultrasonic inspection of an impact damaged CFRP.
same piezoelectric device used as the ultrasonic transmitter. The piezoelectric sensor was attached to the same place as the FBG sensor had been attached. FBG and piezoelectric sensor signals were recorded under the same condition for data acquisition as the ultrasound detection test was performed. 4.2. Experimental results
Fig. 10. A photograph showing the damaged area of CFRP employed in the ultrasonic inspection test.
Fig. 12 shows the response signals from the broadband light system. Signal amplitude is normalized by the maximum value of response signal in intact area. In this section, all response signals were normalized in the same way and the time was set to be zero when the pulser emitted spike signal to the ultrasonic transmitter. Response to S0 wave
13.1µs
13.3µs 12.4µs
0.8
reflectivity of the FBG sensor Bragg wavelength = 1550.18nm
0.6
transmissivity of the FBG filter Bragg wavelength = 1550.17nm 0.4 lasing wavelength = 1550.28nm
Normalized response
Normalized reflectivity and transmissivity
1 1
0 propagated through damaged area
10.6µs -1 1
6.8µs
0 0.2
0 1549.5
12.8µs
1550
1550.5
1551
Wavelength, nm Fig. 11. Wavelength characteristics of FBGs and lasing wavelength in the ultrasonic inspection.
-1 -20
0
propagated through intact area
20
Time, µs
40
60
80
Fig. 12. FBG sensor response from the broadband light system in the ultrasonic inspection.
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propagated through damaged area demonstrates different features compared with that in the intact area. First, the initial response starts earlier from 10.6 ls. Second, the first cycle period nearly doubles to 13.1 ls. Third, the peak-topeak value in the first cycle decreases by about 80%. Reasons for these different features in response signal were discussed in the previous paper [9] and are summarized as follows. S0 wave propagates in 0 and 90 layers separately within the damaged area where delamination exists as shown in Fig. 10. S0 wave travels faster in 0 layers within the damaged area than in the intact area because ultrasonic waves travel faster in materials with a higher YoungÕs modulus. Accordingly, S0 wave propagated through damaged area reaches the sensor earlier than that in intact area. The longer period and lower amplitude in the first cycle response would result from the wave dispersion and attenuation caused by the presence of damage. Look at the onset of response more detail. A sinusoidal response appeared in intact area propagation whereas the response to S0 wave propagated through damaged area showed a bit more complex initial behavior. The growth rate in response descends once after the onset of response and then a sinusoidal response appears. These features will be discussed later with the response from the tunable laser system. Fig. 13 shows response signals from the tunable laser system. Both response signals demonstrate a well-defined waveform and the respective starting points of response agree with those in response from the broadband light system. Response to S0 wave propagated through damaged area has nearly double period in the first cycle and approximately 70% peak-to-peak value compared with that in intact area. These are similar features in response signal from the broadband light system. Note the onset of response to S0 wave propagated through damaged area. Response begins at 10.6 ls and then starts to decline slightly from 12.4 ls. This point agrees with time to lower the growth rate in response from the broadband light system. Then, a sinusoidal response
intact area
15 damage area 65
Fig. 14. A schematic illustrating propagation routes of S0 wave in and around the damaged area.
starts from 13.3 ls when the same behavior starts in response from the broadband light system. The initial response from 10.6 ls corresponds to S0 wave propagated straight through 0 layers within the damaged area and the remaining intact area. Wave bypassing the damaged area as shown in Fig. 14 reaches the sensor later. The circumference of ellipse, P is approximately given by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ða bÞ P p 2ða2 þ b2 Þ ; ð5Þ 2 where a and b are longer and shorter radius of the ellipse, respectively. From Eq. (5), the wave bypassing the damaged area travels 3.7 mm longer than that propagating straightway. The extra time for the bypass is calculated to be 0.5 ls from the S0 wave velocity of 7320 m/s. Response propagated through only intact area starts from 12.8 ls. The significant sinusoidal response from 13.3 ls, therefore, corresponds to the S0 wave bypassing the damaged area. Fig. 15 shows response signals detected by the piezoelectric sensor. Compared with intact area propagation, response to S0 wave propagated though damaged area shows a similar behavior as observed in FBG sensor response: earlier onset of response, longer period and lower amplitude in the first cycle of response. In the beginning of response, however, piezoelectric sensor shows a clear sinusoidal response to either wave propagation. This implies that it is impossible from the piezoelectric sensor response to distinguish S0 wave bypassing the damaged
1 11.9µs
1
13.1µs
0 10.6µs
propagated through damaged area
13.3µs -1 1
Normalized response
Normalized response
12.4µs
6.3µs
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10.5µs
propagated through damaged area
-1 1
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0
propagated through intact area
12.8µs
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11.4µs
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propagated through intact area
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Time, µs
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60
80
Fig. 13. FBG sensor response from the tunable laser system in the ultrasonic inspection.
-1 -20
0
20
Time, µs
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60
Fig. 15. Piezoelectric sensor response in the ultrasonic inspection.
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H. Tsuda / Composites Science and Technology 66 (2006) 676–683
area from that propagating straight. It is evident, therefore, that FBG sensors are more suitable for ultrasonic inspection than conventional piezoelectric sensors. 5. Conclusions Two types of FBG ultrasonic sensing system including a tunable laser or a broadband light source were constructed. Detectability for ultrasound and damage of respective systems was examined. The following conclusions can be drawn: (1) Response of FBG sensor to S0 wave propagated through a cross-ply CFRP was recorded at several transmitter-sensor intervals. Wave velocity measured from experimental results agreed well with that estimated theoretically. Both FBG sensing systems possessed ultrasonic sensitivity high enough for practical use and the signal-to-noise ratio seemed to be independent of the transmitter-sensor interval up to 250 mm. Compared with the broadband light systems, the tunable laser system had higher sensitivity to ultrasonic wave. (2) FBG sensing systems were applied to ultrasonic inspection of a cross-ply CFRP with visual impact damage. Ultrasonic propagation through the damaged area divided into two routes: straight propagation and bypass around the damaged area. FBG sensor in either system can detect respective waves propagated in different route whereas piezoelectric sensors cannot distinguish between them. Response from the tunable laser system to S0 wave propagated through damaged area was distinct from that in intact area. (3) FBG sensors in combination with the tunable laser system prove to be an alternative of conventional piezoelectric sensors in ultrasonic inspection of composite structures.
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References [1] Schmerr L. Fundamentals of ultrasonic nondestructive evaluation. New York: Plenum Press; 1998. p. 1–13. [2] Alcoz J, Lee C, Taylor H. Embedded fiber-optic Fabry–Perot ultrasound sensor. IEEE Trans Ultrason Ferroelectr Freq Control 1990;37(4):302–6. [3] Dorighi J, Krishnaswamy S, Achenbach J. Stabilization of an embedded fiber optic Fabry–Perot sensor for ultrasound detection. IEEE Trans Ultrason Ferroelectr Freq Control 1995;42(5):820–4. [4] Pierce S, Philp W, Gachagan A, McNab A, Hayward G, Culshaw B. Surface-bonded and embedded optical fibers as ultrasonic sensors. Appl Opt 1996;35(25):5191–7. [5] Kwon I, Kim C, Hong C. Simultaneous sensing of the strain and points of failure in composite beams with an embedded fiber optic Michelson sensor. Compos Sci Technol 1997;57(12):1639–51. [6] Tsuda H, Koo J, Kishi T. Detection of simulated acoustic emission with Michelson interferometric fiber-optic sensors. J Mater Sci Lett 2001;20(1):55–6. [7] Kim D, Koo B, Kim C, Hong C. Damage detection of composite structures using a stabilized extrinsic Fabry–Perot interferometric sensor system. Smart Mater Struct 2004;13(3):593–8. [8] Perez I, Cui H, Udd E. Acoustic emission detection using fiber Bragg gratings. Proceedings of SPIE, vol. 4328. Washington (DC): SPIE; 2001. p. 209–15. [9] Tsuda H, Toyama N, Urabe K, Takatsubo J. Impact damage detection in CFRP using fiber Bragg gratings. Smart Mater Struct 2004;13(4):719–24. [10] Takahashi N, Yoshimura K, Takahashi S, Imamura K. Development of an optical fiber hydrophone with fiber Bragg grating. Ultrasonics 2000;38(1–8):581–5. [11] Betz D, Thursby G, Culshaw B, Staszewski W. Acousto-ultrasonic sensing using fiber Bragg gratings. Smart Mater Struct 2003;12(1):122–8. [12] Fomitchov P, Krishnaswamy S. Response of a fiber Bragg grating ultrasonic sensor. Opt Eng 2003;42(4):956–63. [13] Othonos A. Fiber Bragg gratings. Rev Sci Instrum 1997;68(12): 4309–4341. [14] Tsuda H, Toyama N, Takatsubo J. Damage detection of CFRP using fiber Bragg gratings. J Mater Sci 2004;39(6):2211–4. [15] Rose J. Ultrasonic waves in solid media. Cambridge: Cambridge University Press; 1999. p. 101–31. [16] Prosser W. The propagation characteristics of the plate modes of acoustic emission waves in thin aluminum plates and thin Graphite/ Epoxy composite plates and tubes. NASA Technical Memorandum 104187; 1991. p. 97.
Ultrasound and damage detection in CFRP using fiber Bragg grating sensors Hiroshi Tsuda
*
Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 2, Tsukuba 305-8568, Japan Received 10 November 2004; accepted 18 July 2005 Available online 10 October 2005
Abstract Ultrasound detection using fiber Bragg gratings (FBGs) and its application to damage detection were investigated. Two types of FBG ultrasonic sensing system were constructed using different light source: a broadband light source and a tunable laser source. Ultrasonic waves generated with a piezoelectric transducer were propagated through a cross-ply CFRP and were detected by the FBG sensing systems at several ultrasonic source-sensor intervals. Furthermore, the FBG sensing systems were applied to ultrasonic inspection of an impact damaged cross-ply CFRP in order to examine their capability for damage detection. Both FBG sensing systems produced better detection of the damage in the composite than piezoelectric sensors. The FBG ultrasonic sensing system including a tunable laser possessed high sensitivity to ultrasonic wave and proved to be an alternative to conventional piezoelectric sensors in ultrasonic inspection of structural composite materials. 2005 Elsevier Ltd. All rights reserved. Keywords: Fiber Bragg gratings; D. Non-destructive testing; Ultrasonics
1. Introduction Ultrasonic inspection has extensively been conducted in structural health monitoring. Damage in structures affects propagation of ultrasonic wave. Thus, the presence of damage can be identified when the detected ultrasonic signal deviates from the reference signal of undamaged structure. So far ultrasonic inspection of structures has been performed using piezoelectric devices as ultrasonic transmitters and sensors [1]. Piezoelectric sensors, however, have a serious drawback of electromagnetic interference. Fiber-optic sensors that are immune to electromagnetic interference have been expected to be an alternative ultrasonic sensor. Several studies on ultrasonic detection with optical interferometric sensors have been reported [2–7]. Although optical interferometric sensors allow sensitive ultrasonic detection, a phase control system is required to maintain *
Tel.: +81 29 861 92 84; fax: +81 29 861 58 82. E-mail address: [email protected].
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.07.043
the optimum sensitivity. In the past years, ultrasonic detection using fiber Bragg grating (FBG) sensors have been reported [8–12]. An FBG is a wavelength encoding sensor thereby making the sensor self-referencing, rendering it independent of fluctuating light levels and the system immune to source power and connector losses that plague many other types of fiber-optic sensors. From the advantages mentioned above, FBGs are considered to be promising ultrasonic sensors in structural health monitoring. FBG ultrasonic sensing systems can be classified into two types according to light source employed. One is a system including a broadband light source and an optical filter [8,9]. Ultrasonic wave can be detected through an optical filter processing of the light reflected from FBG sensor. Another is a system with a tunable laser source in which the intensity of light reflected from FBG sensor directly corresponds to ultrasonic response [10–12]. To the authorÕs knowledge, however, capability for ultrasonic detection and damage monitoring of FBG sensing systems is not well investigated.
H. Tsuda / Composites Science and Technology 66 (2006) 676–683 broadband light source
optical circulator
FBG sensor
FBG filter photo detector
a
b
Reflectivity / Transmissivity
In the present study, two types of FBG sensing system with either a broadband light source or a tunable laser source were constructed. Using the systems constructed, ultrasonic wave propagating through a cross-ply CFRP was detected at several ultrasonic source-sensor intervals. Sensitivity to ultrasound of the two systems was evaluated. Then, these systems were applied to ultrasonic inspection of an impact damaged CFRP. Ultrasonic wave was propagated through either damaged area or only intact area and the resultant responses were recorded. The influence of damage on response behaviour was investigated. Furthermore, ultrasonic inspection using a piezoelectric sensor was conducted to compare with damage detectability of the FBG sensing systems.
677
compression
strain free
tension
wavelength
Fig. 1. FBG ultrasonic detection system with a broadband light source and its principle of operation. (a) FBG ultrasonic detection system with a broadband light source, (b) a schematic illustrating the variation in reflectivity of the FBG sensor with applied strain. Solid and dotted curves correspond to the reflectivity of the FBG sensor and the transmissivity of the FBG filter, respectively.
2. Ultrasonic detection using FBGs 2.1. Fundamentals of FBGs [13] An FBG has periodical variation in the refractive index within the core of an optical fiber and acts as a narrowband reflection filter. The central wavelength of light reflected from an FBG is called the Bragg wavelength, kB and is given by
An imposed strain of 1 · 10 would lead to a 1.22 pm shift in the Bragg wavelength of an FBG whose Bragg wavelength at strain free is 1550 nm. The shift in the Bragg wavelength is positive when the FBG expands. Conversely, the Bragg wavelength shifts to negative when the FBG contracts.
the figure are simplified forms than the actual characteristics. Solid and dotted curves in the figure represent the reflectivity of the sensor and the transmissivity of the filter, respectively. The area enclosed by the reflective curve corresponds to distribution of light reflected from the sensor. The light that can be transmitted through the filter is represented by the area enclosed by the transmissive curve of the filter. Hence, the area where the reflectivity of the sensor overlaps with the transmissivity of the filter corresponds to the intensity of light transmitted through the filter. The area is shaded in Fig. 1(b). The Bragg wavelength of the sensor shifts to a longer wavelength kt when the sensor is elongated. Then, the shaded area grows and so the intensity of light detected with the photodetector increases. Conversely, the Bragg wavelength shifts to a shorter wavelength kc and the intensity of light transmitted through the filter decreases when the sensor is compressed. Thus, ultrasonic wave can be detected from an FBG sensor in combination with the broadband light system.
2.2. Ultrasonic detection by a broadband light system
2.3. Ultrasonic detection by a tunable laser system [11,12]
Details about principle of ultrasonic detection by a system including broadband light source were described in the previous paper [9]. A brief explanation is given here using Fig. 1. The system is schematically shown in Fig. 1(a). Broadband light travels to the FBG sensor via an optical circulator and light reflected from the sensor passes another FBG for filtering. Light transmitted though the filter reaches a photodetector where the intensity of light is converted into voltage signal. Here, we assume that the Bragg wavelength of the filter kf stays constant and the FBG sensor in strain free state has a slightly longer Bragg wavelength k0 than kf. Fig. 1(b) shows the transmissivity of the FBG filter and the change in reflectivity of FBG sensor with varying strain applied to the FBG sensor. These characteristic curves shown in
An ultrasonic sensing system including a tunable laser source is shown in Fig. 2(a). The laser emission wavelength is set to kout where reflectivity of the sensor at strain free is reduced by half as shown in Fig. 2(b). The solid curve in the figure represents the reflectivity of the sensor at strain free. Laser light is launched into the FBG sensor via an optical circulator. The light reflected from the sensor is transmitted to the photodetector without filter processing. When the sensor expands, the Bragg wavelength shifts to a longer wavelength. Then, the reflectivity of the sensor at the lasing wavelength rises as shown in Fig. 2(b) and so the intensity of light reflected from the sensor increases. Conversely, intensity of light reflected from the sensor decreases when the sensor contracts. In the system including a tunable laser source, the intensity of light reflected from
kB ¼ 2nK;
ð1Þ
where n and K are the effective refractive index of the fiber core and the grating period, respectively. The relative shift in the Bragg wavelength, DkB for an applied strain along the fiber axis by e under a constant temperature condition is given by DkB ¼ 0:787kB e.
ð2Þ 6
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tunable laser
optical circulator
FBG sensor
Reflectivity
a
photo detector
b
Wavelength
Fig. 2. FBG ultrasonic detection system with a tunable laser source and its principle of operation. (a) FBG ultrasonic detection system with a tunable laser light source, (b) a schematic illustrating the variation in reflectivity at the lasing wavelength when the FBG sensor expands.
the FBG sensor directly corresponds to ultrasonic response. 3. Detectability for ultrasound of FBG sensing systems 3.1. Experimental procedure Ultrasonic wave was detected at several ultrasonic source–sensor intervals in order to evaluate ultrasound detectability of FBG sensing systems. Ultrasonic waves were propagated through a 340 · 340 · 1 mm3 CFRP (T800H/3631) whose stacking sequence was [0/90]2s. Ultrasonic waves propagating through thin plates are referred to as the Lamb waves. Lamb waves can be categorized into two modes by the displacement with regard to the plate plane: symmetrical and asymmetrical modes which are, respectively, called S and A modes for short. A previous paper reported that FBG sensors were more sensitive to S mode waves than A mode waves [14]. Thus, a piezoelectric shear wave transmitter (Panametrics, V-150) which generates S mode waves was used as ultrasonic source. The ultrasonic transmitter has a diameter of 25 mm and a central frequency of 250 kHz. Lamb wave propagation is characterized by the product of specimen thickness and the wave frequency. Only fundamental S mode waves called S0 waves can spread over cross-ply CFRP when the product is less than 1 MHz mm [15]. The product is 0.25 MHz mm under the present experimental condition. Hence, only S0 waves can propagate through the specimen. Two types of ultrasonic sensing system were constructed. Experimental setup employed is shown in
Fig. 3. The broadband light system works when optical switches 1 and 2 in the figure are set to port A. Then, the light reflected from FBG sensor passes the FBG filter and the light transmitted through the filter arrives at the photodetector. When both optical switches are set to port B, the tunable laser system works. Then, the light reflected from FBG sensor streams into the photodetector directly. The ultrasonic transmitter was placed in contact with the CFRP plate using a highly viscous gel as a couplant and was driven by a spike signal emitted from a pulser (PAC, C-101HV). FBG sensor was glued with adhesive for strain gauges to the CFRP surface 50 mm away from the edge. S0 waves were detected at a transmitter-sensor interval ranging from 50 to 250 mm every 50 mm. The photodetector output was recorded at a sampling rate of 100 MHz and the average from 512 waveform acquisitions was taken. Wavelength characteristics of FBGs employed and the lasing wavelength are shown in Fig. 4. Both FBGs for sensing and filtering has a grating length of 10 mm, a half-value width of approximately 0.2 nm and their Bragg wavelengths in strain free state are 1550.28 and 1550.17 nm, respectively. In the tunable laser system, the laser emission wavelength was set to be 1550.38 nm where the reflectivity of FBG sensor was nearly 50%. Here, we note the relation between the polarity of response and the deformation of FBG sensor. In the broadband light system, the Bragg wavelength of the sensor is slightly longer than that of the filter as shown in Fig. 4. This is the same Bragg wavelength condition as shown in Fig. 1(b). In the tunable laser system, the position of the lasing wavelength in the reflective curve of the sensor is
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A optical switch 1 fiber direction
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tunable laser B
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A FBG filter photo detector
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/mm
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0 -1 1
0.8 reflectivity of the FBG sensor Bragg wavelength = 1550.28nm 0.6
transmissivity of the FBG filter Bragg wavelength = 1550.17nm lasing wavelength = 1550.38nm
0.4
Normalized response
Normalized reflectivity and transmissivity
Fig. 3. Experimental setup in the ultrasonic detection test.
0.2
200mm
0 -1 1
150mm
0 -1 1
100mm
0 -1 1
50mm
0 0.0 1549.5
1550
1550.5
-1
1551
0
Wavelength, nm
the same as shown in Fig. 2(b). Consequently, the relation between them accords with the explanation described in Sections 2.2 and 2.3. In either ultrasonic sensing system employed, the response signal increases and decreases when the FBG sensor expands and contracts, respectively. 3.2. Experimental results Figs. 5 and 6 show the evolution of response signals with transmitter-sensor intervals received from the broadband light system and the tunable laser system, respectively. The intensity of response signals was normalized by the maximum value of response signals from respective systems. The abscissa, time was set to be zero when the pulser emitted spike signal to the ultrasonic transmitter. Response from either system demonstrated rising behavior on S0 wave arrival. As mentioned in the end of Section 3.1, the response in either system grows when the FBG sensor expands. Thus, the FBG sensor must have been stretched on the first arrival of S0 wave. Beginning of response delays in proportion to the transmitter-sensor interval. The velocity of S0 wave was mea-
40
60
80
Time,µs
Fig. 5. Response to S0 wave received from the broadband light system.
transmitter-sensor interval = 250mm
1 0 -1 1
Normalized response
Fig. 4. Wavelength characteristics of FBGs and lasing wavelength in the ultrasonic detection test.
20
200mm
0 -1 1
150mm
0 -1 1
100mm
0 -1 1
50mm
0 -1 0
20
40
60
80
Time, µs
Fig. 6. Response to S0 wave received from the tunable laser system.
sured experimentally from the relation between the time of response onset and the transmitter-sensor interval. The measured velocities were 7170 and 7270 m/s from the broadband light system and the tunable laser system,
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Table 1 Unidirectional YoungÕs moduli, density and PoissionÕs ratio of the CFRP employed 158 8.67 1570 0.38
Another PoissonÕs ratio m21 is given by m21 ¼ EE21 m12 .
respectively. Wave velocity can theoretically be estimated from mechanical properties of material where the wave propagates. Eq. (3) gives the velocity of S0 wave in direction of 0 or 90 of cross-ply composites [16] sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E1 þ E 2 ; ð3Þ V ¼ 2qð1 m12 m21 Þ where E, q and m are YoungÕs modulus, density and PoissonÕs ratio, respectively. The unidirectional YoungÕs moduli of CFRP employed and other properties are listed in Table 1. S0 wave velocity is estimated from Eq. (3) to be 7320 m/s. The velocity estimated theoretically agrees well with experimental results. Influence of the transmitter-sensor interval on the peakto-peak value, Vpp and the signal-to-noise ratio (S/N ratio) given by the following equation was investigated: S=N ratio ¼ 20 log
V pp ; N rms
ð4Þ
where Nrms denotes the root mean square value of signal prior to the ultrasonic response. Figs. 7 and 8 show the change in peak-to-peak value and S/N ratio of response signals from the broadband light system and the tunable laser system, respectively. The value of Vpp should decrease with increasing transmitter-sensor interval while the experimental results showed different behavior. Fluctuation in Vpp would result from the scatter in contact condition between the ultrasonic transmitter and the specimen. The S/ N ratio seems to be independent of the transmitter-sensor interval up to 250 mm in either system and exceeds 40 and 70 dB in the broadband light system and the tunable 80
60
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40
peak-to-peak value, Vpp
S/N ratio, dB
Peak-to-peak value, Vpp
2.0
20
S/N ratio 0.0
0 50
100
150
200
250
Transmitter-sensor interval, mm
Fig. 7. Peak-to-peak value and S/N ratio of response signals from the broadband light system.
Peak-to-peak value, Vpp
Longitudinal YoungÕs modulus, E1 (GPa) Transverse YoungÕs modulus, E2 (GPa) Density, q (kg/m3) PoissonÕs ratio, m12
2.0
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peak-to-peak value, Vpp
S/N ratio, dB
680
20
S/N ratio
0.0
0 50
100
150
200
250
Transmitter-sensor interval, mm
Fig. 8. Peak-to-peak value and S/N ratio of response signals from the tunable laser system.
laser system, respectively. In the broadband light system, the sensitivity is limited by a low optical power reflected by the narrowband FBG from a broadband light source. In the tunable laser system, on the other hand, the intensity of light measured with the photodetector amounts to the product of the laser output and the reflectivity of the sensor. Because of the strong light, the tunable laser system permits sensitive ultrasonic detection with a higher S/N ratio. Response signal from the broadband light system, however, has an S/N ratio high enough for practical use. 4. Damage detection using FBG ultrasonic sensing systems 4.1. Experimental procedure Experimental setup employed in the damage detection is shown in Fig. 9. The monitored material was a 290 · 190 · 1 mm3 CFRP (T800H/3631) whose stacking sequence was [0/90]2s. This CFRP plate contains elliptically shaped 65 · 15 mm2 visible damage introduced by a ball dropping at an impact energy of 7.35 J. Fig. 10 shows the damaged area of the CFRP. Splitting and delamination can be seen in the damaged area. The experimental setup except the monitored material was the same employed in the ultrasound detection test shown in Fig. 3. An FBG sensor was attached on the CFRP surface using adhesive for strain gauges. Wavelength characteristics of FBGs employed are shown in Fig. 11. The Bragg wavelengths of the sensor and the filter were 1550.18 and 1550.17 nm, respectively. The laser emission wavelength was set to be 1550.28 nm where the normalized reflectivity of the sensor was approximately 50%. A spike signal emitted from the pulser was input to the ultrasonic transmitter to generate S0 wave. The ultrasonic transmitter was put in two places so that the generated S0 wave could pass the damaged area or only intact area before reaching the FBG sensor. The interval between transmitter and sensor was 100 mm. In addition, ultrasonic inspection using a piezoelectric sensor was also performed for reference. The piezoelectric sensor employed was the
H. Tsuda / Composites Science and Technology 66 (2006) 676–683 A
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broadband light source
optical switch 1 optical circulator
tunable laser B
A
FBG filter
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photo detector optical switch 2 B
ultrasonic transmitter
damaged area
recorder
ultrasonic transmitter 15
65
FBG sensor
100
100
[0/90] 2s CFRP
fiber direction /mm
Fig. 9. Experimental setup for ultrasonic inspection of an impact damaged CFRP.
same piezoelectric device used as the ultrasonic transmitter. The piezoelectric sensor was attached to the same place as the FBG sensor had been attached. FBG and piezoelectric sensor signals were recorded under the same condition for data acquisition as the ultrasound detection test was performed. 4.2. Experimental results
Fig. 10. A photograph showing the damaged area of CFRP employed in the ultrasonic inspection test.
Fig. 12 shows the response signals from the broadband light system. Signal amplitude is normalized by the maximum value of response signal in intact area. In this section, all response signals were normalized in the same way and the time was set to be zero when the pulser emitted spike signal to the ultrasonic transmitter. Response to S0 wave
13.1µs
13.3µs 12.4µs
0.8
reflectivity of the FBG sensor Bragg wavelength = 1550.18nm
0.6
transmissivity of the FBG filter Bragg wavelength = 1550.17nm 0.4 lasing wavelength = 1550.28nm
Normalized response
Normalized reflectivity and transmissivity
1 1
0 propagated through damaged area
10.6µs -1 1
6.8µs
0 0.2
0 1549.5
12.8µs
1550
1550.5
1551
Wavelength, nm Fig. 11. Wavelength characteristics of FBGs and lasing wavelength in the ultrasonic inspection.
-1 -20
0
propagated through intact area
20
Time, µs
40
60
80
Fig. 12. FBG sensor response from the broadband light system in the ultrasonic inspection.
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propagated through damaged area demonstrates different features compared with that in the intact area. First, the initial response starts earlier from 10.6 ls. Second, the first cycle period nearly doubles to 13.1 ls. Third, the peak-topeak value in the first cycle decreases by about 80%. Reasons for these different features in response signal were discussed in the previous paper [9] and are summarized as follows. S0 wave propagates in 0 and 90 layers separately within the damaged area where delamination exists as shown in Fig. 10. S0 wave travels faster in 0 layers within the damaged area than in the intact area because ultrasonic waves travel faster in materials with a higher YoungÕs modulus. Accordingly, S0 wave propagated through damaged area reaches the sensor earlier than that in intact area. The longer period and lower amplitude in the first cycle response would result from the wave dispersion and attenuation caused by the presence of damage. Look at the onset of response more detail. A sinusoidal response appeared in intact area propagation whereas the response to S0 wave propagated through damaged area showed a bit more complex initial behavior. The growth rate in response descends once after the onset of response and then a sinusoidal response appears. These features will be discussed later with the response from the tunable laser system. Fig. 13 shows response signals from the tunable laser system. Both response signals demonstrate a well-defined waveform and the respective starting points of response agree with those in response from the broadband light system. Response to S0 wave propagated through damaged area has nearly double period in the first cycle and approximately 70% peak-to-peak value compared with that in intact area. These are similar features in response signal from the broadband light system. Note the onset of response to S0 wave propagated through damaged area. Response begins at 10.6 ls and then starts to decline slightly from 12.4 ls. This point agrees with time to lower the growth rate in response from the broadband light system. Then, a sinusoidal response
intact area
15 damage area 65
Fig. 14. A schematic illustrating propagation routes of S0 wave in and around the damaged area.
starts from 13.3 ls when the same behavior starts in response from the broadband light system. The initial response from 10.6 ls corresponds to S0 wave propagated straight through 0 layers within the damaged area and the remaining intact area. Wave bypassing the damaged area as shown in Fig. 14 reaches the sensor later. The circumference of ellipse, P is approximately given by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ða bÞ P p 2ða2 þ b2 Þ ; ð5Þ 2 where a and b are longer and shorter radius of the ellipse, respectively. From Eq. (5), the wave bypassing the damaged area travels 3.7 mm longer than that propagating straightway. The extra time for the bypass is calculated to be 0.5 ls from the S0 wave velocity of 7320 m/s. Response propagated through only intact area starts from 12.8 ls. The significant sinusoidal response from 13.3 ls, therefore, corresponds to the S0 wave bypassing the damaged area. Fig. 15 shows response signals detected by the piezoelectric sensor. Compared with intact area propagation, response to S0 wave propagated though damaged area shows a similar behavior as observed in FBG sensor response: earlier onset of response, longer period and lower amplitude in the first cycle of response. In the beginning of response, however, piezoelectric sensor shows a clear sinusoidal response to either wave propagation. This implies that it is impossible from the piezoelectric sensor response to distinguish S0 wave bypassing the damaged
1 11.9µs
1
13.1µs
0 10.6µs
propagated through damaged area
13.3µs -1 1
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11.4µs
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propagated through intact area
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Time, µs
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60
80
Fig. 13. FBG sensor response from the tunable laser system in the ultrasonic inspection.
-1 -20
0
20
Time, µs
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Fig. 15. Piezoelectric sensor response in the ultrasonic inspection.
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area from that propagating straight. It is evident, therefore, that FBG sensors are more suitable for ultrasonic inspection than conventional piezoelectric sensors. 5. Conclusions Two types of FBG ultrasonic sensing system including a tunable laser or a broadband light source were constructed. Detectability for ultrasound and damage of respective systems was examined. The following conclusions can be drawn: (1) Response of FBG sensor to S0 wave propagated through a cross-ply CFRP was recorded at several transmitter-sensor intervals. Wave velocity measured from experimental results agreed well with that estimated theoretically. Both FBG sensing systems possessed ultrasonic sensitivity high enough for practical use and the signal-to-noise ratio seemed to be independent of the transmitter-sensor interval up to 250 mm. Compared with the broadband light systems, the tunable laser system had higher sensitivity to ultrasonic wave. (2) FBG sensing systems were applied to ultrasonic inspection of a cross-ply CFRP with visual impact damage. Ultrasonic propagation through the damaged area divided into two routes: straight propagation and bypass around the damaged area. FBG sensor in either system can detect respective waves propagated in different route whereas piezoelectric sensors cannot distinguish between them. Response from the tunable laser system to S0 wave propagated through damaged area was distinct from that in intact area. (3) FBG sensors in combination with the tunable laser system prove to be an alternative of conventional piezoelectric sensors in ultrasonic inspection of composite structures.
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