Monitoring of water absorption in CFRP laminates using embedded fiber Bragg grating sensors

Composites: Part A 61 (2014) 163–171

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Monitoring of water absorption in CFRP laminates using embedded fiber Bragg grating sensors Shin-ichi Takeda a,⇑, Takuhei Tsukada b, Sunao Sugimoto a, Yutaka Iwahori a a b

Institute of Aeronautical Technology, Japan Aerospace Exploration Agency, 6-13-1 Osawa, Mitaka-shi, Tokyo 181-0015, Japan Graduate School of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan

a r t i c l e

i n f o

Article history: Received 3 October 2013 Received in revised form 12 February 2014 Accepted 17 February 2014 Available online 25 February 2014 Keywords: A. Polymer–matrix composites (PMCs) B. Environmental degradation D. Non-destructive testing Fiber-optic sensors

a b s t r a c t Practical monitoring technologies are needed for evaluating water absorption/desorption in actual CFRP structures. In this study, the monitoring of water absorption in laminated CFRPs was demonstrated by a simple evaluation using fiber Bragg grating (FBG) sensors. The swelling strain and coefficient of moisture expansion (CME) of unidirectional (UD) laminates were estimated from the axial strain of FBGs. The measured swelling strains of cross-ply laminates agreed with that obtained from transient swelling analysis using the results of UD laminates. The swelling analysis showed that the change in the axial strain of FBG represents the swelling strain of the laminated CFRP. Moreover, another evaluation method employing changes in the FBG spectrum was proposed, and noted its possibility for laminated CFRP with almost 0 in-plane swelling strain. The present results suggested that the FBG measurement technique is useful for monitoring of water absorption in CFRP. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction CFRP laminates have been routinely used in airframe structures because of their excellent fatigue and corrosion resistance properties as compared with metallic materials. However, both external loads and environmental conditions are significant factors that decrease strength in CFRPs. In general, the internal state of a CFRP will change depending on temperature or humidity, even without external loading. Thus, it is important to understand these changes, which may have potential implications for hot spots in CFRP structures. On the other hand, CFRP laminates may be used for lightweight mirrors in space applications, which require high dimensional accuracy and long-term stability. The dimensions of CFRPs in this situation can change due to not only relaxation of thermal residual stress and physical aging but moisture uptake on earth and moisture discharge in space. Nevertheless, the position of such a mirror is adjustable if these dimensional changes can be detected in real time. Therefore, suitable health monitoring technologies are needed to determine changes in the internal states of CFRP aerospace structures. Fiber-optic sensors are candidate sensors for such heath monitoring technology because of their various advantages; they are lightweight, and they possess immunity to electromagnetic ⇑ Corresponding author. Tel.: +81 50 3362 6554; fax: +81 422 40 1451. E-mail address: [email protected] (S.-i. Takeda). http://dx.doi.org/10.1016/j.compositesa.2014.02.018 1359-835X/Ó 2014 Elsevier Ltd. All rights reserved.

interference. Moreover, they exhibit high sensitivity to strain and temperature. The evaluation of non-uniform strain using a fiber Bragg grating (FBG) sensor has proven useful for the detection of damage in CFRPs [1–3]. However, some concerns remain before such devices can be applied to actual structures. This is because the signal from an FBG can change over long periods of operation caused by factors other than strain imposed by damage to a CFRP. Specifically, resin shrinkage caused by physical aging or resin swelling caused by moisture/water absorption can distort the actual measured strain by FBG. Thus, the relationship between strain change and an interfering secondary factor must be verified in advance so as to draw a clear distinction from actual damage occurrence. FBG sensors were applied to evaluate the shrinkage of a unidirectional (UD) CFRP due to physical aging, and the strain change in a direction perpendicular to the carbon fibers was about 250 le in the experiment [4]. The measured strain values were validated by comparison with results of creep tests. On the other hand, this study focused on the evaluation of water absorption in CFRP, which is another important issue and practically inevitable. Swelling behavior in CFRPs has been reported in past studies [5–13]. Typically, the amount of water absorption has been evaluated by weight measurement, and the moisture content has been usually calculated on the basis of Fick’s law, because the penetration of water is classified as diffusive behavior. In addition, the swelling behavior of a CFRP has been expressed as a coefficient of moisture expansion (CME), which is measured by direct

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methods [8,9], by calculation from the warpage of an asymmetric laminated CFRP [10,11], or by interferometric methods [12,13]. The studies aim to not just directly monitor the swelling in actual CFRP structures but also evaluate the material properties that affected such swelling. The monitoring of swelling in a resin by embedded FBG sensors has been reported by Lai et al. [14]. They clarified that FBG can measure the moisture induced strain precisely in view of water concentration and viscoelastic properties of the resin. The monitoring of swelling in CFRP by surfaceattached Fabry–Perot interferometric fiber-optic sensors has been reported by Vaddadi et al. [15]. The latter authors predicted the hygrothermal parameters by inverse analysis and evaluated the swelling strain. These two studies using fiber-optic sensors have demonstrated excellent results; however, these techniques cannot be easily applied practically to laminated composites that are not unidirectional. This study demonstrates a simple method of using FBG to evaluate the swelling behavior of a CFRP. It also describes the validation of the measured swelling strain by the finite element analysis including the influence of an embedded FBG. Technically speaking, strain is induced by the combined effects of swelling, physical aging, and viscoelasticity in a resin. Nonetheless, the results represent the strain that is specifically due to resin swelling, which dominates the other types of strain. UD and cross-ply laminated CFRPs were both immersed in distilled water. Although the rate of water absorption is different from that of moisture absorption, the swelling of a resin will occur under both conditions. Thus, if the present evaluation approach is applied to a CFRP immersed in water, it may also be applied to moisture absorption. An FBG with a polyimide coating was used for the consideration of practical handling and to prevent strength deterioration in the optical fibers. The applicability of the FBG was thus examined by immersion in hot water.

fabrication (cutoffs). Each cutoff specimen had six flat surfaces that water could penetrate. The specimens were stored for 2 weeks in desiccators to protect the moisture ingress before each test. The specimens were placed in a tank with distilled water at 71 °C (HWT: Hot Water Temperature). The specimen was taken from the tank at an appropriate time, and the weight was measured by an electronic precision balance immediately after removing the water from the specimen surfaces. The measurement interval was 2 h at first, but it gradually lengthened by the end of the test. 2.2. Weight gains in CFRP Fig. 2 shows the relationship between weight gain and immersion time for the specimens with and without the optical fiber. Each result is the average value for three specimens with an error bar of the standard deviation. In this figure, OF(0) and OF(90) represent the directions of an embedded optical fiber, which are parallel or perpendicular to the axis of the carbon fibers, respectively. The thickness of the specimen affected the increasing rate of weight gain at an early stage of immersion. The influence of embedding and the direction of the optical fiber are not observed as shown in the overlap of the error bars with each other. Furthermore, although water penetrates through the polyimide coating of the optical fiber, any increase in weight due to it is negligible. Fig. 3 shows the influence of cutoff on the weight gain for 8-ply and 24-ply specimens. The weight gain did not reach saturation levels toward the end of this measurement; however, the results showed the characteristics of water absorption in CFRPs. In particular, the weight gain of the cutoff specimens was greater than that

1.2

The specimens were manufactured from epoxy-based CFRP UD prepregs (IMS60/#133, Toho Tenax Co., Ltd.). The stacked prepregs of 50 mm length and 50 mm width were cured at 180 °C for 2.5 h using an autoclave. The stacking sequences were [0]x (x = 4, 8, 24) and [08/908/08]. In the 40 mm long square area shown in Fig. 1, the thickness was almost uniform due to the use of a flat pressure plate. Single-mode optical fibers were embedded into some of the specimens to determine the effect of such embedding on weight gain of the specimens. The optical fiber with a diameter of 150 lm is compatible with the ITU-T G.652 standard, and a glass cladding of 125 lm was coated with polyimide resin. The embedding location of the optical fiber was in the middle of the specimens with respect to its thickness direction. Some of the specimens were prepared with cuts removed from the edges after

OF (Optical Fiber) 50 mm

0.8 [04]

0.6

[08] [0 /OF(0)/0 ] 2

0.4

4 2

Fig. 1. Photograph of a specimen with an embedded optical fiber. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2

[0 /OF(90)/0 ] 4

4

0 0

100

200

300

400

500

Time (h) Fig. 2. Weight gain of the specimens with and without the optical fiber. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1.2 1 0.8 0.6 Cutoff

0.4

[0]

[0]

[0]

[0]

[08/908/08]

[08/908/08]

8 24

8 24

0 0

50 mm

4

[0 /OF(90)/0 ]

0.2

40 mm × 40 mm

2

[0 /OF(0)/0 ]

0.2

Weight gain (%)

2.1. Experimental setup

Weight gain (%)

1

2. Water immersion tests

200

400

600

800

Time (h) Fig. 3. Weight gain of the specimens and cutoff specimens. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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3. Monitoring of swelling behavior in CFRP 3.1. Experimental setup FBG sensors were embedded into various CFRP specimens to monitor their swelling behavior. The used FBG (Fujikura Ltd.) was fabricated using the same type of single-mode optical fiber as that used in the prior water immersion test. This FBG has a 15 mm grating length that is the sensing element for strain or temperature. The sensitivities of FBG were 1.2 pm/le and 12.26 pm/°C that were preliminarily measured and largely similar to common FBG sensors [17,18]. The specimens with an embedded FBG were manufactured in the same way as those used for the water immersion tests. They were prepared without cutting off their edges, because the optical fiber was inserted from the edge of each specimen. After preparation, the specimens were stored in the same way as for the water immersion test. The monitoring system was composed of an ASE light source, a spectrum analyzer, a circulator, and an optical switch, as shown in Fig. 4. After the spectra were measured once at 23.5 °C (RT: Room Temperature), the specimens were immersed in a tank with distilled water at HWT. The FBG sensor itself was simultaneously immersed to investigate its long-term applicability and the effect of any coating swelling. All the spectra were captured at hourly intervals during the tests. 3.2. Influence of coating swelling on signal in FBG

0.015

Optical power (W)

2.5 με

1550

1551

1552

Fig. 5. Spectral changes for the immersed FBG. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

water. The spectrum maintained its shape during immersion, and its center wavelength shift was 0.003 nm, or 2.5 le in strain terms. The influence of coating swelling on the spectrum was very small. Therefore, we believe that the FBG presents sufficient potential for use. In this study, the reason for choosing an embedded FBG was to avoid the debonding of the FBG from the specimen surface and the swelling of the adhesive required to attach it. In addition, we attempted to use an electrical strain gage and a FBG, which were attached to the surface by adhesives and water-proof coatings, but the both measured strain were found to be sensitive to bonding conditions. 3.3. Reflection spectrum from embedded FBG The spectrum gradually changed during 800 h of immersion. Fig. 6 shows the changes in spectra for [04/FBG(0)/04] and [012/ 0.015 RT 0h 100 h 400 h 800 h

0.427 nm

0.01

0.005

0 1552

1553

1554

1555

Wavelength (nm)

(a) [04/FBG(0)/04] 0.015 RT 0h 100 h 400 h 800 h

0.425 nm

0.01

0.005

0 1550

Water bath (71 o C)

increase 0.003 nm for 0 to 800 h

0.005

Wavelength (nm)

Optical power (W)

Circulator

RT 0h 100 h 400 h 800 h

0.01

0 1549

The spectral changes in the immersed FBG are shown in Fig. 5. The center wavelength of a spectrum is defined as that at maximum optical power for a spectrum with one peak. The center wavelength shifted 0.5475 nm with an increase in temperature from RT to HWT. The calculated temperature change was 44.66 °C using the temperature sensitivity of the FBG, which is slightly smaller than the set value of 47.5 °C, i.e., the temperature difference between RT and HWT. This is believed to be due to the difference in temperature between the core of the FBG and hot

Optical channel switch

0.5475 nm

44.66 °C

Optical power (W)

of the uncut specimens for all the stacking sequences. One reason for this is that water diffusivity in the direction of the carbon fibers is greater than in other directions, as is well known [7,10,11,16]. The weight gain of the 8-ply specimens rapidly increased by 200 h, but then increased only slightly approaching 800 h. Similarly, the weight gain of the 24-ply specimens gradually increased by 800 h. The increasing rate of weight gain depended on specimen thickness as with Fig. 2. This effect is also observed in previous studies [8,10,11]. The weight gain of the cross-ply specimen was slightly greater than that of the 24-ply UD specimen. It is assumed that this is due to the dependency of water diffusion on the internal strain state, which varied between specimens.

1551

1552

1553

Wavelength (nm) Spectrum analyzer ASE light source

Fig. 4. Monitoring system for water swelling in CFRP specimens. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(b) [012/FBG(0)/012] Fig. 6. Spectra from the FBG embedded in the UD specimen. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

S.-i. Takeda et al. / Composites: Part A 61 (2014) 163–171

Optical power (W)

0.015

0.01

2.262 nm

0 1545

RT 0h 100 h 400 h 800 h

0.563 nm

0.005

0 1550

1551

1552

1553

Wavelength (nm)

(a) [08/904/FBG(0)/904/08] 0.015

0.01

RT 0h 100 h 400 h 800 h 0.76 nm

0.005

0 1549

1550

1551

1552

Wavelength (nm)

(b) [08/904/FBG(90)/904/08] Fig. 8. Spectra from the FBG embedded in the cross-ply specimen. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.4. Swelling strain

RT 0h 100 h 400 h 800 h

0.005

0.01

Optical power (W)

FBG(0)/012]. The initial spectrum had only one sharp narrow peak. The spectrum maintained its shape, while the wavelength shifted to a higher value due to the temperature increase. With an increase in temperature from RT to HWT, the wavelength shift was about 0.43 nm, which is smaller than the shift for the temperature change in Fig. 5. This decrease in the wavelength shift is caused by the shrinkage of the specimen, because the coefficient of thermal expansion (CTE) of the CFRP is negative in the direction of the carbon fibers. However, the spectrum did not show a significant change during immersion. Fig. 7 shows the changes in spectra for [04/FBG(90)/04] and [012/ FBG(90)/012]. The initial spectrum had two peaks unlike the case for the embedded FBG in the direction of the carbon fibers. This unique shape is caused by a birefringence effect in the FBG under a non-axisymmetric strain state, as reported in previous studies [19,20]. The spectrum shifted to a longer wavelength with increase in temperature and immersion time. The wavelength shift with an increase in temperature of RT to HWT was about 2.27 nm, which is significantly larger than the shift observed during the temperature change in Fig. 5. The increase in the shift is caused by the expansion of the specimen, because the CTE of the CFRP in a direction perpendicular to the carbon fibers is positive. The shape of the spectrum gradually changed from two peaks to only a single peak, implying that the birefringence effect is progressively eliminated. Fig. 8 shows the changes in spectra for [08/904/FBG(0)/904/08] and [08/904/FBG(90)/904/08]. While both spectra shifted to longer wavelengths as the temperature increased, the magnitude of the shifts were different depending on the embedding direction of the FBG. The initial spectra at RT represent the residual strain states of the specimens, and these residual strains are introduced during the fabrication of the specimens.

Optical power (W)

166

1550

1555

The swelling strain was evaluated from the wavelength shifts in FBGs. Fig. 9 shows the measured swelling strain that was derived from the strain sensitivity of the wavelength shift in the FBG. All the strains were compensated by the wavelength shift in a FBG for temperature measurement, which is immersed in hot water directly. The swelling strain in the direction of the carbon fibers was quite small, while the transverse strains increased to about 3000 le by 800 h of immersion. The strain of the cross-ply specimens was small due to the in-plane mutual constraint of

Wavelength (nm)

(a) [04/FBG(90)/04] 4000

RT 0h 100 h 400 h 800 h

0.004

0.002 2.278 nm

[04/FBG(0)/04]

[04/FBG(90)/04]

[012 /FBG(0)/012]

[012 /FBG(90)/012]

[08/904/FBG(0)/904/08]

[08/904/FBG(90)/904/08] 3246 με 2899 με

3000

Strain (με)

Optical power (W)

0.006

2000 1000 384 με 102 με

0 0 1545

1550

1555

Wavelength (nm)

(b) [012/FBG(90)/012] Fig. 7. Spectra from the FBG embedded transversely in the UD specimen. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

-1000 0

200

400

600

800

Time (h) Fig. 9. Measured strain by the FBG during water swelling. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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the 0° and 90° plies. The thickness effect on the rate of the swelling strain was very similar to that of the weight gain in Fig. 3. Fig. 10 shows the CMEs in the UD specimen evaluated by the swelling strain of the FBG and the measured weight gain. The drift of the measured CME was directly influenced by the variation in the measured weight gain. The CME in the direction of the carbon fibers was almost 0 over the entire immersion time, whereas the CME in a direction perpendicular to the carbon fibers steadily increased until about 400 h before approaching saturation. The reason for the latter is that while the measured weight gain is an overall average for that of the entire specimen, the measured strain is localized in the middle of the specimen. The CMEs at 800 h of immersion were 3226  10 6/% and 3343  10 6/% for [04/ FBG(90)/04] and [012/FBG(90)/012], respectively. These values were appropriate for comparison with similar measured results of previous studies [8–13].

20 mm

20 mm

Water ingress

(Z-symmetry in the back)

X-symmetry

Y-symmetry

z y

x

[08/904/FBG(90)/904/08]

0° ply

90° ply

4. Analysis 4.1. Finite-element model A steady-state analysis was conducted first using the 8-node solid temperature–displacement coupling elements in ABAQUS software. A resin pocket is generated by the embedding of the FBG in a direction perpendicular to the carbon fibers [21]. However, the present model did not account for a resin pocket, because its effect on the axial strain of the FBG was very small. Descriptions of this effect are provided in Appendix A. The model and mesh partitioning of the cross-ply specimen are shown in Fig. 11. They are equivalent for specimens of the same thickness, and only the properties in each partition are different depending on their stacking sequences and the embedding direction of the optical fiber. The calculated strain values obtained by the model during simulated temperature changes corresponded with those measured in the experimental procedure. The temperature changes from cure temperature to service temperature and cure shrinkage are major factors affecting residual strains in CFRPs. Thus this calculation only considers the residual strains caused by temperature change and water absorption, and neglects those induced by cure shrinkage. Subsequently, transient swelling analysis was carried out by replacing material properties with an imported stress state from the prior steady-state analysis. Water absorption is modeled as heat conduction, because both phenomena can be described in the form of diffusion equations. The CTE and thermal properties were changed to simulate the CME and water diffusion, respectively, while the mechanical properties were maintained constant throughout the whole analysis. This analysis discounted the effect of aging on epoxy matrix which causes time-dependent changes in mechanical and diffusional properties. The saturated weight gain

Glass (Cladding and Core)

Coating

Fig. 11. Finite-element model and mesh partition of the cross-ply specimen with the FBG. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

was assumed to be 1.0% and 1.2% for the 8-ply and 24-ply specimens, respectively, based on previous test results. The temperature boundary condition was set to simulate the weight gain of the specimen. The swelling of the coating was ignored since it was relatively insignificant, as described in Section 3.2. 4.2. Calculated results The material properties used in the transient swelling analysis were obtained by fitting the measured weight gain data in the [04/OF(90)/04] specimen. The comparison between the calculated and measured weight gain is shown in Fig. 12. The poor fitting were observed at an early stage of diffusion because the invariant diffusional properties are used in the swelling analysis as shown in Table 1. Nevertheless, the simple calculation is useful for verification of the measured results roughly. The descriptions of the fitting are provided in Appendix B. Fig. 13 shows a comparison of the swelling strain measured by the FBG and the calculated strain. Except for strains measured at an early stage, the measured and calculated strain provides good agreement in both the change ratio and final value. Thus, this 1.2

4000

Weight gain (%)

1

CME (×10−6/%)

3000 [0 /FBG(0)/0 ] 4

2000

4

[0 /FBG(0)/0 ] 12

12

[0 /FBG(90)/0 ] 4

1000

4

[012/FBG(90)/012 ]

0.8 0.6

[04/OF(90)/04] [0 /OF(90)/0 ]

0.4

12 8

0.2

0

12

[0 /90 /OF(90)/90 /0 ] 4

4

8

Calculation, 8 ply Calculation, 24 ply

0 0

-1000 0

200

400

600

800

100 200 300 400 500 600 700 800

Time (h)

Time (h) Fig. 10. Measured CMEs in the UD specimen. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 12. Measured and calculated weight gain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Material properties used in the analysis [25,26]. Steady-state analysis CFRP

FBG, OF

CFRP

Glass

Coating

FBG, OF Glass

Coating

Elastic modulus (GPa)

E1 E2, E3 G12 G13 G23

152 8.21 4.36 3.99 2.52

73.1

1.5

152 8.21 4.36 3.99 2.52

73.1

1.5

Poisson’s ratio

m12 m13 m23

0.334 0.346 0.536

0.16

0.25

0.334 0.346 0.536

0.16

0.25

a11 a22 a33

0.5 22.5 22.5

0.5

15

0* 3226* 3226*

0*

0*

Heat conductivity

k11 k22 k33

2* 1* 0.5*

2*

2*

2* 1* 0.5*

0*

0*

Density

q

1.5*

1*

1.5*

1*

CTE( 10

6

/°C)

Specific heat

SH

Temperature boundary condition (°C) T

*

Transient analysis

250

*

1*

250

*

250

*

250

*

250

1* *

250*

On the whole specimen from 180–23.5 (RT) to 71 On surfaces of the specimen 71–72.0 (8 ply specimens) 71–72.2 (24 (HWT) ply specimens)

Properties denoted by an asterisk are not realistic and only used in the analysis.

analysis can effectively simulate the swelling strains measured by the FBG for all the specimens. The calculated strains throughout the whole analysis are summarized in Table 2. The axial strains of the FBG, ex, and ey, were almost the same as the corresponding strains in the specimens without embedded FBGs. These calculations suggest that the axial strain of an FBG represents the corresponding strain of the CFRP. Therefore, the axial strain measured by an FBG can be used to evaluate the swelling strain of a CFRP. The results also showed the advantages of embedding an FBG, since the measured strain has been reported to change depending on the bonding conditions for surface-mounted FBGs [22,23]. Another evaluation method for swelling is to use the width in the spectrum measured by the FBG. A full-width half-maximum (FWHM) has served as a simple indicator of the birefringence effect of an FBG [24]. Fig. 14 shows the normalized decreases in FWHM by initial value at 0 h of immersion. The FWHM increased with the progress of swelling, except for the case of the UD laminates with the FBG embedded in the direction of the carbon fibers. The swelling freely changes the through-thickness strain but not the in-plane strain depending on the stacking sequences. Thus,

[04/FBG(90)/04]

[08/904/FBG(90)/904/08]

[012 /FBG(90)/012 ]

[08/904/FBG(0)/904/08]

the use of FWHM has the potential to evaluate the swelling progress when the in-plane swelling strain is almost 0, e.g., in-plane balanced laminates. However, the FWHM increased at an early stage for cross-ply laminates. Moreover, FWHM cannot be confidently determined from the steady-state analysis since the latter ignored the cure shrinkage and viscoelastic properties of the resin, and the change in the FWHM will normally be affected by residual strains apart from swelling. Other effects on residual strain, including that of transversely anisotropic cure shrinkage that was not quantified in this study, should be considered to more conclusively establish the effectiveness of evaluation method using FWHM for swelling in CFRP.

Table 2 Calculated strains throughout the whole analysis. [04/FBG(0)/04] ex* [04/FBG(90)/04] ey* [08]

ex

ey

0 78 55 0 55

0

3507 2443 +3213 770

[012/FBG(0)/012]

[012/FBG(90)/012]

[024]

ex*

ey*

(a) UD specimens, 8 ply 180 °C, Curing 0 23.5 °C, RT 78 71 °C, HWT 0 h 54 (Swelling) 0 HWT 800 h 54

4000

0

Calculation

(b)UD specimens, 180 °C, Curing 23.5 °C, RT 71 °C, HWT 0 h (Swelling) HWT 800 h

Strain (με)

3000 2000 1000

24 ply 0 78 54 +3 57 [08/904/FBG(0)/ 904/08] ex*

0

(c) Cross-ply specimen 180 °C, Curing 0 23.5 °C, RT 69 71 °C, HWT 0 h 48 (Swelling) +110 HWT 800 h 62

-1000 0

200

400

600

800

Time (h) Fig. 13. Measured and calculated strain of the FBG. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

*

Axial strain in the FBG.

0 3517 2449 +2949 500

0 3521 3521 2453 2453 +3226 +3226 773 773

ex

ey

0 78 55 +2 57

0

[08/908/08]

0

0

324 225 +340 115

ez

0 3521 3521 2453 2453 +2953 +1986 500 467

[08/904/FBG(90)/ 904/08] ey*

ex

ez

ey

0 69 323 48 225 +110 +340 62 115

ez 0 5193 3617 +3359 258

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[0 /FBG(0)/0 ]

[0 /FBG(90)/0 ]

[012/FBG(0)/012 ]

[012 /FBG(90)/012]

[08/904/FBG(0)/904/08]

[08/904/FBG(90)/904/08]

Normalized decrese of FWHM

4

1

4

4

4

0.8 0.6 0.4 0.2 0 0

200

400

600

800

Time (h) Fig. 14. Normalized decreases of FWHM in the measured spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5. Conclusions FBG sensors were embedded in various UD and cross-ply CFRP laminates to evaluate their swelling strain due to water absorption. Some concerns about the embedded FBG were investigated to determine their suitability for practical use. The effects of embedding FBGs, including the embedding direction, on the accuracy of the measured weight gain were quite small. The reflection spectra were also not much affected by water penetrating the polyimide coating of the optical fibers. The practical applicability of these FBGs was therefore confirmed during the immersion time. The swelling

strain and coefficient of moisture expansion (CME) in the UD laminates were estimated by the measured axial strain of the embedded FBGs. The swelling strain was dependent on the embedding direction: it was quite small in the direction of the carbon fibers, and about 3000 le at 800 h of immersion in a direction perpendicular to the fibers. The CME was estimated to be about 3300  10 6/% in a direction perpendicular to the carbon fibers. These measured results were comparable with results of previous studies. The swelling behavior in CFRP was simulated by transient analysis using the measured results for UD laminates, and the obtained swelling strain was compared with the FBG experiment. It was difficult to fit the weight gains at an early stage based on Fickian diffusion; however, the measured results and calculations gave good agreement with respect to both the change ratio and the saturated swelling strain. The analysis showed that the axial strain change in the FBG represents the swelling strain of the laminated CFRP. When the swelling strain is measured by the FBG, it will be used as a reference for monitoring under complex environments, including humid conditions. Moreover, another evaluation method of using the width of the spectrum was proposed, and noted its possibility to evaluate the swelling progress when the in-plane swelling strain is almost 0, e.g., in-plane balanced laminates. The proposed simple method of using FBG is to evaluate the swelling strain and the swelling behavior by axial strain and FWHM, which is an indicator of non-axisymmetric strain of the FBG. The present result suggests that the evaluation approach is useful for monitoring of water absorption in CFRP. Specifically, the approach has the potential to monitor the water absorption in complicated or thick composite structures that are considered too complex for the application of conventional methods.

z D

y x A C

B

r0 a = 0.375 mm

Combinations of material properties 27)

Region on the model

Properties of resin

A

B

C

D

Case I (Resin pocket)

Glass

Coating

Resin

CFRP

Case II (Resin pocket w/o coating)

Glass

Glass

Resin

CFRP

Case III (No resin pocket)

Glass

Coating

CFRP

CFRP

Case IV (No resin pocket w/o coating)

Glass

CFRP

CFRP

CFRP

Resin (# 133) Elastic modulus

E

3.52

(GPa)

Poisson’s ratio

ν

0.376

CTE

α

45.0

−6

(× 10 /°C)

Fig. A1. Specifications of analysis [27]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table A1 Summary of calculated strain in the [04/FBG(90)/04]. 71 °C

ex ey* ez

0 0 0

985 3516 159

686 2449 111

Case II

ex ey* ez

0 0 0

1192 3511 445

830 2445 310

Case III

ex ey* ez

0 0 0

482 3507 271

335 2443 189

Case I

ex ey* ez

Case IV

*

23.5 °C

0 0 0

306 3507 177

213 2443 123

Axial strain in the FBG.

Appendix A. Influence of resin pocket on strain in FBG Fig. A1 shows the model with its mesh portioning including the resin pocket region surrounding the FBG. The model shown is that for the [04/FBG(90)/04] specimens. The average sizes of the resin pockets 2a were about 10 times greater than the radius (r0) of the FBG, with reference to the previous experiment result [21]. Other boundary conditions and the size of the specimen were the same as those used in the steady-state analysis of Section 4.1. The calculations were conducted to investigate the influence of the resin pocket and coating under temperature change. The combination of the material properties and used resin properties are tabulated below Fig. A1. The used material properties of the CFRP and FBG for the steady-state analysis are shown in Table 1. The calculated strains of the FBG are summarized in Table A1. It can be seen that the axial strains were less influenced by the resin pockets and coating. The results suggest that the resin pocket should be introduced in the model when the non-axisymmetric strain |ex ez| was examined precisely. Since the axial strain of the FBG was the main output used to evaluate the swelling strain, the present finite-element models were simplified, as shown in Fig. 11.

Appendix B. Fitting of thermal properties The fitting of thermal properties in this study are shown for reference. Since temperature simulates weight gain in the transient analysis, the weight gain in the specimen was evaluated from the average of all the temperature outputs at the element centroid except for those of the coating and cladding of the FGB. The

1.2

Weight gain (%)

1 0.8

SH = 250

0.6

k11 = 2, k22 = 1, k33 = 0.5 k11 = 4, k22 = 1, k33 = 0.5 k11 = 0, k22 = 0, k33 = 0.5 k11 = 0, k22 = 0, k33 = 1 k11 = 0, k22 = 0, k33 = 0.25

0.4 0.2 0 0

200

400

600

800

Time (h) Fig. B1. Influence of heat conductivities on weight gain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1

Weight gain (%)

180 °C

1.2

0.8

Experiment 0.6

SH, 150 SH, 250 SH, 350

0.4 0.2

k11 = 2, k22 = 1, k33 = 0.5 0 0

200

400

600

800

Time (h) Fig. B2. Influence of specific heat on weight gain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

diffusion velocity is influenced by heat conductivity and specific heat capacity, which are both fitting parameters, while the mass density is constant. A fitting of data was conducted for the [04/ OF(90)/04] specimen. Fig. B1 shows the influence of the heat conductivities (k11, k22, k33) on the weight gain. The heat conductivity in the thickness direction significantly changed the value for weight gain. Fig. B2 shows the influence of specific heat capacity (SH) on weight gain. The measured weight gain is also plotted here. The weight gain increased with increasing specific heat capacity. From these two fittings, the thermal properties are determined, as shown in Table 1. References [1] Takeda S, Minakuchi S, Okabe Y, Takeda N. Delamination monitoring of laminated composites subjected low-velocity impact using small-diameter FBG sensors. Composites Part A 2005;36(7):903–8. [2] Yashiro S, Okabe T, Takeda N, Sekine H. A new approach to predicting multiple damage states in composite laminates with embedded FBG sensors. Compos Sci Technol 2005;65(3–4):659–67. [3] Minakuchi S, Okabe Y, Takeda N. Real-time detection of debonding between honeycomb core and facesheet using a small-diameter FBG sensor embedded in adhesive layer. J Sandwich Struct Mater 2007;9(1):9–33. [4] Takeda S, Koyanagi J, Utsunomiya S, Arao Y, Kawada H. Long-term monitoring of strain changes in CFRP using FBG sensors. In: Proceedings of the 18th international conference on composite materials, Jeju, South Korea; August, 2011. [5] Judd NCW. Absorption of water into carbon fibre composites. Brit Polym J 1977;9(1):36–40. [6] Boll DJ, Bascom WD, Motiee B. Moisture absorption by structural epoxy-matrix carbon-fiber composites. Compos Sci Technol 1985;24(4):253–73. [7] Paplham WP, Brown RA, Salin IM, Seferis JC. Absorption of water in polyimide resins and composites. J Appl Polym Sci 1995;57(2):133–7. [8] Trigo J. Dimensional stability characterization of carbon fiber with epoxy and cyanate ester resins laminates due to moisture absorption. In: Proceedings of the spacecraft structures, materials and mechanical engineering, Noordwijk, The Netherlands; March 1996. p. 371–6. [9] Scott P, Lees JM. Uptake swelling and thermal expansion of CFRP tendons. Proc Inst Civ Eng Struct Build 2009;162:263–73. [10] Choi HS, Ahn KJ, Nam J-D, Chun HJ. Hygroscopic aspects of epoxy/carbon fiber composite laminates in aircraft environments. Composites Part A 2001;32(5):709–20. [11] Arao Y, Koyanagi J, Hatta H, Kawada H. Analysis of time-dependent deformation of CFRP considering the anisotropy of moisture diffusion. Adv Compos Mater 2008;17:359–72. [12] Rouquiè S, Lafarie-Frenot MC, Pönninger A, Costin W. Thermal cycling under vacuum and CME evaluation of carbon/epoxy laminates. Materialwiss Werkst 2003;34(4):354–8. [13] Poenninger A, Defoort B. Determination of the coefficient of moisture expansion (CME). In: Proceedings of the 9th international symposium on materials in a space environment, Noordwijk, The Netherlands; June, 2003. p. 567–72. [14] Lai M, Botsis J, Cugnoni J, Coric D. An experimental–numerical study of moisture absorption in an epoxy. Composites Part A 2012;43(7):1053–60. [15] Vaddadi P, Nakamura T, Singh RP. Inverse analysis to determine hygrothermal properties in fiber reinforced composites. J Compos Mater 2006;41(3):309–34. [16] Cândido GM, Costa ML, Rezende MC, Almeida SFM. Hygrothermal effects on quasi-isotropic carbon epoxy laminates with machined and molded edges. Composites Part B 2008;39(3):490–6.

S.-i. Takeda et al. / Composites: Part A 61 (2014) 163–171 [17] Kersey A, Berkoff T. Fiber-optic Bragg-grating differential-temperature sensors. IEEE Photon Technol Lett 1992;4:1183–5. [18] Rao Y-J. In-fiber Bragg grating sensors. Meas Sci Technol 1997;8(4):355–75. [19] Gafsi R, El-Sherif MA. Analysis of induced-birefringence effects on fiber Bragg gratings. Opt Fiber Technol 2000;6(3):299–323. [20] Guemes JA, Menendez JM. Response of Bragg grating fiber-optic sensors when embedded in composite laminates. Compos Sci Technol 2002;62(7–8):959–66. [21] Dasgupta A, Wang Y, Sirkis JS, Singh H. Micromechanical investigation of an optical fiber embedded in a laminated composite. Proc SPIE 1990;1370:119–28. [22] Ansari F, Libo Y. Mechanics of bond and interface shear transfer in optical fiber sensors. J Eng Mech 1998;124(4):385–94. [23] Wan KT, Leung CKY, Olson NG. Investigation of the strain transfer for surfaceattached optical fiber strain sensors. Smart Mater Struct 2008;17(3):35–7.

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[24] Okabe Y, Yashiro S, Tsuji R, Mizutani T, Takeda N. Effect of thermal residual stress on the reflection spectrum from fiber Bragg grating sensors embedded in CFRP laminates. Composites Part A 2002;33(7):991–9. [25] Hara E, Yokozeki T, Hatta H, Iwahori Y, Ishikawa T. CFRP laminate out-of-plane tensile modulus determined by direct loading. Composites Part A 2005;41(10):1538–44. [26] Takeda S, Okabe Y, Yamamoto T, Takeda N. Detection of edge delamination in CFRP laminates under cyclic loading using small-diameter FBG sensors. Compos Sci Technol 2003;63(13):1885–94. [27] Hara E, Yokozeki T, Hatta H, Iwahori Y, Ishikawa T. Out-of-plane tensile modulus of laminates by 3-point bending test. J Jpn Soc Compos Mater 2013;39(5):184–92 [in Japanese].