Structural Health Monitoring

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Structural Health Monitoring of an Advanced Grid Structure with Embedded Fiber Bragg Grating Sensors Masataro Amano, Yoji Okabe, Nobuo Takeda and Tsuyoshi Ozaki Structural Health Monitoring 2007; 6; 309 DOI: 10.1177/1475921707081967 The online version of this article can be found at: http://shm.sagepub.com/cgi/content/abstract/6/4/309

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Structural Health Monitoring of an Advanced Grid Structure with Embedded Fiber Bragg Grating Sensors Masataro Amano,1,* Yoji Okabe,2 Nobuo Takeda1 and Tsuyoshi Ozaki3 1

Department of Advanced Energy, Graduate School of Frontier Sciences The University of Tokyo, Takeda Laboratory, Mailbox 311, 5-1-5, Kashiwanoha Kashiwa-City, Chiba, Japan, 277-8561 2 Department of Aeronautics and Astronautics, School of Engineering The University of Tokyo, Takeda Laboratory, Mailbox 311, 5-1-5, Kashiwanoha Kashiwa-City, Chiba, Japan, 277-8561 3 Department of Metal & Ceramics Technology, Advanced Technology R&D Center, Mitsubishi Electric Corporation, 1-1-57, Miyashimo, Sagamihara-City Kanagawa, Japan, 229-1195 The authors focus on the construction of a structural health monitoring (SHM) system with an advanced grid structure (AGS) made of carbon-fiber reinforced plastic (CFRP). AGS is often applied to aerospace structures because the ribs carry only axial forces in the carbon fiber direction, making AGS structurally effective and lightweight, and because the repetition of many ribs in the AGS composition results in damage tolerance. The failure of a single rib hardly affects the fracture of the whole structure, making AGS a fail-safe structure. In this research, the authors have embedded multiplexed-fiber Bragg grating (FBG) sensors into an AGS rib in the longitudinal direction to measure mechanical strains of all ribs in order to detect the existence and regions of AGS rib fractures. Monitoring the change in riblongitudinal strains is the most effective SHM system for AGS. To confirm the proposal, the authors explore the following. First, various damage characteristics under low-velocity impact loading are investigated and it is verified that partial rib cracks are the most typical damage in AGS. An AGS is then fabricated with embedded FBG sensors and verified that the SHM system is able to measure all rib strains. Subsequently, it is analytically determined that the change in longitudinal-rib strains is the most appropriate mechanical feature for damage detection. Moreover, a statistical outlier analysis is introduced into the SHM system for automatic damage detection. Finally, AGS is established with the SHM system and verified experimentally. Results confirm that the existence of damage and its regions in AGS can be detected with the proposed SHM system. Keywords

advanced grid structure (AGS) damage detection fiber Bragg grating (FBG) sensor impact rib fracture finite element analysis (FEA) statistical outlier analysis

*Author to whom correspondence should be addressed. E-mail: [email protected]

Copyright ß SAGE Publications 2007 Los Angeles, London, New Delhi and Singapore Vol 6(4): 0309–16 [1475-9217 (200712) 6:4;309–16 10.1177/1475921707081967]

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Structural Health Monitoring 6(4)

1 Introduction Carbon-fiber reinforced plastic (CFRP) has been used as a prime material for aerospace structures because of its high specific strength and modulus in fiber direction. The use of CFRP dramatically reduces the weight of those structures, which is necessary in the aerospace field. However, the specific strength and modulus of CFRP in the fiber transverse direction is comparatively low. Therefore, CFRP is generally used as laminates or woven-type composites, which results in various complicated damage characteristics. Consequently, safety factors remain high compared with metal materials where weight reduction has not been fully achieved. In recent years, a new type of CFRP structure has been accepted as a possible solution. It is a grid structure made of CFRP unidirectional composites, called an advanced grid structure (AGS). Figure 1 illustrates an example of AGS. It has several advantages compared with other conventional structures [1]. Two specific characteristics, simplicity of damage feature and redundancy (fail-safe), are distinctively important for AGS. Although AGS has been restricted because of its difficult fabrication, recent automatic fabrication technologies overcome the difficulty and AGS is being recognized again. (An overview of fabrication is introduced and the fabrication method is proposed briefly in Section 2.) The preliminary objective of this study is to validate these two characteristics. Since no work has actually investigated the damage characteristics of AGS, this study discusses them under lowvelocity impact loading to verify possible damage characteristics of AGS in Section 3. In addition, redundancy offers exceptional opportunities to

design a fail-safe structure. Several works discuss this feature, especially for energy assumption [2]. In Section 4, this is discussed by proving that a single-rib material failure barely affects structural integrity. On the other hand, due to the increase in use of CFRP, interests in finding a suitable means of monitoring structural health conditions have increased substantially. Especially, during the 1990s, the structural health monitoring (SHM) system has developed as one of the most important research topics. The structure has an integrated sensor system, which is capable of assessing damage and warning of serious structural condition. Fiber-optic sensor technology is the most attractive device currently used in the aerospace and aircraft industry for in situ monitoring of large-scale FRP structures. It is embedded in to a structure to form a novel selfstrain-monitoring system [3]. In order to improve the reliability of AGS, an SHM system is constructed based on the technology into AGS. The SHM utilizes embedded fiber Bragg grating (FBG) sensors [4] for static strain measurement. An FBG sensor is an optical strain sensor that consists of a periodic refractive index change formed in the core of an optical fiber, as illustrated in Figure 2. When broadband light propagates into an FBG sensor, only a narrow component at Bragg wavelength 0 is reflected, which corresponds to grating period d and effective refractive index n. Fiber Bragg grating sensors have often been used for health monitoring of CFRP. Several works discuss health monitoring of CFRP with embedded FBG sensors for static strain measurement [5]. Friebele et al. [6] multiplexed FBGs into one optical fiber and embedded the fiber into CFRP unidirectional laminates to measure static 10 mm

Figure 1 Advanced grid structure made of CFRP.


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SHM of an AGS with Embedded FBG Sensors

strain distribution. In this research, the sensors are embedded into all AGS ribs (one FBG in each rib). There are several advantages to using FBG sensors for SHM of AGS. First, since all AGS ribs are composed of CFRP unidirectional laminates, all optical fibers can easily be embedded parallel to carbon fibers. Therefore, embedding does not degrade the mechanical properties of CFRP but does enable permanent strain-distribution measurement. Second, in any load case, stresses only appear in that direction as loads pass only in the longitudinal direction of the rib. Therefore, it is sufficient for SHM of AGS to diagnose the structural condition by monitoring all rib strains in their longitudinal directions, as discussed in Section 4. Moreover, since all ribs connect beyond every intersection, all rib strains can be measured using a few optical fibers by multiplexing many FBGs into one optical fiber. Subsequently, a statistical inference tool has been developed for damage detection. A statistical inference is concerned with the implementation of the algorithms that operate on the extracted features (in this research, rib longitudinal strains) to quantify the damage state of the structure [7]. In this study, a statistical pattern recognition technique based on an outlier analysis is presented to automate the damage detection in Section 5.

This study is organized as follows. Section 2 explains the fabrication process of AGS. Section 3 details the observation of low-velocity impact-damage in AGS to clarify all possible failure patterns of the grid structure. Section 4 proposes a damage signal for evaluation of damage existence and region with the help of finite element analysis (FEA). Section 5 outlines the development of a statistical damage recognition method. Section 6 describes the multiplexed FBG sensors that are embedded into AGS during automatic fabrication, and the construction of an SHM system of AGS. Section 7 verifies all discussions mentioned above, and a quasi-static loading test is conducted for the AGS with SHM system. Section 8 concludes the study.

2

AGS Fabrication

Since the 1970s, increasing attention has been given to the manufacturing of AGS using composite materials. Many new and innovative manufacturing methods were reviewed in previous papers [8,9], such as the SnapSat concept from Composite Optics Inc., the tooling-reinforced integral grid (TRIG) concept from Stanford University, and the expansion block-tooling and hybrid-tooling concept from the Air Force Phillips Laboratory. The research presented is similar to the expansion block-tooling, and AGS was fabricated from a CFRP unidirectional tape prepreg T800H/E011604-1M (Bryte Technologies, Inc.), 3 mm wide. Table 1 identifies the mechanical properties of the prepreg. At first, the tape was sequentially laid in the rib directions of 0, þ60, and 608. The directionally laminated prepregs overlapped each other at every intersection, as indicated in Figure 3. This figure also demonstrates that all lay-ups were offset at every intersection to reduce the amount of tapes from

Table 1 Mechanical properties of T800H/E011604-1 M.

Figure 2 Schematic of an FBG sensor.

311

E1 G12 ¼ G13 12 ¼ 13

08 Tensile modulus Shear modulus Poisson’s ratio


149 GPa 3.25 GPa 0.281

312


three times to two times. Triangular expansionblock tools made of silicon rubber were then inserted into all triangular spaces to provide lateral compaction to the ribs during the cure process and formation of AGS. This compaction reduces rib width to approximately half, and doubles rib height. Finally, everything was bagged and cured in an autoclave. These steps were automated by Mitsubishi Electric Co. [10]. Figure 4 represents a configuration of fabricated AGS. As displayed in this figure, seven variables, A, B, a, b, l, t, and h, identify the AGS geometry. In this study, the authors refer to Figure 4 to explain the geometry of AGS in some experiments and analyses.

3 Damage Characteristics Advanced grid structure has often been used for space structures, such as satellites and rocket shrouds. Therefore, many researchers have

Figure 3 Schematic of an intersection. Each ply is sequentially laid-up in three different directions automatically. To avoid material concentration, lay-up was offset at the intersection. Therefore, the density of carbon fibers at each cross-section is twice as much as that at the ribs.

focused on the fracture of those structures under operational conditions. However, damage caused by impact loading, such as a dropped tool or debris impact, is another serious problem. Since few works had been done for the damage characteristics of AGS, in this section, they are clarified under low-velocity impact tests for AGS. Figure 5 represents the test configuration. A drop weight impact tester (Dynatup 930-I, Instron Co.) was used. A specimen was cut from fabricated AGS and fixed as illustrated in Figure 5. The photographs in Figure 6 represent impact-damage acquired with (A) a digital camera, (B) an optical microscope, and (C) scanning electron microscopy (SEM). As presented in Figure 6(A) and (B), rib cracking and fiber discontinuity were observed. Moreover, as illustrated in Figure 6(C), since the fracture surface is relatively flat, this damage was determined to be the result of fiber micro buckling. Figure 7 represents damage characteristics in the test with higher impact energy. Compressive fracture, tensile-fiber break, and interlaminarshear delamination were observed. These damage characteristics are generally observed in CFRP unidirectional beam under flexural loading [11]. Rib intersections were also investigated under the same impact loading. However, no damage feature was observed even when maximum loading of the tester was applied. From these results, the damage characteristics of AGS were determined as: . .

Damage occurs only at ribs, not at intersections. All ribs can be considered as CFRP unidirectional beams because damage characteristics of the ribs are the same as those of beams.

b

B

t h A

Figure 4 Configuration of test specimen.


a

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Figure 5 Photographs of test configuration under low velocity impact loading.

Figure 6 Photographs taken with (A) digital camera, (B) optical microscope, and (C) SEM. They represent the damage characteristics of a rib caused by impacts.

The partial crack in the rib is therefore considered as the target of the SHM.

4

Figure 7 Damage characteristics caused by a highenergy impact loading. Compressive cracks, tensile-fiber breaks, and delamination were observed.

.

A partial crack in the rib, a result of fiber micro buckling, is the primary damage characteristic that generates discontinuity of a load path.

Analytical Discussion

The prime objective of this section is to determine the most sensitive signal to identify damage and the damaged region. An analytical model was constructed for AGS based on FEA and strains under intact and damaged conditions were calculated. The strain in the fiber direction was then discussed as a signal for damage detection. In this research, the authors propose a passive-sensing SHM system to monitor strains in the fiber direction. To detect damage, the damage should initiate changes in signals and one should


314


be able to detect the signals. However, mechanical states do not change without loading, even if partial cracks appear in the AGS ribs since AGS inherently has no thermal residual stress after fabrication. Consequently, AGS is given some external loading for damage sensing. Mechanical states between intact and damaged conditions were calculated under one-point loading and were compared with each other. Most analytical models of grid structure have aimed to optimize geometrical rib arrangement for structural stability. There are roughly two approaches in modeling: equivalent-stiffness modeling [12] and exact modeling [13,14]. Previously, the authors proposed a FEA model using 3D beam elements with commercial FEA software, ABAQUS (Hibbitt, Karlsson & Sorensen, Inc.) [15]. In the model, damage was simulated by setting all stiffness components of the corresponding elements in damage points, E1, G12, and G23, about 0. However, this could not evaluate the effect of partial cracks in ribs precisely because beam-element modeling follows the assumption that plane cross-sections initially normal to the beam’s axis remain plane and undistorted [16]. This assumption is no longer applicable near damage points. Therefore, rib modeling was improved as follows. Figure 8 illustrates the schematic of the model with 3D beam elements. At first, every beam element was divided into two elements in the thickness direction, (d) and (i). The thicknesses of the two elements, td and ti, are determined from the length of a partial crack in a rib, as depicted in Figure 8. Five constraint equations, (1)–(5), were then introduced between the two elements with the function of linear-constraint equations, which ABAQUS offers. ti td ui1 ud1 3i 3d ¼ 0 2 2

ð1Þ

ui2 ud2 ¼ 0

ð2Þ

ui3 ud3

ti þ td i 1 ¼ 0 2

ð3Þ

1i 1d ¼ 0

ð4Þ

2i 2d ¼ 0

ð5Þ

where u represents displacement of nodes and represents rotation of nodes. In addition, subscripts represent the local coordinates, displayed in Figure 8, and superscripts represent the corresponding regions. These equations represent a linear combination of nodal variables that define the relative motion of the nodes. Therefore, linear-constraint equations lead to constraint forces at all degrees of freedom appearing in the equations [16]. When assuming a partial crack at a certain location in a rib, td is determined from the length of the crack and all mechanical properties of the crack elements are set to about 0. Generally, when a partial crack exists, the discontinuous surface cannot transmit any loads. Therefore, the load path concentrates in the continuous region by shear deformation. Those five constraints represent this phenomenon. By using this model, the change of strains are calculated along the fiber direction. Figure 9 illustrates the calculation model, where a specimen was loaded at 1000 N at one point, and four edge lines were simply supported (uz ¼ 0). In addition, since the intersections are more rigid than ribs, they could be assumed as rigid body. Therefore, the intersections consist of three crossed ribs and three nodal points representing crossed points, so that everything could be modeled with only the beam elements as shown in Figure 9. In the calculation, the authors assumed two damage points at the marked locations noted in Figure 9 (td ¼ 5 mm). These boundary conditions and structural conditions correspond to the experimental verification discussed in Section 6. In addition, dimensions of the calculation model are determined from the sizes of the specimen examined in Section 6 and are tabulated in Table 2. Mechanical properties of CFRP were the same as in Table 1. At first, strains were calculated under two structural conditions: (1) undamaged, eundamage ; and (2) damaged at two damage points, edamage , where e represents rib longitudinal strains of


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Figure 8 Schematic of damage modeling with 3D beam elements.

Figure 9 Finite element modeling of AGS under one-point loading of 1000 N. Four edges of AGS were simply supported. Two damage points were assumed at the marked positions.

all elements as the vector form stated in Equation (6). 0 1 e1 B e.2 C ð6Þ e ¼ @ . A: . en Here, e represents the rib longitudinal strain of one element and n represents the number of ribs in the model. The difference e between edamage and eundamage was then calculated as:

e¼e

damage

eundamage :

ð7Þ

Table 2 Sizes (in mm) of AGS specimen (All characters correspond to Figure 4).

A B a b l t h

526.8 (0.2) 550.9 (þ0.16, 0.14) 105.0 (0.1) 182.1 (þ1.39, 1.01) 93.2 (þ1.41, 1.09) 1.8 (þ0.33, 0.27) 9.7 (þ0.34, 1.04)

The calculated "e is visualized in Figure 10 as a contour map. As shown in this figure, "e near the damaged ribs largely deviated from 0. In contrast, "e of the other ribs almost equals 0.


316

Structural Health Monitoring 6(4) (mm) Loaded point

600

Damage locations

400 (µε)

y

200 150 100 50 0 −50 −100 −150 −200

200

0 0

200

400

600 (mm)

x

Figure 10

Contour map of e ¼ edamage eundamage .

Therefore, two damage locations can be clearly distinguished because AGS has inherent redundancy (fail-safe) as described in Section 1. Because of its redundancy, a single rib failure affects only circumferential ribs and barely affects the whole structural condition, which results in high concentration of "e only near the damaged location. Consequently, "e was found to be a damage sensitive signal. Therefore, the position where "e is extremely large is determined as the damaged rib. It should be noted that one point loading is enough to detect damage locations. In the previous discussion [17], test loadings were analytically applied at four different locations. The result showed that the loadings at all four loading locations could clarify the same damage locations.

5 Statistical Damage Recognition In the previous section, the change in longitudinal strains of all ribs has been determined as damage-sensitive signals. The next step in constructing the SHM system is to judge from measured data whether the condition of the structure has deviated from its normal operational condition. Therefore, a new statistical damage recognition system for SHM is proposed. 5.1

Basic Assumption

In Section 4, it has been clarified that "es are highly concentrated only near damaged ribs.

Therefore, "es near damaged ribs can be considered as outliers in the total data. Moreover, the others are much smaller than the outliers, so one cannot distinguish them from measurement errors. (In a real situation, a strain-measurement system often has errors in 0–50 me because of environmental factors, such as heat or moisture, or inaccuracies of instruments.) Generally, measurement errors follow a normal distribution. Hence, it is assumed that the others also follow a normal distribution. From this a concept of damage recognition is proposed. When the changes in longitudinal strains of some ribs have largely deviated from others, namely, that they can be considered as outliers, one can determine damage and identify approximate damage locations. To recognize some data as outliers, a statistical discordancy test is utilized.

5.2

Discordancy Test

Recently, the development of statistical inference tools to enhance the discipline of novelty detection has received attention [18]. Novelty detection identifies from measured data whether a structure has deviated from its normal condition. Outlier analysis has been exploited for damage-detection purposes [18,19]. Discordant outliers in a data set are the data that are inconsistent with the rest of the data and therefore are considered to be generated by an alternate mechanism to the other data. They are compared to the rest under some objective criterion to be judged statistically likely or unlikely. There are numerous discordancy tests, but one of the most common tests for a twosided discordancy test of a single outlier in a normal sample is based on deviation statistics and given by: jmax x x j jmin x x j , T ¼ max s s

ð8Þ

where x corresponds to the measured data, x corresponds to the mean, and s corresponds to the standard deviation of the data. If T exceeds a


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threshold value corresponding to a certain significant level, the corresponding data x is determined to be an outlier. In this case, one regards changes in longitudinal strains at all ribs "ei as random variables X1 , X2 , . . . , Xn and a set of measured data x1 , x2 , . . . , xn is the sample of the variables. 5.3

Ordered measured data x 2 ,x 2 ------ x k−1 , x k , x k+1 , ------ , x n

:S1 :S2 :Sk−1 :Sk

T1 T2 ------- Tk−1 Tk Outlier analysis

Procedure

As illustrated in Figure 10, one should look for multiple outliers in the data in order to detect a damaged region. Therefore, ‘a consecutive test of up to k outliers’ is also introduced. Figure 11 presents the test procedure of statistical damage recognition for AGS. First, the number of contaminants k in the sample is estimated, determined by the number of outliers in the most serious structural damage conditions in AGS before operation. Equation (8) is then modified as:

If Tj exceeds λj (β) (100β% significance level), x1, x2, ... , x j are outliers. (where j=k, k−1, ..., 1)

Outliers x1, x2, --------, xj

Figure 11

Normal xj +1, --------, xn

Test procedure of consecutive outlier analysis.

i ðÞ is then determined, where the probability P½Ti 4i ðÞ ¼ (for i ¼ 1, 2, . . . , k) follows

max x x j min x x j , Tj ¼ max sj sj ð j ¼ 1, 2, . . . , kÞ

317

( ð9Þ

P

k [

) ½Ti > i ðÞ ¼ :

ð10Þ

1

where x j and Sj correspond to the mean and standard deviation in subsamples S1 , S2 , . . . , Sk , where Sj is the set of data excluding x1 , x2 , . . . , xj1 . First, T1 is calculated in the complete sample, where T1 determines the most deviated sample as x1. Then T2 is calculated in the sample of n 1 data without x1, and so on. Therefore, x1 , x2 , . . . , xk (for prescribed k) are the samples that have the possibility to be outliers in subsamples S1 , S2 , . . . , Sk . Once k data are removed, no outliers must exist in the rest of the data. This consecutive procedure of reducing size in reverse order aims to protect against masking effects. It is the inability of a testing procedure to identify even a single outlier when several suspected values are present. They sometimes form subgroups, i.e., several outlier values are closer to each other than to the others, which makes procedures for testing the most deviated data insensitive [19].

Subsequently, a level-a test operates as follows. If Tk > k ðÞ then x1 , x2 , . . . , xk are discordant. Otherwise, Tk is concluded to be normal and the test proceeds by examining Tl forl ¼ k 1, k 2, . . . , 1. When Tl > l ðÞ, it is concluded that x1 , x2 , . . . , xl are adjudged discordant at level a, namely, they are outliers. If Tl ; l ðÞ for all l ¼ 1, 2, . . . , k, it is concluded that there are no discordant outliers. Therefore, when the existence of outliers is determined, it is concluded that damage exists. Moreover, the outliers tell the approximate regions of the damage.

6

is

Construction and Verification of the Measurement System The strain-measurement established with an


system of AGS embedded FBG

318


Figure 12 Schematic of strain-measurement system with embedded FBG sensor network.

sensor network. This section explains and verifies this system. Following are the concept of the system. First, FBGs are multiplexed into a few optical fibers and embedded into AGS ribs during fabrication. The optical fiber is placed along carbon fibers and bent at the intersections. This construction was automated by Mitsubishi Electric Co. [10]. Figure 12 depicts the measurement system with the embedded FBG sensors. Broadband light is first generated at a light source, which then propagates in all optical fibers through an optical switch. When it reaches every FBG, each FBG reflects a specific narrow band light at the Bragg wavelength () determined by its average refractive index (n) and grating period (d) as: ð11Þ

marked rib has a pair of embedded FBG sensor and strain gage attached on the lower surface of the rib. Therefore, this AGS specimen has 19 FBGs. They are located at the mid points between intersections. They have 0.3 nm of reflection bandwidth at 3 dB and 10 mm of grating length. Table 3 represents their center wavelengths (CWLs). The heights of FBGs depend on the rib directions of ribs. FBGs in þ608 directional ribs were embedded between 10 and 11 plies; 608 directional ribs were embedded between 11 and 12 plies; and 08 directional ribs were embedded between 12 and 13 plies. In other words, the serial numbers of 4, 8, 10, 13, and 17 FBGs were embedded between 10 and 11 plies; 2, 3, 7, 9, 14, and 18 FBGs between 11 and 12; and 1, 5, 6, 11, 12, 15, 16, and 19 FBGs between 12 and 13 plies.

The reflected light signals are transferred to an FBG monitor. The FBG monitor measures multiplexed Bragg wavelengths. When each FBG extends or shrinks, a corresponding wavelength shifts higher or lower. Therefore, applied longitudinal strains of all ribs are measured by monitoring of the change in wavelength. To verify this measurement system, an AGS was constructed with this SHM system and tested as follows. The sizes of the specimen are listed in Table 2. All characters correspond to Figure 4. Thirty-four plies of CFRP unidirectional prepregs were laminated in each rib (68 plies were laminated at cross points). Figure 13 illustrates the arrangement of FBG sensors and strain gages (KFG-1N-120-C1-11L3M3R, Kyowa Electronic Instruments Co., Ltd.). In this figure, each

Figure 14 schematically illustrates the verification test. Test fixture, measurement systems, and wiring of sensors are depicted in this figure. The specimen was tested under three-point lateral loading with Instron1185 (4400R) at a cross-head speed of 1.0 mm/s. During the test, the changes of Bragg wavelengths of reflected light from the 19 embedded FBGs "i (for i ¼ 1–19) were measured with an ASE light source (LA158D16FSS1, Mitsubishi Cable Industries, Ltd.), an optical switch (AQ8203, ANDO Electric Co.), and a spectrum analyzer (Q8384, ADVANTEST). This spectrum analyzer uses a new monochromator developed by ADVANTEST as spectrum investigation. It offers a 10 pm wavelength resolution and a 20 pm wavelength accuracy in the 1550 nm band. Moreover, longitudinal rib strains at their lower surfaces, eSG i , were

¼ 2nd:



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Figure 13 Arrangements of FBG sensors and strain gages. Each marked rib has an embedded FBG sensor and an attached strain gage. Table 3 Center wavelengths of 19 FBG sensors used in Section 6.

Rib number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Here, corresponds to the Poisson’s ratio and p corresponds to the strain-optic coefficients of FBG sensors listed in Table 4. Therefore, the coefficients Cis between measured "i and must be 0.085 (%e/nm) for all FBGs. applied eOF i SG and e were then compared for verification. eOF i i However, these two strains measure different locations in rib height direction as illustrated in was modified based on Figure 13. Therefore, eSG i Equation (13)

CWL (nm) 1556 1540 1590 1540 1590 1580 1580 1580 1590 1530 1540 1570 1580 1580 1590 1580 1540 1540 1556

¼ emSG i

measured simultaneously. The measured "i were then transformed into applied strains eOF based i on the equation [20], i n2i p12 ðp11 þ p12 Þ eOF ¼ 1 i i 2 1 , eOF ¼h i i : i n2i 1 2 p12 ðp11 þ p12 Þ i ¼ Ci i : , eOF i ð12Þ

ZOF i eSG i ZSG i

ð13Þ

represents positions of each FBG where, ZOF i sensor and ZSG represents strain gages in height i direction, and are summarized in Table 5. eOF i and emSG were then compared. i Figure 15 compares strains measured with embedded FBGs and those measured with attached strain gages. As illustrated in this figure, almost equals to emSG at all 19 points. eOF i i Therefore, it is concluded that applied strains of all ribs could be measured from the wavelength shift of embedded FBG sensors. The slight difference was primarily caused by three factors. First, positions of the FBGs in the rib-height direction differed from the expected positions because FBGs often flow during the cure process. Second, positions of the FBGs in the rib-longitudinal direction differed from the positions of the attached strain gages because it is difficult to determine positions of FBGs in an optical fiber. Third, applied strains in some ribs were relatively small, which resulted in a low signal-to-noise


320


Figure 14 Schematic of three-point bending test for system verification.

Table 4.

Optical properties.

Poisson’s ratio

f

0.16

Strain-optic coefficients

p11 p12

0.113 0.252

Table 5 Positions of embedded FBG sensors in height direction.

Rib axis 30 degree þ30 degree 90 degree

Embedded plies

ZOF

Between 10 and 11 Between 11 and 12 Between 12 and 13

0.412 0.353 0.294

ZOF represents normalized height of the FBG sensor, where ZSG equals 0.5.

ratio of strain gages attached on those ribs. Therefore, further accuracy would be realized if the positions of embedded FBGs were precisely identified in the longitudinal and thickness directions during the manufacturing process.

7 Experimental Verification of the SHM System Finally, the entire SHM system was experimentally verified. Figures 16 and 17 present the experimental setup and serial numbers of the ribs.

The material properties are described in Table 1 and the specimen sizes are described in Table 7. This specimen has 39 FBG sensors multiplexed into seven optical fibers attached on the lower surfaces of the corresponding ribs (Figure 17). These FBGs have 0.3 nm of reflection bandwidth at 3 dB and 10 mm of grating length. Table 6 represents their CWLs. At first, the specimen was one-point loaded on one nodal point with Instron1185 (4400R) at a crosshead speed of 1.0 mm/s. Under the loading conditions, strains eundamage were measured with an ASE light source, an optical switch (OSW8108, THORLABS Inc.), and an FBG sensor monitor (FB200, Yokogawa Electric Co.). This FBG sensor monitor uses a polychromator. Since the architecture of this monitor has no moving parts, it can be compact and highly reliable, and it achieves high-speed measurement. It offers a 1 pm wavelength resolution and a 50 pm wavelength accuracy in the range from 1527 to 1567 nm (or from 1528 to 1607 nm). Subsequently, the specimen was unloaded at the same speed. The 20th and 27th ribs were then partially notched as illustrated in Figure 18. Afterward, the specimen was loaded again, and strains edamage were measured again under the same boundary conditions. Finally, the change of applied strains e ¼ edamage eundamage was calculated from the measurement results. Figure 19 represents measured "e for all ribs. As shown in this figure, "es near the 20th and 27th ribs, which were the damaged ribs, were


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Figure 15 Comparison of measured and "mSG . strains between "OF i i

Figure 16 Experimental setup.

Table 6 Center wavelengths of 39 FBG sensors used in Section 7.

Table 7 Sizes (in mm) of AGS specimen (All characters correspond to Figure 4).

Rib number

A B a b l t h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

CWL (nm)

Rib number

CWL (nm)

1553 1557 1532 1530 1536 1535 1552 1531 1549 1545 1541 1528 1552 1540 1534 1548 1543 1537 1529 1533

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

1537 1528 1544 1544 1533 1542 1541 1551 1546 1542 1538 1532 1529 1536 1545 1550 1549 1530 1534

526.8 (þ0.90, 1.8) 549.6 (þ0.66, 0.34) 104.9 (þ0.66, 0.54) 181.7 (þ1.53, 0.86) 81.6 (þ0.92, 0.82) 3.5 (þ0.54, 0.26) 10.1 (þ0.34, 0.81)

much larger than those of the other ribs and could be distinguished clearly as mentioned in Section 4. Subsequently, the statistical outlier analysis mentioned in Section 5 was conducted for the measured "e. In the beginning, the number of contaminants in the sample was estimated as


322


Figure 19 number i.

Changes of strains i versus rib serial

Table 8 Critical values for 5% consecutive test for up to k ¼ 5 outliers in a normal sample n ¼ 40. Figure 17 Rib serial numbers. Strains of 39 ribs were measured with the attached FBG sensors under one-point loading. In the experiment, the 20th and 27th ribs were partially notched.

Figure 18

Photograph of partial notch.

5 (k ¼ 5), which was determined by this experimental condition. First, T1 in the complete sample was calculated and "e20 was determined as x1. Then, T2 was calculated in the sample of n 1 data without x1. This calculation continued until x5 was determined. Consequently, e20 , e27 , e19 , e21 , and e24 were determined as x1, x2, x3, x4, and x5, respectively. The remaining data without them must have no outliers. The significance level a ¼ 0.05 was then chosen. A corresponding i is shown in Table 8 [19]. First, T5 was compared with 5. Since T545, T5 was normal

i

i (b)

1 2 3 4 5

3.31 2.88 2.69 2.55 2.47

and x5 was not discordant. Then, T4 was compared with 4. In this comparison, since T444, it is concluded that x1, x2, x3, and x4 were outliers. Figure 20 illustrates the result of the analysis representing the ribs that are determined as outliers. From this, one can conclude that this structure has a possibility of damage, and the damage must be near the marked ribs. Since the region estimated from the experimental results was the same as the real damaged region, it is concluded that the SHM system for AGS was realized and verified.

8 Conclusions In this research, the authors proposed and realized a SHM system for AGS with an embedded FBG sensor network. In the beginning, a low-velocity impact test was conducted to define the types of damage on which to focus. According to the test results,


Amano et al.


Figure 20 Result of damage detection with the statistical outlier analysis. Marked ribs were determined as outliers.

it was found that the damage characteristics of AGS were the same as for a CFRP unidirectional beam structure. Therefore, it is concluded that rib partial cracking, which was the result of fiber compressive micro buckling caused by a bending load, was the initial and most serious damage characteristic in AGS. The change in strains induced by damage in AGS was calculated with an improved finite element modeling of AGS to investigate the most damage-sensitive signal. The result confirmed that the change of static strains at all ribs in the fiber direction was a most sensitive signal indicating rib cracking in AGS. In addition, the result validated that AGS is a redundant (fail-safe) structure because the change of strains appeared only near damaged locations and structural integrity was hardly degraded. Moreover, a statistical damage recognition method based on the concept of consecutive outlier analysis was also proposed and introduced in the SHM system to automatically determine the existence and regions of damage. An innovative strain-measurement system was then proposed that measures strains of all ribs with multiplexed FBG sensors embedded in all ribs of AGS. This system was experimentally

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with eSG verified by comparing eOF i i . The result verified that the measurement system was able to measure static strains at all ribs precisely. Finally, it was experimentally discussed whether the proposed SHM system could detect damage automatically. A strain-measurement system was constructed with FBGs and some instruments and the change in strains of all ribs was measured. In this experiment, rib partial notches were introduced in two AGS ribs as impact damage. The consecutive outlier analysis was then conducted for the measured signals to determine whether the specimen was damaged and where the damage existed in the specimen. The result of the experiment proved that the system was able to clarify the existence and approximate region of damage in AGS. Although this measurement technique could identify only approximate region of damage, it is considered that this resolution is enough for preliminary investigation in maintenance activities.

Acknowledgments This study was performed under the contract of ‘‘R&D for Structural Integrity Diagnostics,’’ with RIMCOF, who was entrusted and funded by the Ministry of Economy, Trade and Industry (METI), as a part of the ‘‘Advanced Materials & Process Development for Next Generation Structures’’ project for the ‘‘Civil Aviation FundamentalTechnology Program’’ of METI Japan.

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