Journal of Solid Mechanics and Materials Review Paper. Engineering

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1 and Materials Review Paper Engineering Strain and Damage Monitoring of CFRP Laminates by Means of Electrical Resistance Measurement * Akira TODOROKI **, Masahito UEDA *** and Yoshiyasu HIRANO **** **Department of Mechanicals Sciences and Engineering, Tokyo Institute of Technology, I1-58, Ookayama, Meguro, Tokyo Japan atodorok@ginza.mes.titech.ac.jp ***Department of Mechanical Engineering, College of Science and Technology, Nihon University, 1-8 Kanda-surugadai, Chiyoda, Tokyo, Japan , Japan ueda@mech.cst.nihon-u.ac.jp ****Advanced Composite Technology Center, Japan Aerospace Exploration Agency, Ohsawa, Mitaka-shi, Tokyo , Japan hirano@chofu.jaxa.jp Abstract This review discusses the use of electrical resistance and potential change in various methods for monitoring Carbon Fiber Reinforced Plastic (CFRP) composites. CFRP composites have electrical conductivity as they contain carbon fibers. When fibers and fiber networks break within the structure its electrical resistance changes; the electrical resistance of carbon fiber changes with the applied strain just as physical resistance varies in a conventional strain gage. Using these changes in electrical resistance damage to CFRP composites can be monitored and applied strain measured without implementation of additional sensors. This makes CFRP a self-sensing material. The electrical resistance change method has been applied for decades to detect carbon fiber breakages. As regards the measurement of applied strain, there is divergence of opinion amongst researchers about the piezoresistance of CFRP. This discrepancy is discussed in detail in this review. For monitoring damage such as delamination cracks, the electrical resistance change method can accurately estimate the location and dimension of the damage. This review also discusses electrical potential change and eddy current methods for monitoring CFRP composites. Key words: Composites, CFRP, Carbon Fiber, Electrical Resistance, Damage, Strain, Monitor 1. Introduction *Received 13 Mar., 2007 (No. R ) [DOI: /jmmp.1.947] Since Carbon Fiber Reinforced Polymer (CFRP) materials have high specific strength and stiffness, they are very effective in weight saving in aeronautical structural components. Aeronautical and other applications of CFRP are on the increase. CFRP is usually formed into laminated structures made from unidirectional prepreg or woven fabric sheet. For these CFRP laminated structures, it is, however, quite difficult to detect internal damage such as delamination or matrix cracks because this damage is not visible from the outside of the structure. The inability to inspect CFRP structures visually demands the development of automatic monitoring or damage detection systems for CFRP structures. Although fiber optic sensors are good candidates as damage-monitoring sensors they require further research before they can be considered effective and are high cost. 947

2 Carbon fiber is an excellent electric conductor, and has been used as a strain sensor for decades (1). Strain monitoring of the CFRP structures is very important to confirm integrity of processing. Vibration and applied stress monitoring are also used to determine the effect of external load. Recently, electrical resistance change measurement has been employed to detect and/or monitor internal damage to CFRP laminates by many researchers (2)-(93). Electrical resistance change measurement does not require expensive instruments. Since the method adopts the carbon fiber itself as a sensor, it does not cause a reduction in strength, and can be applied to existing CFRP structures. Further, measurement requires no additional research to fabricate composite structures as it does not require embedded sensors. Although electrical resistance change measurement has a lot of advantages over other methods of structural health monitoring, it has received little attention until recent years. This is because the strong anisotropic electrical resistance of CFRP causes complicated behavior in electrical resistance change when it is measured by conventional methods. Recent research has shed light on many of the problems in this area and enabled identification of damage location and dimension by measurement of electrical resistance change at multiple points within target CFRP structures. This paper reviews all of the significant existing research in this field. The review provides clear answers to questions regarding electrical resistance change measurement as applied to monitoring and analyzing CFRP. 2. Strain monitoring 2.1 Principles of strain monitoring Carbon fibers have high electric conductivity, while the epoxy matrix surrounding them is an insulator. For ideal CFRP composites, electric conductance in fiber direction is very high. The ideal conductance can be easily calculated by multiplying the fiber volume fraction by the electric conductance of carbon fiber. In comparison, electric conductance in the transverse direction vanishes in an ideal carbon fiber structure. In reality, carbon fiber in a unidirectional ply is serpentine. The curved carbon fibers contact around each other, creating a large carbon-fiber network. The contact across this network causes non-zero electric conductance even in the transverse direction. In the same way, the fiber-network produces non-zero electric conductance in the orthogonal direction (to the plane of the ply). The electric conductance in the transverse direction is, however, much lower than the electric conductance of the fiber orientation. The electric conductance in the orthogonal direction (σ t ) is usually lower than the electric conductance of transverse direction (σ 90 ). Although the fiber-network structure in the orthogonal direction is similar to the structure of the transverse direction in a ply, orthogonal conductance (σ t ) is smaller than the σ 90 for normal laminated composites. The prepreg production process causes this reduction in electric conductivity in the orthogonal direction. Several fiber bundles are usually pressed by means of rollers to make unidirectional prepreg. This causes large numbers of fiber contacts in the transverse direction but the roller press reduces the warping of fiber bundles in the orthogonal direction reducing the conductance. The contact between plies causes no-zero electric conductance in the orthogonal direction. Thus the σ 90 is usually larger than the σ t. When a delamination or matrix crack grows in the structure, the crack breaks the fiber-contact-network between plies. This causes an increase in the electrical resistance of CFRP composites. Therefore, delamination or the formation of matrix cracks can be detected by the resulting change in electrical resistance in CFRP composite laminates. Piezoresistance is electrical resistance change due to applied strain. The most popular sensor using piezoresistance is a conventional strain gage. The electrical resistance R of a wire is expressed as follows: 948

3 L R = ρ (1) A where ρ is specific electrical resistance, L is length of the wire and A is the cross sectional area. When the wire deforms, the fraction of electrical resistance change is expressed as follows: R ρ L A ρ = + = (1 + 2ν ) ε + R ρ L A ρ where ε= L/L and ν is Poisson s ratio. The fraction of specific resistance change ( ρ/ρ) is proportional to the fraction of volume change. ρ = m V ρ V where m is a material constant. Substituting equation (3) into equation (2), we can obtain a well-known relation. (2) (3) R = Kε R (4) K = ( 1+ 2ν ) + m(1 2ν ) (5) where K is a proportionality constant called a gage factor. For most metallic materials, ν=0.3 and m=1. Although the gage factor varies depending on materials, the gage factor is approximately 2~4 for most metallic materials and 80~170 or 95~-110 for semiconductor strain gages. Since carbon fiber resembles a conventional strain gage, the gage factor in the fiber direction during tension loading in the fiber direction is expected to be near to 2. The gage factor in the transverse direction, however, is unclear because the fiber-contact network is the main cause of electrical conductivity. 2.2 Controversy between four-probe and two-probe methods Schulte and Baron (3) have reported that the electric resistance of a CFRP laminate in the fiber direction rises with increase in applied tensile load in the fiber direction using a two-probe method. The two-probe method has two electrodes to measure electrical resistance as shown in Fig.1: the probes apply an electric current and measure resistance via the corresponding change in voltage. Other researchers have also reported this positive piezoresistance (positive gage factor) see references (8),(10),(13),(14),(17). Here, the electric resistance in the fiber direction rises with increase in applied tensile strain in the fiber direction. Wang and Chung obtained completely contrary results (26). They employed a four-probe method to measure precise electric resistance change in the fiber direction during tensile loading in the fiber direction of a CFRP laminate as shown in Fig.2. Their results showed negative piezoresistance (negative gage factor.) The electric resistance in the fiber direction is reduced with the increase of applied tensile load in the fiber direction: see Fig.3. The four-probe method uses an outer pair of electrodes to apply electric current and uses an inner pair of electrodes to measure resistance from the voltage drop between them. 949

4 Fig. 1 Typical two-probe method Fig. 2 Typical four-probe method Fig. 3 Discrepancy between the two-probe method and the four-probe method Wang and Chung s paper (26) describes the mechanism of the discrepancy between the two opposite results: positive and negative piezoresistance of unidirectional CFRP. They conducted measurements using both four and two-probe methods, and concluded that the true piezoresistance of a CFRP laminate in the fiber direction is negative and the apparent positive piezoresistance was obtained due to the increase in electric resistance at the probes for the two-probe method. They also explain that the negative piezoresistance is due to the realignment of carbon fibers during tensile loading in the fiber direction. Angelidis and others (60) have reported that positive piezoresistance has been obtained using the four-probe method with electrodes made from silver paste and negative piezoresistance has been obtained with electrodes made from carbon paste. They showed that the low reliability of electrodes made from carbon paste caused the negative piezoresistance: electrical contact is attained only at several discrete points with the carbon paste electrodes causing a complicated electrical current path in the specimen. This complicated electric current path, they argue, is the cause of the apparent negative piezoresistance. They propose a model for apparent negative piezoresistance and test it 950

5 using irregularly placed dot shaped electrodes; successfully reproducing negative piezoresistance.. Todoroki and Yoshida have investigated the effect of contact at the electrodes in the four-probe method (66). They conducted tensile tests and measured electrical resistance changes in single ply CFRP specimens using the four-probe method. The single ply specimen did not show fiber misalignment between plies although the ply had small fiber undulation. Figure 4 shows the results of the fraction of electrical resistance change during cyclic loading as per Todoriki and Yoshida s study (66). The figure shows the single ply specimen has positive piezoresistance. Fig. 4 Measured piezoresistance of a small 0-tensile-0-charge single ply specimen.todoroki and Yoshida (66) (Solid symbols represent loading and open symbols represent unloading). Fig. 5 Measured piezoresistance of a test tensile load as applied to fiber direction and electrical resistance measured in the fiber direction (0-tensile-0-current laminated specimen) without polishing the specimen surface. Todoroki and Yoshida (66). (Solid symbols represent loading and open symbols represent unloading) Figure 5 shows the results for a unidirectional multi-ply CFRP specimen under cyclic loading (66). For this test, the specimen surface was not polished before being painted with silver paste to make electrodes. The fraction of the electrical resistance change decreased with increase in applied tensile strain. After this test, the specimen surface was polished thoroughly with sand paper, and new electrodes were made using silver paste. Electrical resistance change during cyclic loading was measured again using the identical specimen. Figure 6 shows the results of the cyclic loading test (66). The results show that the identical specimen has positive piezoresistance after polishing the specimen surface. 951

6 Fig. 6 Measured piezoresistance of 0-tensile-0-charge laminated specimen (multi-ply). Todoroki and Yoshida (66). (Solid symbols represent loading and open symbols represent unloading) Fig. 7 Effect of surface resin layer at electrodes. Cross-section before and after polishing the surface (79). Fig. 8 Electric potential distribution with a damaged electrode (79). Todoroki has reported the effect of poor electrical contact at electrodes in the four-probe method (79). For conventional metallic materials, the four-probe method is almost completely free from error caused by damage at the electrodes: poor electrical contact does not have significant impact on the measurement of electrical resistance. In actual CFRP samples, electrical contact can be achieved without polishing the surface because some carbon fibers exist on the surface. Without polishing the surface, however, the electrical contact points between carbon fibers and silver paste become sparse as shown in cross section observations in Fig.7 (79). Sparse electrical contact at the electrodes applying electric current has a significant effect on measurement of electrical voltage (79). In CFRP orthotropic conductivity plays an 952

7 important roll in distorting the electric potential field as shown in Fig.8. The distorted potential field due to damaged electrodes causes significant error in resistance measurement for CFRP (79),(84). Wang and Chung claim that ply misalignment (off-axis) causes the apparent negative piezoresistance (83). Although the real mechanism has not yet been definitively discovered, it must surely be accepted that the four-probe method is not free from the effect of damage at electrodes for CFRP. It can be asserted without controversy that reliable electrodes are required for electrical resistance change measurements by both the two and four-probe methods. It should also be noted that a theory describing piezoresistance in CFRP laminates has also been proposed by Xiao et al. (19). Similarly a model of piezoresistance in CFRP is proposed by Ogi and Takao (72). These will, however, not be discussed in detail here. 2.3 Maximum strain history sensor Okuhara and Matsubara (74) have reported an interesting hybrid CFRP sensor that obtains information of maximum stain applied to it. As shown in Fig. 9, a CFRP rod is covered with a GFRP pipe. To make this rod, pretension load is applied to the inner CFRP rod and the outer GFRP pipe maintains the pretension load. When a tensile load is applied to the hybrid rod, carbon fibers in the inner CFRP rod break gradually. This causes a gradual increase in electrical resistance in the hybrid rod. When unloaded, the inner CFRP rod keeps tensile load due to the pretension load. This pretension prevents recovery of electric conductivity in the inner CFRP rod due to unloading. Fig. 9 Schema of hybrid maximum strain sensor (74). Fig. 10 Electrical resistance change of the maximum strain (74). Figure 10 shows the schematic results of the fraction of electrical resistance change during 953

8 a tensile test. The abscissa is the applied strain and the ordinate is the fraction of the electrical resistance change. Due to the pretension load, the electrical resistance maintains a constant value during unloading and reloading up to the maximum load. This enables measurement of the maximum applied load by means of electrical resistance. 2.4 Electrical properties Regarding the electrical properties of CFRP, Arbry et al. (21) and Todoroki et al. (48) have reported on electrical resistance change within the fiber volume fraction and measured the orthotropic resistance of CFRP. In the fiber direction, electric conductivity almost linearly increases with the increase of fiber volume fraction. In the transverse and orthogonal directions, the conductivity increases exponentially with critical fiber volume fraction as per percolation theory. Ezquerra et al. (37) and Shimamura et al (64) have investigated the electrical impedance properties of CFRP experimentally. They found there is no frequency dependency on impedance in CFRP up to 1MHz. This means we can treat CFRP as an orthotropically conductive material for alternating current with frequencies under 1 MHz. Louis et al. (38) have found there is large scatter in the resistance in the orthogonal direction of laminated CFRP. 3. Damage detection Much work has been published on detecting and estimating damage to CFRP using change in electrical resistance measured with electrodes mounted on the specimen s ends. In this section, these results are categorized as three kinds of damage, as follows. 3.1 Monotonic and cyclic loading Schulte and Baron mounted electrodes at the ends of rectangular specimens of CFRP that were not under strain (3). They used unidirectional CFRP laminates and measured the electrical resistance in tension under cyclic loading. They reported that the electrical resistance increased stepwise after carbon fiber breakages. Muto and Yanagida applied this test to hybrid composites (8). Xu et al. conducted tensile tests with various CFRP samples and reported similar results (13). Ceysson et al have investigated the electrical resistance change of CFRP laminates by means of acoustic emission (AE) (14). Arbry et al. compared damage to electrical resistance change under bending loads (21). These papers reported that fiber breakages significantly increase the electrical resistance of CFRP. Moreover, electrical resistance change has a high sensitivity compared to stiffness degradation in the structure. Electrical resistance change methods have been applied to fatigue damage detection by many researchers (3),(8),(15),(15),(17),(22),(34). All applied processes similar to the monotonic tensile test to measure electrical resistance change in CFRP specimens. These tests were applied to unidirectional CFRP and cross-ply CFRP laminates. The electrical resistance increased with decrease of specimen stiffness. Figure 11 shows a schematic representation of the electric resistance change during cyclic loading of cross-ply laminates reported by Seo and Lee (22). 954

9 Fig. 11 Electrical resistance change due to cyclic loading (22). Arbry et al. investigated electrical resistance change in detail using in-situ observation (35). They used cross-ply laminates and conducted pure cyclic bending tests. They observed fiber-matrix debonding and matrix cracking at each cycle and compared this to change in electrical resistance. Alternating current was used to measure electric capacitance change in the orthogonal direction. Since the cross-ply laminates have a resin layer that forms an electric insulator between each ply, CFRP laminates have large electric capacitance. The measured capacitance decreased with increasing damage. Park et al. (49), (53) proposed an interesting model of carbon fiber networks to simulate the gradual electrical resistance increase of CFRP under tensile loading. They modeled the CFRP as a network of resistances, and found several required properties in the model by comparison with the experimental data. Wang and Chung (15) applied an electrical resistance change method to detect delamination of cross-ply laminates. They detected electrical resistance change after the onset of delamination in fatigue loading tests. Fig. 12 Schematic representation of electrical resistance change method to detect matrix cracking (87). Omagari et al. (81) and Todoroki et al. (87) have shown the four-probe method is applicable to matrix crack monitoring of cross-ply CFRP laminates. In these tests, four probes were placed on a single surface of a beam specimen as shown in Fig.12. Tensile tests were then performed to measure matrix crack density. The experimental results revealed that electrical resistance increased with matrix cracking. Even after complete unloading, specimens with matrix cracks retained a residual electrical resistance increase. The effect of specimen thickness on the residual electrical resistance increase was also investigated. When the thickness of the 90-degree ply is larger than the 0-degree ply, the effect of residual stress relief is smaller, and this is matched for resistance. For this case, they proposed a slope of the reloading between the fraction of the electrical resistance change and loading. The method was also used to investigate damage at the cryogenic temperature (89). The electrical resistance change method worked well to detect initiation of damage at the cryogenic temperature. 3.2 Impact damage Wang and Chung (76) have performed impact tests using CFRP composite pipes with a rectangular cross-section. They used 200mm lengths of 10mm width pipes, and mounted 955

10 four or more probes on the specimen surface after polishing. An impact load of up to 5 J was applied at the middle of the specimen. Each impact load caused an increase in electrical resistance. Larger impact loads caused larger increases in electrical resistance. Multiple probes were placed on the specimen and multiple segments were measured between probes. Maximum electrical resistance increase was observed at the segment where the impact load was applied, but large electrical resistance changes were measured even in the adjacent segments. Even though this did not identify the location and dimension of the damage, it proved experimentally that electrical resistance change methods could be used to detect impact damage. 3.3 Bolted-joint damage (80), (90) Shimamura et al. have applied an electrical resistance change method to detection of damage in bolted joints. Figure 13 shows the configuration of a specimen bolted joint. Two electrodes are mounted under the hole at a distance of 9mm. Since most of plies are at 45 or -45degrees, the applied electric current flows in the fiber direction of these angled plies. This enables monitoring of damage with the electrodes slightly removed from the bolt hole as shown in Fig.13. Fig. 13 Schematic representation of electrical resistance change method for damage detection of a bolted joint (90). Fig. 14 Electrical resistance change for delamination detection of bolted joint (90). FEM was used to perform several analyses of different damage types including fiber breakages and delaminations. Figure 14 shows the results for delamination growth. Electrical resistance increases with increase in delamination cracking around the bolt hole. This indicates that electrical resistance change methods can detect bolted joint damage using a pair of electrodes mounted relatively far from the bolt hole. 4. Damage monitoring 956

11 Many researchers have published papers on damage monitoring of CFRP laminates by means of measuring change in electrical resistance or electrical potential. These papers aim to identify the location and the dimension of the damage. In this section, these results are categorized by measurement method as follows. 4.1 Electrical resistance change method Schema of monitoring system Damage monitoring using electrical resistance changes requires measurement of the distribution of electrical resistance changes due to damage. To measure the distribution of the electrical resistance changes, multiple electrodes are usually mounted on the surface of the laminate. The measurement is performed in the same way as in the two-probe method. Electrical resistance change in CFRP laminates due to internal damage is usually very small. Todoroki et al. have applied a modified electrical bridge circuit in order to obtain large output signals from delamination events (49). The modified bridge circuit is schematically shown in Fig.15. Since this only differs from the bridge circuit in a conventional strain gage by a connected electrical resistance, a conventional strain amplifier can be directly applied without any modification to the circuit. Fig. 15 Schematic representation of modified electrical bridge circuit (49). Fig. 16 Schematic representation of temperature compensation bridge circuit (45). Though the measurement mechanism is the same as a conventional strain gauge s, variation in ambient temperature cannot be ignored. The effect of ambient temperature has been experimentally investigated and a simple temperature compensation bridge circuit has been proposed (45). The temperature compensation bridge circuit is schematically shown in Fig.16. Two specimens are connected as in a conventional two-gage method for strain gages. One of the specimens is the target specimen, and the other specimen is used as a dummy. The effectiveness of the temperature compensation bridge circuit has been confirmed experimentally (45). 957

12 4.1.2 Solving the inverse problem using response surface methodology Identification of the size and location of delamination from measured electrical resistance changes is an inverse problem. Todoroki et al. (45), (46), (48), (49) have applied the response surface methodology (RSM) to solve this inverse problem. Quadratic polynomials were adopted as the function for approximation because of their simplicity. The response surface can be described with quadratic polynomials as: j= 1 j= 1 2 j k 1 y = β + β x + β x + β x x (6) 0 k j j k jj k i= 1 j= i+ 1 where k is the number of variables. For the identification of delamination, response y is the delamination location or dimension, and variables x 1..n are electrical resistance changes. The accuracy of the approximation of the response surface can be judged by means of the adjusted coefficient of multiple determinations of R 2 adj as R SSE = 1 S ( n k 1) ( n 1) 2 adj (7) yy where SS E is the square sum of the errors and S yy is the total sum of the squares. Well known Artificial Neural Networks (ANN) can be regarded as a promising option for the solution of RSMs. Three-layer-back-propagation neural networks have been adopted as a tool for solving the inverse problem to identify delaminations using electrical resistance change methods (46). For the ANN, the input information is the electrical resistance changes, and the output information is the delamination location and size. The number of neurons in the middle layer must be decided by the trial and error. The all-link values among the neurons are set using random numbers, and the link values are corrected by means of the least square error method. The structure of the ANN is schematically shown in Fig. 17. ij i j Fig. 17 Schematic representation of the artificial neural network structure (46). The usefulness of the RSM and ANN has been evaluated by performing delamination identification tests on a beam shaped specimen using FEM. It was reported that in comparison RSM was the superior method (46). The performance of new data sets in identifying damage, that were not used for RSM regression and training of the ANN, were compared to those that were, using the square sum of errors (SSE). The RSM produces the best estimations from the new data, and an under-trained ANN produces better results compared to an over-trained one. Further, ANNs require a great deal of tuning, including setting training parameters and defining network structures to tune the regression accuracy, whilst RSMs do not. 958

13 4.1.3 Experimental results Todoroki et al. have successfully applied the electrical resistance change method to identifying the location and size of delamination in beam type specimens (49), (63), (71). Figure 18 shows the configuration of the specimen. Delamination was caused using the interlamina shear test. The estimation of delaminations was performed using the RSM method described above. Fig. 18 Schematic representation of configuration of beam type specimen (49). Fig. 19 Effect of the number of electrodes on the performance of estimation of location and size (49). The effect of number of electrodes and spacing between electrodes for beam type cross-ply specimens has been investigated (49). The result is shown in Fig.19. The estimation performance is improved by increasing the number of electrodes. The effect of electrode spacing has also been analytically investigated in detail (71). The effect of the fiber volume fraction (V f ) was also discussed in this paper. The relationship between inter-electrode spacing and error in estimation of delamination size is shown in Fig. 20. The estimation performance of the delamination length decreases as the inter-electrode spacing is increased. Error in determining delamination length also depends on the fiber volume fraction. For a large V f, a smaller spacing is required to maintain estimation accuracy. The effect of the fiber ply stacking sequence on the accuracy of the electrical resistance method has been examined for beam type specimens (63). Cross-ply and the quasi-isotropic beams were the primary focus of this work. In this paper, use of the normalized method for measuring electrical resistance changes, specifically, using a norm that improves estimation performance was proposed for the first time. The results showed the electrical resistance change method performed well for both stacking sequences. An individual response surface was required for each stacking sequence even when the standardization method was applied. 959

14 Fig. 20 Estimated error bands for different spacings between electrodes (71). Fig. 21 Schematic representation of the configuration of a plate-type specimen (45). The electrical resistance change method has also been applied to plate type specimens (45), (50). An example of the configuration of these specimens is show in Fig. 21. Ten electrodes were co-cured into the plate to measure electrical resistance changes. Delamination was created by means of an indentation test with a cylindrical jig. RSM was applied to estimate the delamination location and size. The estimation of delamination was performed for both cross-ply and quasi-isotropic laminates. The estimation of the delamination size for both types of laminates was quite accurate. In estimating the delamination location, though the results for the cross-ply laminates showed good performance, the results for the quasi-isotropic laminate were not good. It was reported that the method must be improved for practical application involving quasi-isotropic laminates. 4.2 Electric potential change methods Electric resistance change methods are a type of electric potential change method. When the electric potential is measured between two points, i.e. the two-probe method, it is generally described as an electric resistance change method. The Electric potential change method, also well known as the electric potential technique, is used in the nondestructive testing of metallic materials. Electric potential change methods use a pair of current electrodes and a pair of or multiple voltage electrodes, and is thus a form of four-probe method. 960

15 Fig. 22 Schematic representation of unidirectional CFRP under mode 1 testing (12). Todoroki et al. (12) applied an electric potential change method to delamination monitoring of unidirectional CFRP beams as shown in Fig. 22. They performed mode 1 delamination tests and electric potential change was measured using electrodes that were mounted on both surfaces of the beam. The electric potential changes between electrodes A 1 and A 1, A 2 and A 2,, A 7 and A 7 showed an almost linear relationship with delamination length. They showed the applicability of electric potential change methods to delamination monitoring in CFRP. Schueler et al. (39) estimated location and size of a hole in a unidirectional CFRP plate made of a ply of carbon fiber prepreg. They proposed a resister network model and attached 16 electrodes at the edges of the square-shaped specimen as shown in Fig.23. The circular hole is 5mm in diameter. In this study, the conductivity in the fiber direction was supposed as infinite. Hole location and diameter were accurately estimated. The effect of r-ratio, which is a fraction of conductivities of fiber direction and transverse direction, on the estimation accuracy of the method was shown. Fig. 23 Schematic representation of unidirectional CFRP for estimation of hole location and size (39). Fig. 24 Schematic representation of unidirectional CFRP for estimation of hole location and size (53). T. A. Anderson et al. (53) have investigated damage detection in CFRP plates. They performed damage identification analytically assuming a two-dimensional problem. The 961

16 laminate was assumed to be an orthotropic material, in this case, unidirectional CFRP. 16 electrodes were uniformly mounted near the edges of the plate as shown in Fig. 24. Since they performed two-dimensional analysis, the type of damage investigated was a hole. An artificial neural network was applied to estimate the hole s location and radius. The hole s location in the transverse direction (y-direction) and radius were accurately predicted. On the other hand, the comparatively high electrical conductivity in the fiber direction (x-direction) made prediction of x-location of the hole difficult. Fig. 25 Schematic representation of electric potential change measurement for cross-ply CFRP for identification of delamination (61). (27), (61) proposed Todoroki et al. a multi-probe electric potential change method to identify delamination location and size in cross-ply CFRP laminated plates as shown in Fig electrodes were mounted on a surface of the laminate perpendicular to the fiber direction in the surface layer. Electric potential changes between electrodes were measured by applying electric current through the specimen using electrodes mounted to its edges. Response surface methodology was applied to estimate a delamination from the resulting changes in electric potential. Delaminations were successfully estimated experimentally. Fig. 26 Electric current density in the thickness direction of a cross-ply laminate (65). Ueda et al. (65), (77), (91) revealed a problem for the electric potential change method using finite element analysis. The sensitivity of delamination detection remarkably decreases when a delamination is located roughly midway between the current electrodes. The electric potential changes due to delamination are strongly affected by shape. This is because of the progressive reduction of electric current flow in the thickness direction at the mid-point between the current electrodes as shown in Fig. 26. Since delamination is debonding of the in-plane interfaces of plies, electric current in the thickness direction plays an important role in measuring electric potential change due to delamination. This problem for this electric 962

17 potential change method was resolved by a proposed improvement (77), (91). A doubling of the current is required for the electric potential change method to make up for the progressive reduction of electric current in the thickness direction at the center of the current electrodes. N. Angelidis et al. (75) investigated the electric potential field across the surface of cross-ply and quasi-isotropic laminates using arrays of electrical contacts as shown in Fig. 27. The changes in the potential distribution before and after impact events were measured experimentally. FEM analyses were also performed to study electric current flow and potential changes in detail. The maximum potential change was about 45% of the original pre-damaged value. Fig. 27 Schematic representation of CFRP plate on which 121 electrodes are mounted to measure electric potential (75). Fig. 28 Schematic representation of resistance measurement in quasi-isotropic laminate beam (75). S. Wang et al. (76) investigated drop impact damage in unidirectional and quasi-isotropic CFRP beams using the four-probe method. For the quasi-isotropic laminate, the oblique resistance at an angle between the longitudinal and orthogonal directions was found to be more effective than the surface longitudinal resistance in indicating interior drop impact damage as shown in Fig. 28. The oblique direction has a component in the orthogonal direction. As delamination is the main result of impact damage and effects orthogonal more than longitudinal resistance, measuring oblique resistance is more sensitive than simple longitudinal measurement. For the unidirectional laminate, it was found that the electric resistance measured by electrodes mounted with an angle of θ = 45 to the fiber direction, as shown in Fig. 29, was more effective than that measured by electrodes perpendicular to the fiber direction. The transverse separation between fibers due to the impact was measured by oblique contact electrodes, but missed by contacts perpendicular to the fibers. It was also reported that resistance measurement was sensitive to even minor damage associated with negligible indentation from drop impacts. The ultrasonic method failed to 963

18 detect damage from impact at 0.73J, but the oblique resistance change indicated damage. Fig. 29 Schematic representation of resistance measurement in unidirectional laminate beam (75). Fig. 30 Schematic representation of potential measurement of quasi-isotropic laminate (85). Wang and Chung (85) investigated the effect of electrical configuration, i.e. location of the electrodes and the method of applying electric current and measurement, on damage sensing. They performed electric potential measurement using quasi-isotropic CFRP plates. Electrodes were mounted at various locations on both sides and edges of the specimen as shown in Fig. 30. They found that in-plane surface current application at 90 to fiber direction in conjunction with surface voltage measurement was very effective in detecting impact damage, and more effective than methods employing electrodes at 45 and 0. Application of an oblique current in conjunction with oblique potential gradient measurement was also shown to be effective. These results supported the results obtained in reference Eddy current methods Eddy current based nondestructive inspection has been used on metallic materials for crack detection, measurement of thickness, and monitoring of material parameters. An eddy current inspection system primarily consists of a coil. By applying alternating current to the coil, a magnetic field is induced surrounding the coil. When the coil is brought into proximity with the material being investigated, the electromagnetic field induces eddy currents in the material. The eddy currents induce a secondary magnetic field that is opposed to the magnetic field produced by the coil. A variation of the structural integrity of the inspected material due to the presence of a defect causes a change in the eddy current flow, which results in a corresponding modification of the secondary magnetic field. Since these magnetic fields interact, changes to the coil impedance can be used to measure change in induced current. As a result, the defect is detected from changes in coil impedance. A pair of coils, i.e. an excitation and a detector coil, is also often used to allow suitable coil shapes for each device, to decrease the experimental noise, and so on. Damage detection in CFRP laminates using eddy current methods has been extensively studied (2), (4), (6), (7), (11), (18), (28), (41-44), (52), (56-58), (69-70). Inhomogeneity and strong anisotropy in CFRP, however, interfere with application of conventional eddy current methods. C. N. Owston (2) applied eddy current sensors for nondestructive evaluation of unidirectional and cross-ply CFRP laminates. Crack orientations and scanning directions are 964

19 shown in Fig. 31. Fig. 32 shows the difference in indicated width for scanning directions A and B. This indicates an elliptical eddy current path with its major axis in the fiber direction. Fig. 31 Schematic representation of unidirectional composite with three 0.04 inch (1.016 mm) wide saw cuts (2). Fig. 32 Variation in spatial resolution with scanning direction test (2). The penetration depth was also investigated with a cross-ply laminate using a standard cylindrical probe as shown in Fig. 33. Slots of various depths were detected with different frequencies. Penetration depths from in. (1.016 mm) at 56.5MHz rising to 0.65 in. (16.15mm) at 15MHz were considered and showed a fall in spatial resolution. It was shown that working frequency should be high, 100MHz and above, to achieve good spatial resolution when locating cracks. The effect of fiber lay-up on the eddy current method was also investigated; the method was sensitive to fiber lay-up and even for internal misorientations. A. R. Valleau (4) applied the eddy current method to a laminate of nine layers of satin-weave graphite-fiber fabric. The thickness was about 3.5mm. The defect type was flat-bottomed holes that had diameters of about 12.7mm and 6.3mm and various depths. The average estimated depth showed good agreement with the actual depth although it was always larger. If the probe was larger than the hole larger depths were estimated. Images of the damage were also derived using this method. Some images fitted well to the respective hole but images where always larger than the object being measured. D. Placko et al. (6) investigated both the effect of distance between sensor and target and its local conductivity on sensor impedance using an eddy current sensor. The first test indicated the state of the specimen surface and the second indicated the degree of internal damage. From the results, maps of surface state and damage condition of graphite composite materials after impact or compression were created. Damage was easily recognized in these maps. 965

20 Fig. 33 Penetration depth measurement after the reference (2). C. W. Davis et al. (7) performed an eddy current inspection on three kinds of CFRP laminate; thermally exposed unidirectional CFRP, impact damaged graphite composite (the configuration of the laminate was concealed), and [(45/90/-45/0) 6 ] s laminate with a non-conducting insert as a defect. The thermally exposed CFRP was subjected to a temperature of 537 C for a period of 3 minutes before testing. The effect of surface roughness was examined showing that the fibers were not severely damaged due high temperature exposure. For the impact damaged composite, the damage was clearly recognized by measuring impedance change due to fiber displacement and shattering. For the laminate with non-conducting inserts, it was reported that changes in eddy current signal were caused by fiber displacement. Surface roughness measurement showed fiber displacement due to an insert. A non-conductive insert, therefore, cannot be used to represent delamination in eddy current inspection. Fig. 34 Schematic representation of Tedlar-carbon-nida composite for detection of impact damage (11). X. E. Gros (11) has studied the limitations of conventional eddy current inspection for impact damage in CFRP. Impacts of different energies were applied to each specimen using a drop tower mechanism. The specimens were a Tedlar-carbon-nida composite that is used in the primary structure of helicopters and is shown in Fig. 34. The impact energies were from 0.5 to 3.0 J on side 1 and from 0.5 to 7.0 J on side 2. Impacts of energy less than 2.0 J were not visible on side 1 and those below 1.0 J were not visible on side 2. Sensitivity was different on each side because of the asymmetric stacking sequence. X.E.Gros et al. (18) detected damage of a quasi-isotropic CFRP laminate [0/45/-45/90] s. The damage was created using a tensile test. Both delamination and interlaminar cracks were detected. It was, however, reported that it was not possible to distinguish between these types of damage because of the limited resolution of the method. Further, another disadvantage of the method was mentioned; that it detects only surface and subsurface defects. R. Grimberg et al. (28),(42) introduced the eddy current holography method. They detected impacted damage in 2mm thick quasi-isotropic laminates [45 2 /0 2 /-45 2 /90 2 ] s using original transducers. Impact tests were performed with energies up to 4J. In separate studies it was reported that the impacted zone could be detected for energies of more than 2.5 J (28) and 0.75 J (42). 966

21 This team also introduced the eddy current microscopy method (41). [-45 2 /45 2 ] s and [-45 2 /0 2 /45 2 ] s laminates were used and images of the fiber in a focused layer were obtained. For any given layer, fiber direction could be recognized. It was reported that the phase of the electromotive force induced in the reception coil contained enough information to visualize the fiber orientation, which, in turn shows the possibility using this method to find delamination. M. B. Lemistre et al. (43), (56), (70) developed what they have termed a HELP-Layer (Hybrid Electromagnetic Performing Layer). The HELP-Layer is bonded to the CFRP laminate surface. The layer is made of a 100mm thick dielectric substrate with a printed circuit containing a double network of crossed wires spaced at 20mm distance, shown in Fig. 35. The method is designed to detect of both local electric conductivity variation and local dielectric permittivity variations. Fig. 35 Schematic representation of a HELP Layer bonded to CFRP laminate (56). The electric field E r in the laminate caused by eddy current induction is expressed by the following equation. E J = + ( ε r 1) σ r E l where E l is the local electric field induced, ε r the relative dielectric permittivity of the medium, and J and σ the current density and electric conductivity respectively. The induced electric field, is therefore the result of two electric fields that relate to the electric conductivity and dielectric permittivity of the CFRP. Local changes in these two parameters represent damage in the CFRP laminate. The impact tests were performed with impact energies of up to 4J on quasi-isotropic laminate [45 2 /0 2 /-45 2 /90 2 ] s. Burning with a hot object and electric sparks where also used to create experimental damage. The results showed that damage due to impact energy lower than 2.5J could not be detected. This level of impact did not cause fiber breakage, which resulted in no variation of electric conductivity. On the other hand, it was concluded that the method was very sensitive to damage by burning and artificial lightning with and without liquid ingress. Numerical simulation supported the experimental results. It showed that the electric field was more sensitive to variation in dielectric permittivity than electric conductivity. The variation of dielectric permittivity is due burning and liquid ingress while the variation in electric conductivity is caused by delamination and fiber breaks. This study also mentions the possibility of characterizing the type of damage through measurement of variation of the components of the reflected electric field E x and E y ; only a dielectric permittivity change induced a significant effect on the E x component of the electric field (70). A superconducting quantum interface device (SQUID) based eddy current method has also been developed to detect damage in CFRP laminates. Since the electrical conductivity of the CFRP is low compared to metals, a high frequency field is usually required to induce sufficient eddy current (2), (4), (6), (7), (11), (18), (28), (41), (42), (56), (70). For thick CFRP laminates, a low frequency field is, however, required to induce an eddy current at greater depths. Since the (8) 967

22 SQUID has high magnetic sensitivity at low frequencies, it has an advantage in detecting deep-lying defects in electroconductive composite materials. A. Ruosi et al. (44) applied a HTS-SQUID (High Temperature Superconductor -SQUID) based eddy-current method to laminates of [45/-45] s. The thickness of the laminate was about 4mm. The laminate was impact damaged. It was demonstrated that an increase in the SQUID s response corresponded well with increase in impact energy. The threshold energy at which the damage was first detectable was 1J. Fig. 36 Schematic representation of saw cut direction relative to the fiber orientation (57). C. Carr et al. (57), (58) also applied this method to detect both impact damage and saw cuts in the surface of 3mm thick woven fabric CFRP (0/90 fiber orientation) as shown in Fig. 36. The impact energies were 23.4J, 16.5J, and 14.1J. The saw cuts were 13mm long, 0.15mm wide, and 1mm depth. For impact tests, the maximum amplitude of the SQUID s response increased with increase in impact energy. Although some saw cuts were not detected in some coil orientations, the applicability of the method to damage detection in CFRP laminate was clearly demonstrated. (52) Y. Hatsukade et al. proposed an induction coil with a U-shaped high-magnetic-permeability ferrite core to generate a strong induction field in thick CRRP while supplying a low frequency current to the coil. This method was applied to 20mm thick cross texture CFRP fabric with hidden slots at various depths as shown in Fig. 37. The results showed that the peak amplitude of gradient component db z /dy decreased with the depth increase. Slot depth could be roughly estimated from the peak amplitude. Slots up to 17.5mm in depth were in 20mm thick CFRP using this method. D. Graham et al. (69) propose a HTS SQUID-based automated detection system using a neural network to speed the detection process of impact damaged CFRP and to increase the probability of detection in data affected by environmental noise. An image of the damaged area was produced using this method. This image can be used to detect flaws within a relatively small amount of noise. 968

23 Fig. 37 Schematic representation of CFRP cloth for detection of hidden slots (52). 7. Concluding remarks The present review deals with various electrical resistance change based methods for structural health monitoring of CFRP composites. Electrical resistance change methods do not require expensive equipment and adopt the carbon fibers themselves as sensors. For this reason they can be called self-sensing systems. Over recent decades, the measurement of electrical resistance change has enabled measurement of applied strain and to detect breakages in carbon fibers. Recently, these methods have been applied to identify the location and dimension of damage such as delaminations in CFRP laminates. Although some controversy still exists around strain measurement using the four-probe method, electrical resistance change methods are growing in popularity and represent attractive methods of monitoring damage to CFRP composites. These methods have higher precision for monitoring damage than other strain sensing methods because they directly measure internal damage through electrical resistance changes in the carbon fibers themselves. References (1) Coror, P.C. and Owston, C.N., Electrical resistance of single carbon fibres Nature 223, September,1969, p (2) C.N.Owston, Eddy current methods for the examination of carbon fibre reinforced epoxy resins Materials evaluation,34(11),1976,p (3) Schulte, K and Baron, Ch., Load and failure analyses of CFRP laminates by means of electrical resistance measurement, Composites Science and Technology,36 (1), 1989,p (4) A.R.Valleau, Eddy current nondestructive testing of graphite composite materials Materials evaluation, 48(2),1990,p (5) De Goeje M. P. and Wapenaar K. E. D.,Non-destructive inspection of carbon fibre-reinforced plastics using eddy current methods, Composites, 23(3),1992,p (6) D.Placko and I.Dufour, Eddy current sensors for nondestructive inspection of graphite composite materials, Industry Applications Society Annual Meeting, 1992., Conference Record of the 1992 IEEE,2, 1992, p (7) C.W.Davis, S.Nath, J.P.Fulton, and, M.Namkung, Combined investigation of Eddy current 969