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Effect of the crack length on the piezoelectric damage monitoring of glass fiber epoxy composite DCB specimens
Composites Science and Technology 72 (2012) 902–907
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Effect of the crack length on the piezoelectric damage monitoring of glass fiber epoxy composite DCB specimens H.Y. Hwang ⇑ Department of Mechanical Design Engineering, Andong National University, 388 Andong, Kyoungsangbuk-do 760-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 22 November 2011 Received in revised form 18 January 2012 Accepted 26 February 2012 Available online 4 March 2012 Keywords: Piezoelectric properties D. Nondestructive testing C. Crack A. Glass fibers A. Polymer–matrix composites
a b s t r a c t A new nondestructive method using the piezoelectric characteristics of polymer matrix was suggested for the damage monitoring of glass fiber polymer composites, and the feasibility of the use of the method was proven through basic experiments. Heretofore, most studies have focused on basic material properties such as the piezoelectric properties of unidirectional glass fiber epoxy composites with respect to the fiber orientation or the loading speed. In this study, the effect of the crack length on the piezoelectric damage monitoring of glass fiber polymer composites was experimentally investigated. Dynamic tests of mode I were performed using double-cantilever-beam (DCB) specimens, and the relationship between the crack length and the electric-charge signals measured from the electrodes on the DCB specimens was analyzed. The experiment results showed that the magnitude of the electric-charge signals increased very slowly as the crack tip approached the electrodes, rose sharply when the crack tip was passing through the electrodes, and then decreased fast and maintained relatively very low values when the crack tip had completely passed through the electrodes. The investigation of the mechanical behaviors via finiteelement analyses during the dynamic tests revealed that the tendency of electric-charge signals is quite similar to that of the strain changes in glass fiber epoxy composites near electrodes. Based on the results of the experiments and finite-element analyses conducted in this study, it was concluded that piezoelectric damage monitoring can detect crack propagation. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Due to their high specific stiffness and specific strength, fiber-reinforced composites have been widely used in various industries. The in situ detection of the initial defects or damages in-service is essential for improving the structural reliability of such composites because such defects or damages may decrease the strength and durability of the laminated composite structures. There are several nondestructive damage-monitoring methods for composite structures, such as the electric, ultrasonic, optical-fiber, and acoustic-emission methods [1–9]. As self-sensor methods use their own materials or structural characteristics and do not require any embedded or surfacemounted sensors, they can detect cracks or damages and can maintain the strength or structural reliability. Among the aforementioned methods, the electric method, which uses the resistance changes of composite materials due to cracks or damages, is a typical example of a self-sensor-type nondestructive method. The electric method, however, can be used only
⇑ Tel.: +82 54 820 6305; fax: +82 54 820 5167. E-mail address: [email protected] 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2012.02.022
for electrically conductive composite materials such as carbon fiber composites. To detect cracks or damages in polymer matrix composites reinforced with nonconductive fibers or particles such as glass or ceramic, a new nondestructive method using the piezoelectric characteristics of polymeric matrix was recently suggested. The feasibility of the use of the suggested method was experimentally proven in this study through static and dynamic tests using unidirectional glass fiber epoxy composites. Heretofore, most studies have focused on basic material properties such as the piezoelectric properties of unidirectional glass fiber epoxy composites with respect to the fiber orientation or the loading speed [10–12]. In this study, the effect of the crack length on the piezoelectric damage monitoring of glass fiber polymer composite materials was experimentally investigated. Dynamic tests of mode I were performed using double-cantilever-beam (DCB) specimens. The crack length and electric-charge signals were measured during the dynamic tests, and their relationship was analyzed. In addition, the mechanical behaviors near the electrodes were checked via the finite-element method (FEM) to explain the electric-charge signal change trends.
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H.Y. Hwang / Composites Science and Technology 72 (2012) 902–907 Table 2 Specifications of silver paste (ELCOAT P-100, CANS, Japan).
2. Materials and methods 2.1. Materials and specimens DCB specimens with various initial crack lengths, as shown in Fig. 1, were fabricated using unidirectional glass fiber epoxy prepregs (UGN150, SK Chemicals, South Korea), via hot-press molding under the standard cure cycle suggested by the manufacturer, and were then cut with a diamond wheel cutter. The mechanical and piezoelectric properties of the unidirectional glass fiber epoxy composites with 0° fiber orientation are listed in Table 1 [11]. Pre-cracks were made by inserting a nonporous Teflon film between the mid-plane layers while stacking the glass fiber epoxy prepregs, and the initial crack length was changed from 50 to 120 mm at intervals of 5 mm. Two kinds of specimens with different thicknesses (2.0 and 4.0 mm) were used for the investigation of the effect of specimen thickness. Electrodes for measuring the electric-charge signal were fabricated, using electrically conducting silver paste (ELCOAT P-100, CANS, Japan), on the 60-mm (front electrodes) and 100-mm (rear electrodes) specimen surfaces, apart from the loading position. Table 2 shows the specifications of the silver paste. 2.2. Experiments Before the dynamic tests, the magnitude of such tests (P) without the crack propagating statically during the tests was selected. Thus, static tests of the DCB specimens with respect to the crack length were performed on a material-testing machine (Instron 8526, Instron Co., USA). Mode-I-type loads were applied with a
Properties
Data
Viscosity Pencil hardness Density Volume resistance
230 Poise (25 °C) 2H 2300–2500 kg/m3 10 4 Xm
2 mm/min loading speed to the loading jigs of the DCB specimens (see Fig. 1) [13]. Mode I dynamic tests with respect to the crack length were conducted under a 1 Hz sinusoidal load (0P) [14]. Fig. 2 shows the experimental setup for the mode I dynamic test. As the mode-Itype tests of DCB specimens need a relatively large displacement but a small load, an electrodynamic testing machine was specially designed for mode I dynamic tests, with a small dynamic-load capacity (up to 100 N) and a relatively large stroke (up to 50 mm). The electric-charge signals induced from the electrodes of the DCB specimens were measured using a charge-conditioning amplifier (type 2626, Bruel & Kjar Co., Denmark). The measured signals were processed via a computer equipped with an analogto-digital converter, which also calculated the electric-flux density (i.e., the ratio of the magnitude of the measured electric-charge signals to the electrode area) [10–12,15]. 2.3. Finite-element analyses The experiment results were subjected to finite-element analyses using a commercial FEM software (ABAQUS 6.5, Hibbitt, Karlsson, & Sorensen, USA), and the mechanical behavior of the DCB specimens were compared to the electric-charge signals. Fig. 3 shows the finite-element model for the 4.0-mm-thick DCB specimens. A 20-node 3D solid element (C3D20) and the anisotropic material properties listed in Table 1 were used. The elements and nodes numbered 12,000 and 58,597, respectively. The nodes at the center line of the end surface were fixed, and a 1 Hz sinusoidal load (0P) was applied on the surface, for loading the jigs. All the nodes on the crack surface were doubly defined and tied to model the crack surface, and were later released together to represent crack propagation, for maintaining the model’s coincidence with different crack lengths [16–17]. 3. Results and discussion 3.1. Static-test results of the DCB specimens with respect to the crack length
Fig. 1. DCB specimens of unidirectional glass fiber epoxy composites (UGN150, SK Chemicals, South Korea).
Table 1 Mechanical and piezoelectric properties of unidirectional glass fiber epoxy composites (UGN150, SK Chemicals, Korea). Mechanical properties
Dielectric constant
Piezoelectric strain constant
Density (kg/m3) Fiber volume fraction
E1 (GPa) E2 (GPa) G12 (GPa) v12 v23 e1 (F/m) e2 (F/m) e3 (F/m) e13 (C/m2) e23 (C/m2) e33 (C/m2)
43.3 14.7 4.4 0.3 0.4 4.87 10 4.47 10 4.54 10 0.106 0.635 0.272 1980 0.6
8 8 8
Fig. 4 shows the static-test results of the DCB specimens of unidirectional glass fiber epoxy composites with respect to the crack length. As the crack length increased, the maximum load required to propagate the crack decreased for both cases. The test results showed a tendency similar to those of the typical DCB tests presented in ASTM [13]. Within the crack length of 50–120 mm for the dynamic test, the maximum loads for preventing static crack propagation during the dynamic tests were 8 and 15 N for the 2.0- and 4.0-mm-thick specimens, respectively. Thus, it was determined that the magnitudes of the sinusoidal loads were 8 and 15 N for the mode I dynamic tests. 3.2. Electric-flux density with respect to the crack length Fig. 5 shows the measured electric-flux densities (the ratios of the measured electric-charge signals to the electrode areas) of
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H.Y. Hwang / Composites Science and Technology 72 (2012) 902–907
Fig. 2. Photograph of the experimental setup for the mode I dynamic tests.
Fig. 3. Finite-element model of the DCB specimens.
the DCB specimens of unidirectional glass fiber epoxy composites from two electrode pairs on the specimen surface with respect to the crack length under the mode I dynamic load. In Fig. 5, ‘‘Front’’ and ‘‘Rear’’ pertain to the electrode pairs near the initial crack tip (60–80 mm apart from the loading position) and far from the initial crack tip (100–120 mm apart from the loading position), respectively. In the case of the 2.0-mm-thick specimen, the magnitude of the electric-charge signals measured from the front electrodes began to increase when the crack tip reached the front end of the front electrodes, increased sharply until the crack tip passed through the front electrodes, then decreased and maintained relatively small values as the crack tip approached the rear electrodes. It was impossible to measure the electric-charge signals if the crack length was larger than 120 mm because the DCB specimens would have been abruptly fractured then. Therefore, the electric-charge signals could be measured from the rear electrodes, until the point when the crack tip had just passed through such electrodes. The magnitude of the electric-charge signals measured from the rear electrodes, however, had a tendency similar to that of the electric-charge signals measured from the front electrodes. In the case of the 4.0-mm-thick specimen, except for the facts that the measured data in the region of the front electrodes were somewhat scattered and that the magnitude of the measured elec-
tric-charge signals was higher, the general trend was the same as that of the 2.0-mm-thick specimen. Fig. 6 shows the electric-charge signals with respect to the relative positions of the crack tip to the front ends of the front and rear electrodes replotted from Fig. 5. In both cases, the magnitude of the electric-charge signals began to increase when the crack tip got to the front end of the electrodes, and increased sharply while the crack tip was passing through the electrodes, although the electric-charge signals of the rear electrodes were slightly lower than those of the front electrodes. In summary, the measured electric-charge signals significantly changed as the crack tip propagated, and the electric-charge signals measured from different electrodes had similar inclinations but could be distinguished at a certain point. Therefore, the analysis results showed that piezoelectric damage monitoring can detect crack propagation such as the crack tip position and crack length. 3.3. Mechanical behavior of the DCB specimens with respect to the crack length As the piezoelectric damage monitoring of polymeric composite materials uses the phenomenon of the electric-charge output induced by the material deformation under the external load, the parameter that most affects the electric-charge outputs is the
H.Y. Hwang / Composites Science and Technology 72 (2012) 902–907
Fig. 4. Static-test results of the DCB specimens with respect to the crack length when the specimen thickness was (a) 2.0 mm and (b) 4.0 mm.
Fig. 5. Dynamic-test results of the DCB specimens with respect to the crack length when the specimen thickness was (a) 2.0 mm and (b) 4.0 mm.
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Fig. 6. Electric-charge signals with respect to the position of the crack tip relative to that of the electrodes: (a) 2.0 mm and (b) 4.0 mm.
strain between the electrodes. The experiment results were subjected to finite-element analyses using a 3D model of the DCB specimens with respect to the crack length. The strain distributions near the front electrodes are shown in Fig. 7. For the typical behavior of the DCB specimens under mode I loading, e11 (the normal strain along the fiber direction) and e22 (the normal strain along the transverse direction) in the upper part of the DCB specimens were anti-symmetric. Thus, it seems that the sum of the strains between the electrodes was almost zero. On the other hand, e33 (the normal strain along the through-thickness direction) was positive over a relatively large area near the outer surface, and negative over a very small area near the crack tip while the crack tip was passing through the electrodes. Therefore, the through-thickness strain was not balanced and was expected to make the materials between the electrodes more polarized. To explain in greater detail, the changes in the through-thickness strain can be described as follows. As there was a very low strain between the electrodes until the crack tip reached the front end of the electrodes, the electric-charge signal changed very little. While the crack tip passed through the composite specimens between the electrodes, there were large strains, and the induced electric-charge signals increased. When the crack tip has completely passed through the specimens, the strain between the electrodes decreased very fast and maintained relatively small values. The qualitative-analysis results mentioned above are quite concordant with the experiment results. Fig. 8 compares the measured electric-charge signals and the sum of the through-thickness strains between the electrodes of the 4.0-mm-thick DCB specimen. Unlike the results of the qualitative analysis, the results of the quantitative analysis were somewhat different from the experiment results. The sum of the through-thickness strains between the front electrodes was
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H.Y. Hwang / Composites Science and Technology 72 (2012) 902–907
Fig. 7. Distributions of the strains near the front electrode with respect to the crack length, as determined via finite-element analyses, when the specimen thickness was 4.0 mm: (a) e11; (b) e22; and (c) e33.
H.Y. Hwang / Composites Science and Technology 72 (2012) 902–907
Fig. 8. Comparison of the measured electric-charge signals and the sums of the through-thickness strains with respect to the crack length when the specimen thickness was 4.0 mm.
minimally changed until the crack tip approached the front end of the front electrodes, then increased very fast when the crack tip passed through the electrodes. Up to that point, the changing tendency of the sum of the through-thickness strains was in good agreement with that of the measured electric-charge signals. When the crack tip had gotten out of the electrodes, however, the sum of the through-thickness strains rapidly decreased, and although it did not become zero, it became relatively very small. These behaviors were also observed in the rear electrodes. In addition, the sum in the rear electrodes was higher than that in the front electrodes while the magnitude of the electric-charge signals measured from the front electrodes was higher than that measured from the rear electrodes. The difference between the measured electric-charge signals and the results of the quantitative analysis of the strains via FEM might have been due to the assumption that the electric-charge signals were induced by the material deformation only between the electrodes. As the piezoelectric effects, however, are related to the mechanical–electrical interaction, the material deformation around the electrodes may affect the polarization and charge flow between the electrodes. Besides, the electric-charge signals measured from the front and rear electrodes might be interfered with because two electrode pairs were located nearby. More accurate analysis results can be obtained by conducting finite-element analysis considering the piezoelectric properties of glass fiber epoxy composites, and by directly evaluating the induction of electriccharge signals in the future. In the experiments and finite-element analyses, it was found that the measured electric-charge signals changed distinguishably as the crack propagated, and that the mechanical behaviors of the DCB specimens were quite similar to the experiment results even if the quantitative results were somewhat dissimilar due to the assumption that was used for the analyses. Therefore, it can be concluded that piezoelectric damage monitoring can detect crack propagation such as the crack tip position and crack length because the change in the crack length from the crack propagation affects the material deformation between the electrodes, which causes significant changes in the degree of the material polarization. 4. Conclusions In this study, the effect of the crack length on the piezoelectric damage monitoring of glass fiber epoxy composites was experimentally investigated. Mode I dynamic tests were performed using
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double-cantilever-beam (DCB) specimens, and the relationship between the crack length and the electric-charge signals measured from the electrodes in the DCB specimens was analyzed. The experiment results showed that the magnitude of the electriccharge signals began to increase when the crack tip reached the front end of the front electrodes, increased sharply until the crack tip had passed through the front electrodes, then decreased and maintained relatively small values. In addition, the mechanical behaviors obtained via finite-element analysis were in good agreement with the experiment results. Based on the results of the experiments and of the finite-element analyses conducted in this study, it can be concluded that piezoelectric damage monitoring can detect crack propagation such as the crack tip position and crack length because the measured electric-charge signals changed distinguishably as the crack propagated. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20100023918). References [1] Scott IG, Scala CM. A review of nondestructive testing of composite materials. NDTE Int 1982;15(2):75–86. [2] Kitade S, Fukuda T, Osaka K, Hamamoto A. Detection of damages in composite laminates with embedded optical fibers. Smart Mater Struct 1996;4(2):283–90. [3] Wang X, Chung DDL. Real-time monitoring of fatigue damage and dynamic strain in carbon fiber polymermatrix composite by electrical resistance measurement. Smart Mater Struct 1996;6(4):504–8. [4] Todoroki A, Suzuki H. Application of electric potential method to smart composite structures for detecting delamination. JSME Int J Ser A: Mech Mater Eng 1995;38(4):524–30. [5] Schueler R, Joshia SP, Schulte K. Damage detection in CFRP by electrical conductivity mapping. Composites Science and Technology 2001;61(6): 921–30; Chung DDL, Wang S. Self-sensing of damage and strain in carbon fiber polymer-matrix structural composites by electrical resistance measurement. Polymers & Polymer Composites 2003;11(7):515–25. [6] Angelidis N, Khemiri N, Irving PE. Experimental and finite element study of the electrical potential technique for damage detection in CFRP laminates. Smart Mater Struct 2005;14(1):147–54. [7] Wang D, Chung DDL. Comparative evaluation of the electrical configurations for the two-dimensional electric potential method of damage monitoring in carbon fiber polymer–matrix composite. Smart Mater Struct 2006;15(5): 1332–44. [8] Angelidis N, Irving PE. Detection of impact damage in CFRP laminates by means of electrical potential techniques. Compos Sci Technol 2007;67(3– 4):594–604. [9] Todoroki A, Samejima Y, Hirano Y, Matsuzaki R. Piezoresistivity of unidirectional carbon/epoxy composites for multiaxial loading. Compos Sci Technol 2009;69(11–12):1841–6. [10] Hwang HY. Feasibility study of the damage monitoring for composite materials by the piezoelectric method. Trans Korean Soc Mech Eng 2008; 32(11):918–23. [11] Hwang HY. Effect of strain rate on piezoelectric characteristics of unidirectional glass fiber epoxy composites. Compos Mater 2011;45(6): 613–20. [12] Hwang HY. Electromechanical characteristics of unidirectional glass fiber epoxy composites. Polym Compos 2011;32:558–64. [13] ASTM, D 5528. Standard test method for mode I interlaminar fracture toughness of unidirectional fiber-reinforcement polymer matrix, composites; 2007. [14] ASTM, D 6115. Standard test method for mode I fatigue delamination growth onset of unidirectional fiber-reinforcement polymer matrix, composites; 2004. [15] Kwon JW, Chin WS, Lee DG. Piezoelectric monitoring of the reliability of adhesive joints. J Adhes Sci Technol 2003;17(6):777–96. [16] Hwang HY, Lee DG. Diagnosis criterion for damage monitoring of adhesive joints by the piezoelectric method. J Adhes Sci Technol 2005;19(12):1053–80. [17] Hwang HY, Kim BJ, Chin WS, Kim HS, Lee DG. Prediction of the crack length and crack growth rate of adhesive joints. J Adhes Sci Technol 2005;19(12): 1081–111.
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Effect of the crack length on the piezoelectric damage monitoring of glass fiber epoxy composite DCB specimens H.Y. Hwang ⇑ Department of Mechanical Design Engineering, Andong National University, 388 Andong, Kyoungsangbuk-do 760-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 22 November 2011 Received in revised form 18 January 2012 Accepted 26 February 2012 Available online 4 March 2012 Keywords: Piezoelectric properties D. Nondestructive testing C. Crack A. Glass fibers A. Polymer–matrix composites
a b s t r a c t A new nondestructive method using the piezoelectric characteristics of polymer matrix was suggested for the damage monitoring of glass fiber polymer composites, and the feasibility of the use of the method was proven through basic experiments. Heretofore, most studies have focused on basic material properties such as the piezoelectric properties of unidirectional glass fiber epoxy composites with respect to the fiber orientation or the loading speed. In this study, the effect of the crack length on the piezoelectric damage monitoring of glass fiber polymer composites was experimentally investigated. Dynamic tests of mode I were performed using double-cantilever-beam (DCB) specimens, and the relationship between the crack length and the electric-charge signals measured from the electrodes on the DCB specimens was analyzed. The experiment results showed that the magnitude of the electric-charge signals increased very slowly as the crack tip approached the electrodes, rose sharply when the crack tip was passing through the electrodes, and then decreased fast and maintained relatively very low values when the crack tip had completely passed through the electrodes. The investigation of the mechanical behaviors via finiteelement analyses during the dynamic tests revealed that the tendency of electric-charge signals is quite similar to that of the strain changes in glass fiber epoxy composites near electrodes. Based on the results of the experiments and finite-element analyses conducted in this study, it was concluded that piezoelectric damage monitoring can detect crack propagation. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Due to their high specific stiffness and specific strength, fiber-reinforced composites have been widely used in various industries. The in situ detection of the initial defects or damages in-service is essential for improving the structural reliability of such composites because such defects or damages may decrease the strength and durability of the laminated composite structures. There are several nondestructive damage-monitoring methods for composite structures, such as the electric, ultrasonic, optical-fiber, and acoustic-emission methods [1–9]. As self-sensor methods use their own materials or structural characteristics and do not require any embedded or surfacemounted sensors, they can detect cracks or damages and can maintain the strength or structural reliability. Among the aforementioned methods, the electric method, which uses the resistance changes of composite materials due to cracks or damages, is a typical example of a self-sensor-type nondestructive method. The electric method, however, can be used only
⇑ Tel.: +82 54 820 6305; fax: +82 54 820 5167. E-mail address: [email protected] 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2012.02.022
for electrically conductive composite materials such as carbon fiber composites. To detect cracks or damages in polymer matrix composites reinforced with nonconductive fibers or particles such as glass or ceramic, a new nondestructive method using the piezoelectric characteristics of polymeric matrix was recently suggested. The feasibility of the use of the suggested method was experimentally proven in this study through static and dynamic tests using unidirectional glass fiber epoxy composites. Heretofore, most studies have focused on basic material properties such as the piezoelectric properties of unidirectional glass fiber epoxy composites with respect to the fiber orientation or the loading speed [10–12]. In this study, the effect of the crack length on the piezoelectric damage monitoring of glass fiber polymer composite materials was experimentally investigated. Dynamic tests of mode I were performed using double-cantilever-beam (DCB) specimens. The crack length and electric-charge signals were measured during the dynamic tests, and their relationship was analyzed. In addition, the mechanical behaviors near the electrodes were checked via the finite-element method (FEM) to explain the electric-charge signal change trends.
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H.Y. Hwang / Composites Science and Technology 72 (2012) 902–907 Table 2 Specifications of silver paste (ELCOAT P-100, CANS, Japan).
2. Materials and methods 2.1. Materials and specimens DCB specimens with various initial crack lengths, as shown in Fig. 1, were fabricated using unidirectional glass fiber epoxy prepregs (UGN150, SK Chemicals, South Korea), via hot-press molding under the standard cure cycle suggested by the manufacturer, and were then cut with a diamond wheel cutter. The mechanical and piezoelectric properties of the unidirectional glass fiber epoxy composites with 0° fiber orientation are listed in Table 1 [11]. Pre-cracks were made by inserting a nonporous Teflon film between the mid-plane layers while stacking the glass fiber epoxy prepregs, and the initial crack length was changed from 50 to 120 mm at intervals of 5 mm. Two kinds of specimens with different thicknesses (2.0 and 4.0 mm) were used for the investigation of the effect of specimen thickness. Electrodes for measuring the electric-charge signal were fabricated, using electrically conducting silver paste (ELCOAT P-100, CANS, Japan), on the 60-mm (front electrodes) and 100-mm (rear electrodes) specimen surfaces, apart from the loading position. Table 2 shows the specifications of the silver paste. 2.2. Experiments Before the dynamic tests, the magnitude of such tests (P) without the crack propagating statically during the tests was selected. Thus, static tests of the DCB specimens with respect to the crack length were performed on a material-testing machine (Instron 8526, Instron Co., USA). Mode-I-type loads were applied with a
Properties
Data
Viscosity Pencil hardness Density Volume resistance
230 Poise (25 °C) 2H 2300–2500 kg/m3 10 4 Xm
2 mm/min loading speed to the loading jigs of the DCB specimens (see Fig. 1) [13]. Mode I dynamic tests with respect to the crack length were conducted under a 1 Hz sinusoidal load (0P) [14]. Fig. 2 shows the experimental setup for the mode I dynamic test. As the mode-Itype tests of DCB specimens need a relatively large displacement but a small load, an electrodynamic testing machine was specially designed for mode I dynamic tests, with a small dynamic-load capacity (up to 100 N) and a relatively large stroke (up to 50 mm). The electric-charge signals induced from the electrodes of the DCB specimens were measured using a charge-conditioning amplifier (type 2626, Bruel & Kjar Co., Denmark). The measured signals were processed via a computer equipped with an analogto-digital converter, which also calculated the electric-flux density (i.e., the ratio of the magnitude of the measured electric-charge signals to the electrode area) [10–12,15]. 2.3. Finite-element analyses The experiment results were subjected to finite-element analyses using a commercial FEM software (ABAQUS 6.5, Hibbitt, Karlsson, & Sorensen, USA), and the mechanical behavior of the DCB specimens were compared to the electric-charge signals. Fig. 3 shows the finite-element model for the 4.0-mm-thick DCB specimens. A 20-node 3D solid element (C3D20) and the anisotropic material properties listed in Table 1 were used. The elements and nodes numbered 12,000 and 58,597, respectively. The nodes at the center line of the end surface were fixed, and a 1 Hz sinusoidal load (0P) was applied on the surface, for loading the jigs. All the nodes on the crack surface were doubly defined and tied to model the crack surface, and were later released together to represent crack propagation, for maintaining the model’s coincidence with different crack lengths [16–17]. 3. Results and discussion 3.1. Static-test results of the DCB specimens with respect to the crack length
Fig. 1. DCB specimens of unidirectional glass fiber epoxy composites (UGN150, SK Chemicals, South Korea).
Table 1 Mechanical and piezoelectric properties of unidirectional glass fiber epoxy composites (UGN150, SK Chemicals, Korea). Mechanical properties
Dielectric constant
Piezoelectric strain constant
Density (kg/m3) Fiber volume fraction
E1 (GPa) E2 (GPa) G12 (GPa) v12 v23 e1 (F/m) e2 (F/m) e3 (F/m) e13 (C/m2) e23 (C/m2) e33 (C/m2)
43.3 14.7 4.4 0.3 0.4 4.87 10 4.47 10 4.54 10 0.106 0.635 0.272 1980 0.6
8 8 8
Fig. 4 shows the static-test results of the DCB specimens of unidirectional glass fiber epoxy composites with respect to the crack length. As the crack length increased, the maximum load required to propagate the crack decreased for both cases. The test results showed a tendency similar to those of the typical DCB tests presented in ASTM [13]. Within the crack length of 50–120 mm for the dynamic test, the maximum loads for preventing static crack propagation during the dynamic tests were 8 and 15 N for the 2.0- and 4.0-mm-thick specimens, respectively. Thus, it was determined that the magnitudes of the sinusoidal loads were 8 and 15 N for the mode I dynamic tests. 3.2. Electric-flux density with respect to the crack length Fig. 5 shows the measured electric-flux densities (the ratios of the measured electric-charge signals to the electrode areas) of
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H.Y. Hwang / Composites Science and Technology 72 (2012) 902–907
Fig. 2. Photograph of the experimental setup for the mode I dynamic tests.
Fig. 3. Finite-element model of the DCB specimens.
the DCB specimens of unidirectional glass fiber epoxy composites from two electrode pairs on the specimen surface with respect to the crack length under the mode I dynamic load. In Fig. 5, ‘‘Front’’ and ‘‘Rear’’ pertain to the electrode pairs near the initial crack tip (60–80 mm apart from the loading position) and far from the initial crack tip (100–120 mm apart from the loading position), respectively. In the case of the 2.0-mm-thick specimen, the magnitude of the electric-charge signals measured from the front electrodes began to increase when the crack tip reached the front end of the front electrodes, increased sharply until the crack tip passed through the front electrodes, then decreased and maintained relatively small values as the crack tip approached the rear electrodes. It was impossible to measure the electric-charge signals if the crack length was larger than 120 mm because the DCB specimens would have been abruptly fractured then. Therefore, the electric-charge signals could be measured from the rear electrodes, until the point when the crack tip had just passed through such electrodes. The magnitude of the electric-charge signals measured from the rear electrodes, however, had a tendency similar to that of the electric-charge signals measured from the front electrodes. In the case of the 4.0-mm-thick specimen, except for the facts that the measured data in the region of the front electrodes were somewhat scattered and that the magnitude of the measured elec-
tric-charge signals was higher, the general trend was the same as that of the 2.0-mm-thick specimen. Fig. 6 shows the electric-charge signals with respect to the relative positions of the crack tip to the front ends of the front and rear electrodes replotted from Fig. 5. In both cases, the magnitude of the electric-charge signals began to increase when the crack tip got to the front end of the electrodes, and increased sharply while the crack tip was passing through the electrodes, although the electric-charge signals of the rear electrodes were slightly lower than those of the front electrodes. In summary, the measured electric-charge signals significantly changed as the crack tip propagated, and the electric-charge signals measured from different electrodes had similar inclinations but could be distinguished at a certain point. Therefore, the analysis results showed that piezoelectric damage monitoring can detect crack propagation such as the crack tip position and crack length. 3.3. Mechanical behavior of the DCB specimens with respect to the crack length As the piezoelectric damage monitoring of polymeric composite materials uses the phenomenon of the electric-charge output induced by the material deformation under the external load, the parameter that most affects the electric-charge outputs is the
H.Y. Hwang / Composites Science and Technology 72 (2012) 902–907
Fig. 4. Static-test results of the DCB specimens with respect to the crack length when the specimen thickness was (a) 2.0 mm and (b) 4.0 mm.
Fig. 5. Dynamic-test results of the DCB specimens with respect to the crack length when the specimen thickness was (a) 2.0 mm and (b) 4.0 mm.
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Fig. 6. Electric-charge signals with respect to the position of the crack tip relative to that of the electrodes: (a) 2.0 mm and (b) 4.0 mm.
strain between the electrodes. The experiment results were subjected to finite-element analyses using a 3D model of the DCB specimens with respect to the crack length. The strain distributions near the front electrodes are shown in Fig. 7. For the typical behavior of the DCB specimens under mode I loading, e11 (the normal strain along the fiber direction) and e22 (the normal strain along the transverse direction) in the upper part of the DCB specimens were anti-symmetric. Thus, it seems that the sum of the strains between the electrodes was almost zero. On the other hand, e33 (the normal strain along the through-thickness direction) was positive over a relatively large area near the outer surface, and negative over a very small area near the crack tip while the crack tip was passing through the electrodes. Therefore, the through-thickness strain was not balanced and was expected to make the materials between the electrodes more polarized. To explain in greater detail, the changes in the through-thickness strain can be described as follows. As there was a very low strain between the electrodes until the crack tip reached the front end of the electrodes, the electric-charge signal changed very little. While the crack tip passed through the composite specimens between the electrodes, there were large strains, and the induced electric-charge signals increased. When the crack tip has completely passed through the specimens, the strain between the electrodes decreased very fast and maintained relatively small values. The qualitative-analysis results mentioned above are quite concordant with the experiment results. Fig. 8 compares the measured electric-charge signals and the sum of the through-thickness strains between the electrodes of the 4.0-mm-thick DCB specimen. Unlike the results of the qualitative analysis, the results of the quantitative analysis were somewhat different from the experiment results. The sum of the through-thickness strains between the front electrodes was
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Fig. 7. Distributions of the strains near the front electrode with respect to the crack length, as determined via finite-element analyses, when the specimen thickness was 4.0 mm: (a) e11; (b) e22; and (c) e33.
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Fig. 8. Comparison of the measured electric-charge signals and the sums of the through-thickness strains with respect to the crack length when the specimen thickness was 4.0 mm.
minimally changed until the crack tip approached the front end of the front electrodes, then increased very fast when the crack tip passed through the electrodes. Up to that point, the changing tendency of the sum of the through-thickness strains was in good agreement with that of the measured electric-charge signals. When the crack tip had gotten out of the electrodes, however, the sum of the through-thickness strains rapidly decreased, and although it did not become zero, it became relatively very small. These behaviors were also observed in the rear electrodes. In addition, the sum in the rear electrodes was higher than that in the front electrodes while the magnitude of the electric-charge signals measured from the front electrodes was higher than that measured from the rear electrodes. The difference between the measured electric-charge signals and the results of the quantitative analysis of the strains via FEM might have been due to the assumption that the electric-charge signals were induced by the material deformation only between the electrodes. As the piezoelectric effects, however, are related to the mechanical–electrical interaction, the material deformation around the electrodes may affect the polarization and charge flow between the electrodes. Besides, the electric-charge signals measured from the front and rear electrodes might be interfered with because two electrode pairs were located nearby. More accurate analysis results can be obtained by conducting finite-element analysis considering the piezoelectric properties of glass fiber epoxy composites, and by directly evaluating the induction of electriccharge signals in the future. In the experiments and finite-element analyses, it was found that the measured electric-charge signals changed distinguishably as the crack propagated, and that the mechanical behaviors of the DCB specimens were quite similar to the experiment results even if the quantitative results were somewhat dissimilar due to the assumption that was used for the analyses. Therefore, it can be concluded that piezoelectric damage monitoring can detect crack propagation such as the crack tip position and crack length because the change in the crack length from the crack propagation affects the material deformation between the electrodes, which causes significant changes in the degree of the material polarization. 4. Conclusions In this study, the effect of the crack length on the piezoelectric damage monitoring of glass fiber epoxy composites was experimentally investigated. Mode I dynamic tests were performed using
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double-cantilever-beam (DCB) specimens, and the relationship between the crack length and the electric-charge signals measured from the electrodes in the DCB specimens was analyzed. The experiment results showed that the magnitude of the electriccharge signals began to increase when the crack tip reached the front end of the front electrodes, increased sharply until the crack tip had passed through the front electrodes, then decreased and maintained relatively small values. In addition, the mechanical behaviors obtained via finite-element analysis were in good agreement with the experiment results. Based on the results of the experiments and of the finite-element analyses conducted in this study, it can be concluded that piezoelectric damage monitoring can detect crack propagation such as the crack tip position and crack length because the measured electric-charge signals changed distinguishably as the crack propagated. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20100023918). References [1] Scott IG, Scala CM. A review of nondestructive testing of composite materials. NDTE Int 1982;15(2):75–86. [2] Kitade S, Fukuda T, Osaka K, Hamamoto A. Detection of damages in composite laminates with embedded optical fibers. Smart Mater Struct 1996;4(2):283–90. [3] Wang X, Chung DDL. Real-time monitoring of fatigue damage and dynamic strain in carbon fiber polymermatrix composite by electrical resistance measurement. Smart Mater Struct 1996;6(4):504–8. [4] Todoroki A, Suzuki H. Application of electric potential method to smart composite structures for detecting delamination. JSME Int J Ser A: Mech Mater Eng 1995;38(4):524–30. [5] Schueler R, Joshia SP, Schulte K. Damage detection in CFRP by electrical conductivity mapping. Composites Science and Technology 2001;61(6): 921–30; Chung DDL, Wang S. Self-sensing of damage and strain in carbon fiber polymer-matrix structural composites by electrical resistance measurement. Polymers & Polymer Composites 2003;11(7):515–25. [6] Angelidis N, Khemiri N, Irving PE. Experimental and finite element study of the electrical potential technique for damage detection in CFRP laminates. Smart Mater Struct 2005;14(1):147–54. [7] Wang D, Chung DDL. Comparative evaluation of the electrical configurations for the two-dimensional electric potential method of damage monitoring in carbon fiber polymer–matrix composite. Smart Mater Struct 2006;15(5): 1332–44. [8] Angelidis N, Irving PE. Detection of impact damage in CFRP laminates by means of electrical potential techniques. Compos Sci Technol 2007;67(3– 4):594–604. [9] Todoroki A, Samejima Y, Hirano Y, Matsuzaki R. Piezoresistivity of unidirectional carbon/epoxy composites for multiaxial loading. Compos Sci Technol 2009;69(11–12):1841–6. [10] Hwang HY. Feasibility study of the damage monitoring for composite materials by the piezoelectric method. Trans Korean Soc Mech Eng 2008; 32(11):918–23. [11] Hwang HY. Effect of strain rate on piezoelectric characteristics of unidirectional glass fiber epoxy composites. Compos Mater 2011;45(6): 613–20. [12] Hwang HY. Electromechanical characteristics of unidirectional glass fiber epoxy composites. Polym Compos 2011;32:558–64. [13] ASTM, D 5528. Standard test method for mode I interlaminar fracture toughness of unidirectional fiber-reinforcement polymer matrix, composites; 2007. [14] ASTM, D 6115. Standard test method for mode I fatigue delamination growth onset of unidirectional fiber-reinforcement polymer matrix, composites; 2004. [15] Kwon JW, Chin WS, Lee DG. Piezoelectric monitoring of the reliability of adhesive joints. J Adhes Sci Technol 2003;17(6):777–96. [16] Hwang HY, Lee DG. Diagnosis criterion for damage monitoring of adhesive joints by the piezoelectric method. J Adhes Sci Technol 2005;19(12):1053–80. [17] Hwang HY, Kim BJ, Chin WS, Kim HS, Lee DG. Prediction of the crack length and crack growth rate of adhesive joints. J Adhes Sci Technol 2005;19(12): 1081–111.