Experimental Mechanics (2006) 46: 609–618 DOI 10.1007/s11340-006-8734-0
Multiple Crack Detection of Concrete Structures Using Impedance-based Structural Health Monitoring Techniques S. Park & S. Ahmad & C.-B. Yun & Y. Roh
Received: 25 October 2005 / Accepted: 10 April 2006 / Published online: 26 June 2006 # Society for Experimental Mechanics 2006
Abstract This paper presents a feasibility study for practical applications of an impedance-based real-time health monitoring technique applying PZT (Lead– Zirconate–Titanate) patches to concrete structures. First, comparison between experimental and analytical studies for damage detection on a plain concrete beam is made. In the experimental study, progressive surface damage inflicted artificially on the plain concrete beam is assessed by using both lateral and thickness modes of the PZT patches. Then, an analytical study based on finite element (FE) models is carried out to verify the validity of the experimental result. Secondly, multiple (shear and flexural) cracks incurred in a reinforced concrete (RC) beam under a third point bending test are monitored continuously by using a sensor array system composed of the PZT patches. In this study, a root mean square deviation (RMSD) in the impedance signatures of the PZT patches is used as a damage indicator. Keywords Impedance . PZT . Structural health monitoring . Multiple crack detection . Finite element analysis . Concrete structures
S. Park (*) : S. Ahmad : C.-B. Yun Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea e-mail:
[email protected] Y. Roh School of Mechanical Engineering, Kyungpook National University, Daegu 702-701, South Korea
Introduction Concrete structures have been used extensively in civil infrastructural systems. However, as compared with metallic or other composite structures, the non-destructive evaluation (NDE) technologies of concrete structures are relatively undeveloped. Furthermore, due to their extensive and complex nature, conventional NDE methods might be very tedious, expensive, or unreliable. Therefore, more reliable and automated NDE techniques are being investigated for real-time health monitoring of concrete structures. The automated NDE techniques that enable continuous health monitoring of concrete structures while in operation require the development of a built-in diagnostic system. Such a built-in diagnostic system would be placed practically anywhere, even in remote and inaccessible locations to actively monitor the conditions of various types of structures. In particular, an impedance-based damage detection technique which uses a smart piezoelectric ceramic material (PZT) has emerged as a potential tool for the implementation of a built-in diagnostic system [1–5]. This technique utilizes high-frequency structural excitations, which are typically higher than 20 kHz from the surfacebonded PZT patches to monitor the changes in the structural mechanical impedance. A basic principle of the impedance-based damage detection method is to track an electrical point impedance of the PZT patch bonded onto the structure. Physical changes in the structure may cause changes in the structural mechanical impedance, which may induce changes in the electrical impedance of the PZT patch. Those changes in the impedances of the PZT patches are used to identify incipient damage in the structure. Applica-
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tions of the PZT patches to civil structures began relatively recently. Ayres et al. [6], Park et al. [7], Tseng et al. [8], Soh et al. [9], Bhalla et al. [10] and Park et al. [11] have reported successful applications of this method to various civil infrastructural systems. In this paper, first, comparison between experimental and analytical studies for damage detection on a plain concrete beam is made. In the experimental study, progressive surface damage inflicted artificially on the plain concrete beam is assessed by using both lateral and thickness modes of the PZT patches. Then, an analytical study based on finite element (FE) models is carried out to verify the validity of the experimental result. Secondly, multiple (shear and flexural) cracks incurred in a reinforced concrete (RC) beam under a third point bending test are monitored continuously by a sensor array system composed of the PZT patches. In this study, a root mean square deviation (RMSD) in the impedance signatures of the PZT patches is used as a damage indicator.
Impedance-Based Structural Health Monitoring The impedance-based structural health monitoring (SHM) technique utilizes a coupling effect (electromechanical property) between the PZT patch and the host structure. The coupling effect can be conceptually explained by using an idealized 1-D electro-mechanical system, as shown in Fig. 1 [2]. The electrical aspect of the PZT patch is described by its short-circuited impedance, and the host structure is represented by its driving point mechanical impedance, which includes the effects of mass, stiffness, damping, and boundary conditions. The PZT patch is powered by a voltage or a current. The integrated electro-mechanical system may be electrically represented by the electrical impedance, which is affected by the dynamics of the PZT and the host structure. The mechanical impedance, ZA, of the PZT patch in Fig. 1 is defined as the ratio of a harmonic input voltage V(w) at an angular frequency w to a current response I(w) in frequency domain. Similarly, the mechanical impedance, ZS, of the host structure idealized as a SDOF system is defined as the ratio of a harmonic excitation force F0(w) at an angular frequency w to a velocity response xðwÞ in a frequency domain. Then, the apparent electro-mechanical impedance of the PZT patch as coupled to the host structure is obtained as
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Fig. 1. 1-D electro-mechanical impedance system [2]
Ztotal ðwÞ ¼
E wl d23x Yxx ZA ðwÞ tan ðlÞ s ZA ðwÞ þ ZS ðwÞ l 1 E þ "T33 d23x Yxx jw
ð1Þ
where w is a PZT width, l is a PZT length, s is a PZT E thickness, d3x is a piezoelectric strain coefficient, Yxx is an elastic stiffness at a constant electric field, is a E wave number ¼ w cE t , w is an excitation frequency, ct T is a wave velocity, and "33 is a permittivity at constant stress. The electro-mechanical impedance technique enables damage detection, health monitoring, and builtin NDE because it can measure directly the high frequency local impedance, which is very sensitive to local damage. This method utilizes the changes that take place in the high-frequency drive-point structural impedance to identify incipient damage in the structure. Hence, the changes of the mechanical properties of the host structure may be detected by monitoring the variations of the electro-mechanical impedance functions, as shown in equation (1). Herein, an index needs to be defined to quantify the damage. The index should reflect the difference in the electric impedance signatures before and after the damage. For this study, the RMSD in the impedance signatures of the PZT patches caused by the damage is used as a damage indicator, which is given by ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pi¼N 2 i¼1 ðZ ðwi Þ Z0 ðwi ÞÞ RMSDð%Þ ¼ 100 Pi¼N 2 i¼1 ðZ0 ðwi ÞÞ
ð2Þ
where Z(wi) is a post-damage impedance signature at the ith measurement point, and Z0(wi) is a corresponding pre-damage value. An experimental setup for the impedance-based SHM consists of a test specimen, the PZT patches, an impedance analyzer (HP4294A), and a personal computer (PC) equipped with data acquisition software, as shown in Fig. 2. The PZT patches are bonded to the specimen, and they are connected to the HP4294A.
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Fig. 2. Experimental setup and PZT patch attached to host structure
Then, the impedance signatures are extracted as a function of the exciting frequency. The PC is used to control the HP4294A. The sensitivity for damage detection of the impedance-based SHM method is closely related to the frequency band selected. In order to detect the damage effectively, it is necessary for the wavelength of the excitation to be smaller than the characteristic length of the damage [12]. The frequency range for the given structure is commonly determined by a trial-and-error method. In this study, the PZT patches attached to host structures have been scanned over a wide frequency range of 40 kHz to 8 MHz to select a suitable frequency range for acquiring the impedance signature, and two frequency ranges were selected, as shown in Fig. 3: one is 1 to 5 MHz (high frequency range) for the thickness modes of the PZT patch, and
Fig. 3. Lateral and thickness modes of PZT [11]
the other is 20 to 500 kHz (low frequency range) for the lateral modes of the PZT patch [11].
Comparison Between Experimental and Analytical Studies Experimental Study A plain concrete beam was tested for the detection of progressive surface damage, as shown in Fig. 4. One PZT patch was attached to the surface of the specimen at a distance of 50 mm from the left edge. The progressive surface damage on the concrete beam was simulated by making six artificial notches (5 5 mm) with a regular interval of 50 mm. The first notch was located at a distance of 300 mm (case A), and the last one was located at a distance of 50 mm (case F) from the PZT patch, as shown in Fig. 4. First, the impedance signature for an intact case was measured and defined as a baseline. Then, every damage case (from A to F) was assessed by considering both thickness and lateral modes of the PZT patch according to high and low frequency range, respectively. The data and the RMSD charts are shown in Figs. 5 and 6, respectively. Figure 5, which depicts the impedances of the thickness modes of the PZT patch, shows the considerable changes in the impedance signatures due
Fig. 4. Test specimen and progressive surface damage simulated by notches
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3
Impedance(Ohms)
10
A
2
2.05
2.1
2.15
2.2
2.25
2.3
intact case crack at 300mm crack at 250mm crack at 200mm crack at 150mm crack at 100mm crack at 50mm
2.35
2.4
2.45
2.5 4
x 10
Frequency(Hz) 50mm
1.8 1.6 1.4
100mm
RMSD (%)
1.2 1 0.8 0.6
150mm 200mm 250mm 300mm
0.4 0.2 0
Fig. 5. Impedance signatures and RMSD chart for thickness modes of PZT
1
2
3
4
5
6
Locations of surface cracks Fig. 6. Impedance signatures and RMSD chart for lateral modes of PZT
to the surface damage cases D, E, and F, but shows no changes due to the damage cases A, B, and C. On the other hand, Fig. 6, which depicts the impedances of the lateral modes of the PZT patch, shows considerable changes in the impedance signatures of all the surface damage cases (from A to F). Consequently, it can be noted that the cases which use the lateral modeimpedance have a larger sensing area (more than 30 cm) as compared with the cases which use the thickness mode-impedance in the PZT patch.
Analytical Study In order to validate the above experimental results, an analytical study based on finite element (FE) models was carried out. A commercial software, ANSYS6.0, was used to model the smart structural system (Concrete beam with PZT patch) and to compute the electro-mechanical impedance of the system. This SEM
Fig. 7. FE model of PZT and concrete beam with damage
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Table 1 Properties of concrete used for finite element analysis j3
Density (Kg m ) Poison’s ratio Young’s modulus (MPa) Mass damping factor Stiffness damping factor
2,400 0.21 2.25 104 0.001 1.5 10j8
analytical study was performed only at the low frequency range (20 to 25 kHz) corresponding to the lateral modes of the PZT patch. To compute the electro-mechanical impedance, a coupled field analysis, which takes the interaction between two or more disciplines of engineering (i.e., a piezoelectric analysis handles the interaction between the structural and electric fields) into account, was utilized [13]. The FE model for the plain concrete beam was coupled with the PZT patch model for the six damage cases (from A to F), as shown in Fig. 7. The PZT patch was modeled using SOLID5 element which is a three-dimensional (3-D) coupled field solid element with eight nodes and six degree of freedoms (DOF) at each node. This element has an electrical voltage used as an additional DOF. In order to model the plain concrete beam, a SOLID45 element was used. This element is defined by eight nodes and 3 DOF at each node, with translations in the nodal x, y, and z directions. It is noted that exact properties of the concrete used in the experimental study were not used, and the concrete was assumed to be an isotropic material. The properties of the concrete were assumed, as described in Table 1. PZT-5H type piezoelectric material properties were used for the PZT patch model in this analysis. The following three kinds of material properties were required: Permittivity ["] (dielectric constant); Piezoelectric Matrix [e], which is a 6 3 matrix that relates the electrical field to stress; and Elastic Coefficient
Matrix [c], which is a 6 6 symmetric matrix that specifies the stiffness coefficients. These properties were assumed as follows: 2
0
0
6:5
3
6:5 7 7 7 0 0 23:3 7 7; 0 0 0 7 7 7 0 17 0 5 17 0 0 2 1698 6 ½" ¼ 4 1698 1498 2 126 79:5 84:1 6 126 84:1 6 6 6 117 ½c ¼ 6 6 6 6 4 6 6 6 6 ½e ¼ 6 6 6 6 4
0
0
Symmetric
2 C m
3 7 5;
X109 ðF=MÞ
0
0
0 0
0 0
23:3
0 23
0
3
7 7 7 7 7X10ðPaÞ 0 7 7 7 (3) 0 5 23 0 0
where [e] is the piezoelectric matrix, ["] is the dielectric matrix, and [c] is the stiffness matrix, and the density of the PZT patch was assumed to be 7,500 kg/m3. The mesh of the FE model should be fine enough to resolve the waves and to ensure accuracy. A rule of thumb is to have at least ten elements per wavelength along the direction of the waves. The PZT patch was polarized in its thickness direction, but it operates in the plane dimension. Therefore, its input material properties must be orthotropic and characterized by the stiffness, dielectric, and piezoelectric matrices. As shown in Fig. 8, a voltage of +0.5 Vpeak has been given for all the top nodes of the PZT patch, and a voltage of j0.5 Vpeak has been given for all the bottom nodes. The lateral modes of the PZT patch were observed by a harmonic analysis at the low frequency range (20 to
Fig. 8. FE-mesh of PZT patch and lateral mode-vibration at 20 kHz
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Practical Applications for Reinforced Concrete Beams As a second example study, a reinforced concrete (RC) beam (100 15 18 cm) was tested to investigate the practical applications of the proposed method to real-time health monitoring of concrete structures. A sensor array system composed of ten PZT patches was placed on the front, back, and bottom sides of the RC beam, as shown in Fig. 11. The PZT array system was arranged by considering the PZT patch_s sensing area, as defined in the first example study, and the probable crack locations to avoid de-bonding or breaking of the PZT patches. A third point bending test was performed to simulate multiple (shear and flexural) cracks. The RC beam was designed so that it could be loaded up to around 120 kN. The loading steps used in this test were as follows: 20, 60, 100, 120 and 140 kN. These were applied under a displacement control rate of 0.0254 mm/s. At a loading step of 20 kN, micro-cracks were observed,
Fig. 9. Simulated impedance signatures and RMSD chart for lateral modes of PZT
25 kHz). Figure 8 shows the FE-calculated piezoelectric extension (in m scaling) of the upper surface of the PZT patch (not bonded to the concrete beam yet), with its lower surface acting in compression proportional to the electrical voltage at the excitation frequency of 20 kHz. First, the impedance analysis was carried out for an intact case of the FE model. Then, the impedance signatures for all the progressive damage cases (from A to F) were calculated. The data and the RMSD chart are shown in Fig. 9. The results of this analytical study were compared with the experimental results, as shown in Fig. 10. The results were not exactly same, but the comparison shows a good agreement based on the fact that damage nearer to the PZT patch provided a larger deviation value in the RMSD chart. Consequently, it can be noted that the FE model could predict the variations of electro-mechanical impedance signatures according to the progressive surface damage in the plain concrete beam. These results have verified the validity of the experimental results very clearly. SEM
Fig. 10. Comparison between experimental and analytical results
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Fig. 11. Test specimen and PZT patch array system
and hair line shear cracks appeared at 60 kN. Then, visible shear and minor flexural cracks were observed near the supports at 100 kN. The size and width of the shear cracks increased at 120 kN along with an increase in crack density. Finally, shear failure occurred near PZT #8 at 140 kN. Figure 12 shows the shear and flexural cracks incurred near the corresponding PZT patches at a loading step of 100 kN.
The impedance signatures were recorded for each loading step at the high and low frequency ranges, corresponding to the thickness and lateral modes of the PZT patches, respectively. The measurement was repeated 16 times and then averaged. Variations of the impedance signatures measured from the both lateral and thickness modes of the typical PZT patches at the different loading steps are shown in Figs. 13 and 14,
Fig. 12. Shear and flexural cracks incurred near supports at loading step of 100 kN
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Fig. 13. Variation of impedance signatures for lateral modes
respectively. The combined RMSD charts are shown in Figs. 15 and 16. In Fig. 15, which depicts the lateral modes of the PZT patches, the combined RMSD chart shows good damage indication based on the manner by which the initiation and growth of the shear cracks were detected and monitored at the corresponding PZT patches #1, #4, #8, and #10. Flexural cracks were also detected well at the corresponding PZT patches #2, #3, and #6. Figure 16, which shows the combined RMSD chart for the thickness modes of the PZT patches, also shows similar damage indication patterns as the lateral mode chart. Especially, it shows that the flexural cracks were detected much more prominently at the corresponding PZT patches #2, #3, and #6 as compared with the lateral mode results. Therefore, it can be concluded that the impedancebased damage detection method using both lateral and thickness modes of the PZT patches is fairly practical and reliable for real-time health monitoring and multiple (shear and flexural) crack detection in concrete structures. SEM
Conclusions It has been verified that an impedance-based damage detection method using both lateral (frequency range >20 kHz) and thickness (frequency range >1 MHz) modes of PZT patches is fairly practical and reliable for real-time health monitoring and multiple (shear and flexural) crack detection in concrete structures. In the first example study, progressive surface damage inflicted artificially on a concrete beam could be effectively assessed. Root mean square deviation (RMSD) in the impedance signatures of the PZT patch showed good damage indication. In addition, it has been confirmed that cases which use the lateral mode-impedance have a larger sensing area as compared with cases which use the thickness modeimpedance. The analytical study using finite element (FE) models showed successful damage prediction results with a consistent trend in the variations of the impedance signatures of the PZT patches, according to the progressive surface damage on the concrete beam.
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Fig. 14. Variation of impedance signatures for thickness modes
The analytical results have verified the validity of the experimental results by providing a valuable tool for investigating the practical applications of the impedance method. In the second example study, multiple (shear and flexural) cracks incurred in a reinforced concrete beam under a third point bending test could be successfully detected by using a sensor array system
composed of ten PZT patches. The combined RMSD charts for both lateral and thickness modes of all the PZT patches showed clear damage indication based on the manner by which the initiation and growth of the multiple (shear and flexural) cracks were detected and monitored at the corresponding PZT patches. In particular, the prominent alarming peaks which indicate
Fig. 15. Combined RMSD chart of PZTs at all loading steps (lateral modes)
Fig. 16. Combined RMSD chart of PZTs at all loading steps (thickness modes)
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failure situations of the RC beam could be observed from the combined RMSD charts very clearly.
Acknowledgments This study was supported in part by the Smart Infra-Structure Technology Center (SISTeC), sponsored by Ministry of Science and Technology (MOST) and the Korea Science and Engineering Foundation (KOSEF), Korea, and, in part by the Infrastructure Assessment Research Center (ISARC), sponsored by Ministry of Construction and Transportation (MOCT), Korea. Their financial support is greatly appreciated.
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