Identifying technology for structural damage based on the impedance analysis of piezoelectric sensor

Construction and Building Materials 24 (2010) 2522–2527

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Identifying technology for structural damage based on the impedance analysis of piezoelectric sensor Xu Dongyu a,b, Cheng Xin a,*, Huang Shifeng a, Jiang Minhua b a b

Shandong Provincial Key Lab. of Construction Materials Preparation and Measurement, University of Jinan, Jinan 250022, China State Key Lab of Crystal Materials, Shandong University, Jinan 250100, China

a r t i c l e

i n f o

Article history: Received 20 September 2009 Received in revised form 24 April 2010 Accepted 7 June 2010 Available online 7 July 2010 Keywords: Piezoelectric sensor Impedance spectrum Structural damage Health monitoring

a b s t r a c t PZT piezoelectric ceramic was used as sensing element to fabricate piezoelectric sensor. The fabricated PZT piezoelectric sensor was embedded into and affixed to the structure to detect the structural damage, respectively. The structural crack damage was investigated using the impedance spectra of the sensor. The results show that the electric impedance of the PZT piezoelectric sensor with both different arrangements can reflect the variations of the structural crack damage in the testing frequency ranges. In the frequency range of 20–70 kHz, both the impedance value and the resonance frequency of the embedded PZT piezoelectric sensor can show the incipient crack damage and the increase of crack depth clearly. The impedance spectra of the affixed and embedded PZT piezoelectric sensors show the similar variation regularity with increasing the structural crack damage degree. Furthermore, the impedance spectra variation of the PZT piezoelectric sensor with different arrangements in the thickness resonance frequency range is more obvious than that in the planar resonance frequency range. A scalar damage metric is presented based on the impedance spectra of the PZT piezoelectric sensor around the resonance frequency, and the cracks in different positions of the structure were also analyzed using the damage metric. The variation of structural crack damage can be observed effectively and obviously using this damage metric. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, with the rapid development of science and technology, more and more large-scale civil engineering structures and important infrastructures are being constructed. These civil engineering structures usually hold the large-scale and complex design, which will inevitably be influenced by the environment destroy, excessive using, materials aging and so on. Under the effects of these interactions, their abilities to resist natural disaster will decrease, and some catastrophic accidents will also happen, especially under some extreme cases [1,2]. Therefore, it is very important to monitor the change of structural condition to diagnose the destructive location and extent. The development of intelligent materials presents the new research method for the real time health monitoring of civil engineering structures. Piezoelectric ceramic and piezoelectric composite attract people’s attention because of their superior sensing and driving properties. The research shows that the mechanical impedance of the structures will change with the variation of the structural performance. The piezoelectric coupling properties of the piezoelectric materials provide the method to combine the structural mechanical impedance with the electric impedance of * Corresponding author. Tel./fax: +86 531 82767017. E-mail addresses: [email protected] (D. Xu), [email protected] (X. Cheng). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.06.004

the piezoelectric materials. Therefore, the structural damage information can be obtained by investigating the electric impedance of the piezoelectric sensor. In 1993, Liang et al. [3–6] first put forward the impedance analyzing method for the intelligent structures. They deduced the relationship of PZT dynamic impedance with the structural mechanical impedance and proved it by lots of experiments. Subsequently, the electromechanical impedance method (EMI) was studied by many researchers. In 1999–2001, Park et al. [7–11] detected the bolted joint structure, civil structure component and built-in pipeline using EMI technique, and obtained the significant experimental results. In 2003, Park et al. [11] summarized the research work in the past several years, and meanwhile discussed the future research areas of EMI technique. In 2002–2005, Giurgiutiu et al. [12–17] performed the structural online health monitoring in conjunction with the impedance technique and lamb wave transmission by embedding the dynamic piezoelectric sensor into the structure. Furthermore, they also presented an approach to damage metric quantification based on the comparison and classification of high-frequency local impedance spectra. In 2000, Soh and their coauthors [18] monitored the RC bridge using the impedance method, and provided a procedure to interpret a signal from debonding or breakdown of the PZT sensor. In 2002–2004, Tseng et al. [19,20] presented numerical studies, in which surfacebonded impedance sensor was used to monitor two types of

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damage, void ad crack, in a concrete structure. In 2002, Bhalla and Naidu et al. [21–23] characterized damage in concrete structures and demonstrated the capability of the impedance method to detect incipient damage. Besides, they also monitored the concrete strength gain during its curing process and discovered good corelation between the stiffness of the concrete and variations in impedance peaks. In 2008, Yang et al. [24,25] studied the problems involved in real-life applications of the EMI technique and concluded the relationship between PZT bonding thickness and temperature. Furthermore, they also presented various finite element simulations on the interaction between a piezo-impedance transducer and a structure and verified the phenomena observed previously. Although researchers have done much work on the structural health monitoring based on piezoelectric impedance technology, their researches are mainly focused on affixing the piezoelectric sensor to the structural surface to detect the structural damage. In this study, PZT piezoelectric ceramic was used as sensing element to fabricate the piezoelectric sensor. The PZT piezoelectric sensor was embedded into and affixed to the mortar specimens, respectively. The crack damage information of the specimens was obtained by investigating the electric impedance variation of the PZT piezoelectric sensor in different frequency ranges. 2. Experiments 2.1. Raw materials Mortar specimens (40 mm  40 mm  160 mm) were prepared using standard sand and Portland 42.5R cement, and the water cement ratio is 0.5. PZT-51(U20 mm  2 mm) piezoelectric ceramic disk was used as sensing element and a kind of polymer/cement based waterproof material was used as the capsulation materials to prepare the PZT piezoelectric sensor. 2.2. Experimental procedure First, RVVP conducting wire was welded to both surfaces of the PZT piezoelectric ceramic. After welding, the alcohol was used to wash the PZT piezoelectric ceramic, and then the polymer/cement based waterproof material was used to capsulate it. After solidifying of the capsulation material, the PZT piezoelectric sensor was embedded into the mortar specimens. The embedded PZT piezoelectric sensor was located at 30 mm one end of the mortar specimen (Fig. 1a).The epoxy resin was used to affix 1# and 2# PZT piezoelectric sensors to the mortar specimen (Fig. 1b), and these

sensors were affixed to 40 mm at both ends of the mortar specimen, respectively. WSQ 50 diamond cylindrical cutting machine was used to cut the cracks with different depth on the mortar specimen. The arrangements of the PZT piezoelectric sensor and the location of the cracks were illustrated in Fig. 1. Initially, 1# crack was cut with the depth of 3 mm, 5 mm, 7 mm and 9 mm, and then 2# and 3# cracks were cut successively with the same crack depth variation. The crack depth of the specimen with affixing PZT sensor was 3 mm, 6 mm and 9 mm, respectively. The width for all the cracks, that is, the thickness of the cutting machine blade is 0.3 ± 0.05 mm. Agilent 4294A impedance analyzer was used to test the electric impedance of PZT piezoelectric sensor. The PZT piezoelectric sensor was excited by the 500 mV rm alternating voltage from the impedance analyzer. The impedance information of the PZT piezoelectric sensor was collected and analyzed by the computer software. 3. Results and analysis 3.1. Influence of crack depth on electric impedance of PZT piezoelectric sensor Fig. 2 describes the electric impedance variation of the embedded PZT piezoelectric sensor with increasing the 1# crack depth in the mortar specimen. The testing frequency ranges are 20–70 kHz, 80–260 kHz and 600 kHz–2.4 MHz, respectively, which correspond to the frequency ranges of the PZT piezoelectric sensor under different vibration modes. In order to observe the variation of impedance value and resonance frequency more clearly, the impedance spectra in above three frequency ranges are partly zoomed. It can be seen from Fig. 2a that comparing with the condition without crack, in the frequency range of 20–70 kHz, the resonance frequency of the PZT piezoelectric sensor shift towards the lower frequency with increasing the crack depth, and meanwhile the impedance value of the PZT piezoelectric sensor increases gradually. This indicates that the local rigidity of the mortar specimen decreases with increasing the crack depth, which also causes the decrease of systemic resonance frequency and increase of the systemic impedance value. The planar resonance frequency of the PZT piezoelectric sensor appears in the frequency range of 80–260 kHz. Because of the coupling effects from the other vibration modes, there appear many coupling peaks in the impedance spectrum (Fig. 2b). It can be seen from the zoomed area that the planar resonance frequency of the PZT piezoelectric sensor also shifts to the lower

sensor

(a) PZT sensor embedded into the specimen

sensor

(b) PZT sensors affixed to the specimen

Fig. 1. Diagrammatic sketch of the PZT piezoelectric sensor arrangements.

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2750 2800 2700 Impedance/ohm

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(c) 600 kHz-2.4 MHz Fig. 2. Impedance change of the embedded PZT piezoelectric sensor with the crack depth.

frequency, and the impedance value shows the increasing trend. However, because of the appearance of coupling effects, the impedance curves of the PZT piezoelectric sensor overlap with increasing the crack depth, so it is hard to observe the impedance variation in the impedance spectrum. Therefore, it is necessary to establish a kind of damage metric to evaluate the damage in this frequency range.

Fig. 2c shows the impedance versus frequency spectrum of the PZT piezoelectric sensor in the frequency range of 600 kHz– 2.4 MHz. Because the resonance frequency of the PZT piezoelectric sensor under the thickness vibration mode is larger than that under the planar vibration mode, there is little coupling peak in the impedance spectrum. It can be seen from the zoomed area that with increasing the crack depth, the thickness resonance frequency

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PZT piezoelectric sensor is more obvious in the similar frequency range. As mentioned above, the change of impedance and resonance frequency of the PZT piezoelectric sensor can be observed by investigating the impedance spectra in different frequency ranges, which is very useful for the detection of structural crack damage. In this study, a wide frequency range is chosen to observe the impedance variation of PZT piezoelectric sensor with increasing the crack depth. However, because the testing frequency point of the analyzer is limited, the frequency analyzing accuracy will inevitably be reduced. It is known from the above analysis that only the impedance variation of the PZT piezoelectric sensor closed to the resonance frequency is obvious, therefore, the crack damage will be better detected by narrowing the testing frequency range.

drift is not obvious, while the impedance value increases gradually, especially when the incipient crack appears, the impedance value variation is more obvious. Because the affixed PZT piezoelectric sensor has little impedance variation in the frequency range of 20–70 kHz with increasing the crack depth, only the frequency ranges of 50–250 kHz and 600 kHz–2.4 MHz were chosen to be investigated. It can be seen from Fig. 3a that the coupling effect between the planar resonance peaks and the other resonance peaks of the affixed 1# PZT piezoelectric sensor in the frequency range of 50– 250 kHz are obviously weaker than that of the embedded PZT piezoelectric sensor in the similar frequency range. It also can be seen from the zoomed area that the planar resonance frequency of the PZT piezoelectric sensor varies little, and the impedance value increases gradually with increasing the crack depth. However, the impedance spectrum variation of the affixed PZT piezoelectric sensor is not as obvious as that of the embedded PZT piezoelectric sensor in the similar frequency range. Fig. 3b shows the impedance versus frequency spectrum of 1# PZT piezoelectric sensor in the frequency range of 600 kHz– 2.4 MHz. Comparing with the condition without crack, the impedance value of the PZT piezoelectric sensor at the thickness resonance frequency also increases with increasing the crack depth, however, with the further increasing of the crack depth, the increase of the impedance value in the curve is not obvious. In contrast, the impedance spectrum variation of the embedded

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It can be seen from the impedance analysis that the crack damage of the mortar specimen can be preliminarily identified through the variations of resonance frequency and impedance value of the PZT piezoelectric sensor. However, it is significant to evaluate the structural damage quantitatively by establishing a kind of effective damage metric according to the piezoelectric impedance information. At present, many damage metrics based on the impedance method has been adopted. In 1995, Sun et al. [6] presented a simple statistical algorithm referred to as Root Mean Square Deviation

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(RMSD) based on frequency-by-frequency comparisons. In 1998, Raju et al. [26] adopted another scalar damage metric referred to as the cross-correlation metric to interpret and quantify the information from different data sets, and the obtained the consistent results with RMSD. In 1999, Park et al. [7] use a modified RMSD metric to minimize the impedance signature drifts caused by the temperature or normal variations. Subsequently, Tseng and Zagrai et al. [19,27] investigated the performance of RMSD, MAPD, covari-

20–70 kHz

ance and correlation coefficients as indicators of damage. The RMSD and the MAPD were found to be suitable for characterizing the growth and the location of damage. In this study, a kind of damage metric is established based on the impedance spectra of the PZT piezoelectric sensor pre- and post- structural damage (Because the impedance variation of the PZT piezoelectric sensor is obvious only around the resonance frequency, the impedance spectra in the frequency range of 20–70 kHz, 80–150 kHz and 600–1500 kHz were chosen to be considered, respectively). The root mean square deviations of the PZT piezoelectric sensor impedance modulus in different frequency ranges are chosen as the damage metric to evaluate the structural incipient damage and damage development. The calculation formula is as following:

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP n u ½jz j  jz0 j2 i u i u RMSD ¼ ui¼1 n t P 0 2 ½jzi j

ð1Þ

i¼1

where |Zi| is the impedance modulus of the PZT piezoelectric sensor in different frequency ranges. |Zi0| is the impedance modulus of the PZT piezoelectric sensor under the condition without crack. n is the frequency number in the selected impedance spectra. Fig. 4 is the RMSD damage metric of the embedded PZT piezoelectric sensor in different frequency ranges. The depth variations of the cracks in different positions of the specimen were also investigated, respectively. It can be seen that RMSD damage metric in all

80–150 kHz

600–1500 kHz

Fig. 4. Damage metric of the embedded PZT piezoelectric sensor in different frequency ranges.

80–150 kHz

600–1500 kHz

Fig. 5. Damage metric of the affixed PZT piezoelectric sensors in different frequency ranges.

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the three frequency ranges can reflect the variation of crack depth, which increases gradually with increasing the crack depth. The deeper the crack depth, the larger RMSD damage metric is. In the same frequency range, the RMSD variation of the PZT piezoelectric sensor closing to 1# crack is the most obvious. Subsequently, with the appearance of 2# and 3# cracks, RMSD of the PZT piezoelectric sensor increases. RMSD damage metric of the embedded PZT sensor in the frequency ranges of 20–70 kHz and 600 kHz–2.4 MHz can show the crack depth variation more clearly. Fig. 5 is the RMSD damage metric of the affixed 1# and 2# PZT piezoelectric sensors in different frequency ranges. It can be seen that 1# PZT piezoelectric sensor closing to the crack damage has better sensing ability to detect the crack depth variation than 2# PZT sensor. However, the RMSD damage metric of 2# PZT piezoelectric sensor can show the obvious change only when the crack depth is large enough. Therefore, it can be concluded that the detecting ability of the PZT piezoelectric sensor to the crack damage of the structure increases with decreasing the distance between the sensor and crack damage. 4. Conclusions The structural crack damage and its expansion can be identified effectively based on the impedance technology of the PZT piezoelectric sensor. The PZT piezoelectric sensor with different arrangements can well reflect the structural crack damage. With increasing the crack depth, the impedance value of the PZT piezoelectric sensor increases in all the testing frequency ranges. In 20– 70 kHz frequency range, the impedance value of the embedded PZT sensor increases and the resonance frequency shift towards the direction of decreasing frequency. With increasing the structural crack damage degree, the impedance variation of the PZT piezoelectric sensor in the thickness resonance frequency range is more obvious than that in the planar resonance frequency range. A kind of damage metric based on the impedance spectra of PZT piezoelectric sensor is established in different frequency ranges. The structural crack damage can also be effectively detected by means of this damage metric. Although the impedance technology has obtained great development in the past, there are still many problems to be solved, e.g., how to optimize the arrangement of the sensor in the structure to decrease the influence of the coupling vibration on the testing sensitivity; how to establish the accurate qualitative relationship between the electric impedance and the structure damage; how to identify the structural damage utilizing the multigroup monitoring signals, etc. Besides, the influence of complex environment of the surroundings, such as environment temperature, humidity on the monitoring sensitivity of the PZT piezoelectric sensor also needs to be further studied in the future. Acknowledgments This work is supported by National Natural Science Foundation of China (50931160438, 50942022) and Natural Science Foundation of Shandong Province (ZR2009FM018). References [1] Jinping Ou. Research and practice of smart sensor networks and health monitoring systems for civil infrastructures in mainland china. China Sci Found 2005;19(1):8–12.

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