Hindawi Publishing Corporation Smart Materials Research Volume 2011, Article ID 236719, 7 pages doi:10.1155/2011/236719

Research Article

Electrical Response of Cement-Based Piezoelectric Ceramic Composites under Mechanical Loadings

Biqin Dong,1 Feng Xing,1 and Zongjin Li2

1 Department of Civil Engineering, The Key Laboratory on Durability of Civil Engineering in Shenzhen, Shenzhen University, Shenzhen, Guangdong 518060, China

2 Department of Civil Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Correspondence should be addressed to Feng Xing, xingf@szu.edu.cn Received 1 December 2010; Accepted 26 January 2011 Academic Editor: C. R. Bowen

Copyright © 2011 Biqin Dong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Electrical responses of cement-based piezoelectric ceramic composites under mechanical loadings are studied. A simple high order model is presented to explain the nonlinear phenomena, which is found in the electrical response of the composites under large mechanical loadings. For general situation, this nonlinear piezoelectric effect is quite small, and the composite is suitable for dynamic mechanical sensor as holding high static stability. The experimental results are consistent with the relationship quite well. The study shows that cement-based piezoelectric composite is suitable for potential application as dynamic mechanical sensor with excellent dynamic response and high static stability.

1. Introduction

Civil infrastructural systems age and deteriorate during their service life due to factors such as ageing of materials, excessive use, overloading, environmental conditions, and deficient maintenance. As a result, structural health monitoring of civil infrastructures is of considerable importance in view of the immense loss of life and property that may result from structure failure. The unique feature of large size and complexity of most civil infrastructures renders visual inspection very tedious, expensive, and sometimes unreliable. The need for quick assessment of the state of health of civil infrastructures has necessitated research for the development of an automated, real-time, and in situ health monitoring technique. This kind of technique allows the system to monitor its own structural integrity while the infrastructures are in service, and also the monitoring can be performed throughout the service life of the infrastructures. Such a structural health monitoring system is useful not only to improve reliability but also to reduce the costs of maintenance and inspection for infrastructural systems [1].

In such a structure, sensor is an essential component for structural health monitoring purposes. Among the techniques used in sensors, piezoelectricity has been proved to be

one of the most efficient mechanisms for most applications in smart structures [2-5]. For a typical piezoelectric material sensor, its two-part model with electrical-mechanical phase can be revealed as shown in Figure 1; whereas, in civil engineering, cement-based material concrete is the most popularly used structural material. But normal piezoelectric composites (such as polymer-based piezoelectric composites) are not suitable for civil engineering application because of their distinct differences in the volume stability from concrete. To meet the requirement of civil engineering structures, a 0-3 cement-based piezoelectric ceramic composite has been developed and studied in HKUST for the first time in the world [6-9]. The experimental results show that the composite is effective and applicable both in piezoelectric properties and compatibility, and it can be suitable for potential sensor application in concrete structures.

In this paper, electrical responses of cement-based piezoelectric ceramic composites under mechanical loadings are studied. Moreover, a simple high-order model is presented to explain the nonlinear phenomena of the electrical response of the specimens under large mechanical loadings. Actually, the generalized Hamilton's principle, which incorporates different electric boundary conditions as well as mechanical boundary conditions, is utilized to obtain the governing

I (current)

Piezoelectric technologies

V (voltage)

X (displacement)

F (force)

Electrical side Mechanical side

Figure 1: Schematic representation of two-part model of piezoelectric sensor.

Figure 2: The specimens of cement-based piezoelectric ceramic composite (left: uncoated with silver paint; middle: coated with silver paint; right: after poling).

Figure 3: Illustration of MTS for mechanic-electrical response measurement of cement-based piezoelectric ceramic composite.

equations of motion. And deduction of the governing equations for piezoelectric beams, plates, and circular cylindrical shells from the high-order piezoelectric generic shell theory is discussed by many researchers [10-13]. The study will help us state the nonlinear turbulence of cement-based piezoelectric composites with large mechanical boundary.

2. Experiment

Lead zirconate titanate (PZT) piezoelectric ceramic powders (Hong Kong Functional Ceramic Co. LTD.) and cement (H.S.L. Enterprises Co. LTD.) were used to prepare cement-based piezoelectric composites.

Piezoelectric ceramic powder and white cement (white cement is chosen as major matrix in our experiment, and take Portland cement as a comparison) are mixed together to make 0-3 type cement-based piezoelectric ceramic composites. In order to improve the fluidity of the fresh mixture, a superplasticizer (W19, W. R. Grace) was used. Using a normal mixing (mixing duration is about 2min), piezoelectric ceramic powder could be incorporated into the composites with different ratios of ceramic/cement. To achieve a uniform mixture, cement and piezoelectric ceramic particles were mixed thoroughly first, then water and superplasticizers were added into the mixture. The mixing process was continued

Figure 4: The frame for mechanic-electrical response measurement.

until the mixture became uniform. Then the mixture was compacted into the 13 mm * 13 mm * 3 mm model. After casting, the specimens were put in the curing room with a temperature of 650C and relative humidity of 98% for 24 hours.

-500 _ -1000 -1500 g -2000 sp -2500 « -3000 -3500 -4000

Time (s)

800 600 > 400 (m 200

& -200

1100 1200 1300 1400 1500 1600 1700 1800

-400 -600 -800

1100 1200 1300 1400 1500 1600 1700 1800

Time (s)

-500 _ -1000 £ -1500 g -2000 cp -2500 « -3000 -3500 -4000

3000 3100 3200 3300 3400 3500 3600 3700

Time (s)

800 600 > 400 (m 200

H -200

o -400 O-600

3000 3100 3200 3300 3400 3500 3600 3700

Time (s)

-500 -1000 £ -1500 t( -2000 ^ -2500 -S -3000 -3500 -4000

4900 5000 5100 5200 5300 5400 5500 5600

Time (s)

800 600 400 200 3 0 & -200 o -400 -600 -800

4900 5000 5100 5200 5300 5400 5500 5600

Time (s)

Figure 5: Mechanical loading scheme for different mean loading levels (1500 N, 2000 N, 2500 N) and corresponding electrical output of cement-based piezoelectric ceramic composites.

A key technique for fabrication of piezoelectric materials is polarizing procedure. Polarizing was carried out in a silicon oil bath with a temperature of 1600C. After polarization the specimens were immersed in cold silicon oil to get a fast cooling in order to maintain the status of polarization. Then the polarized specimens were wrapped with aluminum foil to eliminate remnant charge generated during polarizing process. The specimens can be seen in Figure 2.

The mechanic-electrical response measurement of cement-based piezoelectric ceramic composite are carried out with MTS. The illustration of test can be seen in Figure 3. Different types of mechanical dynamic loadings are applied by MTS, and the output voltage is collected by PC through a charge amplifier. In order to eliminate the influence of environmental electric magnetic wave (normally, for 50 Hz AC), a metal frame is designed as shown in Figure 4.

Different sine function loadings are applied by MTS, and corresponding electrical outputs of composites are measured, along with the influence of all parameters for mechanic-electrical response. Figure 5 shows the mechanical loading scheme for different mean loading levels

(1500 N, 2000 N, 2500 N) and corresponding output voltage of cement-based piezoelectric ceramic composites. For every segment, different magnitude is varied from 200 N to 2000 N (peak to peak value, 200 N, 400 N, 600 N, 800 N, 1000 N, 1200 N, 1400 N, 1600 N, 1800 N, 2000 N); the details can be seen in Figure 6. In addition, the applied frequency is changed from 1 Hz to 10 Hz. For all variables, five cycles loading are executed, and the relationship between mechanical force input signal and electrical voltage output signal is studied (see Figure 7).

3. Results and Analysis

A linear constitutive equation of piezoelectric materials with mechanical clamped motion and electrical short circuit is expressed as [14-17]:

cfiSj ejnEn,

Dm emiSi + £mnEn,

) -1000 - 200 N

-1500 - tué

a, -2500

-3000 -3500 4000

)0N 800N 1000N 12°0N 1400N g ||

1600N 1800 N 2000 N

1440 Time (s)

800 i 600 )V 400 (m 200

jt -200

-400 -600 -800

léia WW

■111

1440 Time (s)

Figure 6: Sinusoidal mechanical loading scheme with different magnitude (from 200 N to 2000 N, peak to peak) and corresponding electrical output of cement-based piezoelectric ceramic composites.

-500 _ -1000 g. -1500 g -2000 sp -2500 « -3000 -3500 -4000

5004 5007

Time (s)

800 t 600 > 400 J= 200

it -200

¿5 -400 O-600

5001 5004 5007 5010

Time (s)

Figure 7: Sinusoidal mechanical loading and its electrical output of cement-based piezoelectric ceramic composite.

where j is elastic stiffness coefficient with constant electrical field, eSmn is dielectric coefficient with constant strain, emj and ejn are piezoelectric stress coefficient, Tj is stress, Dm is electrical displacement, Sj is strain, and En is electrical field. This means that any effect, including mechanical or electrical

jk 500 -

> 400 m

.a 100 -

200 400 600 800 1000 1200 1400 1600 1800 2000 Mechanical input (magnitude (N), peak-peak)

-■- Mean loading =1500N, 1 Hz -•- Mean loading = 2000 N, 1 Hz -A- Mean loading = 2500 N, 1 Hz

ft 700 -É

st 600

200 400 600 800 1000 1200 1400 1600 1800 2000 Mechanical input (magnitude (N), peak-peak)

-■- Mean loading =1500N, 10 Hz -•- Mean loading = 2000 N, 10 Hz —A— Mean loading = 2500 N, 10 Hz

Figure 8: The influence of mean loading on magnitude of mechanical-electrical response of cement-based piezoelectric ceramic composite (a) 1 Hz, (b) 10 Hz.

excitation, will change the value of the electrical displacement. In cement-based piezoelectric ceramic composite, the piezoelectric property is not affected by cement material and only determined by the kind of piezoelectric ceramic and the volume proportion of ceramic in the composite [18]. As a result, it is feasible to apply the constitutive equation of piezoelectric materials to cement-based piezoelectric ceramic composites. In order to characterize the mechanic-electrical response of composite, we only apply mechanical loading, and (1) can be rewriten as:

Dm emiSj•

0 -2 -4

« -8 ¿3

12 -14 -16 -18

Mean loading = 1500 N Mean loading = 2000 N -A- Mean loading = 2500 N

Figure 9: The influence of static loading on phase shift of mechanical-electrical response of cement-based piezoelectric ceramic composites.

Mechanical branch

Electrical branch

Î I Î

s. ,s '

î i î

Figure 10: The inducing procedure of initial stress field and electrical field in cement-based piezoelectric ceramic composites.

For one-dimensional (33 direction) input-output relationship

D3 — e33S3.

Then, an electrical potential U calculated at a distance along the 3 directions will now be nonzero as shown in,

U — E ■ d,

where U is the voltage and d is the distance of dipole moment, which are directly relative to the electrical displacement D. From (3) and (4) and basic stress-strain relationship,

we can obtain a linear relationship between electrical output and mechanical input:

U(t) — a ■ F(t),

where a is the convert constant of mechanical signal F(t) to electrical signal U(t).

According to basic electrical knowledge, it is defined that any voltage output is attributed to the variety of electrical potential (V = AU). Then for dynamic mechanical loading, if we only consider the magnitude, (5) evolves to

F — AU — a ■ AF,

where V is voltage output, AF is the magnitude of dynamic loading force. Different mean loading level (static) is applied to test the stability of the composite specimens under the mechanic loads. Varying the mean loading at different frequencies is added, and the results are shown in Figures 8 and 9. All plots indicate that mechanic-electrical dynamic response of cement-based piezoelectric composites match a linear relationship very well. And for all mean loading we measured, the curves show no much difference in both magnitude and phase angle.

The experimental data reveal that the response of cement-based piezoelectric ceramic composite holds a similar behavior despite the mean loading level. This result is consistent with a linear relationship, whereas a small voltage output difference (see Figure 8) appears in our measurement, especially, in large input signal. This phenomenon is called nonlinear piezoelectric effect (or high-order piezoelectric effect), which is studied by many researchers and lots of papers are published [10-13]. A simple explanation is that after polarization, an electrical equilibrium appears between dipole moment and ambient charge in cement-based piezoelectric ceramic composites, whereas, if a stress field (initial, zero order) is applied, the electrical equilibrium will break, and an induced electrical field (one order) will come forth, which is opposite to the direction of the stress field. Similarly, the one-order electrical field will cause a second stress field. Based on the above, we can know that (n - 1)-order stress field induces n-order electrical field. If initial electrical field is applied, the same circs will come forth. The whole procedure can be seen in Figure 10.

So, for single mechanical loading, the total electrical output (voltage) of cement-based piezoelectric ceramic composite can be derived from

Vtota! — F [a + E(1) + E(3) + ■■■ + E(n))

where n is odd value, and from Figure 9 we can know that the direction for all orders of induced electrical field is opposite to the initial stress field. And the direction for all orders of induced stress field is the same to the initial stress field. Moreover, for higher initial stress field applied, corresponding induced electrical field will be larger, which means that electrical output (voltage) will decrease but mechanical output (strain) will increase with improving stress field level. So comparing (7) to the linear constructive equation (5), it is clear that

Vtotai < V — F (a ).

Figure 11: Multiple sine wave inputs and corresponding electrical output for cement-based piezoelectric ceramic composite.

4000 -3600 -3200 -2800 -2400 2000

Input signal

36.5 37

Time (s) (a)

> -2400n Ê -2200 ts -2000 -1800 o -1600 -1400

'5 -1200 J -1000-

Output signal

Time (s)

Figure 12: Random wave inputs and corresponding electrical output for cement-based piezoelectric ceramic composite.

For general situation, this nonlinear piezoelectric effect is quite small and can be neglected. It shows that the composite is suitable for dynamic mechanical sensor as holding high static stability.

With the linear system, the principle of superposition states that a linear combination of input signals produces an output signal that is simply that linear addition of the separate output signals that would result if each input term had been applied separately [19]. The extensive studies are carried out with multiple sine input function, according to the following form:

Table 1: The components of multiple sine function signals.

F(t) = 3000+ 1000 sin(0.1nt) + 400 sin^0.4nt + 3

+ 300sin^2nt + + 200sin(10nt + n) (9)

+ 200sin^40rct + 4^).

Every component of multiple sine function signals is listed in Table 1. The relationship of input/output waveform is expressed in Figure 11. The plots show that electrical output signal can reproduce the mechanical input signal with quite complex waveform without any visible distortion.

More practical measurement is focused on the random wave test, which is reciprocal to the common dynamic loading, such as earthquake. The waveform is generated by PC and is added to composite specimen through MTS. Both input signal and output signal are plotted in Figure 11. Comparing with input/output waveform in Figure 12, we

No. Frequency (Hz) Mean loading (N) Peak-peak amplitude (N) Phase (degree)

1 0.05 3000 2000 0

2 0.2 3000 800 60

3 1 3000 600 120

4 5 3000 400 180

5 20 3000 400 240

can know that electrical signal is quite close to the mechanical signal due to the instinct of composite as its linear mechanic-electrical relationship.

4. Conclusion

In this paper, mechanic-electrical response of cement-based piezoelectric ceramic composite is studied. As a typical linear smart material, it possesses an excellent dynamic performance to mechanical loading within whole frequency range of civil engineering application. Some conclusions can be drawn as follows.

(1) A high-order response of cement-based piezoelectric ceramic composite exists under mechanic loads. For general situation, this nonlinear piezoelectric effect is quite small and can be neglected. It shows that the composite is suitable for dynamic mechanical sensor as holding high static stability.

(2) The experimental results show that electrical output signal can reproduce the mechanical input signal with quite complex waveform without any visible distortion.

(3) Measurement with multiple function inputs indicates that cement-based piezoelectric ceramic composite holds an excellent performance to dynamic signals in frequency range for general civil engineering applications.

(4) The study shows that cement-based piezoelectric composite is suitable for potential application as dynamic mechanical sensor with excellent dynamic response and high static stability.

Acknowledgments

The authors would like to acknowledge the financial support provided by National Natural Science Foundation of China (nos. 50778114 and 50908149), Natural Science Foundation for the team project of Guangdong Province, 2009, and The team Programs for Science and Technology Development of Shenzhen University, 2009.

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