Effect of adhesive on the performance of piezoelectric elements used to monitor structural health
International Journal of Adhesion &Adhesives 26(2006)622–628
Effect of adhesive on the performance of piezoelectric elements used to
monitor structural health
Xinlin P.Qing a,?,Hian-Leng Chan a ,Shawn J.Beard a ,Teng K.Ooi b ,Stephen A.Marotta b
a Acellent Technologies,Inc.155C-3Moffett Park Drive,Sunnyvale,CA 94089,USA b
US Army Aviation and Missile Research,Development and Engineering Center,USA
Accepted 27October 2005Available online 10January 2006
Abstract
The effect of adhesive thickness and its modulus on the performance of adhesively bonded piezoelectric elements for the purpose of monitoring structural health has been experimentally investigated.All the piezoelectric elements were adhesively bonded to aluminum plates.Experimental results revealed that an increase in adhesive thickness alters the electromechanical impedance and the resonant frequency of the piezoelectric elements as well as the amplitude of the sensor signal.When the modulus is within a certain range,the modulus of adhesive slightly affects the impedance of PZT element and the amplitude of sensor signal at lower frequency,while at high frequency,the impedance response and sensor signal are more sensitive to the modulus of adhesive.r 2005Elsevier Ltd.All rights reserved.
Keywords:Adhesive;Bonding;Electromechanical impedance;Piezoelectric element;Structural health monitoring
1.Introduction
Safety,reliability,and life-cycle cost are of great concern to the aircraft manufacturing and maintenance industries.Current nondestructive inspection (NDI)methods are labor-intensive and therefore result in high maintenance and life-cycle costs.Furthermore,these techniques are often limited to areas that are accessible.To overcome these limitations,there is an increasing need to develop a cost-effective,in-service structural health monitoring (SHM)system to monitor and assess the condition of aircraft structures.Recent advances in sensor technology,material processing,damage modeling,and system integra-tion have enabled new developments in structural evalua-tion and inspection technologies to overcome the shortcomings of existing inspection systems.Among them is the utilization of a piezoelectric sensor network integrated onto structures to monitor their condition throughout their lifetime [1,2].
Typically,the piezoelectric sensor network used for structural health monitoring is permanently bonded with adhesive to the host structure to be monitored.The adhesive forms an interfacial layer of ?nite thickness between the piezoelectric element and host structure.The adhesive interface provides the necessary mechanical coupling needed to transfer the forces and strains between the piezoelectric element and the host structure.To develop an effective structural health monitoring system using such a network of sensors,it is important to have a compre-hensive understanding on the adhesive effects on the interaction of piezoelectric elements with the structure.Over the last few years,a large number of studies on wave propagation method and electromechanical impe-dance technique using piezoelectric elements had been carried out [3–8].While the structural dynamics are always accounted for in theoretical analysis,most authors ignored the effect of adhesive layer.Few investigations on the effects of a bonding layer on the dynamical interaction between a piezoelectric element and a host structure can be found in literature.A modi?ed electromechanical impedance model,which treats the bonding layer bet-ween the piezoelectric element and host structure as a
https://www.sodocs.net/doc/917419901.html,/locate/ijadhadh
0143-7496/$-see front matter r 2005Elsevier Ltd.All rights reserved.doi:10.1016/j.ijadhadh.2005.10.002
?Corresponding author.Tel.:+4087451121;fax:+4087456168.
E-mail address:peter@https://www.sodocs.net/doc/917419901.html, (X.P.Qing).
spring-mass–damper system was proposed by Xu and Liu [9].A step-by-step derivation to integrate the shear lag effect into impedance formulations was presented by Bhalla and Soh[10].The shear-layer coupling between the piezoelectric wafer active sensor and structure was also analyzed by Giurgiutiu[11].
However,the physics of the electromechanical interac-tion between the piezoelectric element,adhesive,and the host structure has not been fully understood.Many fundamental issues related to its practical implementation have not yet been fully addressed.The proposed models also need to be veri?ed experimentally and further improved.In this paper,a series of parametric tests are performed to identify the effect of the thickness and modulus of adhesive layer on electromechanical impedance of a piezoelectric element and on the sensor signal.
2.Theoretical principle
2.1.Electromechanical impedance of piezoelectric element Lead-zirconate-titanate(PZT)operates on the piezo-electric principle and achieves direct transduction of electric energy into elastic energy and vice versa.The constitutive equations that couple the electric and mechan-ical variables take the form[12]:
D i?e T
ij
E jtd im T m,
S k?d jk E jtS E
km
T m,e1T
where D i is the electric displacement,S k is the mechanical strain,E j is the electric?eld and T m is the mechanical stress.
e T ij is the complex electric permittivity o
f the piezoelectric
material at zero stress,d im and d jk represents the piezo-electric coupling effect,S E km is the mechanical compliance of the material measured at zero electric?eld.
The theoretical development of the impedance measure-ments to structural health monitoring was?rst proposed by Liang et al.[13].When a PZT is bonded to a structure and is driven by an alternating voltage,the PZT exerts an alternating strain on the surface of the structure.This causes the structure to vibrate.In turn,the vibration causes the PZT to generate a current to?ow through the PZT transducer.The interaction between a PZT and its host structure can be described by a simple one-dimensional model shown in Fig.1.The apparent electromechanical impedance of the PZT as coupled to the structure is
ZeoT?V
I
?i o aˉe T
33
à
Z streoT
Z aeoTtZ streoT
d2
3x
Y E
xx
à1
,
(2)
where V is the input voltage to the PZT and I is the output
current from the PZT.a,d3x,Y E
xx andˉe T
33
are the geometry
constant,the piezoelectric coupling constant,Young’s modulus,and the dielectric constant of the PZT at zero stress,respectively.Z a and Z str are the mechanical impedances of the PZT and the structure,respectively.
Assuming that the mechanical property of the PZT does not change over the monitoring period of the structure,the above equation shows that the electromechanical impe-dance of the PZT is directly related to the mechanical impedance of the host structure.Any change caused by damage to the structure will change the mechanical impedance of the structure,which will manifest as a change in the electromechanical impedance of the PZT sensor.Speci?cally,the real part of the impedance is mainly used for monitoring structural health,because the real part is more indicative to the damage or changes in the structure’s integrity than the imaginary part[14].
2.2.Wave-propagation-based monitoring of structural health
Guided waves such as Lamb and Rayleigh waves are most widely used for damage detection in metallic and composite structures.Guided waves used for damage detection are introduced into a structure at one point by a piezoelectric element(actuator)and are sensed by another piezoelectric element(sensor).A wave-propaga-tion-based structural health monitoring system consists of three major components:(1)a network of piezoelectric actuators/sensors,(2)integrated diagnostic hardware,and (3)data analysis software.To collect health monitoring data throughout the service life of the structure,the sensor network is permanently bonded on the structure using an adhesive.The diagnostic hardware required for interfacing the sensor network with a computerized diagnostic system comprises of sensor/actuator ampli?ers,?lters,function generator,data acquisition card and laptop computer.The diagnostic software is used for signal processing,to generate control and command signals;and to detect location and extent of damage.
The structural diagnostic technology relies on the use of a piezoelectric element to emit a sonic signal to the structure,which is then again recorded by neighboring piezoelectric elements in the actuator/sensor network.The diagnostic system actively instruct the actuators to generate pre-selected diagnostic signals and transmit them to neighboring sensors whose response can then be inter-preted in terms of damage location and size or material property changes within the structure.The methodology
I = i sin( t + )
V =
Fig.1.One-dimensional model used to represent a PZT-driven dynamic structure system.
X.P.Qing et al./International Journal of Adhesion&Adhesives26(2006)622–628623
used in the diagnostic process is based on comparing the current sensor responses to previously recorded ‘‘baseline’’sensor responses.The differences between the two sets of signals contain information about structural changes.3.Experimental investigation 3.1.Specimen and material properties
There were three Al 2024-T4rectangular plates used in the study.Each plate was 305mm wide,610mm long and 3.17mm thick.A total of 144piezoelectric elements made of PZT material were mounted on the surfaces of three specimens with different adhesives.The PZT elements were circular disks,each having a diameter of 6.35mm and a thickness of 0.25mm.Fig.2shows a schematic of one specimen bearing PZT elements adhesively bonded with three different adhesives.The piezoelectric elements in Groups I and II were mounted on the aluminum plate with Hysol EA 9396at thicknesses of 10and 40m m,respectively.The piezoelectric elements in Groups III and VI were mounted on the aluminum plate with Hysol EA 9395at thicknesses of 10and 40m m,respectively.The piezoelectric elements in Groups V and VI were mounted on the aluminum plate with Hysol EA 9361at thicknesses of 10and 40m m,respectively.
The mechanical properties of the adhesives,PZT and Al 2024-T4are listed in Tables 1and 2.3.2.Impedance measurement
A HP electrical impedance analyzer (model HP 4194A)was used to measure the electric impedance of PZT
elements.The measurement for each PZT was made over a frequency range of 50–600kHz.The measured impedance data was transferred from the impedance analyzer to a laptop through a GPIB bus for post processing.3.3.Wave propagation measurement
As shown in Fig.3,Acellent active diagnostic system was used to generate Lamb wave and measure the response of sensor [2].The function generator in the diagnostic system sends in an electric diagnostic signal to the PZT actuator on the structure,and in turn,the actuator mechanically excites the structure via the converse piezoelectric effect.A stress wave propagates through the structure,toward the sensor.The sensor picks up the stress wave via the piezoelectric effect.A 5-peak sine wave modulated by a Gaussian envelope is used to drive actuators because of its narrow band signal.
A ?1
2sin o t 1àcos o t 5 h i ,(3)
where o ?2p f and f is the center frequency of the waveform in Hz.4.Test results
Fig.4shows the comparison of the real part of impedance of PZT elements over a frequency range of 50–600kHz when adhesives with three different elastic moduli and two different thicknesses were used,respec-tively.Fig.5shows the effect of adhesive thickness (Hysol EA 9396)on the real part of impedance of PZT elements.Fig.6shows the comparison of sensor signals for an actuator–sensor path with 155mm sensor spacing when adhesive bondlines about 10and 40m m thick were used,respectively.Fig.7compares the amplitude of the ?rst wave packet of the sensor signals at three typical frequencies when adhesives with three different elastic moduli and two different thicknesses were used.Fig.8shows the effect of adhesive thickness (Hysol EA 9396)on the amplitude of the ?rst wave packet of the sensor signals.Figs.9and 10show the effect of elastic modulus of the adhesive on impedance of PZT elements at three typical frequencies for adhesive thicknesses of 40and 10m m,
Fig.2.Schematic of Al 2024specimen with PZT elements mounted with different adhesives.
Table 1
Mechanical characteristics of adhesives Material name
Hysol s EA 9361Hysol s EA 9395Hysol s EA 9396Tensile modulus (MPa)72349402750Shore D hardness
709080Elongation at break (%)40 2.6 3.4Peel strength (N)*
11122–36
111Tensile lap shear strength (MPa)24.127.624.1Service temperature (1C)60177177Density (g/ml) 1.28
1.27
1.14
*Width was 25mm.
X.P.Qing et al./International Journal of Adhesion &Adhesives 26(2006)622–628
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respectively.Figs.11and 12show the effect of elastic modulus of the adhesive on the amplitude of the ?rst wave packet of the sensor signals for adhesive thicknesses of 40and 10m m,respectively.5.Discussions and conclusions
As shown in Fig.5,an increase in adhesive thickness alters the electro-mechanical impedance and the resonant frequency of the piezoelectric elements.With increasing the adhesive thickness within the whole test range of 10–120m m,the real part of the impedance of the PZT elements at resonant frequency signi?cantly increases.However,as shown in Fig.8,after the adhesive thickness reaches a certain value ($40m m),the amplitude of the sensor signal at the frequencies around 500kHz changes only slightly with increasing adhesive thickness.
It is clear from Figs.6and 7that the thickness of adhesive bondlines signi?cantly affects the amplitude of sensor signal when the thickness of adhesive is less than 40m m.At lower frequency,around 50kHz,the signal for actuator–sensor PZTs with thin adhesive bondlines is stronger than for those with thicker adhesive bondlines.At higher frequency,around 500kHz,the amplitude of the signal for actuator–sensor PZTs with thin adhesive bondlines is smaller than for those with thicker adhesive
Fig.3.An active diagnostic system.
Frequency (kHz)
R e a l P a r t o f I m p e d a n c e (?
)
200
400600800
Frequency (kHz)
200
400600800
Frequency (kHz)
200
400600
800
18016014012010080604020
https://www.sodocs.net/doc/917419901.html,parison of the real part of impedance of PZTs bonded with different adhesives.
Table 2
Mechanical characteristics of PZT and Al 2024-T4Material name Density (g/ml)Modulus (MPa)Tensile strength (MPa)Elongation (%)PZT-APC 8507.763000700.11Al 2024-T4
2.78
72400
470
19
X.P.Qing et al./International Journal of Adhesion &Adhesives 26(2006)622–628
625
bondlines.It can also be seen from Figs.4and 5that the real part of impedance of PZT element decreases with increasing bondline thickness at lower frequency,around 50kHz,while the real part of impedance of PZT element increases signi?cantly with increasing bondline thickness at higher frequency,around 500kHz,because of the effect of resonant frequency.
Based on these results,it is obvious that there is better mechanical coupling between the PZT element and host structure at higher frequency (around 500kHz )with a
10020025030035040050150
Frequency (kHz)
R e a l P a r t o f I m p e d a n c e (?)
Fig.5.Effect of adhesive thickness on the real part of impedance of PZTs.
S e n s o r O u t p u
t
500
200
300
500
400
600
700
800
900
1000
300350450400500550600650700750800
100015002000250030003500
4000
4000300020001000
0-1000-2000-3000-4000
4000300020001000
0-1000-2000-3000-4000
8000600040002000
0-2000-4000-6000-8000
300 kHz
Time ( x 0.1 μsec)
Time ( x 0.1 μsec)
Time ( x 0.1 μsec)
https://www.sodocs.net/doc/917419901.html,parison of sensor signals for different adhesive bondline thicknesses.
0.0
0.20.40.60.81.01.2 1.41.6
Hysol EA 9396
Hysol EA 9395
Frequency (kHz)
50
300
500
50
300
500
S e n s o r O u t p u t (v o l t s )
50
300
500
40-μm thick adhesive 10-μm thick adhesive
Hysol EA 9361
https://www.sodocs.net/doc/917419901.html,parison of the amplitude of sensor signals for different adhesives.
0.0
0.2
0.4
0.6
0.81.01.21.41.61.8
50
300500
S e n s o r O u t p u t (v o l t s )
Frequency (kHz)
Hysol EA 9396
10 μm 30 μm 40 μm 60 μm 90 μm 120 μm
Fig.8.Effect of adhesive bondline thickness on the amplitude of the sensor signals.
X.P.Qing et al./International Journal of Adhesion &Adhesives 26(2006)622–628
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40-m m adhesive bondline than with 10-m m adhesive bond-line,while there is better coupling at lower frequency (around 50kHz)with thin adhesive bondline than thick adhesive bondline.
From Figs.9–12,it is observed that the elastic modulus of the adhesive only slightly affects the impedance of the PZT element and the amplitude of the sensor signal at lower frequency,when the modulus is within a certain range.However,the responses of impedance and sensor signal are more sensitive to the modulus of adhesive at high frequency than at low frequency.Acknowledgments
The authors would like to acknowledge the ?nancial support of the US Army.The authors also gratefully acknowledge Professor Fu-Kuo Chang at Stanford Uni-versity and Dr.Amrita Kumar at Acellent Technologies,Inc.for their support and technical discussion.References
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20406080100120140R e a l P a r t o f I m p e d a n c e (?)
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20406080100120
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Low tensile modulus Median tensile modulus
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