Use of Piezoelectric Materials for Strain Measurements …structurallab.egr.uh.edu/sites/structurallab.egr.uh.edu/files/... · Use of Piezoelectric Materials for Strain Measurements

Use of Piezoelectric Materials for Strain

Measurements and Wave Propagation Analysis

Alexandra Woldman – Undergraduate Researcher

Dr. Haichang Gu – Postdoctoral Mentor

Dr. Gangbing Song – Faculty Mentor

University of Houston, Houston, TX

1

Table of Contents:

Abstract 2

Introduction 2

Experimental Setup 4

Results 7

Conclusions 12

Future Work 14

Acknowledgements 14

References 15

Appendices 16

2

Abstract

This paper covers several uses for piezoelectric material. Piezoelectric materials deform when a

voltage is applied to them and inversely will produce a voltage when they are deformed. For this

reason, they can be used as both sensors and actuators. Piezoelectric sensors should theoretically

be able to give an accurate measurement of the stress and strain in an object if the output voltage

is measured accurately and the relationship of output voltage to strain is known. This paper

explains an attempt at calibrating a charge amplifier so the voltage output can be used to

determine the strain in the material. The experiment succeeded on an aluminum beam but failed

to work similarly for sensors embedded in concrete. A different type of piezosensor is necessary

for implementation in concrete. This paper also discusses the use of piezoceramic sensors and

actuators to test the propagation of waves in concrete under static loading. Two concrete

cylinders with embedded piezoceramics were tested in compression. One cylinder was tested

until failure. The voltage at the sensor was converted to energy to show the change in the amount

of energy that propagates to the sensor. There is a dramatic drop in energy during the loading of

the first 10% of the load. The change in energy at higher loads is small. When the concrete

cylinder is broken, the energy begins to decrease as small fractures form in the cylinder. There is

a tremendous energy drop-off once the cylinder breaks.

Introduction

Concrete is a popular and widely used material in structural engineering. Throughout the life of a

structure, concrete often becomes damaged from repeated loading. Currently, the most

widespread concrete health monitoring tests are destructive tests. One method of testing concrete

is by casting cylinders of the same concrete as used in a structure and periodically destructively

3

testing these samples to monitor the strength of the concrete. Since the cylinders and structural

members are made from the same mix and cast at the same time, they should have the same

properties over time. However, casting cylinders does not accurately portray to state of the

concrete, since structural members are of a different shape and therefore cure differently than a

cylinder with the same concrete mix (Limaye, 2002). Moreover, the structural members are

experiencing stresses that the cylinders are not, which may have a large effect on the strength of

the concrete. A more accurate method of destructive testing is coring. This process removes a

cylinder of concrete from the actual structure (Limaye, 2002). While more accurate, this method

directly damages a structure. Piezoelectric sensors are the newest method for monitoring the

health of concrete structures throughout their lifetime. Piezoelectric materials produce a voltage

when they are stressed and also deform when a voltage is applied to them. These materials are

made by a process called poling. During poling, an electric field is applied to the material,

aligning the random electric dipoles of the atoms. Once the material is removed from the electric

field, the dipoles retain some their alignment. When a voltage is applied to the material, the poles

realign, causing a shift in the shape of the material. Likewise, when the shape is changed, a

voltage is produced because of the induced realignment of the atoms (Piezo, 2008). The

piezoceramic used in this specific study is lead zirconate titanate (PZT), a fairly common and

inexpensive piezoelectric material. Patches of PZT are much more durable than strain gauges and

can be cast into concrete and used to nondestructively monitor the health of a structure.

Moreover, piezoelectric sensors allow for continuous structural monitoring, rather than

monitoring at one arbitrary point in time. Piezoelectric sensors can detect damage before it is

significant enough to detect visually. This allows maintenance on structures before they become

dangerous and must be closed for repairs.

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1. Previous studies have been done on impact detection through the use of PZT sensors and

actuators (Song, Gu, Mo, 2008). However, these studies do not give any specific

numerical data about the stress and strain in the structure. Numerical data regarding the

stress and strain within structural members will indicate whether any one member is

under an unexpected amount of stress, signaling a problem with the distribution of the

load in the structure. This can prevent the catastrophic failure of a structure. This project

aims to calibrate a charge amplifier to get accurate measurements for the stress and strain

in concrete under static loading.

2. Wave propagation is a nondestructive method for testing concrete. This method uses the

velocity of the waves through the concrete to determine its strength. It can also detect the

presence of cracks and voids before they are visible. Most research using wave

propagation has focused on isolated structural members (Gassman & Tawhed, 2004).

This experiment analyzes the change in wave propagation under static loading. The static

loading conditions mimic the load conditions the member would experience when in is

embedded in a structure. The results should indicate whether loading of a structural

member may lead to an inaccurate judgment of the health of the member or possibly be

confused with a damaged sensor (Overly, 2007).

Experimental Setup

Two different experimental setups existed in this experiment. One was for the calibration of the

charge amplifier while the other aimed to check wave propagation in concrete. In future

experiments, the results of the two should be combined for accurate numerical results. Both

experiments use the piezoceramic material PSI-5A4E. This material can be used as both a sensor

5

and an actuator. The first experiment focuses on sensors, while the second utilizes both

capabilities of the material.

1. The first experimental setup aimed to

calibrate the charge amplifier so that it

would generate a quantitative measure of the

stress and strain in concrete. This test

features a thin aluminum beam (5cm x 61

cm x .08 cm), with a PZT patch mounted on

one side 11 cm from the end, and a strain

gauge mounted on the other side 11 cm from the end. This placement assured that the

strain on the two sides was equal but opposite in sign. Then the beam was fixed by a

clamp at one end. The strain gauge was connected to Vishay Signal Conditioning

Amplifier System (Model A2). The piezoceramic was connected to the Kistler Charge

Amplifier (Type 5073). The output voltage from both devices was fed into a computer,

saved and graphed. This was done to get develop a conversion from the voltage read by

the charge amplifier to the voltage read by the strain

gauge. The signal conditioner attached to the strain gauge

was set so that 1 mV was equivalent to 1 microstain.

After this initial setup, the charge amplifier was

connected to various samples of piezoceramics embedded

in concrete and the voltage readings were taken manually

with a multimeter. Many different samples were tested (Figure 2).

Figure 2: Several different samples

of concrete with embedded

piezoceramic sensors.

Figure 1: Cantilever beam used to calibrate the charge

amplifier. The piezoceramic patch and strain gauge were

mounted at the same spot on opposite sides of the beam.

6

Figure 4: The same PZT patches were used as sensors

and actuators in the cylinders, on top and on bottom.

Figure 3: Hydraulic load on

cylinder on testing surface.

2. The wave propagation tests were set up as shown in Figure 3. A concrete cylinder of

height 12” and diameter 6” was placed on the testing surface. The tests were performed

on two different cylinders cast at the same time. Each cylinder had two piezoceramic

patches embedded inside of it, each at the center of the

circular cross section 1” from the bottom and top of the

cylinder (Figure 4). The hydraulic load was lowered onto

the cylinder and increased as the test proceeded. The first

measurement for the test was always taken at zero load. At

least 10 measurement, and sometimes several more, were

taken as the load on the cylinder increased from 0 to just

over 40 kips. Each data set recorded the voltage at the

sensor from the signal sent by the actuator. The signal from

the actuator was the same of every data point of every test.

3 tests were performed for each cylinder in each configuration drawn in Figure 4.

Piezoceramic patches were used

for the sensor and the actuator.

When the tests with the actuator on

top and the sensor on bottom were

complete, the cylinder was turned

upside down so the actuator would

be on the bottom and the sensor on

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the top. This was done so the piezoceramic patch that acted as the actuator and sensor

remained the same, for consistent results. After all 6 tests were completed on the first

cylinder, it was loaded to failure. This test was done with the PZT on actuator on top.

Results

1. Charge Amplifier Tests:

The first step in the charge amplifier calibration was

to work with a vibrating aluminum beam with a

strain gauge on one side and a piezoceramic sensor

on the other side. The strain on both sides is of the

same magnitude, but of different signs. In order to

simplify visual comparison, the data from the

charge amplifier was immediately multiplied by -1.

Data was collected while the beam was manually

moved over a 30 second time period, allowing both

devices to record voltage measurements

simultaneously. Figure 5(a) shows the voltage

measured by the two devices before any transformation had been applied for a given test.

Figure 5(b) shows the two data sets after the charge amplifier data is transformed to

match the strain gauge data. The linear transformation used for this data is:

Vstain gauge = 1.577341 * Vcharge amp + 0.132023. (1)

Figure 5: (a) The voltage from the charge

amplifier and strain gauge before

transformations are applied and (b) after

transformation.

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Since the charge amplifier can never be completely reset to zero, the first value (when the

aluminum beam was under zero strain) was afterwards subtracted from every data point,

making the complete transformation, which was used to make Figure 5(b)

Vstain gauge = 1.577341 * Vcharge amp + 0.132023-V0,charge amp . (2)

Once the charge amplifier voltage is converted into the equivalent strain gauge data, the

known value of 1 mVstrain gauge = 1µε is used to determine the strain at any point on the

graph. A slight drift was observed in the charge amplifier data, as seen by the difference

in endpoints of Figure 5(b). Matlab was used to load, transform and graph data. The code

used can be found in Appendix 1.

The next stage of the experiment was to implement the charge amplifier transformation

data to determine the strain in concrete. Several concrete samples with embedded

piezoceramics (a variety of which are shown in Figure 2) were used to test out the

transformation determined in Equation 2. Unfortunately, there was a dramatic drift in the

voltage every time an attempt was made to gather data. The drift in the data was so

dramatic, that no accurate data collection was possible. Using an oscilloscope as a visual

aide, the charge amplifier did give an obvious indication of when an impact occurred.

However, this is no improvement over the work done by Song, Gu and Mo, which did not

use a charge amplifier, but measured voltage directly (2008). The application of a static

load was too small to be registered by the charge amplifier as compared to its own drift.

Several different insulation methods were used to try to minimize the drift. These

methods include a water insulation coating and spray on electrical tape insulation coating.

The drift with these insulated samples was less dramatic than the drift noticed in samples

with no insulation. However, the insulation was not enough stop the drift from occurring.

9

2. Wave Propagation:

The data collected for wave

propagation in a concrete cylinder

under static loading gave the voltage

at the sensor from the signal output

by the actuator. Figure 6 shows a

typically graph for one test. 40 000

voltage measurements were taken

over the span of 10 second at each

given load, approximately 4 – 5 kips

apart. Theoretically, the voltage measurements should be centered at zero. However, due

to interference, all of the measurements had an offset. The data was detrended before any

calculations were done (see Appendix 2 for applicable Matlab code). Each voltage vector

was then converted into the energy at the sensor with the equation

Energy = [x] * [x] T . (3)

Figure 7 and 8 show the energy at different loads for the 6 tests done on each cylinder.

The plots indicate that the energy at the sensor decreases drastically as soon as the load

changes from zero. However, for loads larger than approximately 10 kip, the energy level

does not change much with increased load. For both cylinders, the tests with the sensor

on top lead to slightly higher energy levels than the analogous tests with the actuator on

top. Noting the scale on the two figures, one can see that the energy at the sensor depends

dramatically on the cylinder itself. Each cylinder has its own trend. The energy levels at

the sensors for destructive test of the first cylinder are shown in Figure 9. After the 45

Figure 6: A typical data sample for the wave propagation test

at a specific load. The plot consists of 40 000 individual data

points of the voltage at the sensor at some time during the 10

second sampling time.

10

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 104

50

60

70

80

90

100

110

120

130

Load Applied (lbs)

Energy

Act-Top 1

Act-Top 2

Act-Top 3

Sen-Top 1

Sen-Top 2

Sen-Top 3

Figure 7: The energy for different load for the first test cylinder. The blue data points are the samples with the sensor on top

while the warm colors represent the tests with the actuator on top.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 104

10

15

20

25

30

35

40

45

50

55

60

Load Applied (lbs)

Energy

Act-Top 1

Act-Top 2

Act-Top 3

Sen-Top 1

Sen-Top 2

Sen-Top 3

Figure 8: The energy for different load for the second test cylinder. The blue data points are the samples with the sensor on top

while the warm colors represent the tests with the actuator on top.

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Figure 9: The energy at different loads for the destructive test of the first cylinder. The energy dropped as soon as the load

changed from zero, and stayed relatively steady as more load was applied. The load began to decrease at 90 kips until failure.

kip mark used as the cutoff for all the previous tests, the energy at the sensor increases

slightly until it reaches a maximum at approximately 90 kips. The energy then decreases

until failure. Failure occurred at 129 kips. The primary crack formed in between the

sensor and the actuator (the images from this test can be found in Appendix 3). When the

cylinder broke, the hydraulic jack immediately stopped adding to the load. Since the

material had rearranged, the load was no longer 129 kips. A reading was taken before the

hydraulic jack was moved up, removing all load from the cylinder. The load read 6 kips

when the final reading was made (labeled as "Broken” in Figure 9). The energy at the

sensor from the signal sent by the actuator is substantially smaller after the cylinder broke.

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Conclusions

1. Although the calibration of the charge amplifier only succeeded for the aluminum, the

experiment showed a need for better sensors to be used in conjunction with the charge

amplifier. Even in the aluminum experiment, where the PZT was mounted on the surface

of the metal, there was still some drift. The drift is most likely the effect of low insulation

resistance, which is a common characteristic of ceramic piezosensors (Fialkowski, 2008).

With a better sensor, the charge amplifier voltage output can be used to give stress and

strain measurements for concrete used in a structure. This knowledge would allow much

more accurate quantitative monitoring of structures for fatigue and wear. Instead of

monitoring purely for impact, this monitoring method would allow for analysis of gradual

changes and shifting of weight in a structure over time.

2. The propagation of waves through concrete changes dramatically as soon as any load is

applied to the cylinder. This is due to the change in boundary condition. When the load is

zero, the boundary condition at the top can essentially be modeled as a free end. Once

any load is applied, the top of the cylinder can no longer move freely. This suppresses the

vibrations in the concrete, leading to a significant drop in energy that propagates through

the concrete. As compared to the ultimate load on the concrete cylinders (129 kips), the

dramatic drop in energy continues until a load of approximately 10% of the ultimate load

is reached. This could be considered the settling range for the samples. Taking wave

propagation measurements in concrete that is loaded to 0 – 10% of its ultimate load,

especially completely unloaded, could cause misleading results. Once the sample is

loaded to a significant portion of its ultimate load (>10%), the energy at the sensor

remains fairly constant. The use of wave propagation testing should therefore not be

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affected by static loading as long as the initial measurements are taken after the load has

been applied and the load is a significant percentage of the ultimate load.

3. When the load on the cylinder comes close to the ultimate load, the energy at the sensors

declines. In the experiment documented in Figure 9, the decline begins at 2/3 of the

ultimate load. This is due to the formation of small fractures in the concrete. Small

fractures cause a loss of energy, since some of the signal is reflected by the cracks. Figure

10 is a close up image of the signal during the destructive test for two different loads. The

first is at a load of 40 kips, while the second is at 126 kips, the last sample point before

failure occurred. The 40 kip image has clearly defined boundary while the 126 kip image

has ample noise. This is caused by the fractures in the concrete, which disrupt the signal.

Figure 10: The close-up views of the voltage sensed at the sensor are from load 40 kips and 126 kips respectively. The 126 kips

signal has visible noise caused by small fractures in the cylinder.

4. The energy at the sensor for any given test has some dependency on the placement of the

sensor in the sample. When the sensor is closer to the hydraulic load, the energy levels

are higher than in the case where the sensor is at the bottom. This is caused by a

difference in boundary conditions at the loading end and the table surface. When

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piezocermaics are embedded in concrete, placement should be considered carefully as it

could affect the results.

Future Work

Structural monitoring is an extremely useful field. Structures can be monitored for damage

effectively maintained without waiting for visible signs of damage first. Piezosensors can be a

very effective way of monitoring structures. In future work, sensors should be picked to work

well with the charge amplifier to create an accurate measuring system for the stresses in concrete.

This research showed that wave propagation energy decreases significantly during the loading of

the first 10% of the ultimate load. To better understand the reason for the decrease in the energy

that propagates through the concrete, Scanning Electron Microscope (SEM) imaging of the

piezoceramic in its embedded state could be useful. The SEM images, taken before and after

testing, will show if any microscopic changes occur that influence the behavior of the

piezoceramic patches.

Acknowledgements

The research study described herein was sponsored by the National Science Foundation under

the Award No. EEC-0649163. The opinions expressed in this study are those of the authors and

do not necessarily reflect the views of the sponsor. This research was conducted under the

supervision of Dr. Gangbing Song of the University of Houston. Dr. Haichang Gu and Claudio

Olmi were of great help in understanding the material and conducting the research in this paper.

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References

Fialkowski, L. (2008). (Discussion of Kistler charge amplifier ed., pp. 1). Houston.

Gassman, S. L., & Tawhed, W. F. (2004). Nondestructive Assessment of Damage in Concrete

Bridge Decks. Journal of Performance of Constructed Facilities, 18(4).

Limaye, B. R. (2002). Need for Non-Destructive Testing (NDT) of Reinforced Concrete &

Various ND Tests. Madras.

Overly, T. G. S. (2007). Development and Integration of Hardware and Software for Active-

sensors in Structural Health Monitoring. University of Cincinnati, Cincinnati.

Piezo Systems Inc. (2008). Frequently Asked Questions Retrieved 1 Aug, 2008.

Song, G., Gu, H., & Mo, y.-L. (2008). Smart aggregates: multi-functional sensors for concrete

structures - a tutorial and a review. Smart materials & structures, 17(3), 17.

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Appendices

Appendix – 1

load calibration;

% 'calibration' is data gathered through DataDesk

% it is a structure

% X is the time

% Y(1) is the charge amp data

% Y(2) is the strain gauge data

time = calibration.X.Data;

Z = calibration.Y(1).Data;

S = calibration.Y(2).Data;

% Plot two original sets of data

subplot(2,1,1)

plot(time,Z,'-r',time,S,'-b')

xlabel('Time (s)')

ylabel('Voltage (V)')

legend('Strain Gauge','Charge Amp (Original)')

title('(a)','Fontsize',16)

% Transform charge amp data

Z = 1.577341*Z+.132023;

% Get rid off starting offset

Z = Z-(Z(1)-S(1));

subplot(2,1,2)

plot(time,Z,'-r',time,S,'-b')

legend('Strain Gauge','Charge Amp (Transformed)')

xlabel('Time (s)')

ylabel('Voltage (V)')

title('(b)','Fontsize',16)

% end

Appendix – 2

% Cylinder 2 – draw plot of energy at different loads

% SET UP SCREEN

scrsz = get(0,'ScreenSize');

figure('Position',[1 scrsz(4) scrsz(3) scrsz(4)])

axes('FontSize',16)

hold on

% axis([0 45000 10 130])

% ACTUATORS ON TOP, 3 TRIALS

lbs = [0 198 650 5400 10100 14400 18400 22900 27100 ...

31500 36100 40400];

17

E=[];

for i=1:12

x=['test2a',num2str(i)];

S=load(x);

y=detrend(S.(x).Y(2).Data);

E(i)=y*y';

end

scatter(lbs,E,'filled','CData',[1,0,0],'SizeData',100)

xlabel('Load Applied (lbs)','Fontsize',20)

ylabel('Energy','Fontsize',20)

lbs = [0 260 2800 8500 13100 17700 21900 26900 31800 ...

37600];

E=[];

for i=13:22

x=['test2a',num2str(i)];

S=load(x);

y=detrend(S.(x).Y(2).Data);

E(i-12)=y*y';

end

scatter(lbs,E,'filled','CData',[1,.3,0],'SizeData',100)

lbs = [0 120 1300 7100 11400 16300 20900 25800 30000 ...

34500 38700 0];

E=[];

for i=23:34

x=['test2a',num2str(i)];

S=load(x);

y=detrend(S.(x).Y(2).Data);

E(i-22)=y*y';

end

scatter(lbs,E,'filled','CData',[1,.6,0],'SizeData',100)

% SENSORS ON TOP, 3 TRIALS

lbs = [0 10 54 366 4800 9700 14200 18700 23100 27500 ...

31900 36200 40200];

E=[];

for i=1:13

x=['test2s',num2str(i)];

S=load(x);

y=detrend(S.(x).Y(2).Data);

E(i)=y*y';

end

scatter(lbs,E,'filled','CData',[0 0 1],'SizeData',100)

18

lbs = [0 36 435 5200 10600 17600 21900 26700 30900 ...

35100 39400];

E=[];

for i=14:24

x=['test2s',num2str(i)];

S=load(x);

y=detrend(S.(x).Y(2).Data);

E(i-13)=y*y';

end

scatter(lbs,E,'filled','CData',[0 .4 1],'SizeData',100)

lbs = [0 0 85 770 6300 10300 15600 20000 24600 29200 ...

33700 37000 0];

E=[];

for i=25:37

x=['test2s',num2str(i)];

S=load(x);

y=detrend(S.(x).Y(2).Data);

E(i-24)=y*y';

end

scatter(lbs,E,'filled','CData',[0 .8 1],'SizeData',100)

% LABEL

legend('Act-Top 1','Act-Top 2','Act-Top 3','Sen-Top 1','Sen-Top 2',...

'Sen-Top 3','Fontsize',18)

Appendix – 3