Use of Piezoelectric Materials for Strain. Measurements and Wave Propagation Analysis

Transcription

1 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

2 Table of Contents: Abstract 2 Introduction 2 Experimental Setup 4 Results 7 Conclusions 12 Future Work 14 Acknowledgements 14 References 15 Appendices 16 1

3 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 2

4 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. 3

5 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 4

6 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 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. 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 Figure 2: Several different samples of concrete with embedded piezoceramic sensors. with a multimeter. Many different samples were tested (Figure 2). 5

7 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. Figure 3: Hydraulic load on cylinder on testing surface. 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 Figure 4: The same PZT patches were used as sensors and actuators in the cylinders, on top and on bottom. upside down so the actuator would be on the bottom and the sensor on 6

8 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 Figure 5: (a) The voltage from the charge amplifier and strain gauge before transformations are applied and (b) after transformation. 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: V stain gauge = * V charge amp (1) 7

9 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) V stain gauge = * V charge amp V 0,charge amp. (2) Once the charge amplifier voltage is converted into the equivalent strain gauge data, the known value of 1 mv strain 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. 8

10 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 voltage measurements were taken over the span of 10 second at each given load, approximately 4 5 kips Figure 6: A typical data sample for the wave propagation test at a specific load. The plot consists of individual data points of the voltage at the sensor at some time during the 10 second sampling time. 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 9

11 Act-Top 1 Act-Top 2 Act-Top 3 Sen-Top 1 Sen-Top 2 Sen-Top Energy Load Applied (lbs) x 10 4 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 Act-Top 1 Act-Top 2 Act-Top 3 Sen-Top 1 Sen-Top 2 Sen-Top Energy Load Applied (lbs) x 10 4 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. 10

12 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. 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. 11

13 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 12

14 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 13

15 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 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. 14

16 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 Activesensors in Structural Health Monitoring. University of Cincinnati, Cincinnati. Piezo Systems Inc. (2008). Frequently Asked Questions Retrieved 1 Aug, 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 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 = *Z ; % 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([ ]) % ACTUATORS ON TOP, 3 TRIALS lbs = [ ]; 16

18 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 = [ ]; 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 = [ ]; 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 = [ ]; 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) 17

19 lbs = [ ]; 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 = [ ]; 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 18