Alexandra Woldman – Undergraduate ResearcherDr. Gangbing Song – Faculty Mentor

Dr. Haichang Gu – Postdoctoral Mentor

� Certain materials, when placed in an electric field will align the dipoles of their ions

� When the electric field is removed, some of the alignment remains

http://www.physikinstrumente.com/tutorial/4_15.html

� When a strain is applied to the material, changing the alignment of the poles, a voltage is produced

� Conversely, when voltage is applied to the relaxed material, the poles realign, inducing a strain

Direct Effect

Converse Effect

http://www.bostonpiezooptics.com/?D=6

� Sensors◦ Output a voltage when strained

� Actuators◦ Change shape when voltage is applied to them

First Experiment

Second Experiment

� Measures charge and converts to voltage

� More accurate than direct measurement

� Could provide accurate strain data for piezoceramic embedded in concrete

www.kistler.com

� Used aluminum beam with piezoceramic sensor mounted on one side and strain gauge on other side

� Strain gauge and piezo sensor experience equal and opposite strain and beam is flexed◦ Piezoceramic wired to charge amplifier

◦ Strain gauge connected to signal conditioner

◦ Both devices produce voltage

Clamped EndStrain Gauge

(opposite side, piezo patch)

Aluminum Beam

� Use the readings from both devices to create a transformation from Voltagestrain gauge to Voltagecharge amp

� For strain gauge voltage1 mV = 1 mircostrain

� Thereby, the voltage from the charge amplifier, can be converted to strain

The voltage produced by a piezosensor is proportional to the strain, therefore linear relationship can be developed from the strain gauge output voltage to the piezosensor output voltage

Vstain gauge=1.577341*Vcharge amp + 0.132023 - V0,charge amp

� Piezoceramic patch embedded in concrete

� Try to apply same transformation and compare to theoretical results for static loads

� Drift – increase or decrease in signal without load application

� “The signal drift is probably caused by low insulation resistance which is characteristic of ceramic piezo elements.” -Leon P. Fialkowski

� Increase insulation:◦ Water insulation

◦ Liquid electric tape

◦ Combination

� Drift still too large for static loads to register

� For further testing with the charge amplifier, better sensors are necessary

� Used for:◦ Determination of strength

◦ Detection of cracks and void

� Send signal through material◦ Velocity of signal related to strength

◦ Cracks and voids reflect signal

◦ Changes the signal gathered by receiver

� Loaded concrete cylinders under compression

� Two cylinders, each with two piezoceramic patches embedded inside

Actuator

ActuatorSensor

Sensor

12”

1”

Load Load

� Test Procedures◦ Take first measurement at zero load

◦ Take reading every 4-5 kips increase in load

� Each reading◦ Sends signal from piezoceramic actuator

◦ Collects voltage data at sensor for 10 sec

� To get an understanding for the change in wave propagation as the load increases, calculate the energy for each reading

Energy = [x] * [x] T

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

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

� Test conducted until cylinder failed under compression

� Failure occurred at 129 kips

� Primary crack between sensor and actuator 0 2 4 6 8 10 12 14

x 104

100

120

140

160

180

200

220

Load Applied (lbs)

Energy

Local Max

Broken

� Close up scatter plot of voltage data collected by sensor indicates that small fractures had developed in the cylinders

� “Noise” in data close ultimate load

40 kips

126 kips

� The highest energy is at zero load◦ Boundary condition different from any non-zero load

◦ Top end free can vibrate freely, so no suppression of vibration

� As soon as any load is applied, energy drops

� After first 10% of ultimate load is applied, the energy remains relatively level

� The tests with the sensor on top always showed higher energy readings

� Slight energy drops as cylinder comes close to failing

� Tremendous drop in energy once cylinder fails

� For future tests with wave propagation:

◦ Calibrate tests under loading, for accurate results under loading

◦ Above 10% of the load, the energy of the wave does not change much, so safe to adjust loading up and down without compromising tests results

◦ Always take note of where the sensors are placed in the specimen and consider how that will affect results

� Piezoelectric materials are the future of structural monitoring

� A better understanding of these materials will allow for large savings in repairs in structures◦ Notice damage earlier

◦ Repair before damage it is catastrophic

Special thanks to:

Dr. Song, Dr. Mo

Dr. Gu, Claudio