ERIC EJ906154: Course Modules on Structural Health Monitoring with Smart Materials

2009·0.29 MB·English

by  ERIC

Course Modules on Structural Health Monitoring with
Smart Materials 65
Hui-Ru Shih, Wilbur L. Walters, Wei Zheng, and Jessica Everett T
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Abstract course modules have been developed to intro- lo
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Structural Health Monitoring (SHM) is an duce structural health monitoring concepts to Te
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emerging technology that has multiple applica- undergraduates. The field of SHM is vast. These hn
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tions. SHM emerged from the wide field of teaching modules are not designed to give an lo
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smart structures, and it also encompasses disci- encyclopedic coverage of all the SHM tech- y
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plines such as structural dynamics, materials and niques. Instead, the primary objective of these u
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structures, nondestructive testing, sensors and efforts is to provide students with background ies
actuators, data acquisition, signal processing, information and knowledge of principal tech-
and possibly much more. To stimulate students’ nologies involved in SHM using smart materials
desire for pursuing advanced technologies and and Lamb waves. Lamb waves are guided elastic
to ensure that they are well prepared for their waves. Lamb wave techniques have been emerg-
future careers, technology educators need to ing as one of the most effective methods to
dedicate their efforts to educate the students detect structural damage.
with this emerging technology. At Jackson State
University (JSU), three course modules (Smart From these modules, students can learnto
Materials and Structures, Data Acquisition use software, hardware, and instrumentation
Systems, and Lamb Waves Generation & properly both theoretically and in the context of
Detection) were added to the existing course to real-world applications. All three modules were
help undergraduate students develop hands-on offered together for the first time during the
experience for understanding this technology. spring semester of 2009. Results of these new
The course modules did not require prior knowl- modules are very encouraging. Being able to see,
edge of software and hardware, and they all fol- touch, and interact with entities that demonstrate
lowed an applied, hands-on approach. These complexbehavior is exciting and appealing to
three course modules allowed students to gain students. Students become verymotivated by the
insight into the SHM as well as to become diversity and creativity of the course content.
knowledgeable users of the instrumentation.
Course Modules Development
Introduction Three new course modules (Smart Materials
Structural Health Monitoring (SHM) is fast &Structures, Data Acquisition Systems, and
becoming a highlight of both research and appli- Lamb Waves Generation & Detection) were
cations to ensure the safe operation of various added to the existing Technology course. This
structures, such as bridges, ships, pipelines, and addition is intended to present students with a
aircrafts (Giurgiutiu, 2008; Staszewski, Boller, comprehensiveviewof SHM with smart materi-
&Tomlinson, 2004). The structure under inves- als. The newcourse modules havehelped stu-
tigation is energized using actuators. The dents understand the concept of a smart structur-
response to the excitation is sensed at various al system; it also has helped them to understand
locations throughout the structure. The response its application to performance monitoring of
signals are collected and processed. Based on structural systems.
the processed data, the state of the structure is
Course Module 1:SmartMaterials and Structures
diagnosed.
Smart structures utilize active (smart) mate-
rials as sensors and actuators to sense and
Asuccessful understanding and application
respond to their environment (Gabbert&Tzou,
of structural health monitoring depends on an
2001; Srinivasan & McFarland, 2000). Smart
in-depth appreciation of structural modeling,
materials (e.g., electrostrictives, shape memory
smart materials, signal processing, data acquisi-
alloys, and piezoelectrics) are nowused in
tion techniques, and more. JSU’sDepartment of
numerous applications. However, most of
Technology has developed significant teaching
today’s technology students are not aware of the
capabilities and the experimental facilities for
remarkable properties of smartmaterials or the
structural control and health monitoring. Three
applications of smart-structure technology. behind the extraordinary ability for piezoelectric
Therefore, it is desirable to prepare future tech- materials to behave as both a sensor and as an
66
nologists for the cutting-edge technologies in actuator. Materials delivered in the lectures also
es smart structures, which they will see in broad cover the analysis and design of a control sys-
di
u application during their careers. This proposed tem, as well as MatLab, Simulink, and dSpace.
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course module aims to prepare technologists to The experimental setup is depicted in Figure 1.
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o meet this demand. The test apparatus consists of an aluminum can-
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hn tilever beam 24.75 inches long, 1.5 inches wide,
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Te This module demonstrates the properties of and 0.075 inch thick. The commercially avail-
of piezoelectric materials, and it reveals the basic able PZT patches (QP15W and QP20W from
nal principles of intelligent structures and structure Midé Technology) are used. QP15W works as a
r
ou control. The electromechanical property of sensor and QP20W acts as an actuator. A PZT
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e piezoelectric materials has both a direct and a patch is bonded near the clamped end on each
h
T converse effect (Shih, 1999). The direct effect is side of the beam. This position was chosen
described as the generation of an electric charge because the greatest strains occur at the fixed
in a material when it is subjected to a mechanical end of the beam. The controller is implemented
stress. The converse effect is described as gener- on a dSpace 1104 controller board using
ation of a mechanical strain in a material in MatLab and Simulink software. The Model
response to an applied electric field. 7500 power amplifier from Krohn-Hite
Piezoelectrics are available in polymer Instruments is used in the test.
(polyvinylidene fluoride or PVDF) or ceramic
(lead zirconate titanate or PZT) form. Once the cantilever beam is forced to
Piezoceramics are stiff and brittle, whereas vibrate, the piezoelectric sensor will continue to
piezopolymers are compliant and soft. There are generate the signal. When the controller to the
twopossible ways to utilize piezoelectric materi- actuator patch is on, the sensor signal will be
als. First, piezoelectric materials can be used as preceded by a control algorithm, and then the
sensors (direct effect). Second, piezoelectric control signal will be amplified and fed back to
materials can be used as actuators (converse the actuator to suppress the vibration. This can-
effect). Because piezoelectric materials are appli- tilevered beam system is a simple form of a
cable to both sensors and actuators, their use is smartstructure since both the sensor and actua-
verypopular in smartstructures and systems. tor are integrated parts of the structure. This
smart beam has the ability to sense and to
This module consists of lectures and labora- respond to vibrations.
toryexperiments. The laboratories are developed
first; the lectures then support the laboratories. Alaser vibrometer (VibroMet 500) is also
The lectures introduce students to the piezo- used to measure the tip displacement and obtain
electric effects and the fundamental mechanism the frequency response of the beam’s vibration.
Controller
(dSPACE )
Sensor
Beam
Actuator Signal
Amplifier
Figure 1. Experimental setup.
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uncontrolled e
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time (sec)
Figure 2. Responses of the beam after an initial disturbance at the tip.
Thus, important parameters of the beam, such as behavior of a cantilever beam can be represented
fundamental modal frequencies can be obtained by a second-order linear transfer function. The
and then used to design the controller.The open- fundamental frequencyand damping ratio can
loop response can be validated with analytical be obtained from the experiment as previously
results. The dotted line in Figure 2 shows the described. Next, these parameters can yield the
free vibration of the beam in an open loop after transfer function. In Figure 3, the block
an initial disturbance to its tip. The damping “Transfer Fcn1” represents the beam’s transfer
ratio zcan be determined from the observation function.
of this decay.
Flexible structures consist of a large number
In the next step, a control method is needed of highly resonant modes. In general, PPF can
todampen the vibration of the beam (Song, offer quick damping for a particular mode. The
Qiao, Binienda, & Zou, 2002). The proper con- input value of the PPF controller can be consid-
trol method can be chosen with help of comput- ered as the modal displacement. The resonant
er simulations. Students usuallychoose the posi- frequencyof a controller is set to the designated
tive position feedback (PPF) algorithm. PPF is modal frequency of the structure to be con-
applied bysending the structural position coor- trolled. Around the resonant frequency,the
dinate directly to the controller. Next, the prod- phase lag of the controller is about 90 degrees.
uct of the controller output and a scalar gain are The output of the controller is the modal veloci-
fed back to the actuator. Students can adjust the ty feedback signal. Thus, the controller can
gain to optimize the control performance. As reduce resonant responses of the structure by
mentioned previously, the controller is imple- increasing the system damping at that modal fre-
mented on a dSpace 1104 controller board using quency.
MatLab and Simulink software, illustrated in
Figure 3. Simulink is a graphical extension to An important issue in designing a controller
MatLab for modeling and simulation of systems. for a flexible structure is whether the developed
closed-loop system will have sufficient robust-
In Figure 3, the block “ADC” which is the ness to deal with uncertainties in the structure.
analog-to-digital converter, receives the input One more advantage of the PPF algorithm is
signals from sensor. The block “DAC” is the that the controller can function as a special
digital-to-analog converter. The controller output bandpass filter. Frequencies that are lower or
is sent to the amplifier through DAC. The higher than the frequencyofthe filter will be
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Figure 3. PPF controller.
depressed. Thus, the controller can guarantee recent years, LabVIEW has been widely used in
closed-loop stability in the presence of uncon- applications where engineers, scientists, and
trolled modes. technologists want to acquire, analyze, and pres-
ent data. LabVIEW is a graphical programming
The control performance has been observed language developed by National Instruments
in the time domain by monitoring the transient (NI). LabVIEW is used in this class to write
behaviors of the beam. The decaying behaviors programs for the acquisition, processing, and
are presented in Figure 2 for both the controlled presentation of data. Thus, the additional benefit
and uncontrolled cases. From this figure, the is that students are also introduced to program-
effectiveness of the present vibration control can ming concepts. The graphical programming
be clearly understood. environment of LabVIEW has proven to be a
suitable teaching mechanism for students to use
Smart material systems exhibit very com-
to learn the basic concepts of loops, case struc-
plex coupled physical behavior. From this exper-
tures, etc. in a shortperiod of time and to gener-
iment it can be seen that, in piezoelectrics, the
ate working programs (Bishop, 2007; King,
coupling is in between mechanical stresses and
2009; Strachan, Oldroyd,&Stickland,2000;
electrical potentials. Smartmaterials and struc-
Sumali, 2002; Zhao, 2006).
tures are a rapidlygrowing interdisciplinary
technology embracing the fields of materials The LabVIEW environment consists of two
and structures, sensor and actuator systems, and main windows: the block diagram (Figure 4) and
information processing and control. Study of the front panel (Figure 5). The front panel is
smart materials can increase the interdiscipli- where the developer can build the user interface
nary experiences of the students. through the use of various GUI objects. Students
build the front panel with controls and indica-
Course Module 2:Data Acquisition Systems
tors, which are the interactive input and output
The broad field of SHM encompasses many
terminals, respectively. Controls are knobs, push
advanced technologies that, when integrated,
buttons, dials, and other input devices.
provide a system that can potentiallyidentify
Indicators are graphs, LEDs, and other displays.
and characterize the performance and/or possi-
ble deterioration of a structural system. For a
When a front panel item is dropped on the
SHM system, a data acquisition subsystem is
screen, a terminal is also created on the block
required to record a structure’s response to
diagram automatically. The block diagram is
ambient and external loads.
where the functionality of the LabVIEW pro-
gram (known as a virtual instrument or VI) is
Acourse module has been developed to
defined. Awide variety of function blocks (also
allow students to gain a basic knowledge of data
referred to as VIs) are available to allow the user
acquisition with hands-on experiences. The focus
to define any functionality that they want. In
of this module is to give students a starting point
addition, the VIs that allowthe user to build
for developing the data-acquisition system. In
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Figure 4. Block diagram of a LabVIEW Figure 5. Front panel of a LabVIEW St
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program. program. d
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his/her program, such as while loops, for loops, Course Module 3:Lamb Waves Generation and
and case structures, are also available as in any Detection
programming language. SHM techniques are being developed to
reduce operations and support costs, increase
This module is designed so that the knowl- availability, and maintain the safety of various
edgeand techniques that are introduced can later structures, such as bridges and aircraft.
be used in further studies and applied in SHM. Conventional ultrasonic methods, such as pulse-
The module is a mixture of lectures and practi- echo techniques, have been used successfully to
cal works. In lectures, the basic features of inspect structural integrity. However, the appli-
LabVIEW are first covered along with the data- cation of these traditional techniques has been
acquisition hardware. The procedures of limited to testing relatively simple geometries or
LabVIEW data flow programming are intro- inspecting the region in the vicinity of the trans-
duced. The comparison between LabVIEW and ducer (Yang & Qiao, 2005). Anewultrasonic
the traditional programming languages is pre- methodology which uses Lamb waves to exam-
sented. The block diagram and front panel for a ine the structural components has been devel-
LabVIEW program are discussed. In addition, oped. Lamb waves are guided elastic waves,
the editing features of functions palette and con- which can travel relatively large distances with
trols palette are introduced. A brief discussion of very little amplitude loss; they offer the advan-
the hierarchyof the organization of various pan- tage of large-area coverage with a minimum of
els on those palettes is also presented. Due to installed sensors (Giurgiutiu, 2003a). Thus, in
the limited time available, LabVIEW is not be contrast to the conventional methods, the guid-
covered in detail. ed-wave technology can be used to inspect the
entire structure in a single measurement, and it
In the laboratorysessions, students are can also inspect inaccessible regions of complex
instructed to perform the tutorial from the components. This module is designed to provide
LabVIEW Hands-On Course Manual (2005), students with the foundational knowledge for
the manual that is also used byNI in its training understanding SHM using Lamb waves.
courses. For all of the exercises, the instructor
provides common electronic tools and compo- For Lamb waves, at a given frequency, at
nents (e.g., multimeters, oscilloscopes, bread- least twomodes (one symmetric and one anti-
boards, function generators, and data-acquisition symmetric) are generated. As frequency increas-
systems). A data-acquisition system includes es, the number of simultaneously existing wave-
acomputer, a LabVIEW, an NI PCI 6251 data- forms also increases. Lamb wavepropagation is
acquisition (DAQ) board, and an NI BNC 2110 usually highly dispersive. Therefore, how to
connector block. Through these practices, stu- choose the best frequency is a major issue. In
dents should gain an understanding of the capa- Lamb wave inspection, mostly the S (symmet-
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bilities and structure of a computer-based data- ric) and A (anti-symmetric) modes are used.
o
acquisition system as well as the ability to write So and A are referred to as the fundamental
o
simple programs to acquire, process, and store modes. These twomodes are the most important
data. because theyexist at all frequencies, and they
carry more energy than the higher order modes
in most situations.
So and A modes are often called the exten- Among the various transducers available
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sional mode and flexural mode, respectively. for damage detection, piezoelectric materials
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The A Lamb mode has a much lower wave are particularly attractive because they can act
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dies speed than the SoLamb mode, and therefore it simultaneously as transmitters and receivers.
u has a smaller wavelength, making it more sensi- These transducers can be bonded or embedded
t
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tive to smaller levels of damage. However, the on the structure to be analyzed. Their reduced
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o symmetric waveform has a substantially greater thickness, low weight, and low cost make them
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hn group velocity and would therefore arrive at a very useful when designing an integrated dam-
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Te sensor well before the anti-symmetric waveform. age monitoring system (Hong, et al., 2005). In
of Kessler, Spearing, Atalla, Cesnik, & Soutis the experiments, piezoelectric transducers
al
n (2001) recommended use of the A mode (APC-850, 8 mm x 8 mm x 0.3 mm in size)
r o
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o because of lower attenuation. Alternatively, are used to generate and receive the wave.
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e selective excitation of the S mode has also been
h o
T reported (Giurgiutiu, 2003b). At low frequen- Students have learned how to operate a
cies, below 1 MHz, only A and S Lamb modes function generator and DAQ board, as well as
o o
are present. Frequency excitation ranges above write the LabVIEW program. This module is
1.5 MHz would produce other modes that will designed so that it can integrate the knowledge
make Lamb mode selection more difficult. In they gained and tools they acquired from the
order to generate only a few Lamb wave modes, first two modules. The experiment in this mod-
narrowband techniques are necessary. Thus, a ule enables students to test their abilities in
frequency range of up to 500 kHz is usually using a data-acquisition system to acquire data.
chosen. Giurgiutiu (2005) has also shown that,
This module is also laboratoryintensive.
byadjusting the excitation frequency,it is possi-
An aluminum beam (or plate) was used in the
ble to tune certain transducers to excite a single
experimental measurements. Several square
mode (either S or A )dominantly.
o o
piezoceramic elements were bonded on the
structure surface at the different positions. One
1.5
can serve as the actuator to generate the Lamb
1 waves. The others perform as sensors to detect
the waves. Figure 7 shows the test setup, which
0.5
includes an arbitrary function generator, a digi-
0
tal oscilloscope, and a computer. The piezoce-
-0.5 ramic actuator was excited byasignal construct-
ed and transferred to an arbitraryfunction gen-
-1
erator (Agilent 33220A). This function generator
-1.5
could read the digital signal and output the cor-
0 0.2 0.4 0.6 0.8 1
responding analog signal to the actuator. The
Time (ms)
measured signals were obtained with a digital
Figure 6. Excited waveform.
oscilloscope or recorded in a computer with a
According to four studies (Giurgiutiu, LabVIEW program.
2003a; Hong, Sun, & Kim, 2005; Ullate,
In experiments, both extensional and flexural
Saletes, & Espinosa, 2006; Yang & Qiao, 2005),
modes can be excited and detected. Figure 8
using the tone burst smoothed pulses can
shows the wave signals received at the sensor
improve the feedback signals and enhance the
for the 150 kHz, 250 kHz and 350 kHz excita-
reliability of damage detection. In the experi-
tion, respectively. The wave excitation used is
ment, the excited transient signal is made up of
a5-count tone burst. From the first row of wave
awindowed five-cycle sine tone burst (shown in
in Figure 8, it can clearly be seen that A is
the Figure 6). The center frequency is varied. o
dominant at 150 kHz. When the tone burst is
This signal is windowed in order to get a single
tuned to 250 kHz, both S mode and A mode
wave traveling at the desired driving frequency. o o
can be excited. This can be seen from the
Because the pulse has a finite duration, a range
second rowof the wave. At 350 kHz (the third
of frequencies is excited rather than a single
row), S mode is maximized, whereas A mode
frequency, and dispersion distorts the shape of o o
is still slightly excited.
the pulse as different frequencycomponents
travel at different velocities.
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function oscilloscope h
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generator J
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aluminum beam h
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S
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Figure 7. Experimental setup for Lamb wave propagation.
This experiment can show students that the good introduction to the basics of smart materi-
transmission and reception of Lamb waves in als, data acquisition, structural monitoring, and
thin structures can be effectively implemented in particular, laboratory and experimental works.
using small-size surface-bonded piezoelectric
sensors and actuators. The Lamb waves can be The ratings, relative to a maximum rating of
used to scan the structures and are capable of 10.0 on key categories pertaining to the course
detecting internal damage through ultrasonic content, are presented in Table 1. The responses
techniques (Yang & Qiao, 2005). The piezoelec- of the evaluation showed highly supportive evi-
tric transducers are inexpensive and unobtrusive. dence toward the intended course outcomes. In
Theycan be deployed over large structural areas order to gain additional insight regarding stu-
to create active sensor arrays. dents’perceptions of the modules, students were
also submitted written comments. According to
The learning exercises described above aim the comments, students increased their percep-
to equip students with the knowledge, skills, and tion of the value of technology, and their self-
understanding necessaryfor tackling problems efficacyin performing SHM and DAQtasks.
inSHM-related professional fields. The lecture Although a few students complained about too
materials cover the core concepts. The laborato- much time involved with the laboratory assign-
ryexperiments expose students to the hands-on ments, most of the students agreed that the
techniques. The three teaching modules present- amount of time and work required to complete
ed in this article can easily and seamlessly be the assignments was reasonable. The majority of
integrated to existing technology courses. students enjoyed working in the lab. Several stu-
dents indicated that having additional opportuni-
Student Feedback ties to “playwith the technology” would greatly
These three course modules were delivered
enhance their learning. Some students also
for the first time during the spring 2009 semes-
expressed an interest in being provided with
ter. An important outcome of any course is what
additional information regarding the smartmate-
students think about various aspects of the class.
rials themselves, such as where they can be pur-
The initial outcome for these modules has been
chased and how much they cost. Overall, most
assessed. All students who took the course mod-
students commented positively about the knowl-
ules outlined previously were asked whether they
edge theygained. They agreed that these mod-
thought the structure of the course gave them a
ules were a valuable learning experience.
The student feedback provides valuable
150 kHz information to the instructors for further improv-
ing the modules. Even though student learning
So Ao
250 kHz and perception were positivelyaffected by the
developed teaching materials, there are challenges
350 kHz in future development. Because the laboratory
experiments were custom-made, the major chal-
Figure8. Wave signals received. lenge is preparing enough experimental systems
Table 1. Survey results.
Acknowledgments
The authors would like to thank the
72 Organizes and plans course effectively. 8.2
National Science Foundation (under grants
Ifound the course intellectually stimulating. 8.5
s
e EEC-0634279 and EEC-0741532) for its sup-
di Ilearned a great deal in this course. 9.2
u port of this project. The authors also wish to
t
S Rate overall value of this course. 8.9
y acknowledge financial supports from the
g
olo at a low cost to support a larger student Institute for Multimodal Transportation. The
n
h population. institute is sponsored by the Department of
c
Te Transportation – University Transportation
of Conclusions Centers Program. The assistantship provided by
al
n Keeping curricula and lab resources current JSU's Science and Technology Access to
r
u
o with respect to the fast pace of technological Research and Graduate Education (STARGE)
J
e advances is a challenge. A module-based Project is also appreciated.
h
T
approach is a new learning paradigm that offers
an effective way to address the challenge of Dr. Hui-Ru Shih is a Professor of
emerging technologies. Smart structures and Technology at Jackson State University,
SHM have become increasingly important tech- Mississippi, and is a member of Delta Beta
nologies to ensure the safe operation of various chapter of Epsilon Pi Tau.
structures. In this work, we advocate a module-
Dr.Wilbur L. Walters is an Associate
based approach for teaching and learning. Three
Professor and Interim Chair of the Department
course modules were designed to provide
of Physics at Jackson State University,
students with a firm grasp of the smart struc-
Mississippi.
tures and integrated health-monitoring tech-
niques. The modules balance theoryand applica-
Dr. Wei Zheng is an Assistant Professor of
tion, classroom lectures and laboratory experi-
Civil Engineering at Jackson State University,
ments. These modules also were designed to
Mississippi.
enable students to gain a competence with tools
they can use in their future jobs. These teaching
Miss Jessica Everett currentlyis an under-
modules havebeen successfullytested in the
graduate technology major at Jackson State
three-credit Capstone course offered in the sen-
University,Mississippi.
ior year to all the Industrial Technology majors.
Extending this learning concept to other practi-
cal courses could broaden the students’practical
and advanced skills, which are much desired by
industryleaders.
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