STRUCTURAL HEALTH MONITORING OF ALUMINUM

PLATE USING ACOUSTIC SENSORS

Nishanth.Ra, Maheshprabhu.R b, Malolan.Vc

a. Post Graduate Student, Dept of Mechanical Engineering, Anna University of Technology Coimbatore, Coimbatore, India
b. Assistant Professor, Anna University of Technology Coimbatore, Coimbatore, India
c. Scientist C Advanced System Laboratory, DRDL, Hyderabad, India

Abstract- Structural health monitoring (SHM) is an evolving technology being developed for monitoring of air and space
systems. The information gathered on a systems structural integrity through SHM detection methods may result in reduced
costly maintenance inspections, enhanced safety, and system failure predictions. This study evaluates Lamb wave approaches
used to detect simulated cracks in laboratory experiments on thin plates to detect realistic damage in a test article representing the
rocket motor casing. The main objective of the project is to measure the damage evaluation in Aluminum plate using pzt sensors
by generating Lamb wave because Lamb wave methods need little power for exciting transducers and is suitable for large plate-
like structures because of its characteristic of the long-range propagation.PZT sensors are used for emitting and receiving lamb
waves. Also this technique reduces signal processing time to extract damage information from the sensor signals because it does
not require continuous monitoring during operations unlike other Piezo-based methods. This work is carried out at Advanced
System Laboratory, DRDO, Hyderabad.

Keyword: Piezo, Lamb Wave, Transducers, SHM.

1.1 INTRODUCTION not require continuous monitoring during operations

unlike other piezo-based methods.
Structural Health Monitoring (SHM) is an emerging
Because all vibration-based damage detection
technology that has multiple applications.SHM
processes rely on experimental data with inherent
emerged from the wide field of smart structures, and it
uncertainties, statistical analysis procedures are
also encompasses disciplines such as structural
necessary if one is to state in a quantifiable manner
dynamics, materials and structures, nondestructive
that changes in the vibration response of a structure
testing, sensors and actuators, data acquisition, signal
are indicative of damage as opposed to operational
processing, and possibly much more[1].The main
and environmental variability. A damage detection
objective of this research work is to measure the
system based on Lamb waves for Structural Health
damage evaluation in Aluminum plates using Lamb
Monitoring (SHM) requires transducers for both
wave since minimum power is needed for exciting
excitation and sensing of elastic waves to interrogate
transducers and are suitable for large plate-like
the structures in order to detect, locate and
structures because of its characteristic of the long-
characterize defects and damage[26]. The issues of
range propagation[19] and Lamb waves are guided
concern in this system are the behavior of the
elastic waves. Lamb wave techniques have been
integrated transducers for generation of Lamb waves
emerging as one of the most effective methods to
in the structure, and also the multi-modal and
detect structural damage [1]. Also this technique
dispersive nature of Lamb waves [25].
reduces signal processing time to extract damage
information from the sensor signals because it does
The broad field of SHM encompasses many advanced (200*400*2) mm with horizontal, vertical and angular
technologies that, when integrated, provide a system defects and a defect less aluminum plate is used for
that can potentially identify and characterize the testing figure1.1a. For this lamb wave testing
performance and possible deterioration of a structural techniques with different lamb modes are generated
system [5]. For a SHM system, a data acquisition using pzt as transmitter and receiver have been used
subsystem is required to record a structures response and the sensitivity of these methods has been
to ambient and external loads. This is monitored by compared with each other [19, 25]. The response
means of MATLAB. This is used as additional signals are collected and processed. Based on the
software to monitor the graphs obtained. processed data, the state of the structure the defect is
diagnosed. This is mainly done by three methods [1].
A new ultrasonic methodology which uses Lamb
waves to examine the structural components has been
developed. Lamb waves are guided elastic waves,
which can travel relatively large distances with very
little amplitude loss they offer the advantage of large-
area coverage with a minimum of installed sensors
The Lamb waves can be used to scan the structures
and are capable of detecting internal damage through
ultrasonic techniques. The piezoelectric transducers
are inexpensive and unobtrusive. They can be
deployed over large structural areas to create active
sensor arrays. The experimental Setup is give on
Figure1.1 Experimental Setup for Lamb Wave
Figure 1.1 Propagation

The potential direct benefits from such systems are 1.2.1 Smart Materials & Structures.
enormous. These include: 1.2.2 Data Acquisition Systems.
1.2.3 Lamb Waves Generation &Detection.
1. Real-time monitoring and reporting maintenance
cost savings
1.2.1 Smart materials and structures
2. Minimum human involvement labor, downtime,
Smart structures utilize active (smart) materials as
and human error reduction
sensors and actuators to sense and respond to their
3. Automation safety and reliability improvement environment Smart materials (e.g., electrostrictives,
shape memory alloys, and piezoelectric) are now used
1.2 METHODOLOGY in numerous applications. The electromechanical
The field of SHM is vast. The structure under property of piezoelectric materials has both a direct
investigation is energized using actuators. The and a converse effect. The direct effect is described as
response to the excitation is sensed at various the generation of an electric charge in a material when
locations throughout the structure. The aim of present it is subjected to a mechanical stress.
study is to evaluate defects in thin Aluminum plates
The converse effect is described as generation of a MATLAB is a programming language developed by
mechanical strain in a material in response to an Math Work. It is used in this class to write programs
applied electric field. Piezoelectrics are available in for the acquisition, processing, and presentation of
polymer (polyvinylidene fluoride or PVDF) or data.
ceramic (lead zirconate titanate or PZT) form.
1.2.3 Lamb Waves Generation & Detection
Piezoceramics are stiff and brittle, whereas
piezopolymers are compliant and soft. There are two SHM techniques are being developed to reduce
possible ways to utilize piezoelectric materials. First, operations and support costs, increase availability, and
piezoelectric materials can be used as sensors (direct maintain the safety of various structures, such as
effect). Second, piezoelectric materials can be used as bridges and aircraft. Conventional ultrasonic methods,
actuators (converse effect). Because the piezoelectric such as pulse echo techniques, have been used
materials can be used as both sensors and actuator. successfully to inspect structural integrity. However,
The converse effect is described as generation of a the application of these traditional techniques has been
mechanical strain in a material in response to an limited to testing relatively simple geometries or
applied electric field. inspecting the region in the changes of the transducer.
A new ultrasonic methodology which uses Lamb
There are two possible ways to utilize piezoelectric
waves to examine the structural components has been
materials. First, piezoelectric materials can be used as developed. Lamb waves are guided elastic waves,
sensors (direct effect). Second, piezoelectric materials
which can travel relatively large distances with very
can be used as actuators (converse effect). Because little amplitude loss they offer the advantage of large-
piezoelectric materials are applicable to sensors and
area coverage with a minimum of installed sensors
actuators, their use is very popular in smart structures
[22].
and systems.

Thus in contrast to the conventional methods, the

1.2.2 Data Acquisition Systems guided-wave technology can be used to inspect the
entire structure in a single measurement, and it can
The broad field of SHM encompasses many advanced
technologies that, when integrated, provide a system also inspect inaccessible regions of complex
components. For Lamb waves, at a given frequency, at
that can potentially identify and characterize the
performance and/or possible deterioration of a least two modes (one symmetric and one

structural system. For a SHM system, a data antisymmetric) are generated (figure1.2). As

acquisition subsystem is required to record a frequency increases, the number of simultaneously

structures response to ambient and external loads. existing wave forms also increases. Lamb wave
propagation is usually highly dispersive. Therefore,
These functions provide basic services for common how to choose the best frequency is a major issue. In
SHM related data acquisition tasks. Currently the Lamb wave inspection, mostly the so (symmetric) and
software supports basic functionality for MATLAB Ao (anti-symmetric) modes are used. So and Ao are
DAQ. In recent years, MATLAB has been widely referred to as the fundamental modes. These two
used in applications where engineers, scientists, and modes are the most important because they exist at all
technologists want to acquire, analyze, a present data.
frequencies, and they carry more energy than the Where k is wave number of lamb wave and is given
higher order modes in most situations. by: k=

P and q are calculated by: and

q=

f = linear frequency of lamb wave;

angular frequency = ;

= longitudinal ultrasonic bulk wave velocity;

= Shear ultrasonic bulk wave velocity;

d= thickness of plate

Group velocity ( ) predicts as followed:

Figure1.1a Defect and Defect less Plate

The both group velocity (Cg) and phase velocity (Cp)

is obtained by using the lamb wave equations for both
symmetric and antisymmetric modes (appendix A)
and obtained as shown in figure (1.3 & 1.4). The
normal way to describe the propagation characteristics
is by the use of dispersion curves based on the plate
mode and phase velocity as a function of the product
of frequency and thickness. A particular lamb wave
can be excited if the phase velocity of the incident
longitudinal wave is equal to phase velocity for the
particular mode [24]

The followed equation can be used for predict the

lamb wave phase velocity ( ) in a plate:

For symmetric mode: Figure 1.2 Symmetric and Asymmetric Waves

1.3 RESULTS AND DISCUSSION

=
For Lamb waves, at a given frequency, atleast two
For anti symmetric mode: modes (one symmetric and one antisymmetric) are
generated. As frequency increases, the number of
= simultaneously existing waveforms also increases.
Lamb wave propagation is usually highly dispersive.
Therefore, how to choose the best frequency is a
major issue. In Lamb wave inspection, mostly the So
(symmetric) and Ao (anti-symmetric) modes are used.
So and Ao are referred to as the fundamental modes.
These two modes are the most important because they
exist at all frequencies, and they carry more energy
than the higher order modes in most situations.

The entire thickness of the laminate can also be

interrogated by various Lamb modes, affording the
possibility of detecting internal damage as well as that
on surface In general, a Lamb wave-based damage
detection approach features

Figure1.4 Phase Velocity Dispersion Curve

Optimization experiments provided a procedure

capable of easily and accurately determining the
presence of damage by monitoring the transmitted
waves with piezoceramic sensors (PZT). Lamb wave
techniques have been proven to provide more
information about damage type, severity and location
than previously tested methods, and may prove
suitable for structural health monitoring applications
since they travel long distances and can be applied
with conformable piezoelectric actuators and sensors
that require little power.

Figure1.3 Group Velocity Dispersion Curve The phase and group velocity for antisymmetric and
symmetric are obtained by generating new MATLAB
(1) The ability to inspect large structures while
codes. The lamb wave equations used for obtaining
retaining coating and insulation, e.g. a pipe system
the Ao and So modes are given on appendix A. Of
under water.
these the Ao mode is chosen for testing since it can
(2) The ability to inspect the entire cross sectional travel a long distance up to 1meter and So mode is
area of a structure (100% coverage over a fairly long highly attenuate in nature. The lamb wave is generated
length. by using the functional generator and the obtained
wave form is monitored in the oscilloscope instrument
(3) Excellent sensitivity to multiple defects with high
both Ao and So is identified in at 300 kHz
precision of identification and Low energy
consumption and great cost-effectiveness
 Figure1.5 Comparison of All Type of Defects

1.3.1 Acoustic emission wave form Similarly the wave form datas of all the plates are
collected and compared as shown in fig 1.5
This wave form of acoustic emission is obtained by
using Agilent functional generator and oscilloscope As the frequency of the transducer increases we can
for frequency 300 kHz and 10 amp for 10 cycles. It is observe additional modes like a1, s1, a2, s2 in the
applied in a plate of dimension 400*200*2 (mm) with subplots. Because of these modes detection of defect
horizontal, vertical and angular defect and a defect becomes difficult. To avoid the unwanted noises and
less plate. Thus the obtained value of boundary is disturbances filtering using the golay filter is done to
30us.so the crack can be identified with in 30 us. the signal using MATLAB program and the filtered
Since the wave dimension between two peaks in the signal is obtained as shown in fig1.7.
defect less plate is 30us.

The obtained wave form is saved as a text file and

imported in to MATLAB for further iteration.
Figure 1.6 FFT of Angular and Vertical Defect plate
 Figure 1.7 Filtered Angular and Vertical Defect Wave Form

has also brought out the advantages and disadvantages

of the various systems. The primary motivation is to
Fast Fourier Transform can be programmed using
detect the damage. Significant cost benefits can be
MATLAB and a Time versus amplitude plot can be
achieved, in terms of scheduled and unscheduled
obtained as shown in fig 1.6. From the obtained plots
maintenance, as well as design tolerance.
we can observe that the amplitude range for nano PZT
is much higher. So the detection and analysis of SHM represents the next step in the evolution of air
defects using Nano sensor will be optimum. transport safety builds on lessons learned from
previous incidents, and the processes put in place to
1.4 SCOPE OF THE PROJECT
minimise the loss of lives.The ultimate goal is to be
In the present scenario the SHM is done by able to differentiate between various different types of
fundamental mathematical formulation of guided damage and to be able to calculate where the damage
wave (GW) propagation generated by surface- is located and finally to estimate the size of the
mounted piezoelectric actuators. In this project lamb damage.
waves are used for crack detection and PZT sensors
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