Investigation into Metal Wire Based Variant of EMI Technique for Structural Health Monitoring

Abstract

Electro mechanical impedance method (EMI) is a newly non-destructive evaluation method which is becoming very famous in the field of structural health monitoring. In this article a new approach is being proposed to effectively detect the initiation and progression of structural damage by the global dynamic electro-mechanical impedance (EMI) techniques. In this context the PZT patches are being used to determine the natural frequency and the strain mode shapes and the electro mechanical admittance signature to facilitate an improved damage assessment. Nowadays the safety issues for the case of composite building materials are getting more importance. The main problem of using EMI method is its brittleness so to overcome from this problem we are using this method by coupling a metal wire with a PZT element. In this method we created progressive damages and deterioration scenarios and we evaluated with the application of the proposed metal wire EMI method.

Full text

International Journal of Engineering Research and Technology.
ISSN 0974-3154 Volume 6, Number 6 (2013), pp. 795-800
© International Research Publication House
http://www.irphouse.com
Investigation into Metal Wire Based Variant of EMI
Technique for Structural Health Monitoring
Susmita Naskar1 and Suresh Bhalla2
Department of Civil Engineering, Indian Institute of Technology Delhi
New Delhi-110016, India.
Abstract
Electro mechanical impedance method (EMI) is a newly non-
destructive evaluation method which is becoming very famous in the
field of structural health monitoring. In this article a new approach is
being proposed to effectively detect the initiation and progression of
structural damage by the global dynamic electro-mechanicalimpedance
(EMI) techniques. In this context the PZT patches are being used to
determine the natural frequency and the strain mode shapes and the
electromechanical admittance signature to facilitate an improved
damage assessment. Nowadays the safety issues for the case of
composite building materials are getting more importance. The main
problem of using EMI method is its brittleness so to overcome from
this problem we are using this method by coupling a metal wire with a
PZT element. In this method we created progressive damages and
deterioration scenarios and we evaluated with the application of the
proposed metal wire EMI method.
Keywords: Structural health monitoring, PZT, Electromechanical
impedance, compositematerial.
1. Introduction
The process for implementing a damage identification strategy for civil, mechanical
and aerospace engineering infrastructure is referred as Structural Health Monitoring.
Though the mechanical and aerospace industry started to concentrate on the structural
health monitoring aspects earlier, the civil engineering community had a little delayed
start in this area. Civil engineers studied vibration-based damage assessment of bridge

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796
structures and buildings since the early 1980s. Modal properties and quantities derived
from these properties, such as mode shape curvature and dynamic flexibility matrix
indices, have been the primary features used to identify damage in civil structures.
Damage in structure is defined as the change of material or its geometric
properties of the system, including the changes in boundary conditions and system
connectivity, which adversely affect the system’s performance [1]. There are so many
methods developed so far for damage detection in structures [2-6]. Recently one
methodology called Electro-mechanical impedance (EMI) method has been
successfully applied for this purpose. EMI method is a relatively new non-destructive
evaluation (NDT) technique in which we can use a single piezoelectric material to act
as an actuator and a sensor simultaneously. As the piezoelectric materials are very light
in weight and we can get in various size and shape so they are being widely use in
structural dynamics applications[7-8]. In general very soft type of material (lead-
zirconate-titanate which is having a very high piezoelectric constant) is being used as
piezoelectric material for the case of EMI method. In the case of brittleness of the
structures we have to use this only for the case of flat surfaces.
Piezoelectricity is the ability of the crystals and certain ceramic materials to
generate a voltage in response to applied mechanical stress. The piezoelectric effect is
reversible in the case of piezoelectric crystals, when subjected to an externally applied
voltage, can change shape by a small amount. In the field of physics the piezoelectric
effect can be described as the link between electrostatics and mechanics for the
infrastructure. A piezoelectric sensor is a device that uses the piezoelectric effect to
measure mechanical signals like pressure, acceleration, strain or force by converting
them to an electric signal and an actuator just does the opposite of this. Generally we
use two main groups for the piezoelectric sensors and actuators and they are
piezoelectric ceramics and single crystal materials. One main disadvantage of
piezoelectric sensors is that they cannot be used very effectively for true static
measurement, however for the dynamic measurements they give a very effective
results. Up to date the application of EMI technique for structural health monitoring
has been developed by various authors, including damage detection of composite
materials, steel and concrete structures, at which place most of the work are being
involved by using PZT patch attached to the structures[9-12]. As mentioned earlier the
brittleness behaviour of the piezoelectric material make the use of electro mechanical
impedance method limited and to improve this method we are using above
methodology which will be useful for the complex geometry also. To overcome from
this problem we will be using metal wire in conjunction with EMI method to
monitor composite structures is proposing here. The main advantage of using a metal
wire with EMI method is the elimination of the need for attaching the brittle PZT
element onto the surface of thehost structure.
In this proposed study we are using metal wire EMI method is using to health
monitor composite structures subjected to progressive damage, de-bonding and
deterioration of the bonding layer between the composite plates. The satisfactory result
proves the effectiveness of the proposed technique on composite structures.

Investigation into Metal Wire Based Variant of EMI Technique for Structural 797
2. Electromechanical Impedance Method
In the EMI technique PZT impedance sensors, in the form of small patches, which do
not measure direct physical parameters like stress, strain or temperature. These are
very new as impedance sensors, barely two decades old. In this method the PZT
patches are usually bonded on the surface or embedded inside the host structure to be
monitored. The main basic concept of EMI method based SHM approach is that the
presence of damage in the host structure will affect its mechanical impedance and thus
the EMI admittance of the PZT patch which can be directly measured by an electrical
impedance analyser or LCR meter.
Fig. 1: LCR meter used for damage detection.
The impedance analyser imposes an alternative voltage signal of to the bonded
PZT patch over the user specified frequency range and acquires admittance signature
of the structure. The changes in the extracted admittance signature are indication of the
presence of structural damages, which can be used for damage assessment. The PZT
admittance signature is a function of the stiffness, mass and damping of the host
structure, and the properties of the PZT patch.
3. Damage Idetification Using Emimethod
Generally when a crack occurs on a composite structure, it may be possibly grows to a
point where the structure fails thus this is very important to able to detect any damage
that is progressing in any composite structures. The value of conductance at a
particular frequency range will have less value in the case of damaged structure due to
the presence of cracks, delamination and due to de-bonding.

Susmita Naskar & Suresh Bhalla
798
Fig. 2: progressive damage plans for the specimen.
So, by comparing thefrequency vs conductance plots of a structure in its damaged
and undamaged condition, we can identify damage in a structure.
4. Results and Discussions
For this experiment , two composite plates of a size 200 mm X 50 mm with 3.0 mm
thickness was used at the temperature of 28 degree centigrade with one metal wire and
two PZT element. The fig 2 shows the test specimens with the experimental plan
subjected to progressive damage. A torque range is using to create artificial damage for
this experiment. The progressive damage first created by giving 8 Nm to 20 Nm at the
centre of the two aluminium plates and we will be measuring frequency verses
conductance through LCR meter. To evaluate the performance of the proposed metal
wire EMI method subjected to some artificially created progressive damage, the
composite specimen was set as the reference signature and all signature subjected to
the damage was compared with the reference signature.
Fig. 3: Damaged condition.
-3.00E-04
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0.00E+00
1.00E-04
2.00E-04
3.00E-04
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5.00E-04
6.00E-04
7.00E
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04
50000 55000 60000 65000 70000 75000 80000
conductance
frequency
Pro
gressive
damage
detection
PZT patch
along the
direction of the
wire

Investigation into Metal Wire Based Variant of EMI Technique for Structural 799
Fig. 4: Undamaged condition.
If we take a close look into figure 3 and figure 4 then it can be noticed that in case
of damaged structure the conductance has lower values compared to undamaged
structure for a particular frequency indicating the presence of damage in the structure.
The proposed metal wire EMI method is showing promising results subjected to
progressive damage of composite plates, as the impedance signature mostly dominated
by the metal wire, damaging or de-bonding.
5. Conclusion
In this experiment the metal wire based EMI method is being introduced for the case of
composite structures subjected to progressive damage, de-bonding and deterioration of
adhesive layer. The major advantage of this proposed metal wire is for complex
surfaces and surfaces with elevated temperature. The biggest factor which will effect
this experiment is the size of PZT patches which has largest effect on temperature.So
metal wire based EMI method is showing promising results.
The whole work and the experiment have been done at Smart Structure Laboratory
at INDIAN INSTITUTE OF TECHNOLOGY DELHI.
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0.00E+00
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2.00E-04
3.00E-04
4.00E-04
50000 55000 60000 65000 70000 75000 80000
conductance
frequency

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