Experimental Investigations of Metal Wire Based EMI Technique for Steel Structures

Abstract

Infrastructure construction is the second largest economic activity in India after agriculture and it is growing
rapidly. Structural Health Monitoring (SHM) aims to provide new solutions for easy and swift inspection of engineered structures. Electromechanical impedance (EMI) technique is a relatively new useful and convenient technique in the area of SHM. Though this technique shows promising results in the area of SHM, the main disadvantage of this technique is
the brittleness of the piezo-electric (PZT) material. To overcome this problem, a new variant has recently emerged wherein a metal wire is coupled with the PZT patch and then attached with the host structure. This paper primarily focuses on evaluating the metal wire based EMI technique for damage assessment of 2D structure. Test results show that the technique can effectively identify and locate damage on a 2D structure.

Full text

EXPERIMENTAL INVESTIGATIONS OF METAL WIRE BASED EMI
TECHNIQUE FOR STEEL STRUCTURES
S. Naskara, S. Bhallaa
a Indian Institute of Technology Delhi, Delhi 110016
Email:susmita630000@gmail.com, sbhalla@civil.iitd.ac.in
Abstract:. Infrastructure construction is the second largest economic activity in India after agriculture and it is growing rapidly.
Structural Health Monitoring (SHM) aims to provide new solutions for easy and swift inspection of engineered structures.
Electromechanical impedance (EMI) technique is a relatively new useful and convenient technique in the area of SHM.
Though this technique shows promising results in the area of SHM, the main disadvantage of this technique is the brittleness of
the piezo-electric (PZT) material. To overcome this problem, a new variant has recently emerged wherein a metal wire is
coupled with the PZT patch and then attached with the host structure. This paper primarily focuses on evaluating the metal
wire based EMI technique for damage assessment of 2D structure. Test results show that the technique can effectively identify
and locate damage on a 2D structure.
Keywords: Structural Health Monitoring (SHM), Piezo-electric material (PZT), Electromechanical impedance (EMI) Technique, Metal wire
based EMI Technique
1. INTRODUCTION
Civil Engineering infrastructure development represents the
spine of growth for any developed country. SHM offers
precise information that can help the engineers to identify the
state of the structures and if needed can target the specific
regions to inspect. In the process of increasing the life span of
any existing engineered structure, the concept of long term
monitoring is evolving as a result of the new sensor
technologies EMI is a relatively new non-destructive
evaluation (NDE) technique, which uses a single PZT patch
to act as an actuator and a sensor simultaneously [1]. On
giving a mechanical stress, the PZT material produces
electrical charge and vice versa. Due to its light weight and
easily availability, PZT materials are used widely used for
structural monitoring through EMI technique. But the main
limitation of the technique is the brittleness of the PZT patch
material and for this reason it is difficult to attach the patch
on surfaces with the complex geometries. In addition, certain
unfavorable situations prohibit direct bonding of PZT patches
on the structure. To overcome the aforementioned problems,
a metal wire attached in between the structure and the PZT
patch has been demonstrated to be effective [2,3]. The main
advantage of using a metal wire is the elimination of the need
for attaching the brittle PZT element onto the surface of the
structure. In this study the metal wire EMI technique is used
to health monitor the steel plate having dimensions
1200X970 mm2, subjected to progressive damage. The
promising results of the experiments indicate the
effectiveness of the proposed metal wire technique on steel
structures.
2. ELECTROMECHANICAL IMPEDANCE
TECHNIQUE
The EMI technique employs a PZT patch surface bonded to
the monitored structure, to produce ultrasonic vibrations (in
30-400 kHz range) so as to derive a characteristic electrical
signature of the engineered structure, which will contain vital
information concerning the phenomenological nature of the
2D structure[4]. Electromechanical admittance can be
decomposed and analysed to extract the impedance
parameters of the structure. In this process, the PZT patch
will act as an impedance transducer, which enables the
structural identification for SHM. The PZT patch acts as
actuator as well as sensor. The main idea of EMI technique is
that the presence of damage in the host structure will affect
the mechanical impedance and then the admittance of the
PZT patch, will directly measured by an LCR meter which is
shown in the Figure 1.
Figure1 Electrical impedance analyzer (LCR meter
Agilent E4980)
The LCR meter imposes an alternate voltage signal to the
bonded with PZT patch over the specified frequency and the
changes in the admittance signaturegives the indication of
strctural damage which can be easily accessed for the
identification of damage assessment. This admittance
signature is a function of stiffness, mass and damping
behaviour of the assessed structure.
The schematic set up of the experimental set up is shown in
the Figure 1 where the Agilent E4980A 20Hz-2MHz
precision LCR meter is connected to a laptop and a specimen
through USB cable. The values of conductance (G) and

susceptance (B) were obtained by connecting in frequency
range 100 to 150 kHz through the Agilent VEE Pro 9.0[5].
Figure2 General Schematic set up of the total system
In order to quantify the changes in signature due to damage,
the index used is root mean square deviation i.e. RMSD.
RMSD was defined as [6],
(1)
Where, ui (i= 1,2,3,……..,N) is the baseline signature
wi is the signature obtained after damage.
3. SPECIMEN PREPARATION
The steel plate shown in Figure3 has dimension of 1270X970
mm2 and is 8 mm in thickness. The chemical composition of
the steel plate includes carbon of 0.23 %, manganese of 1.5
%, sulphur 0.045%, phosphorus of 0.045% and silicon of
0.40% as per IS-2062, 2006. The steel plate was supported on
box section pipe of cross section 38X38 mm2 and 3 mm
thickness and the pipe was welded along the perimeter of the
steel plate.
After the fabrication of the steel plate, metal wires of length
50.8 has been attached with the plate at two sides of it, and
then PZT patches were attached on the plate and the metal
wire with two part Araldite epoxy adhesive. The whole Plate
with metal wire is shown in Figure 4.
The key feature of the proposed arrangement is that it
drastically reduces the total number of sensors employed for
SHM
Figure 3(a) Side view of the Steel Plate of 1270X970 mm2
Figure 3(b) Front view of the Steel Plate of 1270X970 mm2
2
1
2
1
()
RMSD(%) 100
in
ii
iin
i
i
wu
u
=
=
=
=
−
=×
∑
∑
Figure 4 Whole Steel plate with metal wire set up
The plate has been divided in panels and on each panel the
PZTs patches were attached, and for the metal wire the PZTs
patches were attached at one end and the metal wire is
attached along other end to the plate. The main advantage is
this technique is we can remotely control the structure with
the help of this metal wire technique. For the conventional
method the total PZT numbers are 42, and we have to take ( n
x n) measurement which will take a long period. But with the
help of the proposed technique we can easily detect the
damage by 2n sensors where n is the number of the sensor
grids (here 7). So the number of readings also get decreased
and it is time saving too and mainly we can easily further use
the metal wire along with the PZTs for other structures.
4. INDUCTION OF DAMAGE
In this experimental study, pre-emptive damage detection has
been done in both the plate and metal wire is accomplished
by using PZT patches of 10x10 mm2 and LCR meter only [7].
A single hole of diameter 5cm was made at the 15th panel of
the plate which is shown in Figure 5. A cover plate as shown
in the Figure 6 was placed over the damage, as a retrofitting
measure and four 10mm diameter bolts were tightened at 30
N-m torque to get a repaired condition.

Figure 5 Damage location created at the 15th panel of the
plate
For the both proposed metal wire technique and conventional
EMI technique, the base line of the conductance signature
was acquired from the PZT patches, which has been shown in
Figure7 for both damaged and undamaged condition with the
help of VEE Pro. 9.0. The repeatability of the signature is
excellent as shown in the figure 7(a) and 7 (b). Damaged
condition is being created by removing the repairing cover
plate from the hole and then RMSD values from equation 1 is
being calculated for each PZT patch.
Figure 6 (a) Retrofitting plate with four bolts of 10mm
diameter,
Figure 6 (b) PZT patch attached on the steel plate
5. EXPERIMENTAL RESULTS
The RMSD values for conductance signature for each patch
for steel plate have been calculated from the equation 1 and
2D bar graph has been plotted for steel plate and Figure 6 (a)
shows the RMSD plot for steel plate using conventional
approach. The same procedure has been repeated for metal
wire also and the RMSD plot for the metal wire is shown in
the approach
Figure 7(a) Repeatability test signature for 6th PZT patch for
conventional technique
Figure 7(b) Repeatability signature for 6th PZT patch for
metal wire technique
From the Plot we can easily see that for both the cases the
damage has been detected for the panel no 15 where actually
damage has been created. After comparing the two RMSD
plots, both the methods are giving correct result. But for the
metal wire technique, we do not need to calculate 42 panels,
we can easily detect the damage from 14 sensors.

6. CONCLUSION
The results show that the damage detection effectiveness of
the metal wire based EMI technique is comparable to the
conventional EMI technique although much less sensors are
needed. Thus, the proposed metal wire technique, which has
much more advantages than the conventional technique, has
proven its viability as a replacement of conventional
technique. The time taken for using the metal wire technique
is much lesser than the conventional one and these
experimental results shows that metal wire technique can be
used for the larger host structure also.
REFERENCES
[1] Bhalla. S and Soh. C. K. (2004), “Structural Health
Monitoring by Piezo-Impedance Transducers I:
Modeling”, Journal of Aerospace Engineering,
ASCE,17(4),154-165.
Figure 8 RMSD of signature at 30 panels acquired for steel
plate
[2] Na. S, Lee H.K. “Steel wire electromechanical
impedance method using a piezoelectric material for
composite structures with complex surfaces”, Composite
Structures 98(2013).
[3] Naskar, S. and Bhalla, S.(2013), “Investigation into
metal wire Based Variant of EMI Technique for
Structural Health Monitoring”, International Conference
on Advances in Mechanical, Automobile and Aerospace
Engineering (AMAAE-2013),Vol 6, No. 6 pp. 795-800
[4] Na, S. and Lee, H.K. “Resonant frequency range utilized
electro-mechanical impedance method for damage
detection performance on composite structures”.
Composite structure 2012:94(8); 2383-9.
[5] Agilent Technologies (2013)”Agilent VEE pro Quick
start Guide” (www.agilent.com)
[6] Bhalla, S. (2001), “Smart System Based Automated
Health Monitoring of Strcture”, Master Thesis, Nanyang
Technological University, Singapore.
Figure 9 RMSD of signature plot for proposed metal wire
technique [7] PI Ceramic (2013), http://www.piceramic.de.