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Structural Health Monitoring

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 Structural Health Monitoring

Devendra Adhikaria, Anand Prakashb, Sahil Yadavc, Smita Kalonid

Studenta,b,c, Assistant Professord, Department of Civil Engineering, NIT Uttarakhand

Abstract

All civil infrastructure facilities like bridges and highways, deteriorates with time due to various
reasons including fatigue failure caused by repetitive & heavy traffic loads, effects of
environmental conditions, and extreme events such as an earthquake. This requires not just local
or critical event based inspections (such as an earthquake), but rather a means of continuous
monitoring of a structure to provide an assessment of changes as a function of time and an early
warning of an unsafe condition using real-time data. Thus, the health monitoring of structures
has been a good research topic. The interest in the ability to monitor a structure and detect
damage at the earliest possible stage is pervalent throughout the civil, mechanical and aerospace
engineering communities. Damage detection methods are either visual or localized experimental
methods such as acoustic or ultrasonic methods, magnetic field methods, radiograph, eddy-
current methods and thermal field methods. Some smart sensing technologies includes the
application of fibre optic sensors, piezoelectric sensors, magnetostrictive sensors and self-
diagnosing fibre reinforced composites possess capabilities of structural monitoring. This report
contains a review of vibrational based structural health monitoring. The basic premise of
vibration-based damage detection is that the damage will significantly changes the stiffness,
mass or energy dissipation properties of a system, which, in turn, will alter the measured
dynamic response of that system. Although the basis for vibration-based damage detection
appears intuitive, its actual application poses many significant technical challenges. The most
fundamental challenge is the fact that damage is typically a local phenomenon and may not
significantly influence the lower-frequency global response of structures that is typically
measured during vibration tests. This challenge is supplemented by many practical issues
associated with making accurate and repeatable vibration measurements at a limited number of
locations on structures often operating in adverse environments.
Introduction

Structural health monitoring (SHM) is the process to detect the damages in the structure and
characterization strategy for engineering structures. The process involves the observation of a
structure periodically using response measurement from sensors, the extraction of features from
these measurements, and the analysis of these features to determine the current state of health of
the structural system. Based on the monitored state, appropriate repair, rehabilitate, and
strengthening of structures are decided to keep these structures operational and further to
lengthen their lives.
Since the beginning of the 19th century, railroad wheel-tappers have used the sound of a hammer
striking the train wheel to evaluate if damage was present. In rotating machinery, vibration
monitoring has been used for decades as a performance evaluation technique. Two techniques in
the field of SHM are wave propagation based techniques Raghavan and Cesnik and vibration
based techniques. The available localized experimental methods being used include acoustic
emission or ultrasonic methods, electro-magnetic field methods, X-ray, radiographs, eddy current
methods, and thermal field methods,
Based on the amount of information provided regarding the damage state, these methods can be
classified as providing 4 levels of damage detection. 1. Identify that damage has occurred, 2.
Determine the location of damage, 3. Estimate its severity, 4. Determine the remaining useful life
of the structure [3].
The motivation behind this review is, 1. To inspect the performance of structural elements in
fixed time intervals. 2.Very reliable, durable and stable sensor technologies are available, this
technology can easily be implemented. 3. As in future, everyone’s main focus will be on
declining the construction work together with growth in maintenance work, Structural Health
Monitoring will play a very important role. 4. Dynamic monitoring systems have proven to be
particularly suited for systems whose structural behaviour are strongly influenced by their
geometric complexity or the inhomogeneity of their constituent materials.
Damage detection methods
Global damage identification techniques can be classified as Static Response-based method,
Physical Model-based method and Vibration based method.
This review only contains the study of Vibration based methods.
Vibrational based structural health monitoring
The basic principle of using vibration monitoring to assess structural integrity relies on the fact
that dynamic response is a sensitive indication of the physical integrity of any structure [4]. As a
result of damage (or structural distress), local or global, there would be a reduction in stiffness
and a change in the energy stored in the body [4]. Due to the fact that each vibration mode has a
different energy distribution, any localized damage will affect each mode differently depending
on the nature, location, and severity of damage [5].
The study proposes a method of integrating the vibration monitoring system of the axle box
bearing with the underfloor wheelset lathe, where the integration scheme and work flow of the
system are introduced followed by the detailed fault diagnosis method and application examples.
Firstly, the band-pass filter and envelope analysis are successively performed on the original
signal acquired by an accelerometer [5]. Secondly, the alpha-stable distribution (ASD) and
multifractal detrended fluctuation analysis (MFDFA) of the envelope signal are performed, and
five characteristic parameters with significant stability and sensitivity are extracted and then
brought into the least-squares support vectors machine based on particle swarm optimization to
determine the state of the bearing quantitatively. The effectiveness of the method is finally
validated by bench test data [5].

Structural Health Monitoring using Laser Doppler Vibrometer

Laser Doppler vibrometer (LDV) LDV is an optical instrument employing laser technology to
measure velocity based on the Doppler principle [6]. The characteristics of LDV are the
followings: in comparison with conventional transducers such as an accelerometer, non-contact
and long distance measurement is possible without adding mass or stiffness to an object.
Secondly, velocity is measured very high accurately, and frequency bandwidth is very wide.
Thirdly, by attaching a scanning unit of mirror in front of the laser sensor head, scanning
measurement for multiple points can be realized [6].
Tensile force measurement of cables in cable stayed bridge
Researches using LDV have been extensively conducted in order to develop quantitative SHM
technique for civil infrastructures. As one problem in these researches, setting up of equipments
requires time-consuming tasks. Since LDV has been utilized for vibration measurement such as a
car or a hard disk drive in laboratory environment, equipments do not suite for on-site
measurement [5]. Therefore it has been difficult to measure large structures with for lack of
portability. A LDV used in this study is a commercial product (PDV-100) supplied by Polytech
Inc. Its sensor head and controller are integrated into a body and driven by a battery.
Measurement system using the LDV was developed in order to be possible to handle by a laptop
PC [5]. Since the system has very excellent movability, the number of working persons for
measurement becomes few. Another characteristic of the LDV is to have not only analog output
but also digital output. The developed system was applied to tensile force measurement of cables
in the Tatara Bridge. All 84 cables on one-side at the Bridge are measured. Automated remote
measurement system for whole large bridge When measurement points are far away from a
LDV, it is difficult to confirm the irradiation of the laser during on-site measurements. To
improve the situation, the measurement system combining the LDV with a Total Station (TS) for
survey was developed. The LDV is attached on the top of the TS. The TS used in the study
posses 1 mm accuracy at 100 m distance. The system has the ability of positioning measurement
points high accurately as well as remotely [6].

Structural Health Monitoring using MEMS-based Technologies

A wireless LAN accelerometer utilized in the study is a commercial product (DATAMARK
SU100) supplied by Hakusan Corp. in Japan. Although it inevitably lacks measured data due to
no-checking of data transmission [5,6], it has following merits; wireless measurement at about
100 m distance on outside, tri-axial measurement, synchronized measurement between sensors
and battery driven (about 10 hours). Wireless measurement can eliminate a lot of works and time
such as needed in connecting cables. Also, tri-axial measurement can capture local three-
dimensional behavior. Therefore the accelerometer is greatly effective device for quantitative
SHM. At first, in order to verify the applicability of the accelerometer to real structures,
performance evaluation test was conducted. Then, the technique identifying whole mode shapes
of a bridge from partially identified mode shapes using the accelerometers is proposed and
validated by on-site measurement. At first, fundamental performance of the wireless
accelerometer was evaluated. the wireless accelerometer and a servo type accelerometer for
validation were attached on a hanger cable of a suspension bridge. Ambient vibration
measurement was conducted, and then their corresponding Fourier spectral amplitudes (FSAs)
were compared. Frequencies giving peak of FSA correspond to natural frequencies of the cable.
We can recognize the agreement of both natural frequencies. Next, since MEMS-based sensor is
often insufficient in measurement resolution, the performance for ambient vibration
measurement of a bridge was evaluated [6,7]. The wireless accelerometer was put on the Tatara
Bridge, and then ambient vibration measurement was conducted. Measurement directions in the
figures up to down are longitudinal, lateral and vertical directions respectively. We can recognize
from time histories that the wireless accelerometer can acquire ambient vibration of bridge..
Since proposed method enables the identification of mode shapes simply and at a low cost, this
method becomes effective for the validation of analytical model and its updating [5,6].
After knowing that there can be some defects in the structure, the location and type of damages
can be detected using methods given below

Properties Types of change Methods to detect damage

Appearance Defects (surface) Digital still camera,
thermograph
Defects (inside) Sonic, Thermograph, Radar
ultra-sonic, X-ray, Impact
echo
Strength and stiffness Concrete strength Rebound hammer
Modulus of elasticity Ultra-sonic velocity
Distribution Digital still camera,
Thermograph
Cracks and spalling Crack width Digital still camera,
Thermograph
Crack depth Ultra-sonic
Cracking Acoustic emission
Steel corrosion Location Natural potential
Corrosion degree Natural potential,
Electric current analysis

Table1 : Damage detection by conventional methods of structural health monitoring

Concluding Remarks
This paper describes several studies relating to structural health monitoring techniques based on
vibration measurement using laser Doppler vibrometer (LDV) and MEMS-based technologies.
The followings can be concluded.
The scope of MEMS and LDV in structural health monitoring is wide in future as in future our
prime focus will be to reduce use of cement and concrete and also increase life of existing
structures. Other than vibration based monitoring, MEMS can be used for monitoring of
temperature and pressure effect also and LDV is used for velocity measurement. A new wireless
LAN accelerometer based on MEMS technology was evaluated and applied to field
measurement of bridges. At first, fundamental performance of the accelerometer was evaluated.
It was confirmed that the accelerometer made possible to measure ambient vibration of a bridge.
Then, using the accelerometer, dynamic characteristics of a bridge were identified by simple
modal testing. Finally, the method identifying mode shapes of a bridge using only two
accelerometers were proposed. Herein, whole mode shapes are identified from partial mode
shapes. Its validity was verified by onsite measurement.
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