Project Report on Embedded Strain Sensor.doc

8/16/2019 Project Report on Embedded Strain Sensor.doc

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Short Abstract:

Carbon nanotube polymer composite hadbeen embedded to glass bers reinforcedplastic (GFRP) for the structural healthmonitoring of the composite material. The

addition of conductive C T ber to nonconductive GFRP material aims to enhance itsmultifunction ability. The conductivepolystyrene beads !ere e"truded incontinuous !ire form and used asdeformation sensor. This polystyrene sensor!ire !as embedded into GFRP compositeplate specimen and tested.The testspecimen#s response to mechanical load andthe in situ C T composites electricalresistance measurements !ere correlated forsensing. Further using Classical laminate

plate theory (C$PT)% the strain is correlated tothe dimension of plate and hence strainmapping of the plate is being done.

Embedded Strain Sensor

Integrative Multiscale Engineering Materials andSystems (iMEMS) Lab,Department of Aerospace Engineering,Indian Institute of Science,Bangalore !""#$, India

Date% "!&"'&$"#!

repared by%

Sikil Kumar Singh


2/20

Project Summary Sheet

ro ect *itlero ect +oro ect -eport +o (as per pro ect report cover page)

.unding Agency +ame (/rite IISc in case of 0E-)

.unding Agency 1ontactDetails

(2eep blan2 in case of 0E-)

rincipal InvestigatorDetails

Dr. D. Roy Mahapatra Associate Professor ndian nstitute of Science Department of Aerospace Engineering! ndian nstitute of Science! "angalore #$%%&'.

1ollaborating AgencyDetails

ro ect Durationro ect Status (also indicate D1)

*otal .unding

*ype of ro ectSub ect AreasDescription of Scope of

ro ectro ect 0utcome Ac3ieved

Abstract (insert s3ort abstract 3ere)4ey/ords

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(ontent

Abstract '

&. ntroduction

# # Bac2ground# $ 1omposite Embedded System !# 5 0b ective 6# ' ro ect 0vervie/ .lo/ c3art 7

'. ()* strain gauge

$ # Strain gauge /or2ing principle 8$ $ .abrication of M91+*:1B:epo;y composite ribbon sensor #"$ 5 reparation of conductive polystyrene continues sensor ##$ ' .abrication of sensor embedded composite plate ##

+. Structural ,ealth Monitoring of (omposite Plate5 # Structural 3ealt3 monitoring implication #$5 $ Electronic circuits and calibration #55 5 Sensor Deployment #6

-. References #8

Abstract

A carbon nano tube with polymer material was used to form a piezo resistive strain sensor for

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structural health monitoring applications. The polymer sensors uses large multi walled carbonnanotubes which improves the strain transfer, repeatability and linearity of the sensor. The

polymer improves the interfacial bonding between the nanotubes.

Carbon nanotube polymer composite had been embedded to glass fibers reinforced plastic

(GF !" for the structural health monitoring of the composite material. The addition of conductive C#T fiber to non conductive GF ! material aims to enhance its multifunctionability. The test specimen$s response to mechanical load and the in situ C#T compositeselectrical resistance measurements were correlated for sensing. %t is the first time this polymer sensors is used in composite material for sensing purpose. C#T composites easy to be embedand does not downgrade the materials mechanical properties. &arious incremental loadingsteps had been applied to the manufactured specimens in tension, plate bending tests. TheC#T polymer composite wor'ed as a sensor in plate specimen. A direct correlation betweenthe mechanical loading and electrical resistance change had been established for theinvestigated specimens.

ince polymers are often used as the matri) of a composite material, the strain sensitivematri) can be mi)ed on a sub material level and used for self strain sensing device as awhole. ensors made from conductive polystyrene beads and carbon nanotube polymer composite materials were used to form continuous strain sensors for structural healthmonitoring applications. The conductive polystyrene beads were e)truded in continuous wireform and used as deformation sensor. This polystyrene sensor wire was embedded into GF !composite plate specimen and tested. The addition to conductive material to the nonconductive GF ! material aims to enhance its real*time sensing ability. +e are reporting for the first time, the conductive polymer sensor continuously embedded throughout hostcomposite material for structural health monitoring of composite material. The testspecimen s response to mechanical load and the in situ sensors electrical resistancemeasurements were correlated for sensing. &arious loading steps were applied to thefabricated specimens in tension mode and direct correlation between the strains and relativechange in resistance was established for investigated specimens. These sensors are the zig*zag patterned C#T nanocomposite sensors fabricated on a chemically treated polyethylenesheet. Chemical treatment enhances its bonding with epo)y*resin based sensors fabricated onit. The chemical treatment also enhances the bonding with epo)y*resin based matri) in of thecomposite laminate. Connectors and printed circuit are provided on polyethylene sheet suchthat after embedding in the composite, the rosette sensor gets embedded in the laminate andthe connector emerges out of the sample for electrical connections. osette sensor can be

placed at desired layer and at desired location where the strain components are to be

determined. A method to calibrate the embedded sensors in the composite laminate isdemonstrated.

Chapter 1

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Introduction

1.1 Background

train sensors are very important in many fields of science and engineering. -ne of the mainlimitations of e)isting conventional sensors such as gauges is that they are discrete point andfi)ed directional sensors, and are separate from the material or structure that is beingmonitored hence, not embedded at the material level. %t is difficult to implement amanspectroscopy for strain measurement in field applications, due to bul'y hardware, such as themeasurement of strain in an aircraft wing. There is a need to develop new sensors that can beembedded into the material and can be used for multidirectional and multiple locationsensing.

The electrical conductivity of the carbon fibers was first used to monitor damage in carbonfiber reinforced polymers (CF !s", which could be related to fiber brea'age. The electricalmethods have been e)tensively studied and had been used to study a variety of damagemechanism, e.g. delamination, matri) crac'ing, under various loading conditions.

Carbon nanotubes (C#T", due to their electrical conductivity they could be used with non*conductive composite materials in order to enhance their monitoring capabilities. Theaddition of several percentage of carbon nanotubes (C#T" to the polymer matri) of CF !( also called as doped resin ", lead to a significant increase of the electrical conductivityof the epo)y matri). This enabled to fully monitor the structural health of CF !s and

establish correlation between internal damage and increase in resistance.

uto et al. /01 first demonstrated that the dispersion of carbon powder to the matri) of GF !material can be used for self*diagnosing purposes. %n the same wor', carbon fiber was used inthe GF ! material for damage monitoring by measuring its change in electrical resistance.The latter hybrid composite material was not as successive in terms of damage monitoring asthe former, mainly due to the high modulus of the carbon fiber and its brittle nature, whencompared to GF ! material.

Glass fiber reinforced polymers (GF !" are widely used in the aeronautical and the

automotive industry mainly due to their high specific mechanical properties. 2uring the lastdecades, the aerospace industry focus its research in producing multi*functionality materials,driving design parameters being the weight reduction with increased mechanical properties aswell as monitoring their structural health by means of sensing capability. C#T based polymer wire fiber is easy to be embedded and does not downgrade the material$s mechanical

properties.

1.2 Composite Embedded System

!ros of embedded sensors

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• !rotect the sensors• Access to interior measurands• 3nobtrusive

Cons of embedded sensors

• ensor ingress4egress problematic• !ossible detrimental effect on structural integrity• Cannot replace failed sensors

%t is possible to embed sensors into composite components during manufacturing to allowinternal interrogation of the material sensors can be based on acoustic waveguide waves,

piezoelectric and optical fibers.

ensors offering the prospect of continuously monitoring the composite structure at all stages

of its life through fabrication, test 5ualification and service.

Composite materials with embedded sensors and actuators will be only gain acceptance if thestructural integrity of the composite is not significantly reduced by the presence of inclusionwhich are presently significantly larger in diameter then the carbon, Aramide or glassreinforcing fibers which are typically (6*07 micro meter" diameter, if sensors typically of (877*977 micro meter" in diameter, are embedded in composite laminates there is aninevitable disruption of the reinforcing fibers in the vicinity of the fibers sensor.

The nature of this disruption is dependent on both the diameter of the embedded sensors and

the relative orientation of the fibers sensor with respect to neighbouring reinforcing plies. For e)ample sensing fibers lying parallel to the local reinforcement cause a minimum disruption

provided the diameter is less than half the ply thic'ness. einforcing fibers lying orthogonalto the sensors are locally deformed creating a resin rich region around the sensors.

%n order to accept

0. !roduce a minimum perturbation in the distribution of reinforcing fibers

:. #ot significantly reduce the mechanical property of composites.

;. #ot suffer from e)cessive attenuation or damage from the embedding process, such thatthe sensing techni5ue cannot be applied.


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1.3 Objective

To study =ealth monitoring especially strain sensing of the GF ! composite through>mbedded sensing by C#T coated polymer (!olystyrene" sensor.

&.- Project o er ie/ flo/ chart

Strain sensing in


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Material sublevel strain measurement

1onductive polymer (polystyrene) sensor

1omposite design, analysis and fabrication /it3 embedded sensor

Mec3anical strain sensing

.ig # # Sc3ematics representation of pro ect overvie/

Chapter &

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CNT composite strain gauge

strain gauge is an electrical sensor which is used to accurately measure strain in a testspecimen. train gauge is effectively a resistor as the strain increase so the resistance

increases. %n a basic sense a strain gauge is simply a long wire. train gauges usually basedon metallic foil pattern mostly made from copper or aluminium. As the wire in the gauge ismostly laid from end to end, the strain gauge is only sensitive in that$s direction. +hen anelectrical conductor is stretched within the elasticity it will become thinner longer. As it

becomes thinner and longer its electrical characteristics change. This is because resistance isa function of both cable length and cable diameter.

2.1 Strain gauge working principle

A strain gauge consists of a foil of resistive characteristics, which is safely mounted on a bac'ing material. +hen a 'nown amount of stress in sub?ected on the resistive foil, theresistance of the foil changes accordingly. Thus, there is a relation between the change in theresistance and the strain applied. This relation is 'nown by a 5uantity called gauge factor.

+here,

GF @ gauge factor

ε @ strain

C @change in resistance after applying load

C @ initial resistance

2 2 !abrication o" #$CNT%C&%epo'( composite

ribbon sensor

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+C#Ts were obtained from Buantum aterials, angalore, %ndia in the form of powder 99D pure with outer diameter 07*:7 nm and have a length of E07 m. Carbon blac' powder with particle diameter less than 07 m is dispersed in an epo)y resin with hardener. Thismi)ture contains ;;D by volume of carbon blac' /:1, with the remaining volume of epo)y

resin and hardener (9H0 by weight". The mi)ture is stirred mechanically about 077 rpm for anhour to ensure uniform dispersion of the constituent particles. This mi)ture of carbon blac' and epo)y is used as a host matri) to which +C#Ts are added and sensors are prepared.

+C#T at (7, 0, :, ;" weight percentages were dispersed in resin with I*07ml amount of ethanol solution are mi)ed. The +C#T mi)ture is undergone mechanical stirring atconstant rate followed by centrifuge mi)ing at I777 rpm and sonification to07 minutes. The

process of sonification is one that shoots ultrasonic waves at the sample to improvedispersion.The mi)tures further gone through one more processing cycle. %n every stage

probability of mi)ing particle increased there by getting uniform dispersion of constituent.After processing the mi)ture with different weight percentage, the mi)ture is deposited on atemplate in composite lamina. The template made up of Teflon sheet has a dimension(:I7):)7.;mm" to fi) on the lamina care must be ta'en while removing mould withoutaffecting dimension of pattern. ample cured under controlled environment at :I 7C.

2.3 reparation o! conductive polystyrene continues sensor

An cylindrical die cavity of (677" m was filled with carbon blac' polystyrene beads. Thematerial was then compressed under predetermined pressure and heated at a constant rate I 7C

per min, up to ma)imum temperature of


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(a" (b"

Figure :.0H !olymer sensor (a" Conductive !olystyrene beads (b" Conductive !olystyrene

wire.

2." #abrication o! sensor embedded composite plate

aterial system

• Glass 32 fabric 7.:I mm thic'ness• esin system JKII8=K L 9I0 (077H07" with hardener • 0 wtD C#T coated polystyrene sensor wire

Fabrication of composite plate involves several processes such as

0. a'ing template for sensor location:. atri) preparation

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;. %mpregnation


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pecification of !late

• #o of layer H 07•

tac'ing se5uence H /7 : 497: 471s• !late dimensions H :MI ) :MI mm• !late thic'ness H :.I mm

#o of sensors embedded into the composite

• +ires in 7O ply on middle of 0 st and : nd layer from bottom side.• +ires in 97O ply on middle of ; rd and < th layer from bottom side.

3.2 Electronic Circuits and Calibration

The sensitivity of each of these wires was investigated by response to a central load under simply supported plate scheme. . -ne end of every polystyrene wires was connected to ane)ternal resistor to form a voltage divider circuit. This circuit was implemented to measurethe relative change in resistance values at each load. The e)ternal resistors were selected to

have a resistance value comparable with the zero strain resistance of the wire to which theywere connected, in order to have better sensitivity in reading the values. Figure ;.: shows theloading and resistance measurement scheme.

(a"

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(b" (c"

Figure ;.:H (a" Central loading on plate specimen (b" electrical contacts between wires for the

purpose of in*situ C values monitoring on the grid, and (c" measurement of voltagedeveloped across each resistor .

Calibration is necessary to determine the gauge factors of any strain gauge at different strainlevels. Fig. ;.;(a" shows an e)perimental setup where the composite laminate is simplysupported at two opposite edges using two wedges. A static point load is applied at centre ofthe laminate using a tray mounted on a blunt ended spindle. The load is varied by changingthe weights 'ept on the tray. Tensile and compressive strain is induced by changing the

laminate upside down such that the sensors are placed on top or bottom surface.

(a" (b"

Figure ;.;H (a" >)perimental setup showing simply supported composite laminate withcentrally applied load. Connectors of sensors are connected to 2AB through voltage dividerusing ribbon cables (b" &oltage divider circuit diagram

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&arying load changes the strain induced on the strain gauges and nanocomposite sensors.These strained gauges change their resistances proportional to the load which can bemeasured using a voltage divider circuit as shown in Fig. ;.;(b". The voltage divider circuit is

powered by an amplifier. The voltage S V across the standard resistance S R is measured

using a data ac5uisition system. The resistance of the C#T nanocomposite sensor is given by

S S

AC R1V

V R

−= (0"

where, AV is the applied voltage across the voltage divider. Jet SF V be the voltage acrossthe standard resistance after straining of the laminate. The resistance of the strained C#Tnanocomposite sensor CF R is given by

S SF

ACF R1V

V R

−= . (:"

3sing >5. (0" and (:", the chance in resistance per unit resistance C C R / R∆ is found by

( )( )S ASF

SF S A

C

C CF

C

C

V V V V V V

R R R

R R

−

−=

−=

∆. (;"

3sing C C R / R∆ the gauge factor is estimated by

ε

∆= C C R / RGF

(


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y

) y , x( w-z = z) y,v(x, 0

∂∂

(8"

) y , x( w= z) y,w(x, 0(M"

where u , v and w are the displacement components along ), y and z a)is respectively, 0wis the transverse displacement at any point on the mid plane. The strain components are given

by

20

2

xx x ) y , x( w

-z = x

z) y,u(x, z) y,(x,

∂∂

∂∂=ε

(')

2

02

yy y

) y , x( w-z =

y

z) y,(x,v z) y,(x,

∂∂

∂∂

=ε ( )

Jevy s solution procedure is used to solve the present problem of simply supported plate attwo opposite edges with point load at centre. The solution for transverse displacement 0w isgiven by

( *)

+here b / n π=β and b is the width of the laminate. n A , n B , nC and n D are evaluatedusing boundary conditions.

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Also P 0, P : , P ; , P < are the roots of the analytical e5uation which is given by

y using e5uation (6" and (9" and (07", strain along )*a)is and y*a)is of plate is calculated,then gauge factor of each wire along )*a)is and y*a)is is being calculated by using standardformula,

ε

∆= C C R / RGF

For wires parallel to ) N a)is, the gauge factor is given by

+here

b @ width of composite plate

Also, for wires parallel to y*a)is, gauge factor is given by

+here

a @ length of composite plate

3.3 Sensor 'eployment

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2eployment of sensor is very important in order to get the map of strain across the plate,strain mapping is being perform using deployment process. The displacement e5uation isgiven by

+(), y" @ a 0) L a : y L a ; ) : L a


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+here

>0, >:, >;, >< are strain along )*a)is of plate

>I, >8, >M, >6 are strain along yNa)is of plate

-n using different values of ), y and z in above matri) e5uation and on ta'ing inverse ofmatri) through pivoting by using Gauss* Qordan elimination method, we are able to calculatethe constant (a;, a


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/91 U. Bing, A. Sumar, C. Vhang, %. Gonzalez, G. Guo and F.*S. Chang, =igh peed =ybrid!iezoelectric4Fiber -ptic 2iagnostic ystem for tructural =ealth onitoring, mart

aterials and tructures, &ol.0

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Project Report on Embedded Strain Sensor.doc