cure monitoring by fiber optic sensor

8/6/2019 CURE MONITORING BY FIBER OPTIC SENSOR

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CURE MONITORING BY FIBEROPTIC SENSOR

S.Cantoni, A.Calabr

Italian Aerospace Research Center


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Outline

Fiber Optic Integrated Sensor Project

Fresnel sensor system design

Isothermal cure kinetics

Comparison with calorimetric kinetic data

Future developments


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Composites ManufacturingComposites Manufacturing

Process Optimization and ControlProcess Optimization and Control

OFFOFF

LINELINE

OptimalOperative

ParametersONON

LINELINE

RealProcess

Sensors

Process

Status

PhysicalModel Based

Control

CorrectionActuation

ManufacturingTechnologyAnalysis

Energy, Mass,Momentum Balances

ConstitutiveEquations

PhysicalModel

Real TimeMonitoring


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Fiber Optic Sensor

Fiber optic sensor offers a very powerful tool toperform remote, on-line, in-situ monitoring ofcomposite manufacturing processes.

Free from electromagnetic interference andcharacterized by high chemical and hightemperature resistance.

Readily embedded and its small size makes of itminimally intrusive in the composite structures.

Due to the capability of this system to bemultiplexed, this approach can provideinformation from several differently locatedpoints within the composite.


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Curingresin

Fiber Optic Integrated Embedded Sensor

Fiber optic

Laser beam

Fresnelsensor

Bragg

Grating 2Bragg

Grating 1

Integrated

Bragg 1 and Bragg 2 todecouple the effects on the strainmeasurements due to temperaturevariations and reticulation

Fresnel principle based sensor tomeasure the global refractiveindex variation due to temperature

changes and polymerizationadvancements


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Principle of Operation

Fiber Optic Sensor

Incident Light

Reflected Light

Resin

The sensor is based on theFresnel reflection principle:

the reflection coefficient R isrelated to the differencebetween the resin refractiveindex nm, the fiber optic

refractive index nf and theincidence angle.

In the case of a monomode fiberthe reflection coefficient R at fiber

end-resin interface can beexpressed:

2

mf

mf

nnnnR

+=


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LASER

DIODE

Y JOINT

RESIN

PHOTO

DIODE

A pigtailed DFB laseremitting at 1310nm is usedto light a step-index

monomode fiber. The backreflected laser beamamplitude is splitted by an Y joint realized by a coupling1x2 to a pin InGaAs

photodiode.

The laser amplitude and the operation wavelength are heldconstant by a feed back control. The overall size of the fiber is less

than 130 m.


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Sensor system

In order to increase the signal to noise ratio, the use of modulatedsource is suitable.As a consequence, laser beam is externally amplitude modulated

and the signal from the photodiode is filtered by a lock-in amplifier.

70C

LASER

DIODE

Y JOINT

RESIN

PHOTO

DIODE

FUNCT.

GEN.LOCK-IN

AMPLIFIER

Ref.in

PD.curr.

I.out

DAQSYSTEM

HEAT PUMP PELTIER

CONTROL SYSTEM


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Sensor system

Peltier effectheat pump

Temperature controller

Fiber optic

Y joint

Laser diode

Photo diode

Laser controller

Signal analyzer Sensor output

)t(R)t(KP)t(I =


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0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 1800 3600Time, s

I, V

12

3

4

5

6

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 1800 3600 5400

Time, s

I, V

1

2

3

Isothermal Sensor Output

Sensor output is a voltage signal I(t)=KPR(t)I(t)=KPR(t),

where K accounts for system losses, lock-in amplifiergain and photo diode sensitivity, P is the powerinjected by the laser diode and R is the reflectioncoefficient.

7070 CC6060 CC


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The fiber diameter is 124 m.

Scanning Electron Microscope Analysis

The fiber cut induces irregularities of the reflectometer surfaces:as a result, the sensor output I0 may vary at the reaction starting

The K factor [I(t)=KPR(t)] accounts for the effect ofsuch irregularities on the sensor output


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Reflection coefficient based conversion

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.80.9

1.0

0 1800 3600 5400

Time, s

3

2

1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.80.9

1.0

0 1800 3600Time, s

7070 CC6060 CC


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Calorimetric Cure Kinetic

The tested resin: Bisphenol A epoxy based resin (Shell Epon828) and Triethylenetraammine (HY951 Ciba) curing agent.Tg=120C

Due to the exothermalcharacter of polymerizationreactions, the cure kinetic hasbeen characterized through

thermocalorimetric techniqueby the use of the DSC(Differential Scanning

Calorimetry).

Isothermal conversion isreported, based on theisothermal reaction heat H

Calorimetric Isothermal Conversion

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 3600 7200 10800Time, s

DSC

40 C506070

0

0DSC

HH

H)t(H

=


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Lorentz-Lorenz law

The Lorentz-Lorenz law relates the change in the refractive index with

the density and polarizability :

where N is the Avogadro number, M is the molecular weight of polymerrepeat unit and is the permittivity of free space.

=+

M3

N

2n

1n2

2

Some experimental data (De Boer, R. J. Visser, G. P. Melis, 1992), indicate

that polarizability is almost constant during polymerization, i.e.

( )= nn


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=

=0

0

0

0n

)t(

nn

n)t(n

A simplification of the Lorentz-Lorenz law results in a linearrelationship between the refractive index and the densitychanges during polymerization.

In the case under study, the latter approximation leads to anabsolute error of the order of 10-5.

Lorentz-Lorenz law


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Literature experimental data suggested that the relation

between the calorimetric conversion and the volumetricshrinkage of a reacting resin is linear. Thus, the volumereduction is proportional to the increase of crosslinks, andtherefore to the conversion:

( )DSC

0

0

0

0

HH

HtH)t(=

=


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Simplification ofFresnel law

Simplification of

Lorentz Lorenz law

Experimental

evidence

I, sensoroutput

R, reflection

coefficient

n, refractive

index , density

H,

calorimetricconversion

SensorDesign

Linearity Chain Rationale

Direct comparison between FiberOptic Sensor isothermal data andisothermal calorimetric conversion


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Calorimetric vs. Fiber Optic Sensor Conversion


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Conclusion

A Fiber Optic Sensor able to sense isothermal

cure kinetic has been developed

Interpretation of sensor output has been provided

Comparison with calorimetric kinetic data shows

the system capability in cure monitoring


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Future work

Non isothermal cure kinetics

Bragg gratings integration

cure monitoring by fiber optic sensor

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