Application of FBG sensors to monitoring

of CFRP influenced by physical aging

Shin-ichi Takeda a, Jun Koyanagi b, Shin Utsunomiya a,Yoshihiko Arao c, Hiroyuki Kawada c

aAerospace Research and Development Directorate, Japan Aerospace Exploration Agency

bInstitute of Space and Aeronautical Science, Japan Aerospace Exploration Agency

cDepartment of Mechanical Engineering, Waseda University

COMPTESTCOMPTEST 2011 2011

14 – 16 February 2011 at EPFL, Lausanne

Research Background

JAXA will plans to launch some scientific satellite.

Large-scale Mirror (< 5m ) is needed for high resolution observation.

Requirements

Lightweight

Long-term stability

Profile irregularity

(1/10 RMS of laser wavelength )

Surface roughness

(1/100 RMS of laser wavelength )

CFRP (Carbon Fiber Reinforced Plastic) is promising structural

material.

High specific strength

High heat conductivity

Long-term stability of CFRP is affected by some factors.

Water absorption – on ground

Physical aging – on ground, in space

Thermal residual stress relaxation – on ground, in space

Physical aging is known to cause resin shrinkage in CFRP.

Thermal residual strain in CFRP changes in long-term.

Objectives

Strain changes due to physical aging were measured by FBG

sensors experimentally.

Steady state of molecular

Molecular chain

T0 Tg

Non equilibrium State

Equilibrium State

Aging Process

Temperature

Fre

e V

olum

e

Physical aging (PA)

Process of state of molecular chain reach from non-equilibrium

to equilibrium with time.

Cause decrease in free volume and increase in density.

FBG sensors

Small size – 150m including polyimide coating for this study

High sensitivity to strain measurement - 1.2 pm/, 13 pm/˚C

Tension

Compression

TensionCompression

Evaluation of an axial strain by wavelength shift of reflected light.

Experimental Procedures - specimen

100 mm

50 mm

Carbon fiber direction

Optical fiber

100 mm50 mm

FBG

Optical fiber

Thermocouples

CF direction

Plies Embedding locationsThicknesses

(mm)

0o

4 [02/OF/02] 0.501

8 [04/OF/04] 1.010

12 [06/OF/06] 1.580

45o

4 [452/OF/452] 0.507

8 [454/OF/454] 1.018

12 [456/OF/456] 1.593

90o

4 [902/OF/902] 0.509

8 [904/OF/904] 1.021

12 [906/OF/906] 1.598

Materials

Epoxy-based carbon fiber UD prepregs

IMS60/ #133, Toho Tenax Co. Ltd.

FBGs

15mm grating period, polyimide coating, Fujikura Ltd.

Manufacturing: Autoclave

180oC, 2.5hours

90o

0o

Heat flow showed degree of cure in present CFRP is over 95 %.

The present autoclave process was good standard.

Confirmation of degree of cure using DSC

-4

-3

-2

-1

0

50 100 150 200 250 300 350

Hea

t Flo

w (

mW

)

Temperature (deg. C)

-4

-3

-2

-1

0

50 100 150 200 250 300 350

Hea

t Flo

w (

mW

)

Temperature (deg. C)

Exothermal reaction91.5 J/g

Glass transition

Prepreg Cured CFRP

Experimental Procedures – measurement

100oC in vacuum hot oven

Wavelength shift of reflected light: every 1 hour

Temperature: every 5 minutes

PC③

④

②

①

100 ° C

10 hours 10 mins

Hot OvenOptical Switch

Spectrum Analyzer ASE Light

Circulator

Incident light flowReflected light flow

①

②③

④

After 120 minutes, temperature was almost uniform, 98oC to 100oC.

Compressive residual strain after 120 minutes reference value of 0

.

Temperature changes during strain measurement

98

100

102

104

106

108

110

0 8000 16000 24000 32000

Tem

pera

ture

(de

g. C

)

Time (min)

98

100

102

104

106

108

110

0 750 1500 2250 3000

Tem

pera

ture

(de

g. C

)

Time (min)

120 min

0

0.001

0.002

1549 1550 1551 1552 1553

300min3000 mins12000 mins30000 mins

Opt

ical

pow

er (

W)

Wavelength (nm)

0

0.001

0.002

0.003

1549 1550 1551 1552 1553

300min3000 mins12000 mins30000 mins

Opt

ical

pow

er (

W)

Wavelength (nm)

Reflection spectrum changes

While the spectra keeping its shape, the spectra shift to lower wavelength.

Compressive residual strain increased gradually for 90o specimen.

90o

4 Plies90o

8 Plies

0

0.01

0.02

1553.5 1554 1554.5 1555

300min3000 mins12000 mins30000 mins

Opt

ical

pow

er (

nW)

Wavelength (nm)

0

0.01

0.02

1553.5 1554 1554.5 1555

300min3000 mins12000 mins30000 mins

Opt

ical

pow

er (

nW)

Wavelength (nm)

0o

4 Plies0o

8 Plies

Birefringence

Uniform strain

Results of strain changes – UD laminates

-300

-250

-200

-150

-100

-50

0

0 5000 10000 15000 20000 25000 30000

4ply8ply12ply

Str

ain

()

Time (min)

-150

-100

-50

0

50

100

150

0 5000 10000 15000 20000 25000 30000

4ply8ply12ply

Str

ain

()

Time (min)

-200

-150

-100

-50

0

50

100

0 5000 10000 15000 20000 25000 30000

4ply8ply12ply

Str

ain

()

Time (min)

Strain changes were almost same with changes in laminates thickness.

It is important to consider the strain changes due to PA.

Residual strain changes (µε)

0o 45o 90o

4 plies -8.3 -83.5 -242.1

8 plies -8.4 -75.1 -233.8

12 plies -8.3 -91.8 -225.4

90o 45o

0o

Creep test of CFRP

Specimen

Materials

Epoxy-based carbon fiber UD prepregs

(same as previous tests)

Dimensions

210 mm x 25.4 mm x t

Stacking sequences

[9016], t=2.20 mm

[02/908]s, t=2.75 mm

Test conditions

100oC in hot oven

No load (= 0 MPa), 5.8kg (= 10 MPa)

Strain and Temp. measurements

Creep test results of 90o CFRP specimen

-500

0

500

1000

1500

2000

0 5000 10000 15000 20000 25000

Extensometer- 0MPaGage1- 0MPaGage2- 0MPaGage- 10MPaEstimated creep strain

Str

ain

()

Time (min)

-345.6 avg.

Creep strain was estimated on the assumption that

shrinkage due to PA is same as that on 10MPa load.

6000

6200

6400

6600

6800

7000

0 5000 10000 15000 20000 25000

Cre

ep m

odul

us (

MP

a)Time (min)

PAc '

'c

PA

)(tET

Creep test results of cross-ply CFRP specimen

Strain changes were calculated by CLT.

-50

0

50

100

0 5000 10000 15000 20000 25000

Strain-CLTStrain-CLT-PAStrain1Strain2

Str

ain

()

Time (min)

0o modulus, EL 152000 MPa90o modulus, ET(0) 6790.87 MPaLaminate thickness 2.7 mm0o CTE, αL -0.55 E-6 1/K 90o CTE, αT 22.5 E-6 1/K Tem. Change, T -80 KPoisson’ ratio νLT 0.334 Poisson’ ratio νTL 0.018689

Shear modulus, GTZ 2516MPa

k

k

z

z

PAxy

PAy

PAx

xy

y

xn

kxy

y

x

dzT

QQQ

QQQ

QQQ

N

N

N

1

,

,

,

1662616

262212

161211

xy

y

x

xy

y

x

AAA

AAA

AAA

N

N

N

662616

262212

161211

Conclusions

Strain changes due to physical aging were measured by FBGs.

The strain change in 90o CFRP was largest because of resin shrinkage.

The strain changes were almost same with changes in laminates thickness.

Creep test results illustrated that it is important to consider the strain

changes due to PA.

Thank you for your kind attention!

JAXA, Chofu Aerospace CenterAerodrome Branch

Jeju Island, 18th ICCM