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 a Aerospace Research and Development Directorate, Japan Aerospace Exploration Agency b Institute of Space and Aeronautical Science, Japan Aerospace Exploration Agency c Department of Mechanical Engineering, Waseda University COMPTEST – 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 T0T0 TgTg Non equilibrium State Equilibrium State Aging Process Temperature Free Volume 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 – 150  m including polyimide coating for this study  High sensitivity to strain measurement pm/ , 13 pm/˚C Tension Compression TensionCompression  Evaluation of an axial strain by wavelength shift of reflected light.

Experimental Procedures - specimen Optical fiber Thermocouples CF direction PliesEmbedding locations Thicknesses (mm) 0o0o 4[0 2 /OF/0 2 ] [0 4 /OF/0 4 ] [0 6 /OF/0 6 ] o 4[45 2 /OF/45 2 ] [45 4 /OF/45 4 ] [45 6 /OF/45 6 ] o 4[90 2 /OF/90 2 ] [90 4 /OF/90 4 ] [90 6 /OF/90 6 ]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  180 o C, 2.5hours 90 o 0o0o

 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 PrepregCured CFRP

Experimental Procedures – measurement  100 o C in vacuum hot oven  Wavelength shift of reflected light: every 1 hour  Temperature: every 5 minutes PC ③ ④ ② ① ① ② ③ ④

 After 120 minutes, temperature was almost uniform, 98 o C to 100 o C.  Compressive residual strain after 120 minutes reference value of 0 . Temperature changes during strain measurement

Reflection spectrum changes  While the spectra keeping its shape, the spectra shift to lower wavelength.  Compressive residual strain increased gradually for 90 o specimen. 90 o 4 Plies 90 o 8 Plies 0 o 4 Plies 0 o 8 Plies Birefringence Uniform strain

Results of strain changes – UD laminates  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) 0o0o 45 o 90 o 4 plies plies plies o 45 o 0o0o

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 [90 16 ], t=2.20 mm [0 2 /90 8 ] s, t=2.75 mm Test conditions  100 o C in hot oven  No load (= 0 MPa), 5.8kg (= 10 MPa)  Strain and Temp. measurements

Creep test results of 90 o CFRP specimen  avg.  Creep strain was estimated on the assumption that shrinkage due to PA is same as that on 10MPa load.

Creep test results of cross-ply CFRP specimen Strain changes were calculated by CLT. 0 o modulus, E L MPa 90 o modulus, E T (0) MPa Laminate thickness2.7mm 0 o CTE, α L E-61/K 90 o CTE, α T 22.5 E-61/K Tem. Change,  T -80K Poisson’ ratio ν LT Poisson’ ratio ν TL Shear modulus, G TZ 2516 MPa

Conclusions Strain changes due to physical aging were measured by FBGs. The strain change in 90 o 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 Center Aerodrome Branch Jeju Island, 18 th ICCM