Identification of interface properties using Fibre Bragg Grating sensors in a fibre pull-out test Gabriel Dunkel, Laurent Humbert and John Botsis Laboratory of Applied Mechanics and Reliability Analysis Swiss Federal Institute of Technology, Lausanne CompTest 06 April 10-12 2006 1
Outline Problem definition Distributed strain measurements with FBG s Fabrication and measurement processes Experimental results Numerical study Identification of interfacial properties & Conclusions 2
Damage micro-mechanisms Epoxy matrix reinforced-composites 1 1 Fibre breakage 6 2 Crack bridging 3 Crack-Fibre interaction 2 4 Matrix damage 3 5 Fibre fragmentation 5 4 6 Debonding Residual stresses present in the matrix (material consolidation) 3
Pull-out test configuration Simplified fibre composite model How to measure internal strains? Fibre Bragg Grating sensor Versatile, non intrusive Precise location in the composite Sensitive to non-uniform strain fields 4
FBG s working principle Single mode optical fibre: z L core FBG Λ grating length 15 mm (non-uniform) local Bragg wavelength: ( z) 2 n Λ( z) λ = n 0 : core refractive index B 0 5
FBG measurements OLCR-based technique: OLCR apparatus designed at EPFL, time domain measurements Principle: OLCR measurement ht () Fourier transform impulse response r ( ν ) spectral response layer-peeling qz ( ) derivative λ B ( z ) coupling coefficient P. Giaccari, HG. Limberger, RP. Salathé, Local coupling-coefficient characterization in fiber Bragg gratings, Optics Letters 28(8) (2003) 598-600 Opto-mechanical relation: ΔT=0 Homogeneous host material Axisymmetric load configuration Δ λ λ B B0 ( z ) ( z ) where ( 1 p ) ε ( z) = e λ z = λ z λ z p e : optical const ~ 0.22 z ( ) ( ) ( ) Δ B B B0 6
Fabrication of the specimen Reinforcement: Glass fibre with Acrylate coating (Ø = 250 μm) FBG of length 15 mm, 2/3 embedded Matrix: Epoxy resin poured in the mould Curing at 30 C during 24h Post-curing at 70 C during 10h FBG weights DOW resin: 70% DER 730 30% DER 732 13% DEH 26 Cylindrical specimen : length L = 38 mm, Ø ext = 25 mm Mechanical properties: Epoxy Acrylate Glass E [MPa] 2.35 0.7 72 ν 0.38 0.38 0.17 7
Experimental Loading steps Residual stress characterization - Initial OLCR measurement after post-curing Pull-out test for each Δ increment: - OLCR and force F measurements after stabilization - idem after unloading Bragg distributions before loading Δ F OLCR λ B ( z ) free fibre FBGs after post-curing 0 L z Residual stresses effect 8
Residual stresses characterization θ r FGB z Thermo-elastic approach to model the chemical matrix (m) shrinkage: 1 m m ij ( rz + ν, ) ij ( rz ν ε = σ, ) σkk ( rz, ) δij + S m ( rz, ) δij E E m m Shrinkage function F. Colpo, L. Humbert, P. Giaccari and J. Botsis, Characterization of residual strains in an epoxy block using an embedded FBG sensor and the OLCR technique, Composites: Part A 37 (2006) 652-661 9
Optical measurements (pull-out) Recorded Bragg wavelength distributions FBG 0 L z 10
Numerical study (Abaqus) interface Approach: Axisymmetric linear elements Radial mesh refinement at angular points Linear elastic and isotropic material properties Perfect bonding assumed between epoxy and acrylate Introduction of a shrinkage function S m = 6420 (µm/m) 11
Modeling of the glass/acrylate interface Normal hard contact normal pressure: p n Tangential regularized Coulomb friction law total slip: γ tot = γ S + γ A τ lim = μp n τ τ k = μ γ p n lim γ S γ lim A γ μp n friction coefficient μ elastic slip limit γ lim Values of μ, γ lim? 12
Numerical procedure Δ STEP 1 RESIDUAL STRESS INTRODUCTION Matrix shrinkage Initial debonding at the top and the bottom ends STEP 2 APPLIED PULL-OUT DISPLACEMENT Matrix bottom fixed Full debonding when Δ 2r f STEP 3 DISPLACEMENT RELEASED 13
Numerical /experimental results FBG 0 L z Interface Properties Identification friction coefficient: μ = 0.02 elastic slip limit: γ lim = 0.36 for the glass/acrylate interface. L 14
Conclusions FBGs are able to register non-uniform strain changes during the pull-out test. Residual strain accumulation taken into account. Glass fibre / acrylate coating interface properties identified using the Coulomb friction coefficient and an elastic slip parameter. Potential of the FBG to characterize interfaces demonstrated. 15
Acknowledgements The authors would like to acknowledge: The financial support of the Swiss National Science Foundation, Grant no. 103624 Thank you for your attention LMAF Web Site : http://lmaf.epfl.ch 16