Effects of mesostructure on the in-plane properties of tufted carbon fabric composites

Transcription

1 Effects of mesostructure on the in-plane properties of tufted carbon fabric composites CompTest 2011, 14 th February 2011 Johannes W G Treiber Denis D R Cartié Ivana K Partridge j.treiber@cranfield.ac.uk

2 Tufting process - Modified one-sided stitching process - Automated insertion of carbon, glass or aramid thread - For dry composite preforms Automated KSL KL150 tufting head 0.5% carbon tufted NCF Top Presser foot Hollow needle Dry fabric Bottom Tuft thread loop Support foam 2/14

3 Introduction Main purpose: Through-the-thickness reinforcement technique + 460%/+60% mode I/II delamination toughness Tuft bridging for only 0.5% areal tuft density (Cranfield) Delamination crack Drawback: Potential reduction of in-plane properties DCB of 0.5% carbon tufted NCF Stitching: - considerable database Stiffness: -15% to +10% Tensile strength: -25% to +25% Tufting: - to date only 3 experimental studies (KU Leuven, Cranfield) Tensile strengths: -14% to +10% Need for detailed testing database on wider range of tufted materials no agreement 3/14

4 Materials - Carbon Preforms: Uni-weave - [0 ] 7, [0 ] 10 balanced NCF - [(0 /90 ) s ] 2 - Tufted with 2k HTA carbon thread in at s x = s y = 5.6 mm (0.5%) /2.8 mm (2%) Square arrangement - Cross-over - Pattern shift Free loop height: mm - RTM injection of epoxy resin (ACG MVR 444) for dimensional control 4/14

5 In-plane tension behaviour Tensile tests (BS EN ISO 527-4:1997): Normalised elastic modulus (-) UD^ UD NCF Areal tuft density (%) Normalised strength (-) NCF threadless NCF UD UD^ Areal tuft density (%) Property changes depend on fabric and tuft morphology 5/14

6 Meso-structure Thread Tuft Loop In-plane disturbance (x-y): z y x UD 4 Triangular w,φ Resin pocket Tuft Fabric deviation UD: 6 NCF: % 2.0% w Thermal crack Square Square NCF 2φ 7 w,φ UD: 3 NCF: 4 6/14

7 Meso-structure Thread Tuft Loop Out-of-plane disturbance (x-z/y-z): z y x Local fibre volume fraction (%) V f,2d V f = f(w i,t loop, t thread ) Distance between tuft rows (mm) z Surface crimp x Thread layer t th Thread seam Loop layer t l - Surface seam causes local fabric crimp z - Resin rich layers and pockets affect global and local V f y 7/14

8 Numerical Unit Cell model φ = cosine fct. Loop V f = f(t l,t th,w i ) 67.5 wdev Vf (%) y w Tuft Smeared UD Resin channel Thread y z x y x w i ¼ UC 63.0 x Parametric 3D Unit Cell model (Marc): UD, NCF, square and triangular arrangement Isotropic, linear elastic material + Rule of mixtures (Chamis) Failure and degradation: Ply: Puck (FF + 3 modes of IFF) Resin: Maximum strength E G n nt E deg. 1 deg * 0 E 0 / 1.0 E G nt n 0 * n 0 Knops, Comp. Sci. Tech /14

9 Failure prediction NCF 0 Ply Stress exposure IFF σn> A Transverse Shear tension failure Cracks ¼ UC Longitudinal splitting Longitudinal splitting Tensile stress σ (MPa) Experiment FE Model 90 ply cracking A Tensile strain ε (%) 0.5% NCF - Accurate modulus and strength, also for 2% density (error < 2/4%) Longitudinal splitting - Fabric straightening leads to transverse tension failure in fabric and longitudinal splitting of resin pocket 9/14

10 Failure prediction NCF 0 Ply Stress exposure FF B ¼ UC Fibre Initiation failure of fibre initiation failure Rupture Tensile stress σ (MPa) 900 Experiment 750 FE Model Tensile strain ε (%) B 0.5% NCF Tuft 0 - Ultimate fabric fibre failure in close vicinity of tuft 10/14

11 V f distribution Normalised axial properties (-) % NCF NCF - Modulus NCF - Strength ΔV ΔVf=f(wi) f i ) ΔVf=f(Ar.p.) f rp ) ΔV ΔVf=0 f =0 V f distribution models - Local fabric fibre distribution affects both stiffness and strength prediction - Gradient V f model agrees best with true morphology and tension results 11/14

12 Fibre misalignment φ % square Normalised properties (-) Modulus Strength UD NCF 0 ¼ UC Max. Fibre deviation φ dev ( ) w min - Fabric fibre deviation critical on tensile strength, effect on modulus negligible - UD strength more sensitive to fabric deviation 12/14

13 Tuft arrangement 2500 UD Tensile strength S (-) % 80% Model Experiment Square Triangular Tensile strain e (%) 95% 68% - Upper and lower strength bounds defined by square and triangular pattern - Triangular pattern causes most critical strength reduction 13/14

14 Conclusions Tuft introduces structural complexity into Z-reinforced composite Critical meso-structural tuft defects: resin rich pockets, fibre deviation, matrix cracking and local fibre compaction Tufting has no effect on longitudinal tensile stiffness of UD and biaxial NCF composite, but surface loops increase matrix dominated transverse stiffness Reduction in longitudinal tensile strength most prominent for UD (<-19%) Fibre undulation most critical contributing factor, fibre breakage limited effect on tensile strength High quality experimental morphology data allows accurate tensile stiffness and strength prediction of tufted composites 14/14