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

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

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

4. 4/14 Tufting process Exp. database Meso-structure FE-Models 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%) - Cross-over - Pattern shift Free loop height: 3.5 – 5 mm - RTM injection of epoxy resin (ACG MVR 444) for dimensional control Square arrangement

5. 5/14 Tufting process Exp. database Meso-structure FE-Models In-plane tension behaviour Tensile tests (BS EN ISO 527-4:1997): Property changes depend on fabric and tuft morphology

6. 6/14 Meso-structure In-plane disturbance (x-y): Tufting process Exp. database Meso-structure FE-Models Resin pocket Tuft Thermal crack Fabric deviation 0.5% UD NCF Square 2.0% Triangular Square 2φ2φ w w, φ UD: 6° NCF: 10° 4° 7° UD: 3° NCF: 4°

7. 7/14 Meso-structure Tufting process Exp. database Meso-structure FE-Models - Surface seam causes local fabric crimp - Resin rich layers and pockets affect global and local V f Out-of-plane disturbance (x-z/y-z): Surface crimp Thread seam Loop layer t l y z x z Thread layer t th V f = f(w i,t loop, t thread )

8. 8/14 Tufting process Exp. database Meso-structure FE-Models Numerical Unit Cell model Parametric 3D Unit Cell model (Marc): UD, NCF, square and triangular arrangement wiwi V f = f(t l,t th,w i ) ¼ UC Isotropic, linear elastic material + ‘Rule of mixtures’ (Chamis) Failure and degradation: Knops, Comp. Sci. Tech. 2006 φ = cosine fct. Ply:Puck (FF + 3 modes of IFF) Resin: Maximum strength Tuft Resin channel ‘Smeared’ UD Loop Thread x y x y

9. 9/14 0° Ply A Tufting process Exp. database Meso-structure FE-Models A - Accurate modulus and strength, also for 2% density (error < 2/4%) - Fabric straightening leads to transverse tension failure in fabric and longitudinal splitting of resin pocket Failure prediction – NCF Transverse tension failure Longitudinal splitting Cracks Longitudinal splitting 0.5% NCF || ¼ UC

10. 10/14 Tufting process Exp. database Meso-structure FE-Models - Ultimate fabric fibre failure in close vicinity of tuft B Failure prediction – NCF 0° Fibre failure initiation Rupture B 0° Ply 0.5% NCF || Tuft ¼ UC

11. 11/14 Tufting process Exp. database Meso-structure FE-Models V f distribution 0.5% NCF || - Local fabric fibre distribution affects both stiffness and strength prediction - Gradient V f model agrees best with true morphology and tension results

12. 12/14 Tufting process Exp. database Meso-structure FE-Models Fibre misalignment φ 0.5% square - Fabric fibre deviation critical on tensile strength, effect on modulus negligible - UD strength more sensitive to fabric deviation w min

13. 13/14 Tufting process Exp. database Meso-structure FE-Models Tuft arrangement UD || - Upper and lower strength bounds defined by square and triangular pattern - Triangular pattern causes most critical strength reduction

14. 14/14 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 Conclusions