Exploring Mechanical Property Balance in Tufted Carbon Fabric/Epoxy

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Partridge 1 Amir Rezai 3 1 - Composites Centre, Cranfield University, Cranfield, Bedford, MK43

1 Exploring Mechanical Property Balance in Tufted Carbon Fabric/Epoxy Giuseppe Dell Anno 1 Denis D. Cartié 1 Giuliano Allegri 2 Ivana K. Partridge 1 Amir Rezai Composites Centre, Cranfield University, Cranfield, Bedford, MK43 0AL, UK 2 - School of Engineering, Cranfield University, Cranfield, Bedford, MK43 0AL, UK 3 - BAE Systems, Advanced Technology Centre, Filton, Bristol BS34 7QW, UK

2 Slide n 1 Robotic Tufting Table of Contents Tufting Concept and Technology Mechanical testing Tensile test Compression after impact (CAI) test Bridging laws Single-tuft specimen testing Analytical modelling Conclusions

3 Slide n 2 Tufting: the concept Z-pinning Only for pre-pregs Z-pins Plane of composite Stitching Tufting Lock stitches Modified lock stitches High thread tension Two side access Chain stitches Needle Thread Laid-up fabric Loops

4 Robotic tufting of dry preforms Silastic 3481 Silicone rubber from Dow Corning SIL16 Silicone foam from Samco Slide n 3 3mm 3mm

5 Robotic tufting of dry preforms Woven 5 harness satin carbon fibre fabric, 0 /90 lay-up RTM grade epoxy resin Resin Transfer Moulding: ΔP=2bar, 180 C for 185 minutes Glass fibre thread Carbon fibre thread Material EC 9 68x3 S260 T8G Saint Gobain Vetrotex CF Sewing yarn Toho Tenax Specific weight 204g/km 137g/km Filament type Continuous Filament count Filament Φ 9μm <10μm Yarns per thread 3 2 Max strength (unimpregnated) 93N 139.5N Slide n 4

6 Robotic tufting of dry preforms Mesostructure Glass fibre thread Tuft diameter in situ =710μm Slide n 5 Carbon fibre thread Tuft diameter in situ =570μm

7 Slide n 6 Tensile behaviour BS EN ISO 527-4:1997 Glass fibre thread Limess system stereo cameras Checking with a strain gauge VicSnap Cameras Specimen Vic3D Load/Displacement curve Fracture path

Slide n 7 Tensile behaviour Tensile Test Young's modulus Tensile Test Max stress 57 600 GPa 56 55 54 53 52 51 55.0 54.

8 Slide n 7 Tensile behaviour Tensile Test Young's modulus Tensile Test Max stress GPa MPa Control GF tufted Control GF tufted Fracture surface Cross section of a tufted specimen 3mm

9 Tensile behaviour Failure analysis Tufts act as crack initiators Fibre breakage? Resin pockets? Resin impregnation defects? Broken fibres Voids Resin pockets Slide n 8

10 Slide n 9 Compression after Impact Impact at 15J 0 /90 lay-up x152.4mm - Specimen fixture according to Boeing BSS Force (N) Glass fibre tufted Carbon fibre tufted Control Time (s)

11 Slide n 10 Compression after Impact Impact at 15J 0 /90 lay-up x152.4mm - Specimen fixture according to Boeing BSS 7260 Maximum Load Load (N) Control Glass fibre tufted Carbon fibre tufted 0 Control GF tufted CF tufted

12 Slide n 11 Compression after Impact Samples C-scanned before and after impact Images processed with PaintShop Pro (Apparent) total damaged area unchanged by tufting 50mm 50mm Before Impact After Impact Processed Image

13 Slide n 12 Compression after Impact Linear extent of the damage reduced Maximum distance of the damage border from the centre of impact lowered in tufted samples Reduction in tufted samples: Glass fibre 11% Carbon fibre 12% Control After Impact

14 Compression after Impact Number of delamination planes reduced by tufting Cross section of impacted control specimen Delamination sites Slide n 13 Laminate Impactor Φ=20mm

15 Compression after Impact Number of delamination planes reduced by tufting Cross section of impacted tufted specimen Delamination sites Slide n 14 Laminate Impactor Φ=20mm

16 Slide n 15 Compression after Impact Compression response Specimen fixture according to Boeing BSS 7260 Maximum 0.5mm/min 250 Stress at failure (kn) Control Increase 25% Glass fibre tufted Increase 27% Carbon fibre tufted Control GF tufted CF tufted

17 Slide n 16 Single-tuft specimens Aim Determine the mechanical response of a single tuft inserted in a composite laminate Release film Tuft Laminate Mode I Mode II

18 Slide n 17 Single-tuft specimens Mode I rate 1mm/min Mode II rate 0.25mm/min Load (N) Glass fibre tuft Carbon fibre tuft Shear Force (N) Glass fibre tuft Carbon fibre tuft Displacement (mm) Sliding displacement (mm) No evidence of pull-out The tuft fails on the delamination plane SEM of Carbon fibre tuft after failure under mode II

Modelling Analytical model* for through-thickness reinforcement P = χi w S = χii u Bridging stiffnesses χ I and χ II χ = I 2kEAL 2EA 3 χ kl z x 2 II + kzl G( β L) = k EI 4 x β = ( β ) ( ) ( ) 2βL cos

19 Modelling Analytical model* for through-thickness reinforcement P = χi w S = χii u Bridging stiffnesses χ I and χ II χ = I 2kEAL 2EA 3 χ kl z x 2 II + kzl G( β L) = k EI 4 x β = ( β ) ( ) ( ) 2βL cos βl cosh βl + 2 sinh βl sin βl + sin βlcosh βl cos βlsinh βl G L = β L cos βlcosh βl 1 Foundation stiffnesses k z and k x have to be determined experimentally Slide n 18 * G. Allegri, X. Zhang Delamination Bridging Laws for Composite Laminates Reinforced by Through-Thickness Pins submitted for publication to Mechanics of Materials

20 Slide n 19 Mode I Mode II Modelling Z force (N) Experimental Model Total opening displacement (mm) Prediction: Glass fibre tuft X force (N) Experimental Model Total sliding displacement (mm) Prediction: Glass fibre tuft Experimental Model Experimental Model Z force (N) % agreement Carbon fibre tuft Total opening displacement (mm) X force (N) % agreement Carbon fibre tuft Total sliding displacement (mm)

21 Slide n 20 Robotic Tufting Conclusions on effects of tufting 10% drop in failure load in tension In-plane Young s modulus not degraded significantly Tufts act as crack initiators CAI increased by ~26% No tuft pull-out observed under mode I or mode II Analytical model for the bridging law by single tuft established

22 Slide n 21 Thank you for your attention! Further information from g.dellanno@cranfield.ac.uk Research funded by Cranfield University IMRC with industrial contributions

Delamination behaviour of Z - pinned laminates

Delamination behaviour of Z - pinned laminates Denis D.R. Cartié and Ivana K. Partridge Advanced Materials Department, School of Industrial and Manufacturing Science Cranfield University, Cranfield, Bedford,