Lightweighting Opportunities Offered By Discontinuous (Short) Carbon Fibre Composites

Transcription

1 Lightweighting Opportunities Offered By Discontinuous (Short) Carbon Fibre Composites Nick Warrior Composites Research Group Faculty of Engineering University of Nottingham 1

2 Introduction Discontinuous (short) fibre composites offer significant manufacturing advantages over continuous fibre architectures. Mechanical properties and manufacturing technologies differ greatly from more conventional composites and are not widely understood. Presentation covers aspects of our research into short fibre composites, conducted over the last decade at the University of Nottingham. 2

3 3 Processing Routes 1. Preform manufacture for liquid moulding using DCFP Directed Carbon Fibre Preforming (derived from P4, F3P). 2. Compression moulding of DFC Directed Fibre Compounding charge (including hybrid architectures). 3. Fibre architectures from hydrodynamic alignment of recycled fibres. 3

4 DCFP Directed Carbon Fibre Preforming Fibres chopped and sprayed dry onto vacuum table with binder (derived from Owens Corning process, USCAR P4, and Ford F3P). 4

5 Discontinuous Fibres: Effect of volume fraction Stiffness (Young s modulus) and strength dependent on volume fraction Rule of Mixtures gives good approximation for stiffness 5

6 Discontinuous Fibres: Effect of volume fraction 36mm fibre length / 12K T700 50C 95% stiffness retention & 59% strength retention compared to QI NCF V f =45% void level (1.3% ±31%) V f =57% void level (8.6% ±10.6%)

7 Discontinuous Fibres: Effect of specimen thickness 7

8 Discontinuous Fibres: Effect of fibre tow size 8

9 Discontinuous Fibres: Effect of fibre length

10 Discontinuous Composite Properties Performance of meso-scale discontinuous fibre architectures is largely governed by homogeneity Much optimisation can therefore be done without resorting to FEA 24K 30mm tows, 10%vf, 3mm thick specimen 12K 6K 3K

11 Discontinuous Composite Properties Property variation quantified in terms of mass variability and stiffness variation Mass variability Stiffness Variability 90% 13 samples tested Mean = 40GPa, St.dev = 4.6GPa

12 Discontinuous Composite Properties Strength Variability Unstructured B-basis 196MPa Mean = 225MPa Structured (Weibull) B-basis 189MPa Structured (normal) B-basis 176MPa 90% Confidence 13 samples tested Mean = 225MPa, St.dev = 22.9MPa

13 Preform Compaction Filamentised Large Bundle UD NCF (2 layers) 10 Bar 24% 49% 66% Vacuum Pressure 19% 46% 61%

14 Design Considerations Damage tolerance of discontinuous good compared to conventional laminates DCFP vs. NCF (equivalent Vf) Front Rear Tensile Strength! OHT Strength! Compressive Strength! CAI Strength! ILSS! DCFP! 295! 293! 288! 142! 32.1! NCF! 604! 494! 361! 195! 37.4!

15 Fibre Alignment Mechanical alignment mechanism Fibre deposition rate maintained 20kg/hr per 24K tow (lab scale) Process is easily scalable Greatly increased mechanical properties Smaller tows offer greater alignment levels

16 Fibre Alignment Smaller tows offer greater alignment levels at 30mm fibre length Longer fibre lengths offer greater fibre alignment Homogeneity influences final results

17 Micro scale - Bundle Stress Plot Stress Plot Stress Plot Unit Cell Volume Fraction Study - To establish bundle properties for various volume fractions. - Any volume fraction can be modelled to establish its material properties for the bundle. Transverse Loading Longitudinal Loading Shear Loading - Constituent properties can be changed here as new developments are made - The unit cell is loaded in 6 directions Damage Plot Damage Plot Damage Plot

18 Meso-scale: FE model - 1D beams As-deposited state Meshed RVE Model Compressed state

19 Meso-scale: FE model 2D

20 DCFP Process Modelling Integrated optimisation environment Import of tooling geometry Import of robot programs Robot kinematics Fibre flight simulation Homogeneity analysis L f = 10mm 33% COV

21 DCFP Process Modelling Mesh generated from CAD in FE pre-processor Process model displays fibres or coverage level With gravity Robot path vector With external airflow Tool surface

22 DCFP Variability Macroscale Geometrical model Injection Flow front patterns homogeneous filament distribution bundle clustering macroscopic permeability for example l = 28 mm, c f = 24K, V f = 40 % homogeneous K / m ± (± 12 %) clustering ± (± 22 %) balanced filling complete filling? t fill = 494 s injection gate t fill = 1397 s 22

23 Multi-fibre Architectures Stiffened panels using a combination of DCFP, NCF, braid and 3DW Demonstrator panels moulded by VAP process DCFP / DCFP NCF / 3DW DCFP / Braid

24 SMC equivalent DFC Directed Fibre Compounding Bespoke lab-scale SMC line Fibre delivery system (lab-scale) - Deposits 1-4 tows - On-the-fly variable fibre length (6-90mm) - Controllable Vf (30-55%) - Short cycle times - (lab-scale: 2mins for mm) Liquid Resin System - Heated tank and lines to maintain low viscosity - Atomised spray cone Formulated 4-part epoxy - Chemical B-staging required to develop suitable viscosity - B-stage time: 21 o C - Experiments have been demoulded after o C - Capable of demoulding after o C 24

25 Comparison to CF-SMC Isotropic properties for 100% charge coverage - E=36GPa 52%vf * DFC net-shaped charge, 25mm fibre length, 85bar moulding pressure ** DFC 50% mould coverage, 50mm fibre length, 20 bar moulding pressure Flow induced alignment for smaller charges (50%) - E 1 =46GPa σ 1 = 52%vf Near-isotropic DFC properties comparable to commercial carbon fibre moulding compounds

26 1D Flow Initial Mould Filled charge Closing Mould Initial investigation - Degree of achievable flow - No fibre matrix separation - Fibre volume fraction varied within ± 5%

27 Flow induced alignment Increase in fibre waviness 40% 100% Fibre alignment increases as charge size decreases Longitudinal strength shows little change between 40% and 80% mould coverage - Change in failure mode due increased fibre waviness Out-of-plane fibre waviness increases towards plaque edges - increases as charge size decreases - Highest achieved longitudinal strength and stiffness is 480MPa and 48GPa, respectively (fibre length 75mm, mould pressure 20bar)

28 DFC Out-of-Plane Fibre Waviness Left edge (Inside charge region) Right edge (cavity region) 40% 60% 80% 100% Left Right Microscopy specimens 25mm x 10mm 25mm Insignificant level of out-of-plane fibre waviness is observed at the centre of each plaque for all charge sizes Out-of-plane waviness increases towards plaque edges (edge effect) - Highly swirled bundles are observed for non netshaped charges - Waviness is more significant at right edge than left edge, since flow velocity is higher in cavity region than charge region

29 DFC Effect of Moulding Pressure (100% Mould Coverage) Increase in strength and modulus with increase in mould pressure. Void removal by collapsing air bubbles - Increasing pressure reduces void size and void fraction Fibre Volume Fraction 50% Mould coverage 100% Fibre length 25mm

30 Void and Bundle Waviness removal Air bubble collapse Air bubble collapse + resin flow Resin Flow Mould Coverage 100% 50% 50% Pressure 85bar 85 bar 20 bar Compound flow further removes trapped air Flow dominates the air removal Longer fibre lengths are susceptible to increased out-of-plane waviness Fibre Volume Fraction 50% Mould coverage 50% Fibre Length 50mm Tow Size 12K

31 Effect of Fibre Length 100% mould coverage Considered to be isotropic Fibre length appears to be insignificant 50% mould coverage Flow induced alignment decreases with increasing fibre length Reduction in longitudinal and increase in transverse properties as fibre length increases Level of alignment can be controlled by percentage charge coverage and fibre length Fibre Volume Fraction 50% Tow Size 12K Top: Fibre length 15mm Bottom: Fibre length 25mm Press Pressure 85 bar Mould coverage 100% Mould coverage 100%

32 Hybridising UD and SMC 0 UD + SMC Tensile Strength Tensile Modulus Tensile Strength (MPa) % 20% 40% 60% 80% 100% Percentage of UD (%) Longitudinal Transverse Tensile Modulus (GPa) % 20% 40% 60% 80% 100% Percentage of UD (%) Longitudinal Transverse

33 Continuous/Discontinuous Joint design Plane of weakness at interface between SMC/UD laminate design important Shear mechanisms responsible for stress transfer between continuous and discontinuous fibre architectures Evaluate different joint designs

34 Bending of the discontinuous/ continuous joint Increasing the step size, increases the bending strength about the joint Enclosing the steps by a tapered layer of fabric can be used to increase the bending and tensile strength by 11% Positive bending: Fabric on the upper surface (compression) Negative bending: Fabric on the lower surface (tension)

35 Hybrid Interlaminar fracture toughness DFC UD UD UD UD DFC UD UD

36 Effect of resin staging 0% 25% 50% 75% D!50%

37 Effect of charge design U3!0!H2!s50% U2!90 U1!0 H2!s50% Images of U3 90 H2 s50%, the top and bottom of the left side of the plaque

38 Stiffness prediction 1) Binary Image 2) Skeletonisation to erode lines to determine intersections 3) Element rotations 4) Element elongation and translation

39 Stiffness prediction Experimental flexural modulus plotted against theoretical of U3 90 H2 s25% 25% staged Absolute element rotation Element elongation

40 2D flow study UD3/25%/SMC2/60%/s50% and UD2/UD1/25%/SMC2/60%/s50%. (Red 25% UD charge) (Blue 60% SMC charge)

41 3D Flow Tool BigHead Fastener Hybrid Fibre Architecture Ribs Thickness range (1.8mm 20mm) 405mm x 405mm Sharp Corners Deformation dependent on flow with contacting SMC material

42 3D Fibre structure Flow direction perpendicular to image plane Flow along the ribs creates Bridged swirling fibres 100% mould coverage pushes fibre bundles into the ribs. DFC advantages: Locally vary fibre length to reduce bridging during out-ofplane flow Areal density distribution to fully optimise flow paths Controllable local fibre volume fractions to achieve sufficient flow in complex/thin regions 100% mould coverage Charge of uniform thickness Fastener deformation under high pressures

43 3D demonstrator No staging 50% staging

44 SMC Flow Modelling Laboratory validation Weld line formation Compression moulding hybrid architectures Charge separation Automotive demonstrator

45 RCF Recycled Carbon Fibre Carbon fibre recycling facilities at Uni. of Nottingham Laboratory scale fluidised bed 1997 Commercial scale fluidised bed 2016

46 RCF Case Study Case study: recycling of 2013 America s cup yacht Oracle Team USA around 3175kg

47 RCF Random non-wovens Existing commercial wet lay processes can produce random non-woven mat from recovered carbon fibre. Courtesy Technical Fibre Products Ltd

48 RCF alignment studies High volume fractions achieved through fibre alignment continuous process under development

49 RCF alignment studies Standard modulus recovered carbon fibre Moulded at 7 bar (autoclave) MTM57 resin 3mm, 46% vf 12mm, 43% vf

50 Conclusions Discontinuous fibre architectures can provide a low-cost, high manufacturing rate alternative to continuous fibre in many applications The properties can be tailored by locally varying fibre length, areal distribution, degree of flow and fibre volume fraction Joint design between continuous and discontinuous architecture is extremely important and some defect reduction mechanisms have been considered for hybrid architecture and two or three dimensional flow

51 Acknowledgements Thanks to Academic team at University of Nottingham Dr Tom Turner, Dr Lee Harper, Dr Connie Qian, Dr Oliver McGregor, Anthony Evans, David Corbridge, Dr Raj Luchoo, Dr Michael Bond, Dr Critesh Patel, Dr Giridharan Kirupanantham, Dr Shuai Chen Funding EPSRC and Innovate UK Industrial partners Ford, Aston Martin Lagonda, Bentley Motors, Boeing, Technical Fibre Products, Hexcel