The Challenges of Natural Fibres. as Engineering Composite. Reinforcements

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1 The Challenges of Natural Fibres as Engineering Composite Reinforcements Jim Thomason, Fiona Gentles, Jamie Carruthers 19 th Annual BEPS Meeting September 28-3 th 211 Vienna, Austria

2 Introduction Natural Fibre Composites Composite fibre content Thermo-mechanical anisotropy of natural fibres Natural fibre non-circular cross section Conclusions

3 Natural Fibre Composites - Challenges Low cost = technical fibre Fibre natural variability Fibre anisotropy Fibre non-circular Composite fibre content measurement Moisture sensitivity Fibre-matrix interaction

4 Some typical fibre properties are shown in the Table below Why Natural Fibre Composites? Sisal Jute Flax Glass Modulus (GPa) Strength (GPa) >1.5 Density Specific Modulus So some natural fibre may have the potential to replace glass fibres??? E C = L V f E f V m E m

5 Modulus (GPa) Comparison Predicted Composite Modulus For injection moulded long fibre polypropylene Glass Fibre NF 2 GPa(Sisal) NF 4 GPa (Jute) NF 6 GPa (Flax) 4 2 Remember comparison on weight content (i.e. specific fibre properties) means NO weight saving advantage! Fibre Content (% weight)

6 Modulus (GPa) Actual Modulus Injection Moulded Jute-PP ASTM GFPP ASTM Bar Plaque Flow Plaque Cross Flow Glass 4 Jute Fibre Content (% weight)

7 Thermoelastic Anisotropy of Flax and Sisal Fibres Goal Quantify anisotropy of Flax & Sisal fibres Full thermoelastic characterisation Measure UD fibre-epoxy laminates E(q,T), G 12,n 12, n 21,a(q,T) Epoxy matrix E m (T),n m, a m (T) Laminate fibre volume fraction? Flax & Sisal fibre E 1f (fibre cross section?) Calculate E 1f (T), E 2f (T), G 12f (T), n 12f (T), a 1f (T), a 2f (T)

8 Water Absorption (%wt) Water Absorption for Fibre Content 16 Sisal Fibres, Dm m =DM m /M m 12 8 Sisal-Epoxy Composites Dm c =DM c /M c 4 Epoxy Matrix, Dm m =DM m /M m Sqrt Days

9 NF Composite Fibre Volume Fraction W f Δm Δm c f Δm Δm m m V f 1 ρ ρ f m (1 W f ) Wf 1 Sisal fibre density r f = 14 kg/m 3 Flax fibre density r f = 14 kg/m 3 Epoxy matrix density r m = 11 kg/m 3 Sisal composite W f =.46, V f =.4 Flax composite W f =.36, V f =.31

10 Log Storage Modulus (Pa) Composite DMA Results Sisal Sisal 1 Sisal 2 Sisal 3 Sisal 5 Sisal 8 Resin Temperature ( C)

11 Modulus (GPa) Anisotropy of Fibre Modulus Flax E1 Sisal E1 Flax G12 Flax E2 Sisal E2 Sisal G Temperature ( C)

12 Thermal Strain (mm/m) Composite Thermal Strain Epoxy Flax 9 Flax 65 Flax 45 Flax 25 Flax Temperature ( C)

13 CLTE mm/m C Fibre Expansion Coefficients Fibre Transverse Sisal Transverse Flax Transverse Sisal Axial Flax Axial Fibre Axial Temperature ( C)

14 Summary Thermo-Mechanical Properties NF Glass Flax Sisal Longitudinal Modulus (GPa) Transverse Modulus (GPa) Shear Modulus (GPa) Axial LCTE (mm/m. o C) Transverse LCTE (mm/m. o C)

15 Single Fibre Testing 1 mm Fibre Stress = Load/Area = P/A f (= 4P/pD f 2???)

using a transverse image of fibre and assumption of

16 Single Fibre Cross Section Area A f in single fibre testing is almost universally evaluated from D f using a transverse image of fibre and assumption of circular cross-section Is this acceptable for Natural Fibres??

17 mounted on test card windows Single Fibre Measurements Series 1 Series 2 Single flax and sisal fibres Fibre diameter determined by averaging 4 transverse measurements Fibre tensile testing (1,15,2,25,3 mm gauge) Residual fibre ends glued to card tab sectioned in 2 places and true cross sectional area determined 1. Single fibre diameter determined by averaging 4 transverse measurements 2. Fibres embedded, cut and polished 3. true cross sectional area determined 4. Sample ground down 2mm and polished 5. Steps 3-4 repeated 1x

18 Single Fibre Cross Section Area

19 Fibre Cross Section (mm 2 ).3 Single Flax Fibre CSA Variability F1 F2 F3 F4 F5 F6 F7 F8 F9 F1 F11 F12 Average Measurement Position Along Fibre (mm)

20 Fibre Cross Section (mm 2 ).5 Single Sisal Fibre CSA Variability S1 S2 S3 S4 S5 S6 S7 S8 S9 S1 S11 S12 Average Measurement Position Along Fibre (mm)

21 Variability in CSA Determination Average CSA (mm 2 ) % standard deviation of the average CSA Intra-fibre Inter-fibre Sisal % 3.3% Flax % 42.% CSA variability Flax > Sisal Inter-fibre >> Intra-fibre Better to focus on measuring many different fibres rather than many measurement along the same fibre

22 Diameter CSA (mm 2 ) Natural Fibre CSA Evaluation Flax Sisal Y=X y=1.97x y=2.55x y=x Measured CSA (mm 2 )

23 1/Modulus (1 6 /GPa) Single Fibre Modulus Sisal Flax y=1.7x y=11.4x E * f 1 E f C A L Sisal, 1/33=3 GPa Flax, 1/14.1=71 GPa f Fibre CSA/Gauge Length (mm)

24 Diameter CSA/True CSA 5 4 Natural Fibre CSA Evaluation Flax Sisal Average Diameter (mm)

25 Natural Fibre CSA Evaluation Diameter method significantly overestimates CSA Underestimates single fibre modulus and strength Magnitude of error is diameter dependent

26 Effect CSA on Single Fibre Properties CSA method Diameter Actual Flax Strength (MPa) Sisal Strength (MPa) Flax Modulus (GPa) Sisal Modulus (GPa) 2 3

27 Apparent Modulus (GPa) 6 5 Effect of Diameter CSA on Apparent NF Modulus Assume a diameter independent modulus Flax, E1f=71. GPa Sisal, E1f=3.5 GPa Average Diameter (mm)

28 Simple Model of NF CSA Diameter Errors 1mm Flax 2mm NF non-circular simplest model is oval X-section Sisal

29 Simple Model of NF CSA Diameter Errors Due to NF natural twist the oval cross section will be viewed differently at different positions along the fibre Transverse view from microscope

30 Parameteric Ellipse Analysis y True CSA.25pAB B " Diameter" CSA.25pD 2 f t X max x A X(t),Y(t) X(t).5ACos(t)Cos( f).5bsin(t)sin ( f) fibre diameter D Can solve for X max for any f and then average over f=-9 for different A:B ratios

31 CSA Ratio (D 2 /AB) 5 CSA Ratio from Ellipse Analysis A/B 5 A/B 3 A/B 2 A/B 1 Av CSA Ratio Ellipse Major Axis Orientation Angle

32 'Diameter"CSA/True CSA Natural Fibre CSA Evaluation Flax measured Sisal measured Average Fibre "Diameter" (mm)

33 'Diameter"CSA/True CSA Natural Fibre CSA Evaluation 4 3 Lines of fixed CSA and varying ellipse A:B ratio 2 1 Flax measured Sisal measured Flax thin Flax average Flax thick Sisal thin Sisal average Sisal thick Average Fibre "Diameter" (mm)

34 Abaca Other Fibres Coir Kenaf Jute

35 Abaca Other Fibres Ellipse A:B Coir Kenaf Jute Similar issues probable in CSA estimation from fibre diameter

36 Summary Thermo-Mechanical Properties NF Longitudinal Modulus (GPa) 75 Glass Flax Sisal 61.5 (71.) 24.9 (3.5) Transverse Modulus (GPa) Shear Modulus (GPa) Axial LCTE (mm/m. o C) Transverse LCTE (mm/m. o C)

37 What does this anisotropy mean for the reinforcement performance of natural fibres? E C = L V f E f V Comparison NF and GF often assumes isotropic fibre m E m Hence simple Krenchel analysis for cos 4 ( q) NF is more like an orthotropic composite material Apply laminate theory to model reinforcement performance

38 E x x y xy Engineering Stiffness, Off-axis Orthotropic Lamina σ ε x x S S S S S S ε Sσ xy xy set sxy { sx } S S S s x and for all q, S11 S cos q (2S S )sin qcos q S sin hence The terms S 11, etc., are found from S= x S11 1 E ν E E s ν E 1 E x x 4 21 q S 1 G

39 Fibre Modulus Contribution Offaxis Stiffness Contribution of Anisotropic Fibre Flax Krenchel Flax "Laminate" Sisal Krenchel Sisal "Laminate" Off-axis Angle ( )

40 Estimation of natural fibre cross section area via the diameter Conclusions (1) method leads to significant overestimation of CSA. results in significant underestimation of mechanical properties obtained by single fibre testing. also contributes significantly to the variability observed in the measurement of natural fibres properties. since the magnitude of the CSA error is diameter dependent single fibre properties will appear to be diameter dependent. Comparison of the CSA of single Flax and Sisal fibre along their lengths indicated that Inter-fibre CSA variability >> Intra-fibre CSA variability

41 Conclusions (2) A value for the fibre content of NFCs can be obtained from study of their moisture absorption characteristics. Flax and Sisal fibres exhibit very high levels of mechanical and thermomechanical anisotropy. Ignoring natural fibre anisotropy and using only the axial modulus of natural fibres in estimating their composite reinforcing ability will significantly overestimate their potential in any off-axis composite loading scenario.

Engineering Composite Reinforcements

Facing up to the Challenges of Natural Fibres as Potential Engineering Composite Reinforcements Jim Thomason Composites Week @Leuven September 16-20 th 2013 Leuven, Belgium Jim why are you so down on Natural