Recent developments in toughening of composites

Ignaas VerpoestLarissa Gorbatikh, Yentl SwolfsComposite Materials Group,Department of Materials EngineeringKU Leuven (Belgium)

Composite Materials Group

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Composites on micro- and nano- levelLarissa Gorbatikh

Physical chemistry of compositesDavid Seveno

Natural and bio-composites

Aart Van Vuure

Processing and product

developmentJan IvensFrederik

Desplentere

Composites on macro- and meso- level

Stepan Lomov,CMG coordinator

founded 1982, Prof Ignaas Verpoest

Contents of this lecture

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o Two research goals:• Improving toughness in fibre-dominated loading conditions

• � importance:

- Increase ‘work of (total) fracture

- Influence other in-service properties like open hole tension.compression, fatigue, impact, …

• Solutions: 3 concepts: • Intra-layer• Inter-layer• fibre-by-fibre (intra-yarn)

• Improving toughness in other loading/stress conditions• � importance: gradual damage development…

• Solutions:

- Hierarchical / nano-engineered composites

Toughening & hybridisation: the start

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• 2003: discussion with Arno Detaeye, VP R&D of Samsonite:

“ why are (almost) no composites used in suitcases?”o Answer: “yes, composites are stiff, strong and light… but too brittle”

o Major requirements for luggage applications:• Lightness, combined with “reasonable” stiffness• Resistance against impacts (falling from > 1m, in loaded condition)… • … @ -12°C (cargo bay conditions)

o Potential candidate: self-reinforced PP• Combines “just OK” stiffness (~= 4 GPa)• With high toughness:

• Failure strain > 15% (CFRP ~= 1,5%)

• Penetration impact resistance > 15 J/mm (CFRP < 5 J/mm)

• Joint Samsonite - KU Leuven development ofo Processing of woven tapes � laminates � compression moulded shells

o Material optimisation…

o … leading to most successful Samsonite suitcase line ever !

Toughening & hybridisation: the start - 2

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• Next question:

“ can it be a little stiffer, without loosing toughness?”• Answer: hybridisation!

Our approach: hybridization of SRPP with stiff/brittle fibers

Swolfs et al Composites A (2015)

3 hybrid types

Layer-by-layer hybrids: new results

Fundamental differences in behavior of CF/SRPP hybrids based on continuous UD and discontinuous random carbon fiber layers

Pseudo-ductile behavior !

Swolfs et al., Composite Structures 131 (2015)

Fragmentation + delamination

Fragmentation + tape debonding

Collaborations with Toray,University of Bristol and Leeds

Selezneva et al., Proceedings ECCM-17 (2016)

Layer-by-layer hybrids: random CF-PP

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Web

2/2 twill PP tape fabric

10 mm1 mm

SRPP

6 mm long carbon fibres

Polypropylene (PP) films

Internal Vf = 17 %Hybrid

PP fabric

Carbon fibres

PP films

PP fabric

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• Pseudo-ductility was achieved with Vf < 7 %• Stiffness x 3• Yield stress x 3 … while maintaining toughness

Vf = 16.7 % (pure web)

Vf = 9.8 %

Vf = 8.6 % Vf = 6.9 %

Vf = 0 %

(pure SRPP)

Vf = 4.4 %

Layer-by-layer hybrids: random CF-PP

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• Pseudo-ductile samples undergo fragmentation and develop checkered-pattern damage

(a)(b)

(a)

(b)

Web fragmentation

25 mm

Checkered-pattern

Layer-by-layer hybrids: random CF-PP

Tensile results – failure mechanisms

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• Checkered-pattern on the surface is related too Twill meso-structure of the material

o Debonding of the longitudinal tapes

Size of SRPP unit cell

Edge for microscopy

6 mm

Debondedtapes

Debonding is arrested in these

region

Layer-by-layer hybrids with SRPP

• What we learned:

o Distinct fractures in UD-CF layer lead to saw-tooth / unstable stress-strain curve

o More “fuzzy” fractures (either inside or due to ‘fuzzy’ delaminations) seem to lead to a smoother stress-strain curve

• � try to ‘trigger’ more gradual fracture of stiff&brittle layer by…o Introducing discontinuities ☺!

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Meso-scale discontinuities to introduce ductility

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Micro-scale discontinuities yield a more gradual transition

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a combination of aligned short carbon fibre/PP prepregs (from Bristol) with woven SRPP

�no load drops, more gradual transition from CF-failure to SRPP failure

From discontinuous layers to intra-yarn hybrids

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• Micro-discontinuities

o Lead to more gradual transition from CF-failure to SRPP failure

o UD-layers almost look like fine arrays of individually broken fibres

• Investigate potential of

o fibre-by-fibre hybridisation

o (very) thin layer hybridisationo � try to understand their effect by modelling

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Single fibre layer

Four fibre layer Eight fibre layer

Two fibre layer

A versatile model to predict strength of (carbon/glass) hybrids

Swolfs et al., Composites: Part A 69 (2015) 279–287

One of the conclusions:

Thin layer hybrids lead to a higher hybrid effect than comingled ones.

Toughening in thin ply / fibre-by-fibre hybrids

Validation of the models for 1 fibre composites

Collaboration with University of Southampton (H. Morton, A.E. Scott, P.A.S. Reed, I. Sinclair, S.M. Spearing)

Synchrotron radiation computed tomography data

Understanding formation of fiber breaks and their clustering

Swolfs et al, Composites A 2015

To advance state-of-the-art composite failure models to reach the required levels of accuracy and to develop characterization techniques for measuring the input data.

What is next? Innovative Training Network

FiBreMoDFibre break models for Designing novel composite microstructures and applications

Starting date: October 1st 2016

Partners:

Contents of this lecture

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o Two domains:• Improving toughness in fibre-dominated loading conditions

• Solutions:

- 3 concepts: • Intra-layer• Inter-layer• fibre-by-fibre (intra-yarn)

• Improving toughness in other loading/stress conditions• � importance: gradual damage development…• Possible solution:

- Hierarchical / nano-engineered composites

Another “toughness”, another approach…

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• Toughness is more than “stress/strain/energy up to total failure”

• … onset and development of damage is sometimes equally/more important!

Meso-level Micro-level

Ways to include CNTs in composites

“The next level of engineering is at the nanoscale”

Carbon fiber and carbon nanostructure development

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Gorbatikh et al, MRS Bulletin, September issue, 2016

Agglomerated CNTsDispersed CNTs Random CNT forests Aligned CNT forests

Romanov et al, CSTE 2015

Romanov et al., Carbon 2015

Romanov et al, Comp A, 2014

Romanov et al, Comp structures, 2015

Understanding through modelling: transverse loading

Versatile two-scalemodels

Strong heterogeneity in stress fields

Composites with CNT reinforced matrices

Create additional and exacerbate existent μ-scale stress concentrations

Effect of nanotubes agglomerations

Romanov et. al. (2014), Composite Structures

Larger size

Higher density

Poor dispersion

higher

μ-scale stress

concentrations

Stresses at the fiber/matrix interface

Matlab code

Python scriptABAQUS

CNTs - curved 3D solid or hollow cylinders

Developing experimental tools at the micro-scale

Romanov et. al. (2015), Carbon

Spline: CNT in 3DOutput: coordinates of points

Microscopic fibers together with nanotubes

Romanov et al. (2015), Composite Science and Technologies

• High stress heterogeneity

• Suppression of interface stresses

• High stresses in matrix rich zones

Effect of nanotubes on stresses

Conclusion

o Two approaches:• Improving toughness in fibre-dominated loading conditions

• Solutions: 3 concepts for stiffening SRPP while keeping toughness• Intra-layer• Inter-layer• fibre-by-fibre (intra-yarn)

• Investigate potential to transfer concepts to CF-composites!?

• Improving toughness in other loading/stress conditions• Solutions:

- Hierarchical / nano-engineered composites

• Control of CNT-dispersion is crucial!