Energy Efficient Composites for

Automotive Industry

Mariana Rojas

Materials Engineering, master's level (120 credits)

2021

Luleå University of Technology

Department of Engineering Sciences and Mathematics

1

2

Acknowledgements

I am indebted to my supervisors, Roberts Joffe and Sibin Saseendran, for their

guidance, support, encouragement, and patience. Without their persistent help, the

goal of this project would not have been realized.

My gratitude extends to the Luleå University of Technology and RISE (Research

Institutes of Sweden). They have provided me with the materials, equipment,

knowledge, and everything that I needed to carry out this thesis. I must express my

sincere appreciation to Daniel Berglund for belief in me and giving me the chance to

work in RISE.

I am grateful to all of those with whom I have had the pleasure to work during

this thesis. My sincere thanks go to Emil Hedlund, who always made time to help and

support me during my work at RISE. Besides, each member of RISE was willing to help

me and has provided me with valuable teaching opportunities. I am equally grateful to

the Polymers and Composites group for providing me with extensive personal and

professional guidance as well as advice and encouragement throughout these months.

I am immensely grateful to Lars Frisk and Erik Nilsson for giving advice and willing

to help with any problem during experimental work in the laboratory.

I am forever grateful to my mom, dad and brother, whose love and guidance are

with me in whatever I pursue. A very special thank you goes to my friends for being part

of my every day. Finally, I would like to thank my boyfriend Sebastián for your love and

support even at a distance of kilometres.

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Abstract

Hybrid composites play a key role in sustainable development. For many years,

carbon fibres in an epoxy matrix have been an attractive option for many structural

applications because of their higher specific mechanical properties mostly. However,

recycling and sustainability are some of the composite shortcomings; and in that

context, natural fibres have gained popularity.

The present study aimed to design and manufacture short carbon/flax hybrid

composites. Two different arrangements were chosen: random and layers

configuration.

Resin Transfer Moulding (RTM) was used to fabricate these hybrid composites.

Mechanical tests and optical microscopy technique were conducted to understand the

effect of the interaction of these two different reinforcements.

Mechanical tests showed a remarkable difference between the hybrid

configurations under flexural loadings. Furthermore, outstanding property values were

observed in the hybrid configurations compared to single fibre composites. The

resultant materials have seemed an attractive combination of fibres with a remarkable

balance between mechanical performance and eco-friendliness.

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Contents

Acknowledgements .............................................................................................. 2

Abstract ................................................................................................................ 3

Abbreviations ........................................................................................................ 6

1.Introduction ....................................................................................................... 8

1.1 Motivation................................................................................................... 8

1.2 Objectives.................................................................................................... 9

1.3 Scope and limitations .................................................................................. 9

1.3.1 Materials systems................................................................................. 9

1.3.2 Experimental work ............................................................................. 10

1.4 Outline of thesis ........................................................................................ 11

2.Background ...................................................................................................... 12

2.1 Towards sustainable development ........................................................... 12

2.2 Hybrid composites (HCs) ........................................................................... 13

2.3 Hybridization effect .................................................................................. 14

2.4 Natural and carbon fibres ......................................................................... 15

2.4.1 Natural fibres ...................................................................................... 15

2.4.2 Carbon fibres and recycling ................................................................ 18

2.5 Hybridization of Carbon/Flax fibres .......................................................... 20

2.6 Automotive applications of hybrid composites ........................................ 21

2.7 Battery housing for electric cars ............................................................... 24

2.8 Hybridization carbon/glass fibres ............................................................. 25

2.9 Manufacturing of thermoset hybrid composites ..................................... 26

Vacuum infusion .......................................................................................... 27

Resin Transfer Moulding (RTM) .................................................................. 27

2.10 Micromechanical analysis ....................................................................... 28

Rule of Hybrid Mixtures .............................................................................. 29

Halpin-Tsai equation ................................................................................... 31

3.Experimental procedure .................................................................................. 33

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3.1 Materials and methods ............................................................................. 33

3.2 Manufacturing process ............................................................................. 36

3.2.1 Vacuum infusion versus Resin Transfer Moulding ............................. 36

3.2.2 Resin Transfer Moulding (RTM) procedure ........................................ 40

3.3 Characterization techniques ..................................................................... 43

3.3.1 Mechanical Testing............................................................................. 43

3.3.2 Optical microscopy ............................................................................. 46

4. Preliminary analysis ........................................................................................ 47

4.1 Microstructure observation ...................................................................... 47

4.1.1 Longitudinal section micrographs ...................................................... 47

4.1.2 Cross-section micrographs ................................................................. 48

5.Findings and results ......................................................................................... 52

5.1 CC-R and flax hybrid composites .............................................................. 52

5.1.1 Tensile tests ........................................................................................ 52

5.1.2 Flexural tests ...................................................................................... 53

5.2 Carbon/Glass hybrid composites (CG-L) ................................................... 54

5.3 Representative curves of flexural tests .................................................... 55

6.Conclusions ...................................................................................................... 58

7. Future work .................................................................................................... 59

8. References ...................................................................................................... 60

Appendix 1 .......................................................................................................... 67

Calculation of reinforcement weight for manufacturing. .............................. 67

Appendix 2 .......................................................................................................... 69

Studying inhomogeneities through the thickness. ......................................... 69

Appendix 3 .......................................................................................................... 71

Wider and narrow samples ............................................................................. 71

Appendix 4 .......................................................................................................... 73

Representative curves of tensile tests ............................................................ 73

Stress-Strain curves obtaining from bending tests. ........................................ 74

6

Abbreviations

ASTM American Society for Testing Materials

C Carbon fibres

CEN European Committee for Standardization

CFRP Carbon Fibre Reinforced Polymer

CO2 Carbon dioxide

df Diameter of the fibres

E Modulus of elasticity

E random Modulus of randomly discontinuous fibres composites

E11 Longitudinal modulus

E22 Transverse modulus

EC European Commission

EU European Union

f Fibre

F Flax fibres

FRP Fibre-Reinforced Polymers

G Glass Fibres

GLARE Glass laminate aluminium reinforced epoxy

HCs Hybrid Composites

HS High Strength

ISO International Organization for Standardization

L Layer configuration

LCA Life Cycle Assessment

lf Length of the fibres

m Matrix

NCB Neutral Colour Balance

NF Natural Fibres

P Load

PAN Polyacrylonitrile

PET Polyethylene terephthalate

PP Polypropylene

R Random distribution

rCFs Recycled Carbon Fibres

RoHM Rule of Hybrid Mixture

RoM Rule of Mixture

RTM

SMC

Resin Transfer Moulding

Sheet Moulding Compounds

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UD Unidirectional

UP Unsaturated Polyester

V

VARTM

Volume fraction

Vacuum Assisted Resin Transfer Moulding

Vci Relative hybrid volume fraction of the system i

Vt Total reinforcement volume fraction

εc Strain of hybrid material

εci Strain of composite system i

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1.Introduction

1.1 Motivation

There are many types of reinforcements and resins to design thermoset

composites. Their optimization design considers the amount, type and architecture of

the reinforcements. As with any other material, composites have certain drawbacks

which compromise their application in certain areas. Considering this, hybrid

composites which combine two groups of fibres seek to achieve the advantages of both

fibres and reduce the weakness of single fibre composite. [1]

Carbon fibres are one of the main popular constituents in fibre-reinforced

polymers (FRP). These fibres in a thermoset resin achieve the highest mechanical

performance. However, these materials have some limitations related to recycling and

sustainability. The hybrid form with natural fibres (NF) seems a solution to overcome

that because NF derived from renewable resources and is sustainable and

environmentally friendly. Furthermore, this type of configurations tries to balance

mechanical performance and eco-friendliness. [2]

This trend of sustainable materials has gained more popularity over the years

and especially in the automotive industry. Whereas natural fibres have used in interior

cars, it has been trying to use hybrid forms with NF in semi-structural and structural

components. [3], [4]

Being relatively new hybrid forms with natural and synthetic fibres, any studies

of manufacturing and characterization of these materials will contribute to the

development of more sustainable components in a near future.

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1.2 Objectives

This thesis work aims to design and manufacture energy efficient composites for

potential applications in the automotive industry (e.g components of battery housing

for electric cars) using short carbon and flax fibres. The major objectives of this project

are:

✓ To define a suitable composition and configuration of hybrid composites

using micromechanical analysis.

✓ To manufacture different short hybrid composites with multiple grades of

fibres.

✓ To characterize the mechanical performance and the microstructure of the

fabricated composites.

✓ To study the hybridization effect under tension and flexural loads.

This project is beneficial in terms of economic and environmental aspects. Using

NF instead of carbon can greatly reduce the cost of the raw materials and produces

benefits for energy savings and the environment.

1.3 Scope and limitations

While there are many configurations and suitable materials to fabricated hybrid

composites, this work emphasizes hybrid configurations with short carbon and flax

fibres.

1.3.1 Materials systems

Recently, there has been growing interest in natural fibres composites.

Environmental awareness has demanded sustainable materials and natural fibres have

seemed a great candidate for many applications. Natural composites are sustainable,

cost-efficient and renewable resources. However, their lower mechanical performance

and other drawbacks limit their use in many engineering applications.

While on the other hand, carbon fibres are an attractive material for advanced

structural composites because of their high strength-to-weight ratio. However, in that

case, recycling is not trivial and it is one of the main drawbacks of carbon composites.

[2]

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Some advantages of natural fibres represent the drawbacks of carbon fibres and

vice versa. Nevertheless, these materials in the hybrid form could overcome these

disadvantages. Hybrid configurations show an exceptional synergic effect on many

properties which cannot be achieved from non-hybrid composites. In carbon/flax hybrid

configuration, it is replaced a certain amount of carbon with flax to maintain or improve

the good mechanical performance fabricating a sustainable option using natural fibres

in the structure.

The greater part of the literature on carbon/flax reinforced polymer composites

seems to have been based on pre-impregnated and conventional textiles with

unidirectional (UD) fibres orientation. Whereas there are relatively few studies in carbon

and flax as short fibres reinforcements and even less using short fibres without a textile

architecture as we have done. [5]

After several trials, a consistent manufacturing method was developed, and a

different material combination was decided to fabricate. A hybrid composite with short

carbon fibres and a nonwoven of short glass fibres were manufactured. The main aim of

this mixture was to evaluate if the manufacturing process could adapt easier to the new

system and obtain some additional data regarding mechanical properties. Comparing

materials with different morphology is not convenient but at least to contribute with

information for future projects.

1.3.2 Experimental work

Several attempts were carried out to manufacture hybrid composites using the

liquid composite moulding process. Resin Transfer Moulding (RTM) was the most

suitable method to manufacture these hybrid composites. For mechanical

characterization, tensile and flexural tests were performed. While for microstructural

characterization, the optical microscopy technique was used.

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1.4 Outline of thesis

The thesis consists of an introductory scope that provides the background

knowledge related to environmental concerns, hybrid composites, natural and synthetic

reinforcements, micromechanical analysis and manufacturing of composites. The

following sections explain the manufacturing process in depth (details, drawbacks and

gathered information) and characterization procedure. Finding and results describe the

most relevant information obtained from the mechanical characterization. Finally, the

conclusions and future considerations of this thesis work are presented.

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2.Background

2.1 Towards sustainable development

The composites industry has been substantial growth over the last years.

Nevertheless, the recycling of composites has not developed at the same rate. Inherent

features why composites have not been easy to recycle are:

▪ their complex composition (matrix, fibres and fillers);

▪ the crosslinked nature of thermoset resins;

▪ the combination with other materials (metallic inserts, honeycombs, etc). [6]

However, some environmental and economic factors have been contributing to

the development of many recycling routes [7]–[9] and more sustainable products. To

put in numbers, the world production of carbon fibres doubled in the 2009-2014

timespan going from 27 to 53 kt. It is expected 117 kt by 2022. A direct consequence of

that is a strong increase in related wastes. In the current situation, there is no specific

legislation for composites waste treatment. But it is assumed that the manufacturer is

responsible for disposing and the legal landfilling of CFRP (Carbon Fibre Reinforced

Polymer) is limited [6]. There is only a suggestion in the 2000/53/EC EU directive which

required a 95% recovery and 85% recycling extent of total end-of-life vehicle weight by

2015 and limit the use of non-metal components if not complying with the Directive

requirements, but no specific instructions on how to treat the end of life of CFRP [9].

Concerning economic factor is important to highlight that most of the manufacturing

process of composites are expensive. Raw materials are not cheaper (up to 47 £/kg),

and energy consumed is higher (up to 40 MJ/kg) [10].

Regarding sustainable products, natural fibres have been one of the main trends

in the last years and Life Cycle Assessment (LCA) has been developed as a useful tool to

design them. Natural fibre-reinforced composites are an enhanced option for many

industrial applications due to the lightweight, low costs, abundance of renewable

resources, potential recyclability and unique mechanical and physical characteristics.

But like any other materials, natural fibres have several drawbacks which affect their use

in many industrial applications. However, there is the chance to fabricate natural fibres

in a hybrid form to overcome these shortcomings. [11], [12]

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2.2 Hybrid composites (HCs)

Hybrid composites combine two or more materials as reinforcements and/or

matrix system. The purpose of HCs is to formulate the best combination of materials

that maintains the advantages of each constituent and overcomes non-hybrid

composites limitations for a certain application [12], [13]. Nowadays, there is a special

interest in hybrid composites. Being able to combine different materials even on

different scales has increased its popularity. Examples of different hybrid composites

are given below:

✓ Automobile parts such as door panels, instrument panels, seat shells and

others.[12]

✓ Jute fibres and concrete matrix are being developed for structural applications. [12]

✓ Alternating layers of aluminium sheets and glass fibres reinforced epoxy matrix

(GLARE) has used in the fuselage of the superjumbo aircraft A380 of Airbus ®. [14]

✓ Combination of natural fibres (bamboo, banana, coir, cotton, etc) or even natural

fibres with another synthetic material such as glass fibres. Natural fibres are

attractive in environmental concerns. Because they are renewable, sustainable, and

eco-friendly. [15]

✓ The addition of filler particles has a positive effect on properties such as fracture

toughness, strength, and impact properties. (e.g. alumina powder, carbon

nanotubes or graphene oxide in combination with glass fibres and epoxy resin).[16]

✓ On the scale of nano, there are interesting hybrid nanocomposites with biomedical

applications (drug delivery system, dental implants, scaffolds for tissue engineering,

and others). The synthesis of hybrid nanocomposites involves physical and chemical

methods. Physical methods include solution or melt blending while chemicals

methods refer to in situ deposition. [17]

The mechanical performance of hybrid composites could be enhanced with the

proper selection of materials. Furthermore, there could be more advantages of

hybridization such as cost reduction, more eco-friendly behaviour, more corrosion

resistance, and others. [12]

This work will emphasize HCs with two materials as fibres and an epoxy matrix.

Figure 1 shows the most relevant fibres arrangements for this type of hybrid composite:

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Figure 1. Hybrid configurations. (a) Layer-by-layer. (b) yarn-by-yarn. (c) Fibre-by-Fibre. [15]

These three configurations could define as follows:

- Layer-by-layer has a stacking of different types of fibres in different layers. Each

layer has only one fibre type.

- Yarn-by-yarn has both reinforcements in the same layer of fabric, and then layers

can be stacked in many ways.

- Fibre-by-fibre combines both materials on the fibre level resulting in an only layer

with both reinforcements, this configuration has a random dispersion of the

fibres.[15]

Configurations affect the mechanical performance and other properties of the

HCs. That is the reason why researchers study this parameter. [5]

2.3 Hybridization effect

It is a synergic effect presents in a hybrid composite. It is defined in two different

ways; one is based on the fact that compared with low elongation fibre composites, due

to the addition of ductile fibres in the hybrid form, the failure strain of the hybrid

composite is significantly increased. The other definition is based on the deviation

(positive or negative) of a composite property whose value is higher or lower than would

be predicted from a simple application of the rule of mixture. [1], [18]

So, the prediction of hybrid properties is not trivial and reinforcements

configurations are a relevant parameter to understand the hybridization effect. In

literature, there are many studies [19]–[22] related to that and stacking sequences. The

idea to reorganize the locations of the reinforcements and obtains different mechanical

properties is attractive for many engineering applications.

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How well fibres mix is the degree of dispersion. It defines as the reciprocal of the

smallest repeat length. The degree of dispersion is an important parameter to

understand the hybridization effect. Figure 2 shows the dispersion of the two fibres

types.

Figure 2. Degree of dispersion. (a) two layers. (b) alternating layers. (c) bundle-by-bundle dispersion. (d) randomly

dispersion.[15]

Hybrid configuration layer-by-layer (Figure 1-a) could correspond with

‘alternating layers’ in terms of dispersion (Figure 2-b). While fibre-by-fibre arrangement

(Figure 1-c) would be related to random dispersion (Figure 2-c). The latter mentioned

is the best configuration in terms of dispersion. The synergic effect is higher there

because dispersion improves as the number of layers increases and their thickness

decreases. [15]

2.4 Natural and carbon fibres

2.4.1 Natural fibres

Humans have developed natural fibres for industrial use for many centuries.

These products have been used in textiles and ropes mainly. However, the sustainability

of natural materials is well proven and they are cost-efficient so these materials have

gained more popularity in industrial applications. [23]

Raw materials of natural fibres can be derived from animals, minerals and plants.

Table 1 gives a breakdown of natural fibres by these groupings.

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Table 1. Types of natural fibres. (Adapted from [12])

Nat

ura

l fib

res

Cellulose/lignocellulose

Bast Jute, Ramie, Flax, Kenaf,

Roselle, Mesta, Hemp

Leaf Sisal,Banana, Henequen,

Agave, Palf, Abaca

Seed Loofah, Milk weed,

Kapok, Cotton

Fruit Oil palm, Coil

Wood Hard wood, Soft wood

Stalk Rice, Wheat, Barley,

Maize, Oat, Rye

Grass Bamboo, Bagasse, Corn,

Sabai, Rape, Esparto,

Cancry

Mineral - Asbestos

Animal

Silk Tussah, Mulberry

Hair/Wool Lamb, Groat, Angora,

Horse feather.

Table 2 [24] shows the average chemical composition of some plant fibres which

are commonly used to manufacture lightweight components.

Table 2. Natural Fibres chemical composition

Cellulose (%) Hemicellulose (%) Lignin (%) Pectin (%) Waxes (%)

Flax 70.5 16.5 2.5 0.9 -

Jute 67.0 16.0 9.0 0.2 0.5

Kenaf 53.5 21.0 17.0 2.0 -

Hemp 81.0 20.0 4.0 0.9 0.8

Sisal 60.0 11.5 8.0 1.2 -

Cellulose content determines the mechanical properties, higher content of

cellulose means higher mechanical performance. However, hemicellulose and pectin,

increase moist absorption. In addition, the disadvantage of pectin is that it affects the

structure and morphological properties of natural fibres. [24]

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Table 3 shows some mechanical and physical properties and the price of some

natural fibres. Also, the values of synthetic fibres such as carbon and e-glass are

described there.

Table 3. Price and physical and mechanical properties of some natural and synthetic fibres.

Source Price kg-1

(USD)

Density

(g/cm3)

Tensile

Strength

(MPa)

Young’s

Modulus

(GPa)

Elongation

(%)

Moisture

content

(%)

Reference

Flax 2.1-3.11 1.4-1.52 800-940 27.6-80 1.2-1.6 7 [3], [25],

[26]

Jute 0.92 1.3-1.48 393-800 13-26.5 1.16-1.8 12 [26]

Kenaf 0.378 1.2-1.4 284-930 21-60 1.6 6.2-20 [26]

Hemp 1.55 1.48 550-900 70 1.6-4.0 8 [26]

Sisal 0.65 1.3-1.4 390-450 12-41 2.3-2.5 11 [26]

E-glass 1.63-3.26 2.55 1900-2050 72-85 1.8-4.8 - [3], [25],

[26]

Carbon HS (High Strength) 8-14 1.82 2250 200 1.3 - [3], [26]

Even though NF has lower mechanical properties than synthetic reinforcements,

they are less dense. So, NF has outstanding specific properties which make them an

attractive option for several applications.

Flax fibre is another example of bast fibres and has gained popularity in

composite industries. Flax contains a higher content of cellulose with a small amount of

lignin in its structure [27]. It has a similar stiffness to e-glass fibres, but flax has only

slightly more than half of e-glass density. This natural fibre has become an attractive and

potential material for composite applications if it wants something stiff, light and eco-

friendly. [28]

Natural fibres are obtained from a natural source, being natural give them many

advantages. One of the main benefits of natural fibres is their low density,

biodegradable and renewable features. Despite there are many drawbacks to overcome

in terms of durability, strength and moisture absorption. Table 4 summarises the main

benefits and disadvantages of natural fibres. It is important to emphasize that the

combination of natural with synthetic reinforcements seems one of the possible

solutions to overcome their shortcomings. [26], [27], [29]

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Table 4. Advantages and disadvantages of natural fibres. [26], [27], [29]

Advantages Disadvantages

Low density and high specific mechanical

properties i.e stiffness and strength

Lower durability. It could improve considerably

with treatment

Fibres are obtained from renewable resources

(plants, animals and minerals)

Hydrophilic behaviour

Low cost than synthetic fibres Lower values of strength and stiffness

Production energy is less than other synthetic

fibres i.e glass fibres

Natural reinforcements are short fibres mainly

and have heterogenous size

Non-abrasive damage to processing equipment The great variability of properties depends on

cultivation location and weather.

Low emission of toxic fumes during the

incineration at the end-of-life

Lower processing temperature (<200°C)

Remarkable thermal and acoustic performance Lower fire resistance

No skin irritation during handling

As is mentioned in this section, flax fibre has gained popularity in the composite

industry. Flax is one of the natural fibres with the highest content of cellulose (70.5 %)

which is related to its remarkable mechanical performance. For that reason, it has been

decided to work on this project with flax fibres as a second reinforcement in the hybrid

configurations.

2.4.2 Carbon fibres and recycling

Carbon fibres are an attractive material for advanced structural composites due

to their high strength-to-weight ratio. Carbon fibre has a lower density (1.75-2.00 g/cm3)

than steel (7.75 and 8.05 g/cm3) and both share some mechanical properties. Carbon

fibres have excellent strength and stiffness, high fatigue strength, higher resistance to

corrosion, among other features. Due to their high specific properties could replace

conventional materials such as steel or aluminium in a variety of engineering

applications.

Carbon fibres are one of the main popular reinforcements in FRP and combining

with a thermoset resin achieve the highest mechanical performance. Composites stand

out as lightweight materials and industries such as aerospace, automotive, marine and

construction seek materials like that. However, the growing environmental rules and

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ecological issues demand new renewables alternatives. In that context, carbon fibres do

not seem a sustainable alternative because their production requires a higher amount

of energy (around 286-704 MJ/kg [30]) and the predominant precursor is petroleum-

based polyacrylonitrile (PAN). Even though these drawbacks, it has been studying other

potential precursors such as lignin [31] and the use of recycled carbon fibres (rCFs) have

gained interest in recent years.[2]

Recycled carbon fibres (rCFs) are better than virgin carbon in economic and

environmental terms. Regarding mechanical properties, recycled composites reach

lower mechanical properties than virgin fibres but these are enough for many

applications [32]. Table 5 shows retention rates of mechanical properties according to

different recycling routes [33]. The large variation in the pyrolysis technique is due to

the resin residues on the fibre surface after this recycling process.

Table 5. The retention rate of the tensile strength of RCFs depends on a recycling route. [33]

Recycling technology Virgin fibres Retention Rate of Tensile

Strength of RCFs (%)

Pyrolysis AS4-3K 15-98

Steam thermolysis AS4C 95-99

Solvolysis Toho Tenax C124 97-98

Supercritical water Hexcel 48,192 C, 1270 ST 94-98

Fluidized bed Toray T800 82

While carbon fibres retain the mechanical properties after recycling, the fibre

surface usually is altered due to higher temperatures of recycling treatments.

Sometimes, this leads to poor adhesion between fibres and matrix and thus impacts the

mechanical properties of the final components [34]. Besides, recovered fibres often

have short lengths due to the size reduction of components before recycling and the

chopping process during recycling. So, the combination of recovered fibres with carbon

virgin material compensates for the loss in mechanical properties [33]. Another

alternative could be to combine recycled fibres with other synthetic or even natural

fibres. For this purpose, it is required to find and adapt the manufacturing process for

the architecture of these discontinuous fibres and that is one of the main reason that

this project works with them.

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2.5 Hybridization of Carbon/Flax fibres

The growing environmental awareness has increased the interest in natural

reinforcements. Natural composites cannot replace conventional composites in many

applications which require higher mechanical performance. However, the combination

of synthetic and natural fibres has seemed the solution to many issues towards

sustainable composites materials. [12]

The hybridization of carbon and flax could have a powerful effect on many issues.

The addition of flax to carbon fibre reinforced composite could bring some

improvements in flexural and damping properties mainly, without a big degradation of

the rest of mechanical properties. Another way is the addition of carbon fibres to

improve the drawbacks of natural composites such as lower mechanical performance.

An interesting consideration is that configuration could also affect the mechanical

performance of the hybrid composites so, there is a variety of possibilities to study with

the purpose to understand the interaction between these reinforcements and also to

predict the more suitable configuration for a certain application. [35]

The hybridization of natural and synthetic materials has sought to develop new

materials with a balance between properties and environmental issues. In recent years,

this approach has become a trend and it is actively supported by the industry.

Table 6 sums up the chronology of many studied topics regarding hybrid composites

of carbon and flax. [5]

Table 6. Chronological summary of studies on carbon/flax composites. [5]

2012-2013 2015 2016 2017 2018-Present

Mechanical

properties

Mechanical

properties

Damping

properties

Modelling

Mechanical

properties

Damping

properties

Modelling

Dielectric

properties

Mechanical properties

Damage progression

Application-specific construction

parts

Water absorption

Improvement of fibre compatibility

Used of a third type of fibre

Mechanical

properties

Damping

properties

Modelling

Used of

recycled fibres

21

The greater part of the literature on carbon/flax reinforced polymer composites has

been based on pre-impregnated and conventional textiles with unidirectional (UD)

fibres orientation. Whereas there are relatively few studies in carbon and flax as short

fibres reinforcements and even less using short fibres without a textile architecture as

we have done.

The mechanical performance of carbon/flax hybrid composites is mainly governed

by carbon content because carbon properties are much higher than those of flax fibres.

Nevertheless, the flax has an impact on the strain at failure and failure mode. In addition,

the stacking sequence of the configuration and weave art of the flax textiles have also

influence the mechanical properties. Besides, the adhesion between carbon-epoxy and

flax-epoxy is different and in complex plies configurations, the interphase plays an

important role in the mechanical properties. As a result, materials with the same

composition but different arrangements show different mechanical performance. [5]

Although it is useful to use micromechanical models and laminate theory analogies

to predict mechanical properties such as stiffness, these cannot predict the behaviour

of different arrangements with the same composition because these mathematical

models do not consider the interaction between reinforcements materials and

interphase features. So, there are many studies [19]–[22] related to explain the

hybridization effect and how the stacking sequence affect the final properties of the

material. Since the 1970s, many experimental methods and computational models have

attempted to understand the hybrid effect in composites. Unfortunately, there are

several contradictions and gaps in their reports [1]. So, any gathered information would

contribute to the development of more accurate models.

Much of the literature is focused on the positive effect of flax in damping and impact

properties. Controlling parameters such as content and stacking sequence is possible to

fabricate hybrid composites which are competitive with those of the traditional carbon

composites. [5]

2.6 Automotive applications of hybrid composites

One of the main demands in the automotive industry is weight and cost

reduction. Lightweight components imply a reduction in fuel consumption as well as

hazardous emission production. Thus, composite materials have seemed the best

candidate because of their low density and higher mechanical performance. However,

22

in recent years the growth of environmental awareness has pushed towards more

sustainable development in the automotive industry. In this context, the trend of natural

fibres and hybrid composites has grown and increased their applications in many

existing automobile parts.

Natural fibres have started to use only in interior parts of automobile parts due

to their lower mechanical performance, poor fibre/matrix adhesion, and moisture

absorption. Nevertheless, these properties are being improved through surface

treatments, additives, coatings and hybrid configurations. Hybrid configurations,

between natural fibres mainly, has been one of the trends in the automotive industry.

[3], [4]

Table 7 shows some applications of natural fibres in automobiles.

Table 7. Natural fibres in automotive parts.

Automotive markers Automotive parts References

Ford Car seat (soy foams), flex crossover injection moulded

storage bins (wheat form), fuel tank tubes (100% castor

bean oil derived nylon-11)

[36]

Mercedes Benz Door panels (flax/sisal/wood fibres with epoxy resin/ UP

matrix), glove box (cotton fibres/wood moulded,

flax/sisal), instrument panel support, insulation (cotton

fibres), moulding rod/apertures, seat backrest panel

(cotton fibre), trunk panel (cotton with PP/PET fibres), and

seat surface/backrest (coconut fibre/natural rubber)

[3], [29], [29],

[36]–[38]

Audi Boot-liner, spare tire-lining, side and back door panel

(flax/sisal mat reinforced polyurethane composite),

seatback, and hat rack

[37], [39]

Volkswagen Door panel, seatback, boot lid finish panel, boot liner [3], [4]

Seat Door panel, seatback [4]

Fiat Door panels (wood fibres), interior and exterior trims (coir) [29]

Toyota Floor mats and spare tire cover (kenaf), luggage

compartments, speakers and floor mats (bamboo)

[29]

BMW Interior door panels (sisal), door panel lining (flax) [29]

23

Figure 3 depicts the automobile parts manufactured with natural fibres in the

current E-Class Mercedes Benz.

Figure 3. Natural fibres components in Mercedes Benz E-Class.[29]

As mentioned above, natural fibres are mainly used in interior automobile parts.

However, it has been trying to use them in more semi-structural, and as well as

structural components. Mercedes Benz E-Class has taken an important step towards

higher performance applications of natural fibres. In 1994, Mercedes Benz introduced

door panels of jute fibres composites for the E-Class model. Besides, flax, hemp, sisal,

wool, and other natural fibres were used in Mercedes Benz automotive parts. Then, the

new generations of E-Class cars have increased the number of components with natural

fibres as well as have reduced the weight components. Another important milestone in

E-Class automobile was to replace wood fibres materials with a flax/sisal reinforced

epoxy matrix. This hybrid composite has reduced the weight of the structure and has

improved the mechanical properties and passenger protection. [3], [29]

Another trend of natural fibres in automobile parts is to replaced synthetic fibres

such as e-glass fibres. Due to the higher specific properties of natural fibres have seemed

a possible great candidate to replace synthetic fibres. Weight reduction of existing

components without compromising the mechanical performance and as well as

reducing fuel consumption and pollution is one of the main reasons to use natural fibres

in automobile parts. Therefore, components more sustainable would be increasing in a

near future. [4], [29]

24

2.7 Battery housing for electric cars

Lightweight components play an important role in the efficiency of electric

vehicles. The weight reduction of battery housings is one of the steps towards improving

the efficiency of sustainable transport. The main function of the battery housing is to

enclose the battery modules and to enhance vehicle structure and ability to absorb crash

energy during a collision. Indeed, it must fulfil other requirements related to fire

resistance, insulation, being safety mainly. [40]–[44]

Carbon and glass composites have led the solution towards lightweight covers

against traditional materials such as steel or aluminium. Figure 4 shows a scheme of a

battery housing fabricated with carbon fibres reinforced epoxy as covers.

Figure 4. Example of battery housing for electric vehicles.[44]

Some of the most innovative solutions were SGL ® and Evonik® battery housings.

SGL®[45] has designed a battery enclosure with carbon and glass composites covers

whereas Evonik®[41] has made glass fibres- Sheet Moulding Compound (SMC) covers.

While there is an increase of semi-structural and structural components with

natural fibres in vehicles parts, there is not literature related to battery housing with

natural fibres. The hybrid form might be a suitable option but firstly it is necessary to

evaluate one of the simplest parameters, the mechanical performance, and compare

25

with the minimum requirements for this component. Table 8 compares Young’s

modulus of traditional materials for battery cases for electric cars.

Table 8. Properties of common materials for battery housing for electric cars.

Material Young’s modulus

(GPa)

Specific Young’s modulus

(GPa cm3 g-1)

Carbon Composite (Vf=44%)1 40.3 27.5

Steel Reference 200 25

Aluminium Reference 69 26

1 Short fibre randomly or quasi-isotropic laminate

2.8 Hybridization carbon/glass fibres

One of the main purposes of hybrid composites is to overcome the drawback of

single reinforcement composites. Carbon fibres offer many advantages such as high

strength and stiffness but have also certain disadvantages. Recycling was mentioned

above, another is the poor impact strength and high cost of production. Any

combination with other material could lead to a reduction of the cost and glass fibre is

a great candidate to improve the toughness. Indeed, carbon and glass fibres are the

most popular fibres for usage in engineering applications. [46]

Many works [47]–[50] have studied the hybridization of carbon/glass fibres

reinforced polymer. The mechanical properties and the stacking sequence are the most

relevant topics as well as the processing techniques which affects the final mechanical

properties. [46]

Even though glass fibres have a lower cost (compared to other synthetic fibres)

and fairly good mechanical performance, natural fibres have seemed to replace them in

a near future in many applications. Table 9 summarizes some shortcomings of glass

fibres and compares them with natural fibres. [51]

26

Table 9. Disadvantages of glass fibres and comparison them with natural fibres. [51]

Natural fibres Glass fibres

Density Low Twice that natural fibres

Cost Low Low, usually higher than NF

Renewability Yes No

Recyclability Yes No

Energy consumption Low High

CO2 neutral Close to zero No

Abrasion to machines No Yes

Health risk when inhaled No Yes

Disposal Biodegradable Not biodegradable

In terms of mechanical performance, glass fibres have higher properties values

than natural fibres (Table 3). Nevertheless, the density is lower in the natural fibres and

balance this shortcoming because they have acceptable specific properties or even

higher than glass fibres. The most important problem with the natural fibres composites

is the fibre-matrix adhesion which affects their use in many semi-structural and

structural applications. However, there are physical, chemical methods and hybrid

configurations to improve that and natural fibres have gained popularity in a variety of

industrial applications. [23], [51]

Another interesting aspect of natural fibres is that they offer an almost carbon

dioxide (CO2) neutral disposal process based on the captured CO2 in natural fibres during

growth [52]. It is important to highlight that among the NF, flax fibre combines low cost,

lightweight, high strength and stiffness so it has seemed one of the best options to

replace synthetic reinforcements in many applications. [53]

2.9 Manufacturing of thermoset hybrid composites

Usually, traditional techniques which are already used for the fabrication of

thermoset composites, are also alternatives for the manufacturing of carbon/flax hybrid

composites. The next sections will give a very short description of the vacuum infusion

and Resin Transfer Moulding (RTM) process which are the relevant procedures of this

thesis.

27

Vacuum infusion

This process applies a vacuum to drive long-range resin flow and laminate is

enclosed in a bag. It is a process of open mould so the costs are reduced significantly.

Moreover, a high fibre volume fraction (60-70%) is achieved by vacuum infusion and

therefore high structural performance is expected in the component. Figure 5 shows a

typical configuration of vacuum infusion. It is an interesting technique to obtain a

constant higher volume fraction of reinforcements as well as low porosity. [54]–[56]

Figure 5. Scheme of the vacuum infusion process. [54]

Resin Transfer Moulding (RTM)

It is a closed mould process. The mould cavity is loaded with a dry preform, then

the mould is closed, and the resin is injected through an inlet port to impregnate the

reinforcements. RTM is commonly used for complex components with small to medium

size due to the mould close forces necessary when the component size increases. For

components of bigger sizes is recommended to assist the system with vacuum and

enclosing with a vacuum bag the half mould and the preform. This method is commonly

called Vacuum Assisted Resin Transfer Moulding (VARTM) or vacuum infusion without

positive pressure. Figure 6 shows a typical configuration of RTM. This process allowed

to reach 65 % volume fraction content and components with complex geometry. [54]–

[57]

28

Figure 6. Scheme of Resin Transfer Moulding process. [54]

2.10 Micromechanical analysis

Mathematical models try to predict the mechanical properties of materials.

Sometimes, experimental methods are simple and direct. However, it is time-consuming

and often expensive. A set of tests analyse just only a scenario with specific features.

Additional measures are required when any change in a system variable occurs. On that

point, modelling is a powerful strategy because it predicts easier the effects of many

variables in a particular property.

The stiffness is one of the most relevant elastic properties to consider in

component design. There are different approaches with the purpose to estimate this

useful property, micromechanics is one of them. Micromechanical models are based on

the physical characteristics of the constituents (reinforcements and matrix) and

geometrical features. So, this modelling approach helps to study the effects of

constituents on the final properties of composites. [58]–[60]

Several studies [5], [59], [61], [62] of micromechanics in hybrid composites have

reported significant results applying the following models:

o Rule of Hybrid Mixtures (RoHM).

o Halpin-Tsai equation.

Although there are many theoretical models to estimate stiffness, they all are

based on the same basic assumptions [63]:

29

• Fibres and matrix are linear elastic materials.

• Matrix is isotropic, and fibres are isotropic or transversely isotropic

• Fibres are axy-symmetric and identical in shape

• Before and during deformation, fibres and matrix are perfectly bonded at their

interface.

Rule of Hybrid Mixtures

Rule of Hybrid Mixtures is based on the well-known Rule of Mixtures (RoM)

model which is often used in composite materials. RoM approach assumes either iso-

strain or iso-stress conditions to estimate longitudinal or transversal properties,

respectively.

The application of RoHM considers a whole hybrid system as the combination of

singles composites with no interaction between them. Nevertheless, this interaction

exists, and it is a relevant parameter. This could be positive or negative so, the actual

modulus value could deviate from the estimation. This deviation of the rule of mixtures

is called the hybridization effect and it was described above. [5], [59], [61]

To begin the calculation of stiffness in hybrid short fibres composites, the

longitudinal and transversal modulus of a single composite should be evaluated first.

The iso-strain analyse is applied to the unidirectional continuous lamina when it

is loaded in a direction parallel to its fibres. In that case, longitudinal modulus (E11) in

the fibre direction is:

𝐸11 = 𝐸𝑓𝑉𝑓 + 𝐸𝑚(1 − 𝑉𝑓) (1)

where E is the modulus of elasticity and V is the volume fraction. Subscripts f and

m are the individual constituents, fibres and matrix respectively.

When the laminate is loaded in a transverse direction, fibres and matrix are in

an iso-stress condition. As a result, transverse modulus (E22) could estimate by the

following inverse rule of mixtures:

1

𝐸22=

𝑉𝑓

𝐸𝑓+

1−𝑉𝑓

𝐸𝑚 (2)

30

Once E11 and E22 have been calculated, the modulus of randomly discontinuous

fibres composites could calculate using Tsai and Pagano equation. This equation

estimates this elastic property in terms of corresponding oriented moduli [64]:

𝐸𝑟𝑎𝑛𝑑𝑜𝑚 =3

8𝐸11 +

5

8𝐸22 (3)

As it was mentioned before, the analysis using RoHM combine two systems

where there is no interaction between them. If it is applied an iso-strain condition to a

hybrid composite with two types of reinforcements, the strain of each composite system

(εc1 and εc2) and hybrid material (εc) would be the same. Furthermore, force equilibrium

would be [65]:

𝐸𝑐𝜀𝐶 = 𝐸𝑐1𝜀𝑐1𝑉𝑐1 + 𝐸𝑐2𝜀𝑐2𝑉𝑐2 (4)

The modulus of the hybrid composite (Ec) can then be evaluated from the RoHM

as:

𝐸𝑐 = 𝐸𝑐1𝑉𝑐1 + 𝐸𝑐2𝑉𝑐2 (5)

Where Vc are the relative hybrid volume fraction of the individual composite

system. Further, the expressions listed below should be considered valid for the

assumed system:

𝑉𝑐1 + 𝑉𝑐2 = 1 (6)

𝑉𝑐1 =𝑉𝑓1

𝑉𝑡 (7)

𝑉𝑐2 =𝑉𝑓2

𝑉𝑡 (8)

𝑉𝑓1 + 𝑉𝑓2 = 𝑉𝑡 (9)

Vf1, Vf2 are the volume fraction of each reinforcement system. Vt is the total

reinforcement volume fraction. Vt is used as reinforcement volume fraction for

calculation of the elastic modulus of both single composites system (Ec1 and Ec2). [65]

31

Replacing Eq.(6)-Eq.(9) in Eq. (5):

𝐸𝑐 = 𝐸𝑐1𝑉𝑐1 + 𝐸𝑐2𝑉𝑐2 (5)

𝐸𝑐 = 𝐸𝑐1𝑉𝑓1

𝑉𝑡+ 𝐸𝑐2(1 −

𝑉𝑓1

𝑉𝑡) (10)

Halpin-Tsai equation

The classical Halpin-Tsai equation (1976) could be modified to calculate the

elastic moduli of short hybrid composites. This approach considers the length (lf) and

diameter (d) of fibres. Due to this, it is commonly used when it is worked with

discontinuous reinforcements.

The moduli E11 and E22 can be written as follows:

𝐸11 = [1+2(

𝑙𝑓1𝑑𝑓1

⁄ )𝜂𝐿1𝑉𝑓1

1−𝜂𝐿1𝑉𝑓1] 𝐸𝑚 + [

1+2(𝑙𝑓2

𝑑𝑓2⁄ )𝜂𝐿2𝑉𝑓2

1−𝜂𝐿2𝑉𝑓2] 𝐸𝑚 (11)

𝐸22 = [1+2(

𝑙𝑓1𝑑𝑓1

⁄ )𝜂𝑇1𝑉𝑓1

1−𝜂𝑇1𝑉𝑓1] 𝐸𝑚 + [

1+2(𝑙𝑓2

𝑑𝑓2⁄ )𝜂𝑇2𝑉𝑓2

1−𝜂𝑇2𝑉𝑓2] 𝐸𝑚 (12)

𝜂𝐿1 =(

𝐸𝑓1𝐸𝑚

⁄ )−1

(𝐸𝑓1

𝐸𝑚⁄ )+2(

𝑙𝑓1𝑑𝑓1

⁄ ) (13)

𝜂𝐿2 =(

𝐸𝑓2𝐸𝑚

⁄ )−1

(𝐸𝑓2

𝐸𝑚⁄ )+2(

𝑙𝑓2𝑑𝑓2

⁄ ) (14)

𝜂𝑇1 =(

𝐸𝑓1𝐸𝑚

⁄ )−1

(𝐸𝑓1

𝐸𝑚⁄ )+2

(15)

32

𝜂𝑇2 =(

𝐸𝑓2𝐸𝑚

⁄ )−1

(𝐸𝑓2

𝐸𝑚⁄ )+2

(16)

Where subindex 1 and 2 designate the first and second fibre system. In the case

of random distribution, it is used Eq. (11) and Eq. (12) in Eq. (3). [59], [64]

33

3.Experimental procedure

3.1 Materials and methods

The constituents of the different laminates are:

− Short carbon Toray ® T700 SC and flax fibres of 25 mm in length approximately.

− Ahlstrom ® nonwoven of short random e-glass fibres (fibres up to 50mm in

length).

− Resin Araldite ® LY1564 SP and hardener XB 3404-1 (proportions in weight

100:36).

An Aplicator Group ® cutting machine was used to cut carbon and flax rovings.

Figure 7 shows the different reinforcements used it.

Figure 7. Raw materials. (Left to right: carbon fibres, flax fibres and nonwoven glass fibres).

Resin Araldite ® LY1564 SP is a low viscosity resin recommend for vacuum

infusion and RTM process. For this resin system, the mould was preheated at 30°C and

the curing process chosen was 15 h at 50°C, as recommended by the supplier.

Table 10 summarizes the selected properties of raw materials which were

necessary for micromechanical analysis.

34

Table 10. Material data

Constituent Density (g/cm3) Young’s Modulus (GPa)

Carbon fibres Toray ® T700 SC 1.80 230

Flax fibres 1.50 50

Glass fibres 2.58 74

Resin Araldite ® LY1564 SP and

hardener XB 3404-1 1.10 3.3

Note: Carbon and resin system information given by the suppliers. Whereas flax and glass fibres

information are literature values.

According to analytical stiffness modelling (Eq. (10)) and considering the values

of Young’s modulus of common materials for battery cases for electric cars (Table 8),

three different configurations of composites materials of short fibres reinforcements

and thermoset matrices were proposed to fabricate using RTM process (Table 11).

Table 11. Configurations and expected stiffness for the composite materials proposed.

Composite Name Composites Expected

stiffness (GPa)

Pure Carbon CC-R Short carbon fibres randomly (55% Carbon fibres) 52

Carbon/Flax random CF-R Short hybrid composites randomly

(35%Carbon and 20%flax)

39

Carbon/Flax layers CF-L Layers of short hybrid composites randomly [CF]s.

(35%Carbon and 20% flax)

39

Note about sample coding: C is carbon fibres, F is flax fibres, R is random distribution and L is layer

configuration.

After several trials, a consistent manufacturing method was developed, and a

fourth configuration was decided to fabricate. A hybrid composite with short carbon

fibres and a nonwoven of short glass fibres were manufactured:

→ Carbon/ E-Glass layers (CG-L): Layers of short hybrid composite randomly [CG]s

The expected stiffness of CG-L with the real volume fraction of Table 12 (0.24C-

0.14G) is about 30 GPa.

35

One of the main problems in all the plates was to achieve a pre-defined thickness

value. Although the amount of reinforcement was calculated in advance (Appendix 1),

the mould could not close properly, and the mould gap thickness was higher than

desired. As a result, the plates were slightly thicker, and the volume fraction was lower

than expected (the desired thickness was 1.7 mm).

Table 12 shows the amount in weight of reinforcements, dimensions of the plate

and final average thickness obtained. The real volume fraction content was calculated

assuming that the distribution of the reinforcements was homogeneous (the lower

scattering of the mechanical results supported this idea) and there are no voids in the

samples.

Table 12. Features of the fabricated composites.

Name Carbon weight

(g)

2nd reinforcement

weight (g)

Plate

dimensions

(cm)

Thickness

(mm)

Real volume

fraction

CC-R 114.89 - 30x22.6 2.67 0.35

CF-R 68.06 32.48 28x22.6 2.60 0.23C-0.13F

CF-L 67.88 32.41 28x22.6 2.50 0.22C-0.13F

CG-L 67.80 32.87 28x22.6 2.30 0.24C-0.14G

Even with this shortcoming related to the thickness, the quality of the plate

allowed us to study the hybridization and compare the mechanical performance among

the different configurations. To investigate the hybridization effect, two different

configurations of hybrid laminated composites were fabricated using carbon and flax

fibres. In one of them, carbon and flax were mixed randomly and in the other, carbon

was in the outer layers and flax in the inner layer. Both hybrid plates have the same

amount in weight of carbon and flax but the locations of the reinforcements change.

Due to the manufacturing process guaranteed accurate reproduction at good

quality, as mentioned above, it was decided to manufacture one plate of carbon and

glass fibres in the same way. There was a special interest in manufacturing and analysing

36

mechanical performance. In that case, the same amount in weight of flax was replaced

for a nonwoven of short random glass fibres.

3.2 Manufacturing process

When choosing a manufacturing process is essential to consider the size, shape,

desirable properties of the final component, speed of production, and manufacturing

cost involved in the processing [12]. The manufacturing of hybrid composites is complex

and challenging due to the same system should be work for all the reinforcements which

have different morphologies and features.

3.2.1 Vacuum infusion versus Resin Transfer Moulding

Many attempts of vacuum infusion have been done but certain drawbacks led to

a modification of the overall setup and finally, the RTM was the best solution.

For vacuum infusion, the reinforcements were placed onto a heat metal plate

and a plastic film closed it. Two pieces of plastic tubes have lied on the opposite sides of

the preform, commonly called inlet and outlet port. The outlet is connected to a vacuum

pump and the inlet to the resin bucket. In the beginning, the inlet is closed with a clamp

and the preform is evacuated until a vacuum of 50 mbar. When the system is ready and

there are no leaks, the inlet is connected to the resin bucket. The pressure difference

between the vacuum in the cavity and the atmospheric pressure outside the bagging

film facilities the resin flow and the compaction of the preform. Tacky tape is used for

sealing purpose and spiral tubes are used for helping the distribution of the resin.

Sometimes, it is necessary to use a peel ply and a breather layer.

At the beginning of the project vacuum infusion as described above was used it

but a metal plate played the role of a second-sided mould. This metal plate helps to

avoid wavy surfaces. In that case, the plastic film closed all the system. It was decided

to use metal on the top because, in one of the first attempts using only the bagging film,

the laminates were wavy. The most successful configurations using vacuum infusion is

shown in Figure 8 -right and Figure 8-left shows more in detail the area surrounded the

inlet port before closing the system with a bagging film.

37

Figure 8. Vacuum infusion process setup.

Bagging film folds avoid damages in the vacuum bag due to the sharp edges of

the metal plate. It is recommended to do smaller folds at both sides of the inlet and

outlet ports also. These smaller folds allow sealing this area properly where it is easier

to have a leak there during evacuation. The fabric with perpendicular fibres to the resin

flow direction is close to the outlet port with the purpose to reduce the flow resin speed

and promote wetting the preform. Finally, the flow layer enhances the contact between

the feed line and the preform. To summarize, Table 13 shows the vacuum infusion

parameters.

Table 13. Vacuum infusion parameters.

Infused plate dimensions 30 cm x 30 cm

Vacuum pressure 50 mbar

Number of inlet points 1

Number of outlet points 1

There are two main groups of carbon plates manufactured using vacuum

infusion:

38

• Group 1: Traditional vacuum infusion with the reinforcements dispersed as

bundles.

• Group 2: Vacuum infusion with the addition of external load and the

reinforcements dispersed as single fibres using an air compressor.

The following Table 14 and Table 15 describe some process areas that require

refinement and most of these issues were improved using RTM.

Table 14. Group 1: Traditional vacuum infusion with the reinforcements dispersed as bundles.

Design improvement area Description

Control of the composite thickness The thickness was not uniform, and it was

higher than desired. Using a rigid metal plate

on the top has solved partially the issues but

also there are areas which more amount of

fibres than others due to the manual fibre

distribution and the lower compaction during

the vacuum procedure.

Resin rich areas There were some areas without

reinforcements that fill up only by resin. So,

the fibres volume content was not the same

around the plate. This affects the mechanical

performance of the material.

Uneven resin dispersion The dispersion of fibres as bundles make

worse the resin impregnation. The fibres

bundles allow that a set of fibres are near

each other and sometimes it is difficult for

the resin to impregnate all of them. If there is

no resin among the fibres, there is no matrix

to transfer the load properly so the material

will fail at lower loads.

To overcome the drawbacks, fibres distribution has changed using an air

compressor and a rigid thicker metal plate on the top. It has solved the problem related

to the resin-rich areas successfully. Figure 9 shows the change in the morphology

preform.

39

Figure 9. Carbon fibres distribution before (left) and after (right) using an air compressor.

Table 15. Group 2: Vacuum infusion with the addition of external load and the reinforcements

dispersed as single fibres using an air compressor.

Design improvements area Description

Control of composite plate thickness (before

external pressure)

The distribution of the fibres as single fibres

difficult compaction during the vacuum

process. The dry preform volume is higher

than the bundle's arrangement. The vacuum

compaction was not enough, and the

thickness has been higher.

Control of the thickness with external

pressure

External pressure was applied at the

beginning of the curing process and/or during

infusion with the purpose to help the

compaction of the plate and sucking the

excess resin. Three different values of

pressure (0.4, 0.5 and 1 MPa) have not led to

significant differences and the thickness was

comparable among the plates.

Curing conditions In the first attempt using external pressure,

the system was not pre-heat and some

irregularities appeared in the surface of the

plate related to an inappropriate cure cycle.

So, pre-heating at 30°C seems essential for

the resin system.

40

3.2.2 Resin Transfer Moulding (RTM) procedure

After several failed efforts, it was decided to use another liquid composite

moulding process, Resin Transfer Moulding (RTM). One of the main reasons that lead to

taking this decision was that RTM commonly allows better control of the thickness, at

least using traditional reinforcing fabric.

Composites samples were fabricated using RTM according to the detailed stages

as follows:

1. Cutting carbon and flax rovings using a manual cutting machine (Aplicator

Group ®) to 25 mm in length.

2. Applying Zyvax® release agent to the mould for easy removal to the part, at

least twice or three times, ten minutes interval for each waxing.

3. Weighting the reinforcements, dispersing and mixing them using an air

compressor.

4. Displacing dry fibres onto the mould trying a uniform distribution. It should

be avoiding the contact of the reinforcements with the sealant. Otherwise,

there will be leaks during the vacuum procedure (Figure 10-Left).

I

Figure 10. Charging the fibres in the female mould of RTM.

5. Placing adjustment plastic blocks in the mould sideways to allow a minimum

gap thickness mould (Figure 10-right).

6. Closing and clamping the mould.

7. Clamp the inlet, connect the outlet port to the vacuum pump.

41

8. The preform is evacuated until a vacuum of 30-50 mbar is reached. Checking

if there are leaks. For that, shut off the valve which connects the system with

the vacuum pump and check how much differ the vacuum measuring of the

system. If there is a good vacuum, it will not change or at least, it could

change a couple of digits but slower. A scheme of the designed system is

shown in Figure 11.

Figure 11. Scheme of RTM system

9. Pre-heating the mould at 30°C.

10. Preparing the dispensing equipment, connecting it to the pressured air

supply. Dispensing equipment helps the injection of the resin under pressure.

11. Preparing matrix mixing which should be in excess for a reason that will be

explained later. If there are bubbles in the mixture, leave it in the air for a

short time (between five to ten minutes) for bubbles to disappear.

12. The pressure difference between the cavity and the outside in the resin

bucket helps the starting the infusion using only the vacuum pump. For our

resin system, it was used 200 mbar to assist in resin flow. After a couple of

minutes, the resin will be injected under pressure into the mould cavity

helping the impregnation of the resin. It was used a pressure of 2.5 to 3 bar.

13. When the resin reaches the outlet port, an excess of resin will start to

eliminate while the dispensing equipment is keeping inject the resin under

pressure. That is the reason why is recommended to prepare a mixture of

resin in excess. In the beginning, this resin in the outlet might contain air

bubbles then, they should be free of them. In that time, the vacuum pump

can shut off and it is considered that the impregnation was completed. The

dispensing equipment is still injecting resin until it starts to gel. This

42

procedure helps to get a better impregnation of the resin and reduce the

presence of voids in the final plate.

14. Increasing the temperature of the mould, for our system, at 50°C for 15 h. It

is possible to keep on the dispensing equipment as long as there is enough

resin. Otherwise, if the resin runs out before the gel time there is a risk that

some air will enter the cavity.

15. After curing, the composite plate is removed from the mould.

To summarize, Table 16 shows the RTM parameters:

Table 16. RTM parameters

Infused plate dimensions 28 cm x 22,6 cm

Number of inlet points 1

Number of outlet points 1

Vacuum pressure 200 mbar

Injection pressure 2.5 bar-3 bar

The value of vacuum pressure chosen was based on an excessive formation of

bubbles in the inlet port during failed attempts of vacuum infusion using 50 mbar. It is

believed that an excessive pressure difference between the cavity and atmospheric

pressure in the resin bucket helps the formation of air bubbles in this resin system. That

is the reason why 200 mbar instead of 50 mbar was used for RTM process.

The injection pressure of 2.5 to 3 bar is a discrete value. If there is a smaller leak

in the system, a higher pressure value increases the risk of a resin leakage. Indeed, it

was noted a good impregnation process of the resultant composites.

43

3.3 Characterization techniques

3.3.1 Mechanical Testing

Mechanical testing of composite materials involves a range variety of test types.

Tension, compression, flexural and hardness are best known. There is plenty of

standards (ASTM, ISO, CEN) to determine conditions, suggest procedures and criteria

about how to mechanical characterize a material or a component before their

applications. Mechanical properties are essential for design, manufacturing process,

analysis of the product, quality control and application performance requirements. [66],

[67]

Tensile Tests

The unidirectional tension test applies the load in the longitudinal axis of the

specimen. Due to this load, the specimen stretches or elongates to the breaking point.

Such tests produce strain-stress curves which facilities to determine the mechanical

properties such as tensile modulus, ultimate tensile strength, elongation at yield and

others. Tensile testing is presented in Figure 12. [67]

Figure 12. Tension test experimental setup.

44

Tensile tests were carried out following ASTM D3039 [68] using a universal

testing machine (Instron 3366-10kN) at a crosshead rate of 1 mm/min. The gauge length

was 50 mm and the distance between the grips was 100 mm. The applied force versus

extension was recorded until 0.3 % of deformation at room temperature.

The tensile modulus was calculated doing a linear regression of the values

between 0.1% to 0.3% of deformation. For each composite type, six tests were carried

out and the mean values are used for discussion.

Flexural Tests

A rectangular cross-section sample is deflected at a constant rate as follows:

Figure 13. Types of flexural tests (Left: procedure A. Right: procedure B). [69]

The loading (P) is applied in one (Procedure A) or two points (Procedure B). These

configurations produce flexural conditions and some shear loadings. However, shear

strength is constant regardless of the distance between the supports and a higher

support span length produces a negligible effect of the shear strengths. [69]

Flexural tests were performed following ASTM 7264 [69] using a universal testing

machine (Instron 4411-5kN). The rate of crosshead movement was set at 4.48 mm/min

(rate of straining of the outer fibre is 0.01 mm/mm/min), calculated via the method

outline in ASTM D790 for procedure A. The span length to thickness ratio was 32:1,

which it was considered the average thickness of each plate to calculate the span length.

Flexural testing is presented in Figure 14.

45

Figure 14. Flexural testing being conducted.

For each composite type, six tests were carried out and the mean values are used

for discussion. To study inhomogeneities through the thickness, both surfaces in contact

with the mould were tested under elastic bending deformations. There was no

significant difference in flexural modulus values between both sides and the results are

presented in Appendix 2.

Samples preparation for mechanical tests

Samples should be representative of the bulk material. The most common widths

are 20 or 25 mm for tensile tests. However, 25 mm is the length of carbon and flax fibres

in this study. So, it is decided to check how is the effect of wider and narrow samples.

For this purpose, a wider sample (40 mm) had tested in the elastic region, then it was

cut in the middle and tested again (these results are described in detail in Appendix 3).

The tensile modulus changed, so it was worked with the following nonstandard

dimensions:

- Tensile specimens: thickness’s plate x 30 mm x 210 mm

- Flexural specimens: thickness’s plate x 30 mm x 105 mm

Due to the maximum load capacity of the universal testing machine (10 kN), all

tension tests were performed under elastic deformations. So, tensile samples without

any damage were cut lengthwise in the middle to make flexural samples.

The samples were individually machined from the plates using a band saw

(Cocraft ® HB 10L). Then, their edges were ground until P600 or P1200 to ensure smooth

46

surfaces and regular width. No tabs were utilized. Due to the hygroscopic behaviour of

the natural fibres which affects the mechanical performance, flax specimens were not

dried in any stage of the sample preparation.

3.3.2 Optical microscopy

Optical microscopy is a characterization technique of materials that allows

studying the microstructure of the samples. It consists of seeing the plane surface of a

sample, suitably prepared using visible light and a system of lenses to magnify images of

small samples. [70]

Samples for microstructure observation

Pieces of 2 cm x 1 cm with their thickness were cut using a band saw (Cocraft ®

HB 10L). Samples were embedded in an acrylic resin Durocit ®. Grinding was performed

using Buehler® (MetaServ 250) apparatus and using abrasive papers from the same

brand. Several steps were carried on decreasing particle size and gaining good surfaces

status. Regarding polishing, Struers® (Labopol-20) and Buehler ® (Phoenix 4000)

apparatus were used it. The detailed grind and polish guidance are given in Table 17.

Table 17. Polish parameters for microscope observation.

Fine grinding

Polish

parameter Grinding Liquid diamond Final polishing

Abrasive paper from P240 to

P1250 9 μm 3 μm 1 μm

colloidal silica

polishing suspension

Speed (rpm) 300 300 300 300 150

Time (min) - 4 1.5 1 1

Lubricant water Kemet® W2 Kemet®

W2

Kemet®

W2 distilled water

Note: CG-L required 2 min in the final polishing stage.

The stiffness of the constituents is different. So, it is quite difficult to obtain

images with a surface without any scratches. Some damages in the interphase and even

removal of flax were produced during the metallographic preparation.

47

It was observed the transverse and longitudinal section of the plates. For the

cross-section, it was taken some pieces of material across the thickness of the plate from

regions close to the inlet port, middle of the plate and outlet port. It was compared the

microstructure of the material, phases, resin-rich areas and presence of voids. Regarding

the longitudinal section, one of the sides in contact with the mould was ground and

polished, and it was evaluated the orientation of the reinforcements.

For cross-section samples, a Nikon (Eclipse MA 200) equipped with a CCD camera

and connected with a computer was used. Whereas for the longitudinal sections, it was

used a Zeiss axioskop ® optical microscope.

4. Preliminary analysis

4.1 Microstructure observation

4.1.1 Longitudinal section micrographs

Micrographs from the longitudinal section, across the length of the plate, are

shown in the following figures. Figure 15 and Figure 16 show CC-R and CF-R,

respectively. Dark field illumination and NCB (Neutral Colour Balance) filter was used it.

Figure 15. Representative longitudinal section of carbon composite (scale 200μm).

The surfaced of the polished specimen displays a clear delineation between the

constituents. The bright lines correspond to the fibres and the matrix is the opposite.

There are areas with small bundles of carbon sharing the orientation whereas others

have single fibres.

48

Figure 16. Representative longitudinal section of flax hybrid composite with short random fibres. (scale

200μm)

Carbon and matrix look exactly as in Figure 15. The bright areas without any

arrangement of lines correspond to the flax fibres. As before, there some black areas

which are voids.

The fact that most of the carbon fibres look with line morphology implies that

there is a fibres orientation close to 90 ° with respect to the observation plane.

4.1.2 Cross-section micrographs

Figure 17 shows the representative cross-section of CC-R and CF-R. The

regularity of fibre shape, dimension, and distribution between these two fibres are

different. Flax fibres appear mainly as an accumulation of fibres (yarn) which has a

circular geometry and similar contrast to the matrix, and carbon fibres appear are shiny

areas with the architecture of single fibres and small bundles. It was observed some

voids inside the yarns and in the matrix. Due to the porous nature of natural fibres

sometimes is difficult to achieve a complete impregnation of the fibres even using a low

viscosity resin and some voids appear inside the yarns.

Figure 17. Representative cross-sections of CC-R (left) and CF-R (right) composites.

49

The RTM process is assisted with a vacuum pump with the main purpose to help

in the impregnation process. When the resin appears in the outlet port, it is considered

that the impregnation is completed but the vacuum helps to remove this extra resin

through the outlet line and it is recommended to still injecting the resin into the system

with the purpose to reduce the air bubbles in the plate. If the system stills injecting resin

even after the impregnation is completed, the number of air bubbles will be reduced

and there might be a little bit higher number close to the outlet. Most of the cross-

section micrographs showed that the presence of voids is higher in the outlet region as

was expected. Nevertheless, CC-R has shown the presence of multiple voids throughout

the plate (Figure 18).

Turning now to the experimental evidence on quality of compaction, it was

observed that a great distribution of the fibres in the glass hybrid composite (Figure 19).

While the flax hybrid configurations showed more resin-rich areas (Figure 20).

Figure 18. Cross-section micrographs CC-R. (Top-left: inlet region, top-right: middle region, bottom: outlet

region).

50

The main dark areas in Figure 20 corresponds to flax fibres with close to 90 °

orientation to the plane observation which was easily damaged during metallographic

preparation.

Regarding CF-L, it was interesting that in some areas, the inner layer of flax has

been displaced towards the outer surface (Figure 21)

Figure 19. Representative cross-section CG-L.

Figure 20. Representative cross-section CF-R

51

Figure 21. Micrograph of CF-L close to the outer surface.

Unfortunately, it had not been possible to measure the different phases in hybrid

configurations using the micrographs. The contrast between the flax and the matrix is

very similar. Other techniques such as acid digestion or resin burning-off method are not

suitable because these will damage the natural fibres.

52

5.Findings and results

5.1 CC-R and flax hybrid composites

5.1.1 Tensile tests

Figure 22 shows the mean values of tensile modules for flax hybrid composites

and CC-R and also the normalized values. The error bars in Figure 22 (left) correspond

to the deviation standard of the obtained data. Representative curves of tensile tests

are in Appendix 4.

Contrary to expectations [35], the values of hybrid composites are very similar.

So, there was no evidence for the hybridization effect in this mechanical property

between flax hybrid composites with the same composition of reinforcements but

different configurations (random and layers). Unfortunately, these samples could only

test until 0.3 % of deformation due to the maximum load capacity of the tensile machine

(10 kN). So, there is no information about other properties like failure strength/strain,

and it is unclear whether there will be differences between the properties of the hybrid

composites under tension loads.

The tensile modulus of carbon composite is slightly higher than hybrid

composites (27 GPa versus 24 GPa). The performance of composites is governed by

carbon content because carbon fibres tensile properties are much higher than those of

flax fibres. While all the composites have approximately the same volume fraction of

reinforcements, carbon content has a higher effect in Young’s modulus values than flax

amount. The carbon content is ~35% in carbon pure composite while in the hybrid is

27.1

24.3

23.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0

CC-R

CF-R

CF-L

Tensile Modulus (GPa)

Co

mp

osi

tes

1.000.90

0.85

0.00

0.20

0.40

0.60

0.80

1.00

CC-R CF-R CF-L

Figure 22. Tensile modulus of flax hybrid composites and carbon composites (Left: mean values. Right:

normalized values)

53

~23% which could explain the observed differences. However, compared to the

normalized values (Figure 22-right), it is encouraged that Young's modulus of the flax

hybrid configurations is only around 10 to 15% below the non-hybrid configuration. A

possible explanation for this might be that the manufacturing process and the voids

observed in CC-R have a negative impact on its tensile modulus. Nevertheless, all the

composites are manufactured in the same conditions and, interestingly, the tensile

modulus of hybrid composites is at least 85% of carbon composite tensile modulus.

5.1.2 Flexural tests

Figure 23 shows mean values of flexural properties for CC-R and flax hybrid

composites, error bars correspond to the deviation standard of the obtained data.

Figure 23. Flexural properties of CC-R and flax hybrid composites.

What stands out in this figure is the difference between the two flax hybrid

composites. Materials with the same composition show different behaviour under

flexural loadings when the configuration changes from random to layers. There was a

significant increase by about 50% and 25% in modulus and flexural strength respectively,

between these two configurations.

These results agree with the findings of other studies [5], [20], in which the

configuration of the hybrid composites influence the bending properties of the material.

It is expected that the sequence with carbon in the outer layers such as CFFC has higher

27.9 19.0 28.5

366

287

360

0

50

100

150

200

250

300

350

400

450

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

CC-R CF-R CF-L

Flex

ura

l Str

engt

h (

MP

a)

Flex

ura

l Mo

du

lus

(GP

a)

54

flexural properties than vice versa. In this study, CF-L has distributed more carbon than

random distribution in the outer layer. As in tensile, the carbon content mostly governed

the mechanical performance so the presence of flax in the outer layer decreases the

bending properties.

Besides, what is interesting about the data in this graph is that the values of the

flexural modulus of CC-R and CF-L are comparable. It is difficult to explain this result, but

it might be related to manufacturing and the presence of voids in the carbon composite

or just the interaction between the reinforcements in the hybrid configuration. These

findings suggest that using the same manufacturing technique, the performance of the

layer configuration with flax was comparable to the carbon composite even if the latter

had a higher amount of carbon.

5.2 Carbon/Glass hybrid composites (CG-L)

CG-L is similar to CF-L in the following points:

▪ Same amount of carbon.

▪ Same configuration, hybridization by layers.

▪ It was used the same amount of flax and glass reinforcements. In other

words, the layer of flax of CF-L was replaced with a nonwoven of short

glass fibres with the same weight.

Figure 24 shows the mechanical properties under tension (left) and bending

(right) of the CG-L.

27.1

24.3

23.0

28.1

0.0 10.0 20.0 30.0

CC-R

CF-R

CF-L

CG-L

Tensile Modulus (GPa)

Co

mp

osi

tes

27.9 19.0 28.5 24.7

366

287

360411

0

50

100

150

200

250

300

350

400

450

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

CC-R CF-R CF-L CG-L

Flex

ura

l Str

engt

h (

MP

a)

Flex

ura

l Mo

du

lus

(GP

a)

Figure 24. Mechanical properties of manufactured composites (Left: Tensile modulus. Right: Flexural properties)

55

What is interesting about the data in this graph is that CG-L values are

comparable to carbon composite or higher. Even though the amount of carbon is less in

the CG-L, it shows an outstanding value. Using nonwoven, fibres seem better distributed

and compacted, which could explain the observed values.

What is curious about this result is that slight differences could change drastically

the mechanical performance. Although mechanical performance is mostly governed by

carbon content using the nonwoven glass, CG-L could have competed with the reference

sample. These data must be interpreted with caution because there were changes in the

reinforcement’s morphology. Besides, the nonwoven allows achieving better

compaction of the raw materials using the manufacturing setup described above.

5.3 Representative curves of flexural tests

Typical stress-strain curves obtained from three-point bending tests can be seen

in Figure 25. Only one typical curve, with results comparable to the average values per

set of specimens is plotted here. Appendix 4 shows the stress-strain curves for all

batches of composites.

Figure 25. Examples of experimental flexural stress-strain curves for different hybrids and non-

hybrid composites.

The lowest curve corresponds to random flax hybrid composite whereas the two

highest belong to the glass hybrid composite and carbon composite. The curves show

similar linear behaviour at the beginning with the exception of the CF-R. Major

56

differences can be observed in the maximum value of the curves, which corresponds to

the flexural strength. The behaviour of carbon composite and hybrid layer

configurations is dominated by carbon content in the outer surfaces, resulting in linear

load-deflection curves, with failure initiating at significantly lower flexural strains.

CF-R has the lowest mechanical properties but the highest strain at failure. This

highest strain at failure agrees with the finding of other studies [5], in which flax content

is related to a decrease in the flexural modulus and flexural strength but increase the

strain at failure values. However, as mentioned before the stacking sequence is a

relevant parameter to understand the behaviour of hybrid composites. Under bending

loadings, the material is subjected to different loads, compression in the top and tension

in the bottom, and that explains why is relevant the material locates in the outer surface

of the specimen.[5]

There is a higher drop of load in CG-L which means a catastrophic failure after

the maximum load is reached. Nevertheless, the flax hybrid composites curves have

several drops of load (primary and final failure). Hybridization of carbon and flax

introduced further variables, which could explain this behaviour at the end of the stress-

strain curves.

Figure 26 gives an overview of the fracture pattern after bending tests. The

bottom layers under tension suffered delamination and more fibre breakage. There

were observed also damages in the compression side with the exception of the CF-R. CF-

R failed on the tensile side with minimal external damage visible in the compressed

topmost layer.

57

Figure 26. Failed samples under flexural loading. (a) CC-R (b) CG-L (c) CF-L (d)CF-R.

58

6.Conclusions

Growing environmental concerns have led to finding new suitable composites

with more eco-friendly behaviour. One alternative is to use natural fibres instead of

synthetic fibres in composite materials. However natural fibres have some limitations

regarding mechanical performance. With the purpose to achieve a balance between

mechanical performance and environmental concerns is studied the hybrid form.

The thesis was undertaken to design and manufacture carbon/flax hybrid

composites as well as evaluate their mechanical performance. This goal has got partially

because it could not achieve the desire composition (35% carbon-20% flax) to evaluate

the potential application for battery housing for electric cars. However, this study

developed a RTM procedure to manufacture short hybrid composites without a textile

or fabric obtained acceptable quality in the resultant material. Besides, mechanical and

microstructural characterization was performed in the fabricated composites.

The mechanical characterization confirmed the hybridization effect and the

importance of the stacking sequence of the different reinforcements in the hybrid

composite. Flexural properties were found to be significantly influenced by the

configuration of the fibres. The CF-L configuration had higher flexural strength and

modulus than CF-R with the same composition of fibres. Considering that it was used

the same manufacturing process for all the plates is encouraging that there were smaller

differences between hybrid and non-hybrid configurations. For instance, the tensile

modulus of flax hybrid composites was only around 10 to 15% below the pure carbon

composite whereas the flexural properties were comparable between CF-L and CC-R.

The microstructure revealed information about the compaction and distribution

of the fibres. The presence of voids in the CC-R might explain its lower mechanical

properties. Regarding flax hybrid composites, it was observed some voids inside the

yarns mainly.

Finally, the manufacturing process was adapted successfully to a third material

combination, nonwoven of short random glass fibres, which was characterized in the

same way as the others. Glass hybrid composites show outstanding values in tension as

well bending tests. Even though these data must be interpreted with caution because

there were changes in the reinforcement’s morphology using the nonwoven, the

obtained data is valuable and interesting.

59

The information gathered shows the potential of carbon/flax composites decreasing the

dependency on non-renewable materials (e.g by substituting some proportion of the

carbon fibres with flax fibres) and lowering the cost because flax is cheaper.

Furthermore, this sustainable configuration shows interesting mechanical performance

under bending loads

7. Future work

It is recommended that further research be undertaken in the following areas:

→ Improvement of the thickness control during the manufacturing process

(it might use another mould or pre compact the raw materials in a

previous stage).

→ Find a method to control or measure the different phases in the hybrid

composite.

→ Analysis of the failure mechanism in the bending tests, especially the

failure initiation.

→ Study the effect of more layers and configuration using UD

reinforcements and different compositions.

→ Investigate the combination of flax and glass fibres.

→ Working with recycled carbon fibres and see the effect on the mechanical

properties.

Further tension tests until failure need to be carried out with the purpose to

understand the hybridization in this type of loadings.

60

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67

Appendix 1

Calculation of reinforcement weight for manufacturing.

The idea is to determine the weight of each reinforcement (e.g. wf1 and wf2 in

the case of hybrid composites with two types of reinforcements) and resin then it is

needed. For that, it is necessary to know in advance the following:

✓ Dimensions of the composite laminate (pre-defined dimensions).

✓ The desired volume fraction of each constituent (matrix and

reinforcements).

✓ The density of each constituent.

The calculation for reinforcement weight involves the next steps:

1- Calculate the composite density (ρc) using the rule of mixtures as given below:

𝜌𝑐 = 𝜌𝑓1 ∙ 𝑉𝑓1 + 𝜌𝑓2 ∙ 𝑉𝑓2 + 𝜌𝑚 ∙ (1 − 𝑉𝑓1 − 𝑉𝑓2) (17)

Where ρ is the density and the subindex are related to the constituent.

2- Calculate the composite weight (wc) using the composite density and the volume

of the sample.

3- Calculate the weight fraction of each reinforcement (Wfi) using the following

equations:

𝑊𝑓1 =𝑉𝑓1.𝜌𝑓1

𝑉𝑓1∙𝜌𝑓1+𝑉𝑓2∙𝜌𝑓2+(1−𝑉𝑓1−𝑉𝑓2)∙𝜌𝑚 (18)

𝑊𝑓2 =𝑉𝑓2.𝜌𝑓2

𝑉𝑓1∙𝜌𝑓1+𝑉𝑓2∙𝜌𝑓2+(1−𝑉𝑓1−𝑉𝑓2)∙𝜌𝑚 (19)

𝑊𝑚 = 1 − 𝑊𝑓1 − 𝑊𝑓2 (20)

4- The weight of reinforcement (wf) is obtained using composite weight (Wc) and

weight fraction of each reinforcement (Wf) as given below:

68

𝑤𝑓1 = 𝑤𝑐 ∙ 𝑊𝑓1 (21)

𝑤𝑓2 = 𝑤𝑐 ∙ 𝑊𝑓2 (22)

Indeed, the same expression is used for the weight of resin:

𝑤𝑚 = 𝑤𝑐 ∙ 𝑊𝑚 (23)

69

Appendix 2

Studying inhomogeneities through the thickness.

Table 18 shows the results from elastic bending deformations of the

manufactured composites when the load is applied to both surfaces of the specimens.

In this approach, the load was applied on both sides of the samples and was measured

the modulus to ensure homogeneity across the thickness.

Table 18. Flexural results under elastic deformations.

Thickness(mm) Width(mm) Density (g/cm3) Flexural Modulus (GPa)

Side A Side B

CC

-R

2.61 31.6 1.36 31.31 31.42

2.61 39.2 1.20 29.55 29.66

2.65 26.4 1.41 29.33 29.51

2.79 32.9 1.44 30.49 31.22

2.73 33.4 1.43 33.36 33.27

2.81 30.5 1.42 27.33 27.68

2.74 30.8 1.37 20.53 20.17

CF-

R

2.61 31.9 - 22.56 21.98

2.70 40.4 - 25.01 25.18

2.66 30.8 1.35 16.76 16.69

2.74 31.4 1.33 19.95 20.06

2.64 30.6 1.35 18.57 18.39

2.67 31.1 1.35 20.49 20.54

CF-

L

2.23 31.9 1.36 27.13 27.01

2.27 31.7 1.35 22.59 22.56

2.55 31.6 1.33 24.50 24.97

2.45 31.6 1.36 37.06 37.04

2.60 31.9 1.32 26.43 26.48

2.50 31.8 1.36 32.39 32.53

CG

-L

2.17 29.7 1.43 22.73 22.78

2.13 30.8 1.45 23.96 23.74

2.18 31.5 1.45 24.85 24.61

2.20 31.4 1.45 27.40 27.34

2.23 30.9 1.44 22.52 22.57

2.28 31.5 1.45 26.62 26.75

70

Density measurements were calculated based on the dimensions and weight of

the samples. It is not the most precise technique but the information was only used as

an additional quality control method.

The thickness value affects the real volume fraction of the constituents. If the

plate is thicker than was expected, the volume fraction might be lower than pre-defined

or this variation might help a major concentration of fibres in certain areas. Density is

also affected by the variation of the thickness, and by the distribution of the fibres.

Finally, the presence of voids also affect these parameters, and it should be considered

with more detail in future analysis.

71

Appendix 3

Wider and narrow samples

With the purpose to study the effect of the width of the samples in the

mechanical properties of hybrid composites with fibres of 25 mm of length, wider and

narrow samples were tested under elastic deformations.

Regarding tensile tests, a sample of 40 mm was tested under elastic

deformations. Then, this sample was cut in the middle and the two samples of ≈20mm

were tested again. Table 19 summarizes the results.

Table 19. Tensile results of wider and narrow samples.

Turning now to flexural tests, a sample of 30 mm of width had been tested under

elastic deformations. Then, it was cut into two samples of ≈15 mm and tested again.

Another sample of 40 mm of width had been tested. Afterwards, it was cut into samples

of ≈30 mm and ≈10 mm and tested under bending loads. Table 20 summarizes the

bending results.

Thickness (mm) Width (mm) Density (g/cm3) Tensile Modulus (GPa)

C-R

2.51 40.5 1.29 26.58

2.46 19.1 1.22 19.50

2.57 20.1 1.35 26.49

CF-

R

2.53 39.6 1.37 24.11

2.50 19.0 1.33 21.12

2.56 18.5 1.40 24.56

72

Table 20. Flexural results of wider and narrow samples.

Thickness (mm) Width (mm) Density (g/cm3) Flexural modulus (GPa)

CC

-R

2.61 31.6 1.36 31.31

2.59 15.4 1.34 28.12

2.64 14.1 1.34 33.13

2.61 39.2 1.20 29.55

2.54 10.5 1.31 23.94

2.65 26.4 1.41 29.33

2.64 12.5 1.42 27.85

2.65 12.4 1.39 27.08

CF-

R

2.61 31.9 - 22.56

2.60 13.4 1.37 23.33

2.62 15.5 1.36 20.13

2.70 40.4 - 25.01

In both tests, there was a remarkable difference between wider and narrow

samples so it was decided to work with samples of 30 mm of width.

73

Appendix 4

Representative curves of tensile tests

Typical stress-strain curves obtained from tensile tests can be seen in Figure 27.

Only one typical curve, with results comparable to the average values per set of

specimens, is plotted here.

Figure 27. Examples of experimental tensile stress-strain curves for different hybrids and non-

hybrid composites.

74

Stress-Strain curves obtaining from bending tests.

The following figures show the flexural stress-strain curves obtaining from

bending tests.

Figure 28. Flexural stress-strain curves of CC-R.

Figure 29. Flexural stress-strain curves of CF-R.

75

Figure 30. Flexural stress-strain curves of CF-L.

Figure 31. Flexural stress-strain curves of CG-L.