Post on 22-May-2022
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
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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
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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
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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,
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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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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.