Hybridisation of self-reinforced composites: modeling and verifying a new hybrid concept

1. Introduction

A common problem of composites is their brittleness. This is mainly due to the brittle nature of the fibres

and the low off-axis strength of impregnated bundels or laminates. A commonly applied solution for this

problem is toughnening the matrix, but this solution has its limitations. In the seventies an unexpected

alternative popped up. Capiati et al. combined highly-oriented polyethylene (PE) fibres with an un-

oriented polyethylene matrix in the search for low density and easy-to-recycle composites. A new class

of composite materials was born, namely self-reinforced composites (SRC's).

SRC's combine a low density with an exceptional toughness. The most promising SRC up to now is self-

reinforced polypropylene (SRPP). This gives an extremely high tougness (notched Izod impact > 4750

J/m at 20C). On top of that it is one of the few materials which shows a increasing tougness at lower

temperatures (7000 J/m at -40C). The biggest limitation for widespread application of this promising

material is the low stiffness (3-5 GPa). In order to exploit the exceptional properties of SRPP, we will

hybridise the SRPP to increase the stiffness. In the present case, hybridizing means combining two

fibers. On one hand, there's the drawn PP fibre with a stiffness of 10-15 GPa, a strength of 450 MPa and

a failure strain of 15-20%. On the other hand we will use different fibres which have a stiffness ranging

from 30 tot 230 GPa and a failure strain smaller than 3%. These hybrid SRC's will allow us to break

through the toughness-stiffness dilemma (see figure below).

2. The hybrid effect

When combining two fibres synergistic effects can appear. This is called the hybrid effect and is simply a

deviation from simple rules-of-mixture. For stiffness, we can expect a linear rule-of-mixture to be

appropriate, but for other properties, like strength, toughness and failure strain, this isn't always the case.

This hybrid effect has been thoroughly examined in the '70's and 80's for carbon-glass and for carbon-

aramid hybrids. It was explained as a combination of influence of the fracture propagation and process-

induced shrinkage.

a) Fracture propagation

Fracture of unidirectional impregnated composites is a statistical phenomenon. This is because the fibers

do not possess a unique strength, they follow a certain distribution. Most often, this is a Weibull

distribution. At a certain stress level, the weakest fiber will break and locally lose its load transfer

capability. At the fracture location, the matrix is loaded in shear and this ensures the longitudinal fibre

stress in the broken fiber recovers over a certain length. The load, which was initially carried by the now

broken fiber, is transferred to the surrounding fibers. Therefore, the surrounding fibers are subjected to

stress concentrations. The odds that one of the surrounding fibers is loaded to its maximal strength thus

increases. More and more fibers will break when the composite is increasingly loaded. This finally leads

to failure of the entire composite.

Of course, adding a second, ductile fiber will greatly influence this. The stress concentrations will depend

on the kind of neighbouring fiber. On top of that, less brittle fibers will be present. This is simply because

the odds of having weak, brittle fibers in the composite decrease when less brittle fibers are present. To

conclude, the odds that a crack can propagate will most likely decrease when a second, ductile fiber is

added.

b) Process-induced shrinkage

An important difference with the classical hybrid effect lies in the shrinkage. When a stretched polymer

fiber is heated, it will have the tendency to shrink. The shrinkage is irreversible and can be very large, up

to 80% for stretched PP. The presence of the brittle fibers will restrain this shrinkage. Therefore the

polymer fiber will have residual tensile stress. However, this only has a minor influence on the failure of

the hybrid composite. Much more important is the residual compressive stress in the brittle fibers. This

compressive stress will counteract the applied tensile stresses and thus increase the apparant tensile

strain. Because the polymer fibre is taken very close to its melting temperature, the yield stress is very

low and most of the stresses will relaxes. According to previous experiments, this shrinkage stress is on

the order of magnitude of 10 MPa.

Compared to classical hybrids however, the difference in coefficients of thermal expansion (CTE) is a lot

bigger. Compacted PP tape typically has a CTE of 40-70.10-6 K

-1. Classical fibres have CTE's which are

a lot lower:

-glass: 5.10-6

K-1

-carbon: -0,5 - 0.5.10-6

K-1

-aramid: -1 .10-6

K-1

Thus the thermal stresses build up in hybrid SRC's is a lot bigger than in classical hybrids. However, it

can also lead to curved samples or buckled fibers. Therefore, the shrinkage has to be controlled very

carefully.

3. Research strategy

a) Unidirectional composites

At first, we will investigate unidirectional (UD) composites. These composites can be modelled more

easily than multidirectional composites. There are basically two different hybridisation routes. The first

one is intrayarn (shown below).

The second hybridisation routes is obtained by stacking different non-hybrid layers on top of each other.

This is shown below.

The important parameters to be examined are:

Parameter Invloed

Degree of dispersion Better dispersion increases the hybrid effect

Stiffness of hybridisation

fiber

Influences the stress concentrations and buckling of

individual fibres

Parameter Invloed

Volume fraction of

hybridization fiber Influences the hybrid effect and buckling

Volumefraction of matrix Thermal conductivity and heat capacity influence

the amount of molten polymer fiber

Stiffness of matrix Higher matrix stiffness gives less buckling

Yield stress of matrix Higher yield stress gives better load transfer and

increases strength

Matrix-fiber interphase Stronger interfase increases load-sharing and leads

to bigger hybrid effect

b) Multidirectional composites

Based on intrayarn and interlayer hybrids, it's possible to make multidirectional composites. However,

there is a third possibility to make hybrids, namely intralayer or co-weaving.

Most polymer composites contain fibers in at least two different directions. That's why it's useful to set up

models for multidirectional composites. These models can be split up in to three parts:

- Damage initiation in transverse layers

- Damage propagation in transverse layers

- Breaking of longitudinal layers

The first two parts will be modelled using previously developed models of the CMG. For the last part, we

can use the results from our unidirectional model.

4. Conclusion

Hybrid SRC's have a huge potential, but a lot of its features are currently unknown. This research will

help us understand the fundamentals of its behaviour and help us optimise their properties. In the end,

we will be able to develop better hybrid SRC's in a rational, scientific way. On top of that, the developed

models are generic and also applicable on classical hybrids.

3. Review of Literature: .

Acharya and Samantarai (2012) investigated the tribo potential of biomass

based carbon black filler in epoxy composite. They observed that incorporation of

Rice Husk Char in to epoxy significantly reduce abrasive wear loss.

Ndazi et al. (2007) studied chemical and physical modifications of rice husk

for use as composites panels. They found that chemical modification of rice husks by

NaOH improves the adhesion properties of rice husk in composites due to removal of

surface impurities such as silica and carboxylic compounds, which blocks reactive

chemical groups.

Studies are also available on aluminium reinforced by silicon carbide

from rice husk. The reports based on these studies says that the reinforced aluminium

not only has a good combination of room temperature specific strength and modulus

and excellent thermal stability, but it also can be processed by normal metal working

technique. Such materials are increasingly considered for aerospace applications

where high stiffness and strength to weight ratios are additional advantages.

The applications of Rice Husk Ash (RHA) as a filler in plastics is

relatively limited mainly due to polypropylene (PP). As reported for a PP composites

30, with an increase in the RHA loading, its flexural modulus and density increases,

where as its tensile strength, breaking elongation and impact strength decreases, yet

RHA still can replace some commercial fillers.

Navinchand et al. (1987) reported the studies on polyester filled with RHA.

They have mentioned in their study reports that both the tensile and impact strength of

the resulting composites were decreased with the increase in filler loading. they have

also reported that in addition to being used in rubbers or plastics, RHA can also be

used as a filler in rubber/plastic blends.

Rozman H. D. et al. (2000) had made their studies on the effect of chemical

modification of rice husk. They found and reported that with chemical modification in

the rice husk, the reinforcing effect can be increased to an acceptable limit.

Silvia Luciana Favaro et al. (2010) studied the chemical, morphological and

mechanical analysis of rice husk/post-consumer polyethylene (PE) composites. PE and

rice husk were chemically modified to improve their compatibility in composite

preparation. They found improved fibre surface adhesion with matrix and improved

mechanical performance compared to pure polymer matrix, on the other hand no

benefit is observed in the tensile strength over the pure PE.

Garcia et al. (2007) used a combination of waste tire rubber and rice husk with

different size particles as raw materials in their research for obtaining new materials by

sintering technique so that environmental problems could be reduced.

Ayswarya et al. (2012) studied the use of RHA for property modification on

high density polyethylene (HDPE). They found and reported that RHA is a valuable

reinforcing material for HDPE and the environmental pollution arising due to RHA

can also be eliminated.

Rout and Satpathy (2012) studied mechanical and tribo-performance of rice-

husk filled glass-epoxy hybrid composites. They found that hardness, tensile modulus

and impact energy of these new class hybrid composites are enhanced with the rice

husk as filler additive whereas a steady decline of tensile and flexural properties are

also observed.

Bohlooli et al. (2012) analytically investigated the compressive strength of

geopolymers with seeded fly ash and rice husk bark ash by fuzzy logic modelling.

They found that fuzzy logic can be an alternative approach for evaluating the effect of

seeded mixture of fly ash and rice husk bark ash on compressive strength values of

geopolymer specimens.

Kwon et al. (2013) investigated the flexural properties and dimensional

stability of the sandwich-structured composites comprising the rice husk particles in

the core layer and randomly aligned the wood strands in the face layers. They found

that 1040% of the strands into the face layers of the RH particleboards improved the

flexural modulus and strength.

Ahmad et al. (2012)were performed their research on using of rice husk

powder as reinforcing filler in blends of natural rubber(NR) and high density

polyethylene. They observed that the incorporation of radiated Rice Husk into

NR/HDPE blends improved the mechanical properties tensile stress and modulus and

impact strength and hardness.

Mahboobeh Azadi et al. (2011) investigated the influence of the RHA

on different mechanical properties of the cured coatings (wear, hardness, and

elongation). The presence of RHA in epoxy paints can enhance wear resistance,

scratch resistance, and elongation. It seems that this type of filler in epoxy paints

increases paint plasticity. The addition of white ash is better in improving the wear

resistance due to the presence of more silica. Adding 20 wt% black ash to the pure

epoxy paint lowers its friction coefficient with respect to the white RHA. Finally,

using this type of filler, which is cheap and abundant in nature, can modify some

mechanical properties of epoxy paints and also reduce air pollution from burning rice

husks. Thus, a green product can be produced in the paint industry.

S. Mahzan et al. uses natural fibre for studying sound absorption properties.

This study investigates the use of rice-husk waste as the potential element for sound

absorption material of rice-husk reinforced composite. The study of rice husk waste

material for sound absorption purposes has been reported. The optimum percentage of

rice husk was obtained at 25%. The pattern obtained for rice husk was similar to

membrane absorber curves which are predominant at the lower frequencies.

Furthermore the peaks value was obtained at 250Hz. Comparison between virgin

Polyurethane (PU) and the optimum percentage of rice husk (25%) indicated that

value of mixture is higher than virgin PU at low frequency whereas for high frequency

the virgin PU is higher. The comparison between other natural materials also has been

done for recycled rubber and wood shavings. The result demonstrates that rice husk is

superior to both materials for range 0-500Hz. Since, rice husk is available in large

amount, the potential for commercialization, especially for low frequency sound

absorbent material is possible.

Reis et al. (2011) studied experimentally the effect of cork and rice husk ash

micro particles fillers on the mechanical properties (flexural resistance, fracture

toughness, impact absorbed energy, elastic and viscous moduli) of polyester based

hand moulded composite was. Filled materials exhibit fragile behaviour and flexure

strength much lower than polyester matrix, and decreasing significantly when the filler

content increases from 1 to 5%. The resistance loss is more pronounced for cork

powder than for rice husk ash filler. Fracture toughness is also much lower for the

filled composites than for the polymer matrix. Using cork powder the fracture

toughness decreases significantly with filler content, while for rice husk ash filler a

slight increase was observed. Both fillers improve absorbed impact energy, peaking

about 2.5% on filler content. Better improvements were obtained using rice husk ash

powder, reaching about 30%. Both fillers increase glass transition temperature and the

maximum use temperature and also the elastic modulus compared with observed for

the polyester, reaching the modulus a peak for 2.5% of filler content.

Yussuf et al. (2010) investigated and compared the performances of polylactic

acid (PLA)/kenaf (PLA-K) and PLA/rice husk (PLA-RH) composites in terms of

biodegradability, mechanical and thermal properties. It was found that flexural

modulus of pure PLA was increased drastically when filled with both kenaf and rice

husk fibres; however, the flexural and impact strengths declined. For composites, it

was found that kenaf composite shows better mechanical properties compare to rice

husk composite. The thermal stability of the virgin PLA was decreased by addition of

kenaf and rice husk; and the composite with rice husk fibre showed higher thermal

degradation than kenaf composite. From the results of biodegradability, it was found

that addition of natural fibres slightly improves biodegradability of PLA and kenaf has

more significant effect on the biodegradation rate, which exhibits better performances

than rice husk.

Stefani et al. (2005) proposed the use of rice husk as filler for increasing the

value of recycled tire rubber. They observed that the addition of rice husk produces a

decrease in apparent activation energy for low conversions (up to 0.6). For higher

conversions this decrease was not so clearly observed.

Sisir Mantry et al. (2011) fabricated a jute-epoxy composites with

reinforcement of SiC derived from rice husk. They reported that incorporation of

fillers modifies the tensile, flexural and inter-laminar shear strength of the jute epoxy

composites. They also investigated that the presence of particulate fillers (silicon

carbide) in these composites improves their erosion wear resistance.

4. Noteworthy Contribution in the Field of Proposed Work:

D. Siva Prasad et al. (2012) investigated that incorporation of rice husk

particles in aluminium matrix can lead to produce low cost aluminium composites

with improved hardness and strength. These composites can find a wide applications

in automotive components like pistons, cylinder liners and connecting rods. Theses

composites can also find applications where light weight materials are required with

good stiffness and strength.

Acharya et al. (2012) observed incorporation of Rice Husk Char in to epoxy

significantly reduce abrasive wear loss. Nadazi et al. found that chemical modification

of Rice Husks by NaOH improves the adhesive properties of Rice Husk. Rout and

Satpathy studied mechanical and tribo-performance of Rice Husk filled glass-epoxy

hybrid composites.

D. Siva Prasad and A Rama Krishna (2012) studied the effect of T6 heat

treatment on the damping behaviour of aluminium Rice Husk Ash composites. It was

observed that composites exhibit high damping capacities than unreinforced alloy and

increases with increase in weight % and the storage modulus increases with the

addition of RHA particles but decreases with increase in weight %.

5. Proposed Methodology During the Tenure of Research Work:

Fabricating a specimen of a new structure of matrix (resin/epoxy) based low

cost hybrid composite reinforced with the fibres (or whiskers or particles)

obtained from agricultural wastes.

Performing different tests on the composite specimen for different

weight/volume proportions of matrix and reinforcement to study the

mechanical, thermal and tribological performances under different parameters

(viz. types of loading, temperature, environmental factors, etc.) in varying

conditions.

Developing a Finite Element Model (FEM) for the composite under different

parameters and comparing the results obtained with the experiments.

Analysing the results obtained from the above approaches and their validation.

Performing parametric study using optimization techniques, such as Taguchi

Method, to suggest which weight/volume percentage contribute most to give

significant optimized results.

6. Expected Outcome of the Proposed Work:

The mechanical properties of the proposed composite structure depend upon

the type of reinforcement of one or more types of fibres (or whiskers or particles) and

the type of matrix (resin/epoxy) used, the relative percentages of each, and the method

of manufacture. Thus, the expected outcomes will be based on:

The measurement of reinforcement-matrix ratio, since this property is chosen by

the application requirements.

Determination of the relative amounts of reinforcing material(s) and the matrix as

an important measure of the quality and proper processing.

Determination of Void content and density, as these can has detrimental effect on

the mechanical properties of the composite.

Determination of transition temperature (Tg) of the matrix material, as it is one of

the most important properties of a cured matrix. This parameter is both a measure

of the completeness of the curing process and an indication of the maximum

service temperature of the composite. It is the temperature at which a significant

change to the elastic modulus will be observed.

Determination of Elastic Modulus of the composite at the transition temperature.

Determination of thermal conductivity of the composite to define the specific

applications of the composite as insulator, conductor or super-conductor.

Measurement of the operating temperature range of the composite because it is

probably the most important parameter considered in choosing the chemical

nature of the matrix.

Measurement of Moisture content as it tends to plasticize or soften the matrix. As

with temperature effects, the composite properties can be measured after exposure

to water for varying times and at varying temperatures.

Measuring the effects of variable amplitude loading on remaining life of the

composite and fatigue under complex stress states as these have received a

limited attention in the research.

Prediction of operational life of structures can be made of the said materials is

feasible and can be based on measurements of fatigue strength and stiffness

degradation. Thus these properties are to be determined. Also stiffness

degradation can be correlated to the damage accumulated in the material.

Structural response of the composite due to cyclic loads can be studied.

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Biomass Based Carbon Black Filler in Epoxy Composite. International

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Ahmad, I., Ee Lane, C., Dahlam, H. M. and Abdulla, I. (2012). Electron - beam -

irradiated rice husk powder as reinforcing filler in natural rubber / high-density

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8. List of Publications by Candidate: NIL

(AJAY KUMAR VERMA)

Signature of the Candidate

(Dr. S. K. Dhagat) (Dr. M. L. Verma)

Signature of the Supervisor Signature of the Co-Supervisor