PERFORMANCES OF AXIAL ENERGY ABSORPTIONS OF

CYLINDRICAL WOVEN KENAF FIBER REINFORCED POLYESTER

COMPOSITES

NOORHASLINDA BINTI HAJI ABDULLAH

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

PERFORMANCES OF AXIAL ENERGY ABSORPTIONS OF CYLINDRICAL

WOVEN KENAF FIBER REINFORCED POLYESTER COMPOSITES

NOORHASLINDA BINTI HAJI ABDULLAH

A thesis submitted as a partial

fulfilment of the requirement for the award of the

Degree of Master Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

JANUARY 2017

DEDICATION

“ To my beloved parents Hj Abdullah bin Hj Taib and Hjh Hasemah binti Dollah,

Even after especially dedicated to my husband Mohd Aswadi bin Che Abdul Aziz that

always gave me support and encouragement when I feel down, not to forget my siblings,

my friends and for my supervisor, Dr. Al Emran bin Ismail, thanks for giving me some

advices and cooperation to end my research, always share the knowledgement and

guiding me when I’m lost and last but not least to my beloved son Muhammad Aqil

Miqdad bin Mohd Aswadi, your ummi hope that all this can give you an inspiration in

your future life”.

ACKNOWLEDGEMENT

Assalamualaikum w.b.t

In the name of Allah, The Most Generous and The Most Merciful

First at all, I would like to thank and grateful to the Allah s.w.t for his blessings

towards good health, strength and millions of the ideas during the research. I would like

to express my deep thankfulness and appreciated to Dr.Al Emran Bin Ismail for his

utmost inspiration supervisor, initial encouragement to start this project and his valuable

guidance and support throughout the course of my research. His guidance, idea and

expressing an interest in this project give me more motivated to make this project become

reality and perfect.

Apart from that, I would like to highly appreciate the help of technicians in Fkmp

textile and machine laboratory, Mr. Zakaria and Mr. Mahyan because for giving me a

chance and learn to use equipment as a method for my project also not forget master

student who guide me and teach me how to run this project.

Moreover, I would especially like to thank to my husband and son, for their

understanding and love, my family, for their continuing support and encouragement all

this while. They give motivation and dedication which helped me a lot to become what I

am today.

Last, but certainly not least, grateful appreciation to all of those who support me

in any aspect during the completion of the research.

ii

Abstract

Natural fibers more affordable than synthetic fibers and can be replaced in many

applications in which its cost efficiency outweighs the high composite performance

requirements. Nowadays, researchers usually use natural fibers in their research to

investigate whether natural fibers can be used to replace non-biodegrable materials. This

work presents an investigation on a composite that was reinforced with Kenaf fiber, also

known as Kenaf Fibre Reinforced Polyester (KFRP), which capable of absorbing energy.

This research focuses on the method of fabricating cylindrical composite tubes reinforced

with polyester using kenaf. Four different fiber orientations such as 0ᵒ, 15

ᵒ, 30

ᵒ, and 45

ᵒ

were used. The numbers of layers used were two, three and four layers. Later, the

cylindrical tubes were compressed quasi-statically using a constant cross-head vs.

displacement at 5 mm/min. The force versus displacement curves were obtained and the

areas under the curve representing the energy absorption performance were determined.

The observations of the crushing mechanism were viewed in three stages, the first stage

was the pre-crush response of the specimen terminated by the initiation of failure in the

material. In the second stage, which is the post-crush stage, failure spreaded across the

entire specimen, which was characterised by the average crush load. During the final

stage, the specimen became compact and the load increased sharply until the end of the

test. In conclusion, the ideal number of layers was four layer whereas 15ᵒ was shown to

be the optimum fiber orientation for KFRP to be used for energy absorption.

iii

Abstrak

Serat semula jadi adalah lebih murah daripada serat sintetik dan boleh digantikan dalam

pelbagai aplikasi di mana kecekapan kosnya mengatasi keperluan prestasi komposit yang

tinggi. Pada masa kini, penyelidik biasanya menggunakan gentian asli dalam

penyelidikan mereka untuk menyiasat sama ada gentian asli boleh digunakan untuk

menggantikan bahan-bahan bukan boleh dilupuskan. Uji kaji ini menerangkan mengenai

suatu penyiasatan ke atas komposit yang diperkukuh dengan gentian kenaf, yang juga

dikenali sebagai kenaf fibre reinforced poliester (KFRP), yang mampu menyerap tenaga.

Kajian ini memberi tumpuan kepada kaedah reka bentuk tiub komposit silinder

diperkukuhkan dengan poliester menggunakan gentian kenaf. Empat sudut orientasi

gentian yang berbeza digunakan iaitu 0ᵒ, 15

ᵒ, 30

ᵒ dan 45

ᵒ. Jumlah bilangan lapisan yang

digunakan adalah dua, tiga dan empat lapisan. Kemudian, tiub silinder telah dimampatkan

dengan kuasi-statik menggunakan “cross-head” lawan anjakan malar pada kelajuan 5

mm/min. Lengkung daya berbanding anjakan telah diperoleh dan kawasan di bawah

lengkung yang mewakili prestasi penyerapan tenaga bertekad. Pemerhatian mekanisme

penghancuran telah dilihat dalam tiga peringkat, peringkat pertama adalah sambutan pra-

menghancurkan spesimen ditamatkan oleh permulaan kegagalan dalam bahan. Pada

peringkat kedua, yang merupakan peringkat pasca menghancurkan, kegagalan tersebar di

seluruh keseluruhan spesimen, yang dicirikan oleh beban menghancurkan purata. Pada

peringkat terakhir, spesimen menjadi padat dan beban meningkat mendadak sehingga

akhir ujian. Kesimpulannya, jumlah bilangan lapisan yang sesuai adalah empat lapisan

manakala 15ᵒ adalah sebagai sudut orientasi gentian yang optimum untuk KFRP yang

akan digunakan untuk penyerapan tenaga.

iv

CONTENTS

TITTLE i

ABSTRACTS ii

CONTENTS iv

LIST OF FIGURES vii

LIST OF TABLES ix

LIST OF SYMBOL AND ABBREVIATIONS x

CHAPTER 1 INTRODUCTION 1

1.1 Background of study 1

1.2 Problem statement 2

1.3 Objectives of study 3

1.4 Scope of study 3

CHAPTER 2 LITERATURE REVIEW 4

2.1 Introduction 4

2.2 Overview of composites 4

2.3 Merits of composites 5

2.3.1 Polymer matrix composites 6

2.3.2 Polyester (PE) 8

2.4 Characteristics of the composites 9

2.5 Natural fiber reinforced composites 10

v

2.5.1 Classification of natural fiber 10

2.5.2 Kenaf fiber 13

2.6 Mechanical properties of kenaf fiber 14

2.7 Various pattern of woven 16

2.7.1 Plain weave 16

2.7.2 Weave styles-Comparison of properties 17

2.8 Structural crashworthiness 18

2.9 Energy absorption 19

2.9.1 Energy absorber 20

2.10 Factor affecting the energy absorption capability 23

2.11 Testing Modes 24

2.11.1 Quasi-static testing 24

2.11.2 Axial crushing of thin walled tubes 25

CHAPTER 3 RESEARCH METHODOLOGY 28

3.1 Introduction 28

3.2 Material preparation 30

3.2.1 Raw material 30

3.2.2 Weaving process 30

3.2.3 Process of producing kenaf fiber reinforced

polyester (KFRP) composites tubes 32

3.3 Weight of each samples 34

3.4 Quasi-static compression test 36

3.5 Analysis energy absorption of kenaf fiber

reinforced polyester (KFRP) composites tubes 38

3.5.1 Failure modes 40

vi

CHAPTER 4 RESULTS AND DISCUSSIONS 42

4.1 Introduction 42

4.2 Failure mechanism on kenaf fiber reinforced

polyester (KFRP) composites tubes 42

4.3 Effect of numbers of layers on force-displacement

curves 48

4.4 Effect of fiber orientation on force-displacement

curves 51

4.5 Crush force efficiency (CFE) 54

4.6 Initial failure indicator (IFI) 56

4.7 Specific energy absorption on kenaf fiber reinforced

polyester (KFRP) composites tubes 57

CHAPTER 5 CONCLUSION AND RECOMMENDATION 60

5.1 Conclusion 61

5.2 Recommendation 63

REFERENCES 64

PUBLICATIONS

vii

LIST OF FIGURES

2.1 Molecular structure of Polyester 8

2.2 Classification of natural fibers in their categories 11

2.3 Kenaf plant 13

2.4 Plain pattern 17

2.5 Typical load-displacement response 19

2.6 Deforming Tubes (a) flattening tube (b) folding tube 21

2.7 Force-deflection characteristics of ideal energy absorbers 21

2.8 Typical force-deflection characteristics of practical energy

absorbers 22

2.9 Load–displacement curves of axially crushed composite tube

with fiber orientation of (a) 15ᵒ/-75ᵒ and (b) 75ᵒ/-15ᵒ, respectively 26

3.1 Research methodology flowchart 29

3.2 Yarn kenaf 30

3.3 Direction of warps and wefts 31

3.4 (a) The setting of Handloom machine and (b) woven mat of kenaf 31

3.1 Steps producing of tubes 32

3.6 (a) Schematics dimensions of samples and (b) the outside

coverage of samples 33

3.7 Universal testing machine 1000kN 37

3.8 Lay-out of the test set-up used in quasi-static test 38

3.9 Load-displacement curves 39

4.1 Crushing mechanism curve of two layers KFRP tubes on 15ᵒ

fiber orientation. 45

4.2 Crushing mechanism curve of three layers KFRP tubes

on 15ᵒ fiber orientation. 46

4.3 Crushing mechanism curve of four layers KFRP tubes on 15ᵒ

fiber orientation. 47

4.4 Effect number of layers when applying load to different

viii

fibre orientations (a) 15ᵒ, (b) 30

ᵒ, (c) 45

ᵒ 49

4.5 Effect of fibre orientations on KFRP tubes when load is

applied (a) two layers, (b) three layers, (c) three layers and

(d) four layers 52

4.6 Crushing force efficiency (CFE) against numbers of two,

three and four layers 54

4.7 Crushing force efficiency (CFE) against fibre orientations 55

4.8 Initial failure indicator (IFI) 56

4.9 Specific energy absorption of KFRP composites tubes 59

ix

LIST OF TABLES

2.1 Comparison between natural and glass fibers 12

2.2 Kenaf fibers and E-glass fibers properties 16

2.3 Comparison properties 17

3.1 The woven orientation sequence of composites tube for two layers 33

3.2 The woven orientation sequence of composites tube for three layers 34

3.3 The woven orientation sequence of composite tube for four layers 34

3.4 Weight of samples two layers 35

3.5 Weight of samples three layers pattern 1 35

3.6 Weight of samples three layers pattern 2 35

3.7 Weight of samples four layers 36

4.1 The results of two, three and four layers of KFRP tubes 58

x

LIST OF SYMBOLS AND ABBREVIATIONS

AE - Absorbed crush energy

CO2 - Carbon Dioxide

CH - Carbon hidrogen

d - Diameter

Es - Energy in the post crushing zone

F - Force

FEM - Finite Element Method

GPa - Giga Pascal

h - Height

IFI - Initial Failure Indicator

KFRP - Kenaf Fiber Reinforced Polyester

MARDI - Malaysia Agricultural Research and

Development Institute

MPa - Mega Pascal

NaOH - Sodium Hydroxide

PEEK - Polyetherethrketone

PE - Polyester

Pmax - Peak load

Pavg - Average crush load

Pcr - Critical Crushing Load

PLLA - Polylactic Acid

PP - Polypropylene

PVC - Polyvinylchloride

R - Stress Ratio

RCCT - Radial Corrugated Composite Tube

SEA - Specific Energy Absorption

xi

t - Thickness

UD - Unidirectional

UTM - Universal Testing Machine

UTS - Ultimate Tensile Strength

UV - Ultra violet

U.S - United Stated

W - Total Energy Absorption

g/cm3 - Gram per Centimeter cubic

kN - Kilo Newton

m - Mass

mm - Milimeter

mm3 -

Milimeter cubic

Vf - Volume Fraction

Ɵ - Orientation

° - Degrees

% - Percentage

max - Peak compressive strength

CHAPTER 1

INTRODUCTION

1.1 Background of study

Natural fibre reinforced polymer composites have raised great interests among material

scientists and engineers in recent years due to the need for developing an environmentally

friendly material, and partly replacing currently used glass for composite reinforcement

(Li & Mai, 2006). In addition, natural fibers are also more affordable than synthetic fibers

and can be replaced in many applications for which its cost saving feature outweighs the

high composite performance requirements (Oksman et al., 2003). In the past, composite

materials such as coconut fiber and natural rubber latex were extensively used by the

automotive industry. Over the past few years, there has been a renewed interest in using

these fibers as reinforcement materials to some extent in the plastic industry. This

resurgence of interest is due to the increasing cost of plastics, and also because of the

environmental aspects of using renewable and biodegradable materials.

Kenaf (Hibiscus cannabinus, L. family Malvaceae) is seen as an herbaceous

annual plant that can be grown under a wide range of weather condition, for example, it

grows to more than 3 m within 3 months even in moderate ambient conditions with stem

diameter of 25-51 mm. It is also a dicotyledonous plant meaning that the stalk has three

layers; an outer cortical also referred to as (“bast”) tissue layer called phloem, an inner

woody (“core”) tissue layer xylem, and a thin central pith layer which consist of sponge-

like tissue with mostly non-ferrous cells (Sellers Jr et al., 1999 & Ashori et al., 2006).

2

The core and bast fibers from kenaf have been used to make composites for car

components, furniture, and building industries (Akil et al., 2011). The long Kenaf’s bast

fibers have potential to become reinforced polymers due to the high content of cellulose,

ranging up to 70% compared to the kenaf core fiber and the other types of natural fibers

in Malaysia such as banana fiber, coir fiber and others (Aji et al., 2009). Kenaf quite well

known economically. The idea of using cellulose fibers as reinforcement in composite

materials is not a new or recent one. Mankind had used this idea for a long time, since the

beginning of our civilization when grass and straws were used to reinforce mud bricks.

In the present research, there are studies on the effect of fiber orientations and number of

layers of woven kenaf mat on the energy absorption capability on axially crushed

composite tubes that are using kenaf fiber.

1.2 Problem Statements

Production of the product that has a high resistance generally produced using glass or

carbon-based material and the minority it can increase the rate of global warming in

addition to the greenhouse effect. In previous studies, some researchers had explored the

use of natural fiber as replacement for the fiber glass in any methods. It is exploits natural

resources which can reduce production costs as well as environmental pollution. In term

of that, many of researchers not further investigate about the use of woven kenaf fiber

reinforced with polyester as resin in order to study on energy absorption performance.

In this research study, kenaf fiber is used for the experimental study. Kenaf is also

known as the plant that has specific fiber characteristics comparable to normal fiber

which is commonly used in the production of a composite product. This research focused

mainly on the capability of kenaf fiber reinforced polyester composites to absorb energy,

where the polyester selected is known as polyester resin. The usage of long kenaf fiber

(yarn) is aligned with plain structures by producing woven mats, and then a composite is

produced for a technical purpose. The specialization of the parameters has been fixed

geometry in cylindrical form in determining the absorption of energy by using the

compression side. Quasi-static compression was conducted by studying the relationship

between the performances of the crushing energy absorption mechanism itself.

3

Circumferential orientation also varied according to specific angles for a comparison of

energy absorption that occurs during the crushing process.

1.3 Objectives of study

In this study, quasi-static compression tests are carried out on kenaf fiber reinforced

polyester (KFRP) composites tubes. Several specific objectives of the study are as

follows:

i. To investigate the crushing mechanisms behaviors of KFRP composites

tubes under an axial compression force.

ii. To study the effect of fiber orientations of KFRP composites tubes on the

performance of energy absorption.

iii. To study the effect of number of layers on woven kenaf mats on the

energy absorption capabilities of KFRP.

1.4 Scope of study

In order to ensure that the objectives are achieved, there are main scopes that need to be

focused on which are the fiber orientations and the number of layers. The production of

woven mats was done using fiber orientations which are 15ᵒ,30

ᵒ, and 45

ᵒ. Later, the

number of layers which are two, three and four layers are added before they are tested.

The 15 samples including reference samples are then tested using a universal testing

machine (UTM) on quasi-static mode with a consistent 5 mm/min speed and an applied

load. Then, the specific energy absorption (SEA) formula is used to complete the analysis

and to find out the energy absorbing capability of KFRP.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This literature review is the revision and survey on research topics to mainly shed light on

the types of natural fiber reinforced composites which is kenaf fiber. It focuses on kenaf

fiber reinforced polyester composites and energy absorption. Furthermore, to accomplish

the literature review, references from several sources such as books, journal, and online

searches required for analysis and data collection were gathered.

2.2 Overview of Composites

Over the last thirty years, composite materials, plastics and ceramics have been dominant

emerging materials. The volume and number of applications of composite materials have

grown steadily, penetrating and conquering new markets relentlessly. Modern composite

materials constitute a significant proportion of engineered materials market ranging from

everyday products to sophisticated niche applications. While composites have already

proven their worth as weight-saving materials, the current challenge is to make them cost

effective. Efforts to produce economically attractive composite components have resulted

in several innovative manufacturing techniques currently being used in the composites

5

industry. It is obvious, especially for composites, that the improvement in manufacturing

technology alone is not enough to overcome the cost hurdle. It is essential that there is an

integrated effort in design, material, process, tooling, quality assurance, manufacturing,

and even program management for composites to become as competitive as metals. The

composites industry has begun to recognize that the commercial applications of

composites promise to offer much larger business opportunities than the aerospace sector

due to the sheer size of the transportation industry. Thus the shift of composite

applications from aircraft to other commercial uses has become prominent in recent

years.

Increasingly enabled by the introduction of newer polymer resin matrix materials

and high performance reinforcement fibers of glass, carbon and aramid, the penetration of

these advanced materials has witnessed a steady expansion in the uses and volume. The

increased volume has resulted in an expected reduction in costs. High performance FRP

can now be found in diverse applications such as composite armoring designed to resist

explosive impacts, fuel cylinders for natural gas vehicles, windmill blades, industrial

drive shafts, support beams of highway bridges and even paper making rollers. For

certain applications, the use of composites rather than metals has in fact resulted in

savings of both cost and weight. Some examples are cascading for engines, curved fairing

and fillets, replacements for welding metal parts, cylinders, tubes, ducts, blade

containment bands and etc (Tudu, 2009).

2.3 Merits of Composites

A composite material consists of two or more different materials that are combined in a

way to achieve better properties than the two single components by themselves. In

manufacturing, “composite” material typically refers to a two-part substance with high-

tensile fibers and a resin matrix. The fibers have much higher mechanical properties than

the resin, and thus carry the applied loads. The matrix surrounds the fibers, holds them in

place, transfers the load to the fibers, and protects the fibers.

With fiber reinforced composites, meso- and micro-scale variations are possible. These

variations are not possible with isotropic materials like steel. Possible variations include

(Andrew, 2011):

6

1) Alignment of fibers (random, parallel, or any other orientation)

2) Fiber length (short, long, or continuous)

3) Fiber material (glass, carbon, etc.)

4) Matrix system (polyester, epoxy, or thermoplastic)

2.3.1 Polymer Matrix Composites

Most commonly used matrix materials are polymeric. In general the mechanical

properties of polymers are inadequate for many structural purposes. In particular, their

strength and stiffness are low compared to metals and ceramics. These difficulties are

overcome by reinforcing other materials with polymers. Secondly, the processing of

polymer matrix composites not involve high pressure and high temperature. Also the

equipment required for manufacturing polymer matrix composites are simpler. For this

reason polymer matrix composite developed rapidly and soon became popular for

structural applications. Composites are used because overall properties of the composites

are superior to those of the individual components for example polymer/ceramic.

Composites have a greater modulus than the polymer component, but aren’t as brittle as

ceramics. Two types of polymer composites which are Fiber Reinforced Polymer and

Particle Reinforced Polymer will be explained below.

Fiber Reinforced Polymer: Common fiber reinforced composites are composed of

fibers and a matrix. Fibers are the reinforcement and the main source of strength while

the matrix glues all the fibers together in shape and transfers stresses between the

reinforcing fibers. The fibers carry the loads along their longitudinal directions.

Sometimes, fillers might be added to smoothen the manufacturing process, impact special

properties to the composites, and or reduce the product cost. Common fiber reinforcing

agents include asbestos, carbon graphite fibers, beryllium, beryllium carbide, beryllium

oxide, molybdenum, aluminium oxide, glass fibers, polyamide, natural fibers and etc.

Similarly, common matrix materials include epoxy, phenolic, polyester, polyurethane,

polyetherethrketone (PEEK), vinyl ester and etc. Among these resin materials, PEEK is

7

most widely used. Epoxy, which has higher adhesion and less shrinkage than PEEK,

comes in second for its high cost (Grover, 2004).

Particle Reinforced Polymer: The particles used for reinforcement include

ceramics and glasses, such as small mineral particles, metal particles such as aluminium

and amorphous materials, including polymers and carbon black. Particles are used to

increase the modulus of the matrix and to decrease the ductility of the matrix. Particles

are also used to reduce the cost of the composites. Reinforcements and matrices can be

common, inexpensive materials and are easily processed. Some of the useful properties of

ceramics and glasses include high melting temperature, low density, high strength,

stiffness, wear resistance, and corrosion resistance. Many ceramics are good electrical

and thermal insulators. Some ceramics have special properties; some ceramics are

magnetic materials; some are piezoelectric materials; and a few special ceramics are even

superconductors at very low temperatures. Ceramics and glasses have one major

drawback: they are brittle. An example of particle reinforced composites is an automobile

tire, which has carbon black particles in a matrix of poly-isobutylene elastomeric

polymer.

Polymer composite materials have generated wide interest in various engineering

fields, particularly in aerospace applications. Research is underway worldwide to develop

newer composites with varied combinations of fibers and fillers so as to make them

useable under different operational conditions. Against this backdrop, the present work

has been taken up to develop a series of PEEK based composites with glass fiber

reinforcement and with ceramic fillers and to study their response to solid particle erosion

(Tudu, 2009).

8

2.3.2 Polyester (PE)

Figure 2.1 is molecular structure of polyester. Polyester (PE) intrinsic properties of high

stiffness, good tensile strength and inertness toward acids, alkalis and solvents have

secured its position in a wide range of consumer and industrial products. Polyester is also

flexible and can have very high elongation before breaking. The other advantages are:

a) Homopolymer

b) Good process ability

c) Good stiffness

d) Good impact resistance

e) Copolymer

f) High flow index

g) Chemically coupled

Figure 2.1: Molecular structure of Polyester.

Although polypropylene gives a lot of advantages, there are several difficulties for this

thermoplastic such as:

a) Degraded by UV

b) Flammable, but retarded grades available

c) Low temperature

The advantages of using polyester (PE) as matrix are their low cost and relatively low

processing temperature which is essential because of low thermal stability of natural

fibers. Amongst eco-compatible polymer composites, special attention has been given to

polyester composites, due to their added advantage of recyclability.

9

2.4 Characteristics of the Composites

A composite material consists of two phases. It consists of one or more discontinuous

phases embedded in a continuous phase. The discontinuous phase is usually harder and

stronger than the continuous phase and is called the “reinforcement” or “reinforcing

material”, whereas the continuous phase is termed as the “matrix”. The matrix is usually

more ductile and less hard. The matrix functions by holding the dispersed phase and

sharing a load with it. The matrix is composed of any of the three basic material types i.e.

polymers, metals or ceramics. The matrix forms the bulk form or the part or product. The

secondary phase embedded in the matrix is a discontinuous phase. It is usually harder and

stronger than the continuous phase. It serves to strengthen the composites and improves

the overall mechanical properties of the matrix. Properties of composites are strongly

dependent on the properties of their constituent materials, their distribution and the

interaction among them. The composite properties may be the volume fraction sum of the

properties of the constituents or the constituents may interact in a synergistic way

resulting in improved or better properties. Apart from the nature of the constituent

materials, the geometry of the reinforcement (shape, size and size distribution) influences

the properties of the composite to a great extent. The concentration distribution and

orientation of the reinforcement also affect the properties.

The shape of the discontinuous phase (which may be spherical, cylindrical, or

rectangular cross-sanctioned prisms or platelets), the size and size distribution (which

controls the texture of the material) and volume fraction determine the interface area,

which plays an important role in determining the extent of the interaction between the

reinforcement and the matrix. Concentration, which is usually measured as volume or

weight fraction, determines the contribution of a single constituent to the overall

properties of the composites. It is not only the single most important parameter

influencing the properties of the composites, but also an easily controllable

manufacturing variable used to alter its properties (Tudu, 2009).

10

2.5 Natural Fiber reinforced Composites

The interest in natural fiber-reinforced polymer composite materials is rapidly growing

both in terms of their industrial applications and fundamental research. They are

renewable, cheap, completely or partially recyclable, and biodegradable. Plants, such as

flax, cotton, hemp, jute, sisal, kenaf, pineapple, ramie, bamboo, banana, etc., as well as

wood, used from time immemorial as a source of lignocellulosic fibers, are more and

more often applied as the reinforcement of composites. Their availability, reliability, low

density, and price as well as satisfactory mechanical properties make them an attractive

ecological alternative to glass, carbon and man-made fibers used for the manufacturing of

composites. The natural fiber-containing composites are more environmentally friendly,

and are used in transportation (automobiles, railway coaches, aerospace), military

applications, building and construction industries (ceiling paneling, partition boards),

packaging, consumer products and etc (Frederick, 2004).

2.5.1 Classification of Natural Fibers

Natural fiber is class of hair-like materials that are continuous filaments or are in discrete

elongated pieces, similar to pieces of thread. They can be spun into filaments, thread, or

rope. Natural fiber can be classified in three groups which is vegetable (cellulose or

lignocelluloses) fiber, animal (protein) fiber and mineral fibers. Figure 2.2 shows the

example of natural fibers in their categories. Bast fibers refer to fiber gains from stems of

the plant that providing strength to plants. Bast represents the entire length of the stem

and therefore they are very long. Examples of bast fibers are flax, hemp, ramie, jute and

kenaf. Leaf fibers refer to fibers that extract from leaves which rough and sturdy.

Example of leaf fiber is pineapple leaf, abaca, henequen and sisal.

11

Figure 2.2: Classification of natural fibers in their categories (Bismarck et al., 2005).

The use of natural fibers as reinforcements in polymer composites to replace synthetic

fibers like glass is presently receiving increasing attention because of the advantages,

including cost effectiveness, low density, high specific strength, as well as their

availability as renewable resources. Since the 1990’s, natural fiber composites are

emerging as realistic alternatives to glass-reinforced composites in many applications.

Natural fiber composites such as hemp fiber-epoxy, flax fiber-polypropylene (PP), and

china reed fiber-PP are particularly attractive in automotive applications because of lower

cost and lower density. Glass fibers used for composites have density of 2.6 g/cm3

and

cost between $1.30 and $2.00/kg. In comparison, flax fibers have a density of 1.5 g/cm3

and cost between $0.22 and $1.10/kg (Foulk et al., 2000).

Several different natural fiber–polypropylene composites is determined the ability

to replace glass fiber–reinforced materials. Polypropylene with a very high melt flow

index was used to aid in fiber matrix adhesion and to ensure proper wetting of the fibers.

Samples were made with 40% fiber content of kenaf, coir, sisal, hemp, and jute. After the

samples were fabricated, tensile and impact tests were run to compare the properties of

these composites to those made with glass fiber. The tensile strengths all compared well

with glass, except for the coir, but the only sample with the same flexural strength was

hemp. It was shown with kenaf fibers that increasing fiber weight fraction increased

ultimate strength, tensile modulus, and impact strength. However, the composites tested

12

showed low impact strengths compared to glass mat composites. This study demonstrated

that natural fiber composites have a potential to replace glass in many applications that do

not require very high load bearing capabilities (Wambua et al., 2003).

Table 2.1: Comparison between natural and glass fibers (Wambua et al., 2003).

Natural

Fibers Glass Fibers

Density Low Twice that of natural fiber

Cost Low Higher than natural fiber

Renewability Yes No

Recyclability Yes No

Energy consumption Low High

Distribution Wide Wide

CO2 neutral Yes No

Abrasion to machines No Yes

Health risk when

inhaled No Yes

Disposal Biodegradable Not biodegradable

Natural fiber composites are also claimed to offer environmental advantages such as

reduced dependence on non-renewable energy/material sources, lower pollutant

emissions, lower greenhouse gas emissions, enhanced energy recovery, and end of life

biodegradability of components. Since, such superior environmental performance is an

important driver of increased future use of natural fiber composites, a thorough

comprehensive analysis of the relative environmental impacts of natural fiber composites

and conventional composites, covering the entire life cycle, is warranted by Joshi & Ochi

(2003).

13

2.5.2 Kenaf fiber

Kenaf is a 4,000 years old, new crop with roots in ancient Africa. A member of the

hibiscus family (Hibiscus cannabinus L), it is related to cotton and okra, and grows well

in many parts of the U.S. Kenaf grows quickly, rising to heights of 12-14 feet in as little

as 4 to 5 months. U.S. Department of Agriculture studies shows that kenaf yields of 6 to

10 tons of dry fiber per acre per year, are generally 3 to 5 times greater than the yield for

Southern pine trees, which can take from 7 to 40 years to reach harvestable size. There

are many different varieties of kenaf, and certain varieties will perform better in certain

locations, or under certain conditions than other varieties. (Rashdi et al., 2009). Among

the alternative resources, crops like kenaf seems more suitable for that purpose because of

its fibers, especially the outer part which is recyclable and has low density, high

toughness, acceptable strength properties and biodegradability. The stalks consist of two

kinds of fiber: an outer fiber (bast) and an inner fiber (core). The bast is comparable to

softwood tree fibers while the core is comparable to hardwood fibers. After harvest, the

plant is processed to separate these fibers for various products. (Westman et al., 2010).

Figure 2.3 : Kenaf plant

14

Kenaf was first investigated in the U.S. as a source of twine in the 1940’s. Since that,

time it has been used in a wide variety of products that include rope, twine, bagging,

carpet backing, packaging materials, various grades of paper and cardboard, and decking

and fencing products. This versatile plant is also being used by concrete manufacturers

and the plastics industry. The absorbency of Kenaf fibers has made it useful as animal

litter (e.g. Horse bedding) and as a product for cleaning up chemical and oil spills. In

addition, kenaf has been investigated as a possible livestock feed, with very favorable

results. U.S. research into the production and development of kenaf continues primarily

in several southern states (Akil et al., 2011).

The Kenaf plant is composed of many useful components (e.g., stalks, leaves and

seeds) and within each of these there are various usable portions (e.g., fibers and fiber

strands, proteins, oils, and allelopathic chemicals) (Webber et al., 2002). The yield and

composition of these plant components can be affected by many factors, including

cultivar, planting date, photosensitivity, length of growing season, plant populations, and

plant maturity. Kenaf filaments consist of discrete individual fibers, of generally 2–6 mm.

Filaments and individual fiber properties can vary depending on sources, age, separating

technique, and history of the fibers. The stem is straight and unbranched and is composed

of an outer layer (bark) and a core (Ogbonnaya et al., 1997). It is easy to separate the

stem into the bark and core, either by chemicals and/or by enzymatic retting. The bark

constitutes 30–40% of the stem dry weight and shows a rather dense structure. On the

other hand, the core is wood-like and makes up the remaining 60–70% of the stem by

Lee & Edward (2001). The core reveals an isotropic and almost amorphous pattern.

However, the bark shows an orientated high crystalline fiber pattern.

2.6 Mechanical Properties of Kenaf Fiber

The tensile properties of kenaf single fibers have been reported. Kenaf single fibers

exhibit tensile strength and tensile modulus of 11.91 GPa and 60 GPa, respectively.

Kenaf fibers have been used as reinforcement in thermoplastics such as polyethylene,

polypropylene (Sanadi et al., 1994) and thermosetting resins like polyester (Aziz et al.,

2005) and polybenzoxazine (Dansiri et al., 2002).

15

Unidirectional (UD) composites of polyethylene and kenaf fibers have been studied. The

fibers were untreated and coupling agent has been used. Tensile properties of the UD

composites have been tensile tested and it was reported that kenaf fibers enhanced the

tensile properties of polyethylene. Indeed, the tensile modulus of the UD composite with

57 volume % of fibers was seven times as much as the tensile modulus of polyethylene,

and its tensile strength was four times higher (Rashdi et al., 2009). The purpose was to

find an alternative to wood particles, and low or medium density hardboards. The tensile

and flexural moduli increase with increasing fiber content.

Selections of four different polyester resins have been used in composites

containing kenaf fibers (Aziz et al., 2005). One of them was a conventional unsaturated

polyester, the others have been modified to improve the adhesion to natural fibers (e.g.

Make them more polar). An alkaline surface treatment of the fiber was done with the use

of 6 % NaOH solution. Composites with 60 vol. % fibers con- tent have been produced

and tested in bending. Modified polyester exhibited better flexural properties. (Nishino et

al., 2003) investigated the mechanical properties of a composite made of kenaf fiber and

poly-L-lactic acid (PLLA). Young‘s modulus (6.3 GPA) and tensile strength (62 MPa) of

the kenaf/PLLA composite (fiber content 70 vol. %). The effects of the molecular weight

of PLLA, and orientation of the kenaf fibers in the sheet on the mechanical properties of

the composite were also investigated. This composite showed superior mechanical and

thermal properties based on the strong interactions between the kenaf fibers and PLLA

matrix. Both the anisotropies and quasi-isotropic composites could be obtained by

lamination of kenaf sheets with the preferential orientation of the fiber. Table 2.2 shows a

comparison of literature data concerning mechanical properties of kenaf versus E glass

(low alkali borosilicate glass), indicating that kenaf fiber can be a good reinforcement

candidate for high performance biodegradable polymer composites refer to the table

below.

16

Properties

Fibers Density

g/cm3

Tensile

strength,

MPa

E-modulus

GPa

Specific

(E-density)

Elongation

at failure,

%

Cellulose

lignin, %

Micro

fibril

angle, deg

E-glass

Kenaf

26

15

2000

350-600

76

40

29

27

2.6

0.33-0.88

-

75-90

-

9-15

2.7 Patterns of Woven Fibres

Yarn-based fabrics generally give higher strength per unit weight than roving, and being

generally finer, produce fabrics at the lighter end of the available weight range. Woven

roving are less expensive to produce and can wet out more effectively. However, since

they are available only in heavier taxes, they can only produce fabrics in the medium to

heavy end of the available weight range, and are thus more suitable for thick, heavier

laminates.

The most familiar forms of continuous fibers are woven roving and woven yarns,

i.e., fabrics of roving and yarns with various weave patterns. Woven composites are

finding increasing applications since they can be processed via injection molding. The

main advantages of woven materials are that they retain some of their orthogonal

properties when draped over non-flat molds. The weave also acts to enhance their impact

performance. There are several different patterns for the weave including plain weave,

which is the most highly interlaced; a basket weave, which has warped and weft yarns

that are paired up and satin weaves.

2.7.1 Plain weave

The choose of weaves on this research is plain weave. The Figure 2.4 is plain weave

structure.

Table 2.2: Kenaf fibers and E-glass fibers properties (Nishino et al., 2003).

17

Each warp fiber passes alternately under and over each weft fiber. The fabric is

symmetrical, with good stability and reasonable porosity. However, it is the most difficult

of the waves to drape, and the high level of fiber crimp imparts relatively low mechanical

properties compared to other weave styles. With large fibers, this pattern style gives

excessive crimp and therefore it tends not to be used for very heavy fabrics.

2.7.2 Weave Styles - Comparison of Properties

The Table 2.3 shows the comparison between the structures of pattern. They are divided

into six patterns with different criteria.

Property Plain Twill Satin Basket Leno Mock

leno

Good

stability

ᴏᴏᴏᴏ ᴏᴏᴏ ᴏᴏ ᴏᴏ ᴏᴏᴏᴏᴏ ᴏᴏᴏ

Good drape ᴏᴏ ᴏᴏᴏᴏ ᴏᴏᴏᴏᴏ ᴏᴏᴏ ᴏ ᴏᴏ

Low porosity ᴏᴏᴏ ᴏᴏᴏᴏ ᴏᴏᴏᴏᴏ ᴏᴏ ᴏ ᴏᴏᴏ

Smoothness ᴏᴏ ᴏᴏᴏ ᴏᴏᴏᴏᴏ ᴏᴏ ᴏ ᴏᴏ

Balance ᴏᴏᴏᴏ ᴏᴏᴏᴏ ᴏᴏᴏᴏᴏ ᴏᴏᴏᴏ ᴏᴏ ᴏᴏᴏ

Symmetrical ᴏᴏᴏᴏᴏ ᴏᴏᴏ ᴏ ᴏᴏᴏ ᴏ ᴏᴏᴏ

Low crimp ᴏᴏ ᴏᴏᴏ ᴏᴏᴏᴏᴏ ᴏᴏ ᴏᴏ/ᴏᴏᴏᴏᴏ ᴏᴏ

Figure 2.4: Plain pattern

Warp

direction Weft

direction

ᴏᴏᴏᴏᴏ=excellent, ᴏᴏᴏᴏ=good, ᴏᴏᴏ=acceptable, ᴏᴏ=poor, ᴏ=very poor

Table 2.3: Comparison properties (Net composites,

18

The Table 2.3 describes about the comparison between pattern. They include several of

property which are stability, drape, porosity, smoothness, balance, symmetrical and low

crimp. Several of the pattern weaves was good on their own characterization such as satin

have good of the characterization except stability and symmetrical. Differ to plain weave,

they have balance of characterization such as good stability and symmetrical.

2.8 Structural Crashworthiness

Structural crashworthiness describes an investigation into the impact performance of a

structural when the structure collides with another object. Thus, a study into the structural

crashworthiness characteristics of a system is required in order to calculate the forces

during the collision.

The field of structural crashworthiness has received much attention in recent years

due to the enormous public awareness on safety and environmental issues. This led to

increased design pressures and study on weight reduction and optimization. On the other

hand, the impact energies involved in the accidents are very much greater than that of the

maximum amount of elastic energy which could possibly be absorbed by the structure.

Clearly, it is important to understand the various phenomena involved in these highly

nonlinear dynamic structural problems. Over the years, investigations have been reported

on simplified models to capture the essential structural characteristics. Even today, simple

models provide valuable assistance with the availability of highly sophisticated numerical

schemes such as the finite element method (FEM) and it is helpful for preliminary design

purposes by Ramakrishna & Hamada (1998).

In recent times, the crashworthiness has gained considerable importance,

especially in the transportation sector. The traditional design concept of making robust

structures to increase the survivability of passengers in the transportation sector is not

paying off much. So, researchers now are focusing on the concept of structures which are

much more efficient in energy absorption (Schultz, 1998).

19

2.9 Energy Absorption

The ideal energy absorption is fixed compressive load imposed on an ongoing basis

against displacement destruction in a straight graph. It is known as a progressive failure

or destruction table. Here are several energy absorption mechanisms gathered from

previous research.

A schematic typical load–displacement response of a tubular composite under uniaxial

quasi-static compression is displayed in Figure 2.5. It can be divided into three zones.

The first region is from the origin to the peak crush load, known as the pre-crushing zone.

The second region is the post-crushing zone, which is characterized by the average crush

load. The third zone is known as the compaction zone. The parameters for each specimen

can be determined from the load–displacement as displayed in Figure 2.5.

Peak load Pmax is the maximum load neglecting the compaction zone.

Peak compressive strength max is the ratio of the peak load to the initial cross

sectional area of the circular tube.

Figure 2.5: Typical load-displacement response (Yan et al., 2013)

20

Absorbed crush energy, AE is the area under the load–displacement response,

where AE ∫

, P is the crush load (kN) and D is the displacement

(mm). Unit of Joule (J).

Specific absorbed energy, SAE is the absorbed crush energy per unit mass of the

crushed specimen, where SAE = AE/m, m is the mass of the crushed specimen.

Unit of J/g.

Post-crush displacement is the displacement in the post crushing zone.

Average crush load Pavg is the ratio of the absorbed energy in the post crushing

zone (Ec) to the post-crush displacement, where Pavg = Ec/ .

Crush force efficiency, CFE is the ratio of the average crush load to the peak load,

where CFE = Pavg/Pmax (Yan et al,.2013).

2.9.1 Energy Absorber

Energy absorbers are vital in the field of impact engineering and its application can be

found in numerous industries, ranging from automotive structures, aircraft, train,

helicopter skids, satellite recovery, and safety of nuclear reactors, collision damage of

road bridges and offshore structures and oil tankers. An energy absorber is a device that is

functioning to absorb kinetic energy of impact and dissipates it into other forms of

energy, ideally in an irreversible manner (Alghamdi, 2001).

Many practical engineering systems have a requirement for absorbing energy

during impact events. Energy absorbers have been developed to dissipate energy owing to

friction, fracture, shear, torsion, crushing, cyclic plastic deformation, metal cutting,

extrusion and fluid flow. Energy absorbing devices may be classified in one of three

general categories consisting of material deformation, extrusions, and friction. This

classification is made on the basis of the primary energy absorbing mechanism. In many

devices, there is more than one energy absorbing mechanism. In general, however, only

one is dominant. The material deformation category includes a wide variety of energy

absorbers which depend on the deformation of materials for the absorption of energy.

Within the category of material deformation there are several well-defined types of

energy absorbers like deforming tube, which deserves special discussion. Tubes are

21

deformable elements which have a wide variety of uses as energy absorbers as illustrated

in Figures 2.6 (a) and (b).

An ideal energy absorber is defined as one which maintains the maximum allowable

retarding force throughout the stroke, apart from elastic loading and unloading effects. An

ideal energy by Harte & Micheal (2000) absorber has a long, flat loaded-deflection curve:

the absorber collapses plastically at a constant force called the 'plateau force' as shown in

Figure 2.7.

(a) (b)

Figure 2.6: Deforming tubes (a) flattening tube (b) folding tube by Hermann

& Perrone (1972).

Figure 2.7: Force-displacement characteristics of ideal energy absorbers

by Harte & Micheal (2000).

plateau force

Force

22

Practical energy absorbers have a characteristic force, F and displacement response as

illustrated in Figure 2.8. An initial (or subsequent) peak value of force Fmax exceeds the

average value Favg, and leads to an increased acceleration and potential damage of the

object. 'The crush force efficiency', defined by η=Favg/Fmax, is a useful measure of the

uniformity of collapse load; for the ideal energy absorber, η=1. In common, the study of

deformation in energy absorbers accounts for geometrical changes, and interaction

between various modes of deformation such as the concertina (axisymmetric) mode,

diamond (non-axisymmetric) mode or mixed mode of collapse, for axially loaded tubes,

as well as strain hardening and strain rate effects.

Figure 2.8: Typical force-displacement characteristics of practical energy

absorbers by Harte & Micheal (2000).

F

max

Force

23

2.10 Factors affecting the energy absorption capability

The energy absorption characteristics of a crashworthy composite structure can be

tailored by controlling various parameters like fiber type, matrix type, fiber architecture,

specimen geometry, process conditions, fiber volume fraction and testing speed. In this

paper care has been taken to group the various research activities that have been

conducted to understand the effect of a particular parameter on the energy absorption

capability of a composite material.

The effect of a particular parameter (such as fiber type, matrix type, fiber

orientation, specimen geometry, processing conditions, fiber content, test speed and test

temperature) on the energy absorption of a composite material is summarized below:

1. Fiber Type: The density of the reinforced fibers has a lot to do with the energy

absorption characteristics of a composite material. As the density of the fiber

decreased from a higher to a lower value, the specific energy of the fiber

reinforced tubes increased from a lower to a higher value respectively. Tubes

reinforced with fibers having higher strain to failure result in greater energy

absorption properties. Changes in fiber stiffness affect energy absorption

capability less than changes in fiber failure strain, provided the different materials

crush in the same mode.

2. Matrix Type: If one is restricted to discussing the energy absorption capability of

a reinforced fiber thermoplastic matrix material it could be concluded that a

higher inter laminar fracture toughness, GIC, of the thermoplastic matrix material

would increase the energy absorption capability of the composite material. Also

an increase in matrix failure strain causes greater energy absorption capabilities in

brittle fiber reinforcements. Conversely, the energy absorption in ductile fiber

reinforcements decreases with increasing matrix failure strain. The role of

thermosetting resin matrices in energy absorption is not clear and further studies

are essential.

3. Fiber Orientation: Regarding the effects of fiber orientation on the energy

absorption capability of a composite material, the fiber orientations that enhance

the energy absorption capability of the composite material requires them to:

Increase the number of fractured fibers.

Increase the material deformation.

24

Increase the axial stiffness of the composite material.

Increase the lateral support to the axial fibers.

4. Specimen Geometry: Studying the effect of tube dimensions it can be said that

the crush zone fracture mechanisms are influenced by the tube dimensions and

these fracture mechanisms determine the overall energy absorption capability of

the composite tubes. For a given fiber lay-up and tube geometry, the specific

energy follows the order of circular, square, and rectangle.

5. Fiber Content: There has been no systematic study reported in literature on the

effect of fiber content on the energy absorption of composites. It should be noted

that an increase in the fiber content might not always necessarily improve the

specific energy absorption capability. As the fiber volume fraction increases, the

volume of the matrix between the fibers decreases. This causes the inter laminar

strength of the composite to decrease. As inter laminar strength decreases, inter

laminar cracks form at lower loads, resulting in a reduction in the energy

absorption capability. Also, as fiber volume fraction increases, the density of the

composite increases which results in a lower energy absorption capability

(George, 2002).

2.11 Testing Modes

Test methodologies commonly used to analyze the specimen’s behavior under crash are

discussed. Quasi-static and axial crushing of thin-walled tubes are included in further

investigations.

2.11.1 Quasi-static testing

In quasi-static testing, the test member is compressed at a steady rate using the

conventional tensional testing machine. Tubes are axial compressed between parallel, flat

steel platens, one static and one moving at a constant crosshead speed in the range of 1.5

x 10-3

m/s to 0.1 m/s. Quasi-static tests do not represent an actual crash condition because

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