Project Report

FATIGUE CHARACTERIZATION OF A356-SiCp BASED METAL

MATRIX COMPOSITES

A project report Submitted to

P.E.S COLLEGE OF ENGINEERING, MANDYA (AN AUTONOMOUS INSTITUTE AFFILIATED TO VTU, BELGAUM)

In partial fulfillment of the requirement for the award of the degree

MASTER OF TECHNOLOGY

In

MECHANICAL ENGINEERING

(COMPUTER INTEGRATED MANUFACTURING)

2013-2014

Submitted by

SATISH H S [4PS12MCM12]

Under the guidance of

Dr. S.L.AJIT PRASAD M.Tech, Ph.D. Professor and Head of the Department of Mechanical Engineering,

P.E.S.C.E, MANDYA.

DEPARTMENT OF MECHANICAL ENGINEERING

P.E.S COLLEGE OF ENGINEERING

MANDYA-571401

(AN AUTONOMOUS INSTITUTE AFFILIATED TO VTU, BELGAUM)

CERTIFICATE

Certified that, Mr. SATISH H S bearing university seat number 4PS12MCM12 has

satisfactorily completed the project preliminary report entitled FATIGUE

CHARACTERIZATION OF A356-SiCp BASED METAL MATRIX

COMPOSITES in partial fulfillment for the award of degree Master of Technology in

Mechanical Engineering, P.E.S.C.E, Mandya during the year 2013-2014. The Project has

been approved as it satisfies the academic requirements in respect of project work

prescribed for the Degree in Master of Technology.

Signature of the Guide Signature of the HOD

Dr. S.L. AJIT PRASAD Dr. S.L. AJIT PRASAD

Dr. V. SRIDHAR

Principal, P.E.S.C.E, Mandya

Details of Project Work Viva Voice Examination held

Sl. No. Examiners

Date Name Signature

1

2

DECLARATION

I, SATISH H S hereby declare that this dissertation work entitled

FATIGUE CHARATERIZATION OF A356-SiCp BASED METAL MATRIX

COMPOSITES has been independently carried out by me under the guidance of

Dr. S.L.AJIT PRASAD, Professor and Head of the Department Mechanical

Engineering, P.E.S. College of Engineering, Mandya in the partial fulfillment of the

requirement of the degree Master of Technology in Mechanical Engineering (Computer

Integrated Manufacturing).

I further declare that I have not submitted this dissertation either in part or full to

any other university for the award of any degree or diploma.

Place: Mandya SATISH H S

Date:

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DDeeddiiccaatteedd ttoo

MMyy ddeeaarr ppaarreennttss,, tteeaacchheerrss

&&

LLoovviinngg ffrriieennddss

ACKNOWLEDGEMENT

Before introducing my thesis work, I would like to thank the people without whom

the success of this thesis would have been only a dream.

I have a great pleasure in expressing my deep sense of gratitude and indebtedness

to Dr. S.L.AJIT PRASAD, Professor and Head of the Department Mechanical

Engineering, P.E.S. College of Engineering, Mandya for his guidance, constant

supervision and his interest and precious help in the completion of my project work.

I would like to extend my sincere thanks to Dr. V. SRIDHAR, Principal, P.E.S.

College of Engineering, Mandya for permitting me to carry out this project work.

I am sincerely thankful to Mohana kumar K.C, Abhinandan K.S, Ashok

kumar M.S and Vikram C.K for their support and guidance to carry out this project.

I am thankful to Mr. G.C. Krishnappa Naik, Grindwell Norton LTD, for

providing the SiC particles to carry out this project.

I thank all the staff members of Mechanical, Industrial and production department,

P.E.S. College of Engineering, Mandya for their co-operation in the timely completion of

my project work.

I thank Mr. Ravi, Mr. Nagaraju Foreman, Chandru, Chennegowda, Mahesh

and Mr. Y.H. Nagaraju for their co-operation during the project work.

Also I express my deep sense of gratitude to my parents and also to my friends,

who have supported me during the project work.

SATISH H S

ABSTRACT

Composite materials are increasingly replacing traditional engineering materials

because of their advantages over monolithic materials. The development of metal matrix

composite has been one of the major innovations in the materials in the recent times. The

Metal Matrix Composite is a material which consists of metal alloy reinforced with

continuous fibers, whiskers, or particulates of ceramics. These MMCs are widely being

used in the transport, aerospace, marine, automobile and mineral processing industries,

owing to their improved strength, stiffness and wear resistance properties.

Aluminium alloy is the most commonly used matrix for the metal matrix

composites. The ceramic particles reinforced aluminium composites are termed as new

generation material and these materials can be tailored and engineered with specific

required properties for specific application requirements. Among metal-ceramic particle

composite, aluminium-graphite, aluminium-alumina and aluminium-silicon carbide

particles can possess improved wear resistance, high temperature hardness and strength.

In the present study, A356 with 0%, 5% and 10% SiCp MMC material was fabricated

using stir casting (vortex method) method. The vortex method is one of the better known

approaches used to create and maintain a good distribution of the reinforcement material

in the matrix alloy. The cast composites were carefully machined to prepare the test

specimens for hardness, tensile tests, and fatigue test as well as for micro structural

studies as per ASTM standards. Microstructural analysis of cast specimens has been

carried out to investigate the influence of processing parameters.

From the tests conducted for characterization of mechanical properties, composite

material specimens have been found to possess enhanced hardness and tensile strengths

compared to matrix alloy specimens, while at the same time, losing ductility as compared

to matrix alloy.

Also from the fatigue test performed it is found that fatigue life of the composite

with 5% SiCp as reinforcement has longer fatigue life compared with 0% and 10% SiCp.

Also fatigue life has increased with decrease in the neck diameter of composite with 5%

SiCp at identical stress condition.

CONTENTS Page no.

Acknowledgement

Abstract

List of Figures I

List of Tables III

Nomenclature IV

CHAPTER 1: INTRODUCTION 1-4

CHAPTER 2: THEORY AND LITERATURE REVIEW 5-41

2.1 COMPOSITE MATERIAL 5

2.2 CLASSIFICATION OF COMPOSITE MATERIALS 6

2.2.1 Based on the form of reinforcement component 6

2.2.2 Based on the structure of the matrix materials 8

2.3 METAL MATRIX COMPOSITES 9

2.3.1 Merits of MMCs 11

2.3.2 Demerits of MMCs 11

2.4 ALUMINIUM MATRIX COMPOSITES 11

2.5 PROCESSING TECHNIQUES OF MMC 13

2.5.1 Solid state processing 13

2.5.2 Liquid state processing 14

2.6 FACTORS TO BE CONSIDER DURING STIR CASTING 17

2.6.1 Distribution of the reinforcement materials 17

2.6.2 Wettability of reinforcement 19

2.6.3 Porosity in cast metal matrix composites 20

2.7 MECHANICAL CHARACTERISTICS 21

2.8 FATIGUE CHARACTERIZATION 22

2.8.1 Mechanism of Fatigue failure 24

2.8.2 The Stress life approach and The Strain life approach 26

to determine the fatigue life

2.8.3 Factors affecting Fatigue behaviour 27

2.8.4 Establishing S-N curve 29

2.9 LITERATURE REVIEW 30

CHAPTER 3: OBJECTIVE AND METHODOLOGY 42-44

3.1 OBJECTIVE 42

3.2 WORK PLAN 43

3.2 METHODOLOGY 44

CHAPTER 4: EXPERIMENTAL DETAILS 45-55

4.1 WORK MATERIAL DETAILS 45

4.2 PROCESSING DETAILS 47

4.2.1 Fabrication of Al-SiCp metal matrix composites 47

4.2.2 Procedure to fabricate composites 49

4.3 MATERIAL CHARACTERISATION 50

4.3.1 Microscopy 50

4.4 MECHANICAL CHARACTERISATION 51

4.4.1 Rockwell Hardness Number (RHN) 51

4.4.2 Measurement of Tensile strength 52

4.4.3 Fatigue characterization 53

CHAPTER 5: RESULTS AND DISCUSSION 56-68

5.1 MICROSTRUCTURAL STUDY 56

5.1.1 Scanning Electron Microscopy (SEM) 56

5.2 MECHANICAL CHARACTERISATION 57

5.2.1 Hardness 57

5.2.2 Tensile strength 58

5.3 FATIGUE CHARATERIZATION 62

5.3.1 Stress Calculations 62

5.3.2 Fatigue life of the Composites with varying 63

the percentage of the reinforcement

5.3.3 Fatigue life of the Composites with 5%SiC 65

with varying the Neck diameter

5.3.4 Fatigue fractured surface SEM analysis 68

CHAPTER 6: CONCLUSIONS 69

SCOPE OF FUTURE WORK 70

REFERENCES 71

LIST OF FIGURES

Fig

No.

CAPTION

Page

No.

2.1 Classification Based On The Form Of The Reinforcement 6

2.2 Types of reinforcement materials in composites 7

2.3 Classification of composite materials based on matrix materials. 8

2.4 Schematic representation of stir casting process 15

2.5 Different types of stirrer used in stir casting 18

2.6 A sketch of three degrees of wetting and the corresponding contact angles 20

2.7 S-N relationship for ferrous and non-ferrous alloys 25

2.8 Typical S-N relationship 26

3.1 Schematic diagram of work plan 43

4.1 Electrical heating furnace 47

4.2 Permanent spilt mould 48

4.3 Alumina- sodium silicate powder coated stirrer 48

4.4 Al Raw ingot material 49

4.5 Slag Remover 49

4.6 Degasser hexachloroethane C2Cl6 tablet 50

4.7 Cast Aluminium composites 50

4.8 Scanning Electron Microscope 51

4.9 Tensile testing machine 52

4.10 Tensile specimen according to ASTM B557 standard 53

4.11 Tensile test specimen 53

4.12 Rotary Bending machine 54

4.13 Fatigue test Specimen according to ASTM E446 54

4.14 Fatigue testing machine and loading diagram 55

5.1(a) 0%SiC cast-1000X 56

5.1(b) 5%SiC with 23m cast-1000X 56

5.1(c) 10%SiC with 23m cast-1000X 56

5.2 RHN of Base alloy and Composites 57

5.3(a) Load v/s Displacement (elongation) of 0% SiCp 58

5.3(b) Load v/s Displacement (elongation) of 5% SiCp 58

5.3(c) Load v/s Displacement (elongation) of 10% SiCp 59

5.4(a) Stress-Strain diagram of 0% SiCp 59

5.4(b) Stress-Strain diagram of 5% SiCp 60

5.4(c) Stress-Strain diagram of 10% SiCp 60

5.5 Proof Stress of base alloy and composites 61

5.6 Tensile strength of base alloy and composites 61

5.7 Strain to failure of base alloy and composites 61

5.8 Fatigue life of Base Alloy(0% SiCp) 63

5.9 Fatigue life of Composite with 5% SiCp 63

5.10 Fatigue life of Composite with 10% SiCp 64

5.11 Comparision of the Fatigue life of Composite with 0%SiCp, 5%SiCp & 10%

SiCp

64

5.12 Fatigue life of Composite with 5% SiCp having Neck dia 4mm 65




5.16 Comparision of the Fatigue life of Composite with 5%SiCp having varying

neck diameter

67

5.17 Fatigue Fractured Surface of 0% SiC reinforced in A356 Matrix (1000X) 68



LIST OF TABLES

Table

No.

CAPTION

Page

No.

4.1 Mechanical properties of A356 45

4.2 Chemical composition of A356 46

4.3 Mechanical properties of SiC 46

4.4 Technical Specifications of Rotating Bending Fatigue Tester 55

5.1 RHN of as cast and extruded composites 57

NOMENCLATURE

ASTM American Society for Testing Materials

Al Aluminium

SiC Silicon Carbide

PMC Polymer matrix composite

MMC Metal Matrix Composite

CMC Ceramic Matrix Composite

AMC Aluminium Metal Matrix

DRA Discontinuously Reinforced Aluminium

N Load in Newton

Microns

m Meter

Stress

Strain

E Youngs Modulus

M Bending moment

RHN Rockwell Hardness Number

F Imposed load in N

Kg Kilo gram

d Neck diameter of the Fatigue specimen

SEM Scanning Electron Microscopy

1000X 1000 times magnification

FATIGUE CHARACTERIZATION OF A356-SiCp BASED METAL MATRIX COMPOSITES

M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya 1

CHAPTER 1

INTRODUCTION

The engineering fraternity has always been on the lookout for wonder-materials

which would fit the bills for all types of service conditions. It stem from the need to make

progressive discoveries made by scientists, affordable. This affordability quotient has

persuaded many researchers to develop such materials which would satisfy various

hitherto unexplored conditions. In todays world almost all generic materials have been

tried for various uses and their limitations have been met. But the never ending quest of

civilization requires that materials qualify for harsher environments. This unavoidable

situation demands that new materials be created from various combinations of other

compatible materials. It is to be noted here that this method is not new; it has been with

mankind since ages. In every part of the world, various materials have been combined to

achieve some intended properties, albeit each case differs from the others, i.e. one can

create new materials with unique properties, which can be tailor-made and are different

from their base ingredients. This concept holds true for a genre of materials called

Composite materials where in, various types of matrices may be combined with

reinforcements which contribute to the enhancement of the properties.

A composite material is a combination of two or more chemically different

materials with a distinct interface between them. The constituent materials maintain their

separate identities in the composite, yet their combination produces properties and

characteristics that are different from those of the constituents. One of these constituents

forms a continuous phase and it is called as the matrix. The other major constituent is the

reinforcement phase available in the form of fibers or as a particulate in general, added to

the matrix to improve or alter the matrix properties. Reinforcement by a particulate forms

a discontinuous phase uniformly distributed throughout the matrix. Therefore, composites

have improved mechanical properties such as strength and toughness when compared

with monolithic materials.

Neither the matrices nor the reinforcements taken alone can stand up to the

requirement, but the composite may be able to do so. This alteration in properties can be

controlled by many ways, viz. controlling the matrix and reinforcement quality, their



proportion or the fabrication route. This flexibility in manufacturing allows one to

develop composites with varying properties in a precisely controlled fashion.

The main advantage of a composite material over conventional material is the

combination of different properties which are not often found in the conventional

materials. The extraordinary combination properties include high strength to weight ratio,

higher stiffness to weight ratio, improved fatigue resistance, improved corrosion

resistance, higher resistance to thermal expansion, higher wear resistance and fracture

toughness etc. There are a number of situations in service that demand an unusual

combination of properties. Further, the present day trend is to go in for light weight

constructions for easy handling and reduced space, aesthetic appearance and high

resistance to weathering attack. These factors have propelled the modern designers to

develop newer composite materials up to the stage of large-scale production with exacting

requirements.

It is the superiority of properties that has triggered the penetration of composite

materials into all fields of manufacturing. Metal Matrix Composites (MMCs) have

emerged as a class of materials suitable for structural, aerospace, automotive, electronic,

thermal and wear applications owing to their advantages over the conventional monoliths.

They score over in terms of specific modulus, specific strength, high temperature

stability, controlled coefficient of thermal expansion, wear resistance, chemical inertness,

etc. But the down side is populated by inferior toughness and high cost of fabrication in

comparison with Polymer Matrix Composites (PMCs). But MMCs supersede in terms of

higher transverse strength and stiffness, shear strength and high temperature capabilities.

The physical properties that attract are no moisture absorption, non-flammability, high

electrical and thermal conductivities and resistance to most radiations.

Compositionally, MMCs have at least two components, viz. the matrix and the

reinforcement. The matrix is essentially a metal, but seldom a pure one. Except sparing

cases, it is generally an alloy. The most common metal alloys in use are based on

Aluminium and Titanium. Both of them are low density materials and are commercially

available in a wide range of alloy compositions. Other alloys are also used for specific

cases, because of their own advantages and disadvantages. Beryllium is the lightest of all

structural materials and has a tensile modulus greater than that of steel, but it is extremely

brittle, rendering it unsuitable for general purpose use. Magnesium is light, but is highly

reactive to Oxygen. Nickel and Cobalt based super alloys have also found some use, but



some of the alloying elements present in the matrices have been found to have

undesirable effect(promoting oxidation) on the reinforcing fibers at high temperatures.

Aluminium oxide and silicon carbide powders in the form of fibers and particulates are

commonly used as reinforcements in MMCs and the addition of these reinforcements to

aluminium alloys has been the subject of a considerable amount of research work.

Aluminium oxide and silicon carbide reinforced aluminium alloy matrix composites are

applied in the automotive industries as engine pistons and cylinder heads, where the

tribological properties of these materials are considered important. Therefore, the

development of aluminium matrix composites is receiving considerable emphasis in

meeting the requirements of various industries. Incorporation of hard second phase

particles in the alloy matrices to produce MMCs has also been reported to be more

beneficial and economical due to its high specific strength and corrosion resistance

properties.

Aluminium is the most popular matrix for the metal matrix composites. The

aluminium alloys are quite attractive due to their low density, their capability to be

strengthened by precipitation, their good corrosion resistance, high thermal and electrical

conductivity, and their high vibration damping capacity. They offer a large variety of

mechanical properties depending on the chemical composition of the aluminium matrix.

They are usually reinforced by aluminium oxide, silicon carbide, silicon dioxide,

graphite, boron nitride, boron carbide etc., Aluminium based composites, reinforced with

ceramic particles, offer improvements over the matrix alloy: an elastic modulus higher

than that of aluminium, a coefficient of thermal expansion which is closer to that of steel

or of cast iron, a greater resistance to wear and an improvement in rupture stress

especially at higher temperatures and possibly improved resistance to thermal fatigue.

Following successful demonstration and qualification programmes, AMCs are

now being used in the aerospace industry, which represents a major breakthrough in the

growing acceptance of these composite materials in a market with exceptionally high

levels of technical requirements. AMCs are also recognised as having an important role to

play in high speed machinery applications where increased operating speeds of more than

50% have been achieved. Furthermore, their combination of lightness, fatigue resistance,

and stiffness make them ideal for many sporting applications, such as road and mountain

bicycles.



Applications in the defence sector are also varied and make use of some of the

unique properties offered by AMCs. The strength of hoop-wound tubes is exciting, giving

high burst and collapse strengths. Development is also expected for gun barrel overwraps,

missile bodies, rocket blast pipes and submersibles. Other applications for AMCs will

utilise their thermal and electrical properties, especially in dimensionally stable platforms

and electronic packaging.

The major methods to produce aluminium metal matrix composites are: stir

casting, powder metallurgy, liquid metal infiltration, squeeze casting, rheocasting, and

spray deposition technique. Liquid infiltration is a common process to produce metal

infiltration, which involves a melt liquid infiltration into porous preform. However, the

major problem for the production of these materials is to accomplish the wetting of

reinforcement by the liquid metal, which is very poor and is favoured by strong chemistry

bonding at the interface. The poor wetting is because of the presence of oxide film at the

surface of the aluminium. The wettability is a complex phenomenon that depends on

factors such as geometry of interface, process temperature, soaking time, and it

determines the quality of bonding among the systems.

The objective of developing the Al-SiCp metal matrix composite in the present

study is to derive their potential application in the engineering fields. They are prepared

by making use of stir casting technique. These Al-SiCp MMC is then analysed under

SEM to study the SiC particle distribution in the matrix metal Al356 and also the porosity

defects are being considered. Then an attempt has been made to study the mechanical

properties viz. Hardness, Tensile strength and Fatigue life of the cast composite

specimen.



CHAPTER 2

THEORY AND LITERATURE REVIEW

2.1 COMPOSITE MATERIAL

A composite material is a bi-phase or multiphase material whose mechanical properties

are superior to those of the constituent materials acting independently. One of the phases is

usually discontinuous, stiffer and stronger and is called reinforcement where as the less stiff and

weaker phase is continuous and is called matrix. Sometimes because of the chemical interactions

or the other processing effects, an additional phase called interface exists between the

reinforcement and the matrix.

Literally the term composite means- a solid material that results when two or more

different substances, each with its own characteristics, are combined to create a new

substance whose properties are superior to those of the original components for any

specific application. The term composite more specifically refers to a structural material

within which a reinforcement material (such as silicon carbide) is embedded. And the

engineering definition would also go alongside- A material system composed of a mixture

or combination of two or more constituents that differ in form or material composition

and are essentially insoluble in each other. In principle, composites can be fabricated out

of any combination of two or more materials-metallic, organic, or inorganic; but the

constituent forms are more restricted. The matrix is the body constituent, serving to

enclose the composite and give it a bulk form. Major structural constituents are fibers,

particulates, laminates or layers, flakes and fillers. They determine the internal structure

of the composite. Usually, they are the additive phase.

When two or more materials are interspersed, there is always a contiguous region.

Simply this may be the common boundary of the two phases concerned, in which case it

is called an interface. A composite having a single interface is feasibly fabricated when

the matrix and the reinforcement are perfectly compatible. On the other end, there may an

altogether separate phase present between the matrix phase and the reinforcement phase.

This intermediate phase is called an inter-phase. In case there is an inter-phase present,

there are two interfaces, one defining the boundary between the matrix and the inter-

phase, and the other between the inter-phase and the reinforcement. The strength of the

composite in such a case is dependent upon the strength of the weakest of the two



interfaces. There are certain advantages of having a preferred inter-phase. Such a

composite with an inter-phase is fabricated if the matrix and reinforcement are not

chemically compatible or if the wettability of the pair is very poor, such a composite is

materialized, by introducing a third material that has good bonding properties,

individually with the matrix and the reinforcement, which would not be possible

otherwise.

More or less, the strength of a composite is a function of the strength of its

interface between the matrix and the reinforcement. The failure of a functional composite

is essentially a result of the failure of the interface. Hence the strengthening mechanism is

the most dominant parameter in successful fabrication of a high strength composite.

Composites differ by their matrix type, reinforcement type, size and form,

composition, temper state, etc. With such a big window available for fabricating a

composite from different constituent materials, it is not uncommon to experiment with

materials with vividly different properties. There are three broadly classified groups of

composites: Polymer Matrix Composite, Metal Matrix Composite and Ceramic Matrix

Composite.

2.2 CLASSIFICATION OF COMPOSITE MATERIALS

2.2.1 Based on the form of the Reinforcement components

Fig 2.1: Classification based on the form of the reinforcement

Reinforcing Material

Particulate

Or

Whiskers

Fiber Structural

Large particles

Dispersions

Continuous fibers Discontinuous

(Short)

Aligned or Random

Laminates Sandwich

Panels



Fig 2.2: Types of reinforcement materials in composites

1) Particulates

Microstructures of metal and ceramics composites, which show particles of one

phase strewn in the other, are known as particle reinforced composites. The shape of the

reinforcements can be square, triangular, or random as shown in Fig 2.2. The size and

volume concentration of the dispersoid distinguishes it from the dispersion.

The dispersed size in particulate composites is of the order of a few microns. The

reinforcement in the matrix materials reinforces the matrix alloy by arresting motion of

dislocations and needs large forces to fracture the restriction created by dispersion.

2) Whiskers

Single crystals grown with nearly zero defects are termed whiskers. They are

usually discontinuous and short fibers of different cross sections made from several

materials like Graphite, Silicon Carbide, Copper, and Iron etc. Whiskers differ from

particles in which, whiskers have a definite length to width ratio which is greater than

one. Whiskers were grown quite incidentally in laboratories for the first time. Initially,

their usefulness was overlooked as they were dismissed as incidental by-products of other

structure. However, study of crystal structures and growth in metals sparked off an

interest in them and also the study of defects that affect the strength of materials, led to

their incorporation in the composites using several methods.



3) Fiber reinforcement

Fibers are the important class of reinforcements, as they satisfy the desired

conditions and transfer strength to the matrix constituent influencing and enhancing their

properties as desired. Glass fibers are the earliest known fibers used to reinforce

materials. Ceramic and metal fibers were subsequently developed and put to extensive

use, to render composites stiffer and more resistant to heat. Fibers fall short of ideal

performance due to several factors. The performance of a fiber composite is judged by its

length, shape, orientation, composition and the mechanical properties of the matrix. The

different types of fibers in use are Glass fibers, Silicon Carbide fibers, High Silica and

Quartz fibers, Alumina fibers, metal fibers and wires, Graphite fibers, Boron fibers,

Aramid fibers and multiphase fibers.

2.2.2 Based on the Structure of the Matrix materials

Fig 2.3: Classification of composite materials based on matrix materials

1) Polymer matrix composites (PMC) - Also known as FRP-Fiber reinforced

polymers(or plastic)-these materials use a polymer based resin as the matrix and variety

of fibers such as glass, carbon, and aramid as the reinforcement.

2) Metal matrix composites (MMC) - Increasingly found in the automotive industry,

these materials use a metal such as aluminium as the matrix, and reinforce it with

fibers/particles such as silicon carbide.

Matrix Material

Polymer

Matrix

Metal

Matrix

Ceramic

Matrix

Thermoplastic

Thermosets

Light metal &alloys (Al, Mg, Li &Ti)

Refractory Metals (Co, W etc)

Ceramic (oxides,

Carbide etc)

Carbon



3) Ceramic matrix composites (CMC) - Used in very high temperature environments,

these materials use a ceramic as the matrix and reinforce it with short fibers or whiskers

such as those made from silicon carbide and boron nitride.

2.3 METAL MATRIX COMPOSITES

The sustained interest to develop engineering materials which could cope with the

raised performance standards, resulted in emergence of a newer class of materials, called

Metal Matrix Composites (MMCs). They constitute a family of customizable materials

with customizable critical property relationships. Such materials are known for their

exceptional high modulus, stiffness, wear resistance, fatigue life, strength-to-weight

ratios, tailorable coefficient of thermal expansion, etc. With these enhancements in

properties, they pose for strong candidature for replacing conventional structural

materials. But what makes them stand apart is the ability to customize their properties to

suit the service requirement. Such advantages have made this group of materials a nice

pick for use in weight-sensitive and stiffness-critical components in transportation

systems.

MMCs can be described as a group of materials in which a continuous metallic

phase (matrix) is combined with one or more reinforcement phases. The aim of such a

composite material is to enhance the suitability of the end product by selectively

enhancing the complimentary properties, and masking the detrimental properties of the

matrix and the reinforcement. While fabricating the MMC, a solid material results when

two or more substances are physically (not chemically) combined to create a new material

whose properties are superior to those of the original substances for a specific application.

The matrix may be a pure metal or any alloy suitable for the intended application.

The reinforcement may be any other material in the form of particulates, whiskers, fibers,

platelets, etc. The most common reinforcements are ceramics having nominal size in the

range of 0.1 to 100 micrometers. But in fact, just about anything suitable for the

application may be utilized as a potential reinforcement. Even though at times, the matrix

and the reinforcement both can be metallic in nature, MMCs are not fabricated by

conventional alloying methods suitable for metals; since, such a process would mar the

essence of a composite. In alloys the phases are not chemically and physically distinct.

But in a composite, such phases are intentionally kept distinct, to exploit the properties of

the constituents to the fullest.



The reinforcing phase is the nominal constituent of a composite. It is the principal

load bearing component in the system. Hence the reinforcements with better mechanical

properties than the matrix materials are chosen while designing a composite. The matrix

is responsible for holding the load-carrying reinforcement together and retaining the bulk

shape of the composite. It also shares some portion of the total load which is transferred

to the reinforcement via the interface or vice versa. It is the effectiveness of the interface

that decides how much load is transferred to and from the matrix.

In MMCs a high degree of interaction between the matrix and the reinforcement is

inherent. The resulting strength is a direct function of effectiveness of the interface

between the matrix and the reinforcement. The character of the interface depends upon

the chemical and mechanical compatibility of the two phases involved. The chemical

incompatibility constraint can be overcome either by opting for a low-temperature

processing route or by selecting stable constituents. The thermal mechanical

incompatibility problem is sorted out by employing a ductile matrix that accommodates

the strain generated by the thermal alterations. Also it helps to select a pair of matrix and

reinforcement having matching coefficient of thermal expansion. However when it is

chemically or thermo-mechanically not feasible to fabricate a composite from a pair of

constituents, an intermediate phase which is compatible with the matrix and the

reinforcement may be introduced in between the two that masks the incompatibility of the

original pair. This interphase prevents the chemical reaction between the matrix and the

reinforcement and/or aids the matrix in accommodating the strain generated due to any

incongruous strain build-up. A soft precipitate-free layer around the reinforcing

particulates limit the propagation of the crack generated at their surface by effectively

reducing the stress value gradually, thereby increasing the ultimate strength.

Metal matrix composites have been under constant development since the days of

the World War-II. They were intended to be used in the aircrafts as structural materials.

After the war ceased, no longer the purpose was the war, rather MMCs found interest in

civilian uses. Today the composites are extensively used in all aspects of life, be it food

packaging, medical implants, military armours, automotive applications, space

applications or just about anything else. This deep penetration of MMCs in a wide

spectrum of application can be attributed to the previously mentioned advantages

associated with them.



Generally MMCs are classified according to type of reinforcement and the

geometric characteristics of the same. In particular, the main classification groups these

composites into two basic categories:

1) Continuous reinforced composites, constituted by continuous fibers or filaments.

2) Discontinuous reinforced composites, containing short fibers, whiskers or particles.

Both reinforcement and matrix are also selected on the basis of what will be the

interface that unites them. This interface can be as a simple zone of chemical bonds (as

the interface between the pure aluminium and alumina), but can also occur as a layer

composed by reaction (matrix/reinforcement) products.

2.3.1 Merits of MMCS

1. Very high specific strength and specific modulus

2. Low thermal coefficient of expansion

3. Retention of properties at high temperatures

4. Higher operating temperature

5. Better capability to withstand compression and shear loading

2.3.2 Demerits of MMCS

1. Difficulty with processing

2. Reduction in ductility.

However, MMCs are not without some drawbacks either. Their inadequate

fracture toughness and damage tolerance, poor ductility, size limitations, inhomogeneity

of properties, isotropy of properties stand as hindrance to their usability front. Continuous

research works are underway to overcome these limitations and explore new possibilities.

2.4. ALUMINIUM MATRIX COMPOSITES

Aluminium is the most popular matrix for the metal matrix composites. The

aluminium alloys are quite attractive due to their low density, their capability to be

strengthened by precipitation, their good corrosion resistance, high thermal and electrical

conductivity, and their high vibration damping capacity. They offer a large variety of

mechanical properties depending on the chemical composition of the aluminium matrix.



They are usually reinforced by aluminium oxide, silicon carbide, silicon dioxide,

graphite, boron nitride, boron carbide etc. In the 1980s, transportation industries began to

develop discontinuously reinforced aluminium matrix composites. They are very

attractive for their isotropic mechanical properties and their low costs. The properties are

inevitably a compromise between the properties of the matrix and reinforcement phases.

It is clear that the composition and properties of the matrix phase affect the properties of

the composite both directly, by normal strengthening mechanisms, and indirectly, by

chemical interactions at the reinforcement/matrix interface. Aluminium based composites,

reinforced with ceramic particles, offer improvements over the matrix alloy: an elastic

modulus higher than that of aluminium, a coefficient of thermal expansion which is closer

to that of steel or of cast iron, a greater resistance to wear and an improvement in rupture

stress especially at higher temperatures and possibly improved resistance to thermal

fatigue.

Research has shown that the addition of SiCp to Aluminium alloys would result in

an increase of modulus, and may also be accompanied by an increase in yield stress

depending upon the alloy composition, heat treatment, and manufacturing method.

Furthermore it helps in increasing resistance to wear, corrosion and fatigue crack

initiation as compared to the performance of the matrix alloy alone. It has been reported

that addition of SiC particulate reinforcement to Aluminium alloys usually lowers the

fracture toughness. However this drop in the fracture toughness has been found to be

caused by the alterations in flow stress, fracture of SiC particulates, poor dispersion of

SiC and a decrease in tensile ductility. Other factors such as the volume fraction of the

reinforcement, matrix alloy chemistry and processing variables have also been found to

affect the composite character. But the interactions of these parameters are yet to be

quantified to an extent that they can be deciphered.

Al-Si alloys are widely used for applications in the mechanical and tribological

components of internal combustion engines, such as cylinder blocks, cylinder heads,

pistons etc., owing to their good castability, high corrosion resistance and low density.

However, they exhibit poor seizure resistance, which restrict their uses in such

mechanical tribological environments. The wear resistance of these alloys can be

enhanced by incorporation of a ceramic phase in the soft aluminium alloy matrix.

Continuous-fiber-reinforced MMCs exhibit highly anisotropic properties, and this result

in a higher cost for the metal working process. Discontinuous silicon carbide particles



reinforced MMCs are particularly attractive because they exhibit good specific properties

and can be produced by conventional metal working processes. Hence they are being

increasingly used in the automotive industry as materials for pistons, brake rotors,

calipers and liner.

2.5. PROCESSING TECHNIQUES OF MMCs

There is a multitude of fabrication techniques of metal matrix composites

depending on whether they are aimed at continuously or discontinuously reinforced

MMC production. The techniques can further be subdivided, according to whether they

are primarily based on treating the metal matrix in a liquid or a solid form. The

production factors have an important influence on the type of component to be produced,

on the micro-structures, on the cost and the application of the MMCs.

Processing methods of MMCs can be classified into two categories.

1. Solid state processing.

2. Liquid state processing.

2.5.1. SOLID STATE PROCESSING

1. Powder Blending and Consolidation

Blending of aluminium alloy powder with ceramic short fibre/whisker/particle is

versatile technique for the production of AMCs. Blending can be carried out dry or in

liquid suspension. Blending is usually followed by cold compaction, canning, degassing

and high temperature consolidation stage such as hot isostatic pressing (HIP) or extrusion.

AMCs processed by this route contain reinforcement particles in the form of plate like

particles of few tens of nanometers thick and in volume fractions ranging from 0.05 to 0.5

depending on powder history and processing conditions. These fine particles tend to act

as dispersionstrengthening agent and often have strong influence on the matrix

properties particularly during heat treatment.

2. Diffusion Bonding

The diffusion bonding employs the matrix in the solid phase, in the form of sheet

or foil. Composite laminates are produced by consolidating alternate layers of precursor

wires or fibre mats and metal matrix sheets or foils under temperature and pressure. The

precursor wires are collimated filaments held together with a fugitive organic binder. This



is achieved either by winding binder-coated filaments onto a circular cylindrical mandrel

or by spraying the binder on the filaments that are already wound on a mandrel. When the

solvent is evaporated, the fibre-resin combination forms a rolled fibre mat on the mandrel

surface. The binder resin in precursor wires and fibre mats decomposes at a high

temperature without leaving any residue. Under temperature and pressure metal sheets or

foils melt and diffuse through fibre layers to form a laminate. A multilayered laminate

may have any desired stacking sequence. Several complex composite components can be

fabricated by stacking monotapes as per design requirements. The temperature, pressure

and their duration are very critical for making good quality composites. Carbon fibres

have been successfully combined with matrices like aluminium, magnesium, copper, tin,

lead and silver to make a wide range of carbon fibre reinforced metal composites. A

number of products ranging from flat plates to curved engine blades have been fabricated

using the diffusion bonding technique.

3. Physical Vapour Deposition

This process involves continuous passage of fibre through a region of high partial

pressure of the metal to be deposited, where the condensation takes place and a relatively

thick coating of aluminium on the fibre. Composite fabrication is usually completed by

assembling the coated fibres into bundle or array and consolidating in a hot press or HIP

process. Composites with uniform distribution of fibre and volume fraction as high as

80% can be produced by this technique.

2.5.2. LIQUID STATE PROCESSING

1. Stir Casting

This involves incorporation of ceramic particulate into liquid aluminium melt and

allowing the mixture to solidify. Here, the crucial thing is to create good wetting between

the particulate reinforcement and the liquid aluminium alloy melt. The simplest and most

commercially used technique is known as vortex technique or stir-casting technique. The

vortex technique involves the introduction of pre-treated ceramic particles into the vortex

of molten alloy created by the rotating impeller (Fig. 2.4).

Microstructural inhomogeneities can cause notably particle agglomeration and

sedimentation in the melt and subsequently during solidification. Inhomogeneity in

reinforcement distribution in these cast composites could also be a problem as a result of

interaction between suspended ceramic particles and moving solid-liquid interface during



solidification. Generally it is possible to incorporate up to 30% ceramic particles in the

size range 5 to 100 m in a variety of molten aluminium alloys. The meltceramic

particle slurry may be transferred directly to a shaped mould prior to complete

solidification or it may be allowed to solidify in billet or rod shape so that it can be

reheated to the slurry form for further processing by technique such as die casting, and

investment casting. The process is not suitable for the incorporation of sub-micron size

ceramic particles or whiskers. Another variant of stir casting process is compo-casting.

Here, ceramic particles are incorporated into the alloy in the semi solid state.

Fig.2.4 Schematic representation of stir casting process

Major factors to be consider during stir casting

Difficulty of achieving of uniform distribution of the reinforcement materials.

Wettability between the two main substances.

Porosity in cast metal matrix composites

Chemical reaction between the reinforcement material and matrix alloy.



2. Infiltration Process

Liquid aluminium alloy is injected/infiltrated into the interstices of the porous pre-

forms of continuous fibre/short fibre or whisker or particle to produce AMCs. Depending

on the nature of reinforcement and its volume fraction preform can be infiltrated, with or

without the application of pressure or vacuum. AMCs having reinforcement volume

fraction ranging from 10 to 70% can be produced using a variety of infiltration

techniques. In order for the preform to retain its integrity and shape, it is often necessary

to use silica and alumina based mixtures as binder. Some level of porosity and local

variations in the volume fractions of the reinforcement are often noticed in the AMCs

processed by infiltration technique. The process is widely used to produce aluminium

matrix composites having particle/whisker/short fibre/continuous fibre as reinforcement.

3. Spray Deposition

Spray deposition techniques fall into two distinct classes, depending whether the

droplet stream is produced from a molten bath (Osprey process,) or by continuous feeding

of cold metal into a zone of rapid heat injection (thermal spray process). The spray

process has been extensively explored for the production of AMCs by injecting ceramic

particle/whisker/short fibre into the spray. AMCs produced in this way often exhibit

inhomogeneous distribution of ceramic particles. Porosity in the as sprayed state is

typically about 510%. Depositions of this type are typically consolidated to full density

by subsequent processing. Spray process also permit the production of continuous fibre

reinforced aluminium matrix composites. For this, fibres are wrapped around a mandrel

with controlled inter fibre spacing, and the matrix metal is sprayed onto the fibres. A

composite monotype is thus formed; bulk composites are formed by hot pressing of

composite monotypes. Fibre volume fraction and distribution is controlled by adjusting

the fibre spacing and the number of fibre layers. AMCs processed by spray deposition

technique are relatively inexpensive with cost that is usually intermediate between stir

cast and PM processes.

4. In-situ Processing (Reactive Processing)

There are several different processes that would fall under this category including

liquid-gas, liquid-solid, liquid-liquid and mixed salt reactions. In these processes

refractory reinforcements are created in the aluminium alloy matrix. One of the examples

is directional oxidation of aluminium also known as DIMOX process. In this process the



alloy of Al-Mg is placed on the top of ceramic preform in a crucible. The entire assembly

is heated to a suitable temperature in the atmosphere of free flowing nitrogen bearing gas

mixture. Al-Mg alloy soon after melting infiltrates into the preform and composite is

formed.

2.6 FACTORS TO BE CONSIDER DURING STIR CASTING

In order to achieve the optimum properties of the metal matrix composite, the

distribution of the reinforcement material in the matrix alloy must be uniform, and the

wettability or bonding between these substances should be optimised. The porosity levels

need to be minimised, and chemical reactions between the reinforcement materials and

the matrix alloy must be avoided.

2.6.1 Distribution of the reinforcement materials

One of the problems encountered in metal matrix composite processing is the

settling of the reinforcement particles during melt holding or during casting. This arises as

a result of density differences between the reinforcement particles and the matrix alloy

melt. The reinforcement distribution is influenced during several stages including (a)

distribution in the liquid as a result of mixing, (b) distribution in the liquid after mixing,

but before solidification and (c) redistribution as a result of solidification. The mechanical

stirrer used (usually during melt preparation or holding) during stirring, the melt

temperature, and the type, amount and nature of the particles are some of the main factors

to be considered when investigating these phenomena. Proper dispersion of the particles

in a matrix is also affected by pouring rate, pouring temperature and gating systems. The

method of the introduction of particles into the matrix melt is one of the most important

aspects of the casting process. It helps in dispersing the reinforcement materials in the

melt. There are a number of techniques for introducing and mixing the particles including

1. Injection of the particles entrained in an inert carrier gas into the melt with the

help of an injection gun, wherein the particles are mixed into the melt as the

bubbles rise through the melt;

2. Addition of particles into the molten stream as the mould is filled;

3. Pushing particles into the melt through the use of reciprocating rods;



4. Spray casting of droplets of atomised molten metal along with particles onto a

substrate;

5. Dispersion of fine particles in the melt by centrifugal action;

6. Pre-infiltrating a packed bed of particles to form pellets of a master alloy, and

redispersing and diluting into a melt, followed by slow hand or mechanical

stirring;

7. Injection of particles into the melt while the melt is irradiated continuously with

high intensity ultrasound;

8. Zero gravity processing which involves utilising a synergism of ultra-high vacuum

and elevated temperature for a prolonged period of time.

The vortex method is one of the better known approaches used to create and

maintain a good distribution of the reinforcement material in the matrix alloy. In this

method, after the matrix material is melted, it is stirred vigorously to form a vortex at the

surface of the melt, and the reinforcement material is then introduced at the side of the

vortex. The stirring is continued for a few minutes before the slurry is cast. The different

designs of mechanical stirrers are as shown in Fig.2.5. Among them, the turbine stirrer is

quite popular. During stir casting for the synthesis of composites, stirring helps in two

ways: (a) transferring particles into the liquid metal, and (b) maintaining the particles in a

state of suspension.

Fig.2.5. Different types of stirrer used in stir casting



2.6.2 Wettability of reinforcement

Wettability is another significant problem when producing cast metal matrix

composites. Wettability can be defined as the ability of a liquid to spread on a solid

surface. It also describes the extent of intimate contact between a liquid and a solid.

Successful incorporation of solid ceramic particles into casting requires that the melt

should wet the solid ceramic phase. The problem of the wetting of the ceramic by molten

metal is one of surface chemistry and surface tension. The chemistry of the particle

surface, including any contamination, or oxidation, the melt surface and oxide layer must

be considered. The basic means used to improve wetting are (a) increasing the surface

energies of the solid, (b) decreasing the surface tension of the liquid matrix alloy, and (c)

decreasing the solid-liquid interfacial energy at the particles-matrix interface. The

magnitude of the contact angles () in this equation is as shown in fig.2.6 describes the

wettability, i.e. (a) - 0o, perfect wettability, (b) -1800, no wetting and (c) 00 < < 1800,

partial wetting.

Several approaches have been taken to promote the wetting of the reinforcement

particles with a molten matrix alloy, including the coating of the particles, the addition of

alloying elements to the molten matrix alloy, the treatment of the particles, and ultrasonic

irradiation of the melt. In general, the surface of non-metallic particles is not wetted by

the metallic metal, regardless of the cleaning techniques carried out. Wetting has been

achieved by coating with a wettable metal. Metal coating on ceramic particles increases

the overall surface energy of the solid, and improves wetting by enhancing the contacting

interface to metal-metal instead of metal-ceramic. Nickel and copper are well wetted by

many alloys, and have been used for a number of low melting alloys. In general, these

coatings are applied for three purposes viz. to protect the reinforcement from damage in

handling, to improve wetting, and to improve dispensability before addition to the matrix.

The type of coating, in terms of wettability, can be divided into coating which reacts with

the matrix, and coating which reacts with the oxide layer of the metal.

The addition of certain alloying elements can modify the matrix metal alloy by

producing a transient layer between the particles and the liquid matrix. This transient

layer has a low wetting angle, decreases the surface tension of the liquid, and surrounds

the particles with a structure that is similar to both the particle and the matrix alloy. The

composites produced by liquid metallurgy techniques show excellent bonding between

the ceramic and the metal when reactive elements, such as Mg, Ca, Ti, or Zr are added to



induce wettability . The addition of Mg to molten aluminium to promote the wetting of

alumina is particularly successful and it has also been used widely as an addition agent to

promote the wetting of different ceramic particles, such as silicon carbide and mica.

Fig.2.6 A sketch of three degrees of wetting and the corresponding contact angles

2.6.3 Porosity in cast metal matrix composites

The volume fraction of porosity, and its size and distribution in a cast metal

matrix composite play an important role in controlling the material's mechanical

properties. This kind of a composite defect can be detrimental also to the corrosion

resistance of the casting. Porosity levels must therefore, be kept to a minimum. Porosity

cannot be fully avoided during the casting process, but it can however, be controlled. In

general, porosity arises from three causes:

(a) Gas entrapment during mixing,

(b) Hydrogen evolution, and

(c) Shrinkage during solidification.

According to Ghosh and Ray, the process parameters of holding times, stirring

speed, and the size and position of the impeller will influence the development of

porosity. Their experimental work showed that there is a decrease in the porosity level

with an increase in the holding temperature. Structural defects such as porosity, particle

cluster, oxide inclusions, and interfacial reaction are found to arise from unsatisfactory

casting technology. It was observed that the amount of gas porosity in casting depends



more on the volume fraction of inclusions than on the amount of dissolved hydrogen.

Composite casting will have a higher volume fraction of suspended non-metal solid than

even the dirtiest conventional aluminium casting and hence the potential for the

nucleation of gas bubbles is enormous. It has been observed that the porosity in cast

composites increases almost linearly with particle content. The porosity of composite

results primarily from air bubbles entering the slurry either independently or as an air

envelope to the reinforcement particles. The air trapped in the cluster of particles also

contributes to the porosity. Oxygen and hydrogen are both sources of difficulty in light

alloy foundry. The affinity of aluminium for oxygen leads to a reduction of the

surrounding water vapour and the formation of hydrogen, which is readily dissolved in

liquid aluminium. There is a substantial drop in solubility as the metal solidifies, but

because of a large energy barrier involved in the nucleation of bubbles, hydrogen usually

stays in supersaturated solid solution after solidification.

2.7 MECHANICAL CHARACTERISTICS

Mechanical properties of material like strength, hardness, elasticity are of vital

importance in determining the type of fabrication and possible practical application.

a) Strength:

The ability of a material to resist failure under the action of stresses caused by a

load is known as its strength. The load to which a material is commonly subjected to are

compression, tension, shear and bending. The corresponding strength is obtained by

dividing the ultimate load with the cross-sectional area of the specimen.

b) Hardness:

The ability of a material to resist penetration by a harder body is known as its

hardness. It is a major factor in deciding the workability. The hardness bears a fairly

constant relationship to the tensile strength of given material.

c) Ductility:

It is the property of a material which permits a material to be drawn out

longitudinally to a reduced section under the action of tensile force. A ductile material

must be strong and plastic. The ductility is usually measured in terms of percentage of

elongation or percentage of reduction in cross section area of the test specimen.



d) Modulus of elasticity:

Hookes law states that when a material is loaded within elastic limit, the stress is

directly proportional to the strain i.e. the ratio of stress to the strain is a constant with in

elastic limit. This constant is known as Modulus of Elasticity or Youngs Modulus.

Therefore, stress strain

i.e. Stress

Strain = constant

i.e.

= E

Where E = Youngs Modulus.

2.8 FATIGUE CHARACTERIZATION

Fatigue is the condition where by a material fails due to the result of repeated

loading (cyclic stresses) applied below the ultimate strength of the material.

Fatigue failure is phenomenon in which a component fails due to repeated

loading. Repeated loading condition in a compound arrives when the stresses in it due to

the load applied vary or fluctuate between maximum and minimum values. In case of

static loading conditions, the load is applied gradually, giving sufficient time for strain to

develop. Whereas in case of repeated loading this does not hold good. Hence machine

member subjected to repeated loading have them been found to fail at stresses which are

very much below the ultimate strength and very often below the yield strength.

Stress is defined as the intensity of distributed forces that tend to resist change in

shape of a body. In most testing of those properties of materials that relate to the stress-

strain diagram, the load is applied gradually to give sufficient time for the strain to fully

develop. Furthermore, the specimen is tested to destruction and so the stresses are applied

only once. Testing of this kind is applicable, then to what are known as static

conditions. Such conditions only approximate the actual conditions to which many

structural and machine members are subjected. Most failures in machinery are due to time

varying loads rather than to static loads. These failures typically occur at stress levels

significantly lower than the yield strengths of the materials. Thus using only the static

failure theories can lead to unsafe designs when loads are dynamic. However, there are

conditions wherein the stresses vary or fluctuate between levels. For example, surface on

the rotating shaft subjected to the action of bending loads undergoes both tension and



compression for each revolution of the shaft. If, in addition, the shaft is also axially

loaded, an axial component of the stresses is superposed upon the bending component. In

this case, some stresses are always present, but the level of stress will be fluctuating.

These and other kinds of loading occurring in machine members produce stresses which

are called variable, repeated, alternating or fluctuating stresses.

It has been found experimentally that when a material is subjected to repeated

stresses, it fails at stresses below the yield point stresses, and such king of failure of a

material is known as fatigue. Fatigue is the phenomenon of progressive, localized,

permanent structural change occurring in a material subjected to conditions which

produce fluctuating stresses and strains at some point or points and which may terminate

in cracks or complete fracture after a sufficient number of fluctuation. Fatigue failure

begins with a small crack. The initial crack is so minute that it cannot be detected by the

naked eye and is even and is even quite difficult to locate in a magna flux of X-ray

inspection.

The crack will develop at a point of discontinuity in the material, such as change

in cross sectional, a keyway or hole stamp marks, internal cracks or irregularities caused

by machining. Once a crack is initiated, the stresses concentration effect becomes greater

and the crack progresses more rapidly. As the stressed area decreases in size, the stress

increases in magnitude until finally, the remaining area fails suddenly.

A fatigue failure is characterized by two distinct regions. The first of these is due

to the progressive development of the crack while the second is due to sudden fracture.

The zone of sudden fracture is very similar in appearance to the fracture of a brittle

material. When machine parts fail statically, they usually develop a very large deflection

because the stress has exceeded the yield strength, and the part is replaced before fracture

actually occurs. Thus many static failures give visible warning in advance. But a fatigue

failure gives no warning. It is sudden and total and hence dangerous. Therefore the design

of structural members is incomplete without fatigue considerations.

Fatigue of materials is a well known situation whereby rupture can be caused by a

large number of stress variations at a point even though the maximum stress is less than

the proof or yield stress. The fracture is initiated by tensile stress at a macro or

microscopic flaw. Once started the edge of the crack acts as a stress raiser and thus assists

in propagation of the crack until the reduced section can no longer carry the imposed

load. While it appears that fatigue failure may occur in all materials, there are marked



differences in the incidence of fatigue. For example, mild steel is known to have an

endurance limit stress below which fatigue fracture does not occur, this is known as the

fatigue limit. This does not occur with non-ferrous material, such as aluminum alloys,

however, there is no such limit.

To study and analyse the fatigue characteristic of different metals, rotating fatigue

testing machine is used. Fatigue testing machines apply cyclic loads to test specimens.

Fatigue testing is a dynamic testing mode and can be used to simulate how a

component/material will behave/fail under real life loading/stress conditions. They can

incorporate tensile, compressive, bending and/or torsion stresses and are often applied to

springs, suspension components and biomedical implants.

This machine is used to test the fatigue strength of materials and to draw S-N

diagram by research institutes, laboratories, material manufacturers and various

industries. This is a rotating beam type machine in which load is applied in reversed

bending fashion. The standard 5 mm diameter specimen is held in special holders at its

ends and loaded such that it experiences a uniform bending moment.

Specimen acts as rotating beam subjected to bending moment. Therefore it is

subjected to completely reversed stress cycle. Changing the bending moment by addition

or removal of weights can vary the stress amplitude

Basic fatigue testing involves the preparation of carefully polished test specimens

(surface flaws are stress concentrators) which are cycled to failure at various values of

constant amplitude alternating stress levels. The data are condensed into an alternating

Stress (S) verses Number of cycles to failure (N), curve which is generally referred to as a

materials S-N curve. As one would expect, the curves clearly show that a low number of

cycles are needed to cause fatigue failures at high stress levels while low stress levels can

result in sudden, unexpected failures after a large number of cycles.

2.8.1 MECHANISM OF FATIGUE FAILURE

Fatigue failure always begins with a crack. The crack may have been present in

the material since its manufacture, or it may have developed with time due to cyclic

straining around a stress concentration. Virtually all structural members contain

discontinuities ranging from microscopic (



local stress. Thus, it is critical that dynamically loaded parts be designed to minimize

stress concentrations.

There are three stages of fatigue failure; crack initiation, crack propagation and

sudden fracture due to unstable crack growth. The first stage can be of short duration, the

second stage involves most of life of the part, and the third stage is instantaneous.

The figure (2.7-2.8) show typical stress life relationship. The ordinate of S-N

diagram is called the fatigue strength and is always accompanied by a statement of the

number of cycles N to which it corresponds. Endurance limit represents the largest value

of fluctuating stress that will not cause failure for essentially an infinite number of cycles.

In case of steels, a knee occurs in the graph and beyond this knee, failure will not occurs

for any number of cycles. The endurance limit for steel is about 106 cycles. Most non

ferrous alloys do not show knee and have no sharply defined endurance limit. Hence,

limit of 108

cycles is taken to be the endurance limit. The body of knowledge available on

fatigue failure from N=1 to N=1000 cycles is known as low cycles fatigue. High cycle

fatigue is concerned with stress cycles above 103 cycles.

Fig. 2.7 - S-N relationship for ferrous and non-ferrous alloys



Fig. 2.8 Typical S-N relationship

2.8.2 THE STRESS LIFE APPROACH AND THE STRAIN LIFE APPROACH TO

DETERMINE THE FATIGUE LIFE

1. The Stress Life Approach

This is oldest of the three models and is mostly used for high cycle fatigue (HCF)

application where the assembly is expected to last for more than about 103 cycles of

stress. It works best when the load amplitudes are predictable and consistent over the life

of the part. It is the stress based model, which seeks to determine the fatigue strength and

or endurance limit for the material so that the cyclic stress can be kept below that level

and failure avoided for the required number of cycles. The part is then designed based on

the materials fatigue strength (or endurance limit) and a safety factor. In the effect, this

approach attempts to keep local stress in notches so low that the crack initiation stage

never begins. The assumption (and design) is that stress and strains everywhere remains

in the elastic region and local yielding occurs to initiate a crack.

This approach is fairly easy to implement, and large amounts of relevant strength

data are available due to its long-time use. However, it is the most empirical and least

accurate of the three models in terms of defining the true local stress/strain states in the

part, especially for low cycle (LCF) finite life situation where the total number cycle is

expected to be less than about 103 and the stresses will be high enough to cause local

yielding.



2. The Strain Life Approach

The initiation of crack involves yielding; a stress based approach cannot

adequately model this stage of the process. A strain based model gives a reasonable

accurate picture of the crack initiation stage. It can be also account for cumulative

damage due to variation in the cyclic load over the life of the part, such as overloads that

may introduce favourable or unfavourable residual stresses to the failure zone.

Combinations of fatigue loading and high temperature are better handled by this method,

because the creep effect can be included. This method is most often applied to LCF, finite

life problems where the cyclic stresses are high enough to cause local yielding. It is the

most complicated of the three models to use and requires a computer solution. Test data

are still being developed on the cyclic strain behaviour of various engineering materials.

2.8.3 FACTORS AFFECTING FATIGUE BEHAVIOUR

Variables affecting fatigue behaviour are conveniently classified as variations in,

1. Variation in the specimen

2. Surface defects

3. Design factors

4. Surface treatments

5. Size effect

6. Operating temperature

7. Corrosion

8. Stress concentration

9. Overload / Under load

10. Residual stress

1. Variation in the Specimen: The history and geometry of a specimen of a given

material will affect its fatigue behaviour, as will the different processing method

(resulting in variation in grain size, residual stress and surface finish). In general

treatments, which raise the yield strength or tensile strength of a composition, also raise

the fatigue resistance. Thus fine-grained structures have higher fatigue resistance than

corresponding coarse-grained structure.



2. Surface Defects: Since maximum stress within a component or a structure occurs at its

surface. Consequently most cracks leading to fatigue failures originate at surface

positions, specifically at stress amplification sites. Therefore it has been observed that

fatigue life is especially sensitive to condition and configuration of component surface.

3. Design Factors: Design of component can have significant influence on its fatigue

characteristics. Any notch or geometric discontinuity can acts as a stress raiser and fatigue

crack initiation site. These design factors include grooves, holes, keyways, threads etc.

The sharper the discontinuities the more severe are the stress concentration. The

probabilities of fatigue failure may be reduced by avoiding (whenever possible) these

structural irregularities.

4. Surface Factors: During machining operations small scratches and grooves are

invariably introduced into the work surface by cutting tool action. These surfaces marking

can limit the fatigue life. An important method of increasing fatigue performance is by

imposing residual compressive stress within a thin outer surface.

5. Size Effect: Larger specimens and machine parts are observed to exhibit poor fatigue

strength then smaller specimens or machine parts, especially when subjected to cyclic

bending stress. This may be due to the fact that larger specimens have greater volume and

surface area which in turn will have more number of defects when compared to smaller

specimens.

6. Operating Temperature: The temperature of operation has a significant influence on

the fatigue strength. The fatigue strength is enhanced at temperature below room

temperature and diminished at temperature above room temperatures.

7. Corrosion: A corrosive environment tends to lower the fatigue strength of the

engineering material, often by large amount. The use of certain solvents or the presence

of distilled water results in lowering the fatigue strength, especially when the specimens

are operated at elevated temperature.

8. Stress Concentration: The existence of irregularities or discontinuities, such as holes,

grooves, or notches, in a part increase the magnitude of stresses significantly in the

immediate vicinity of the discontinuity due to higher stress concentration. Fatigue failure

mostly originates from such places.



9. Overload/ Underload: The fatigue crack growth life decrease with increasing

overload stress and the fatigue life decreases with compressive underload stress.

10. Residual Stress: The fatigue crack growth behaviour of various types of alloy is

significantly affected by the presence of residual stress induced by manufacturing and

post-manufacturing processes. Residual stress is often a cause of premature failure of

critical components.

2.8.4 ESTABLISHING S-N CURVE

To determine the strength of materials under the action of fatigue loads,

specimens are subjected to repeated or varying forces of specified magnitudes while cycle

of stress reversals are counted to destruction. The most widely used fatigue testing device

is the R.R.Moore high speed rotating beam machine. This machine subjects the

specimens to pure bending by means of weights. The specimens are very carefully

machined and polished, with a final polishing in an axial direction to avoid

circumferential scratches, other fatigue machine are available for applying fluctuating or

reversed stresses, torsional stresses, or combined stresses to the test specimens.

To establish the fatigue strength of a material, quite a number of tests are

necessary because of the statistical nature of fatigue. For rotating beam test, a constant

bending load is applied, and the number of revolution (stress reversals) of the beam

required for failure is recorded. The first test is made at a stress which is somewhat under

the ultimate strength of the material. The second test is made at a stress which is less than

that used for first. The processed is continued, and the results are plotted on the S-N

diagram. In case of the ferrous metal and alloys, the graph become horizontal after the

material has been stressed for a certain number of cycles. Plotting on log paper

emphasizes the bend in the curve, which might not be apparent if the results were plotted

by using Cartesian coordinates.

The ordinate of the S-N diagram is called the fatigue strength S f, a statement of

this strength must always be accomplished by a statement of number of cycles N to which

it correspond.

The abscissa of the S-N diagram is life i.e. the number of cycles of stress reversals

required to cause the fatigue of the specimen.



2.9 LITERATURE REVIEW

Yunhui et.al [1] studied the tilt-blade mechanical stirring of A356-2.5vol%SiCp

liquid which was conducted in a cylindrical crucible to solve the problem of non-

homogeneous radial distribution of SiC particles in conventional straight-blade

mechanical stirring. In this paper, a specially-designed mechanical stirrer with the tilt

blade was used to stir A356-2.5vol% SiCp liquid. In straight-blade mechanical stirring of

A356-SiCp liquid, SiC particles can move from the centre to the periphery of the crucible

under the action of centrifugal force and thus resulting in a non-homogeneous distribution

of SiC particles in A356 liquid along the radial of crucible. In this experimental

equipment, a tilt-blade stirrer which can generate an inward movement of SiC particles is

used. The radial distribution of SiC particles in A356 liquid was studied under the

conditions of 25 for horizontal tilt angle of the blade, 200 RPM for rotating speed of

stirrer and 10 mm/s for speed of moving up and down of stirrer. The results show that the

non-homogeneous radial distribution of SiC particles in conventional straight-blade

mechanical stirring can be eliminated in tilt-blade mechanical stirring of A356-SiCp

liquid by adjusting the circumferential tilt angle of tilt-blade. The reasonable tilt-blade

mechanical stirring parameters of A356-2.5vol%SiCp liquid are 26 for circumferential

tilt angle of blade, 25 for horizontal tilt angle of blade, 200 RPM for rotating speed of

stirrer and 10 mm/s for speed of moving up and down of stirrer.

Sakthivel et.al [2] studied 2618 aluminium alloy metal matrix composites(MMCs)

reinforced with two different sizes and weight fractions of SiCp particles up to 10%

weight were fabricated by stir cast method and subsequent forging operation. The effects

of SiCp particle content and size of the particles on the mechanical properties of the

composites such as hardness, tensile strength, hot tensile strength (at 1200C),and impact

strength were investigated. The density measurements showed that the samples contained

little porosity with increasing weight fraction. Optical microscopic observations of the

microstructures revealed uniform distribution of particles and at some locations

agglomeration of particles and porosity. The results shows that hardness and tensile

strength of the composites increased with decreasing size and increasing weight fraction

of the particles. The hardness and tensile strength of the forged composites were higher

than those of the cast samples.



Abdel Jaber [3] et.al, in this study has aimed to investigate solidification and

mechanical behaviour of Al- Si alloy against both the molding conditions and silicon

content (3%- 15% Si). The pure aluminium matrix and pure silicon with a purity of

99.793% have supplied by the aluminium company of Egypt. The alloy were prepared by

melting the pure aluminium in an oil fired crucible furnace and the required amount of

silicon was added to the molten aluminium in powder form with a particle size about

300m to 500m. Five sets of the casting alloys were prepared with different silicon

content, (3%, 6%, 8%, 12%, and 15%Si). From the results author concluded that with the

increase in silicon content the cooling rate decreased and also a decrease of the liquidus

temperature was observed up to 12% and then increased with increasing Si%. But with

the increase of silicon content the ultimate tensile strength and hardiness increased, and

high coefficient of friction and high wear resistance was produced. The change of mold

thickness affected on the cooling rate of aluminium-silicon casting alloys so on the

microstructure. A pronounced change in the mechanical and tribological properties by the

change of mold thickness was obtained.

Neelima Devi [4] et.al, have studied the mechanical characterization of aluminium

silicon carbide composite. In this paper tensile strength experiments have been conducted

by varying mass fraction of SiC (5%, 10%, 15%, and

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