VISVESVARAYA TECHNOLOGICAL UNIVERSITY
Jnana Sangama, Belgaum-590018
A PROJECT REPORT ON
EXPERIMENTAL DETERMINATION AND ANALYSIS OF
FRACTURE TOUGHNESS OF MMC
Submitted in partial fulfillment of the requirement for the
award of degree of
MASTER OF TECHNOLOGY
IN
DESIGN ENGINEERING
By:
SHIVARAJA.H.B
USN: 1DB12MDE12
Under the guidance of
Mr. B.S. PRAVEEN KUMAR
Associate professor Department of Mechanical Engineering Don
Bosco Institute of Technology
Department of Mechanical Engineering
DON BOSCO INSTITUTE OF TECHNOLOGY
Kumbalagodu, Mysore Road, Bangalore - 560074
ACKNOWLEDGEMENT
I thank to Mr. B S PRAVEEN KUMAR, Associate Professor,
Department of Mechanical Engineering, Don Bosco Institute of
Technology, Bangalore without whom a project of this magnitude
would not have been completed up to this extent.
I express my deep gratitude to my institute, DBIT, Bangalore
which provided an opportunity and platform for fulfilling my
dreams, and desire to reach my goal. I sincerely thank my respected
Principal Dr. T SREENIVASAN who is the constant source of
inspiration, throughout the academics.
I express my sincere gratitude Dr. V S RAMAMURTHY, Professor and
Head of Department of Mechanical Engineering, Don Bosco Institute
of Technology, Bangalore for extending all the support from the
department for granting me the permission to carry out my project
and his immense guidance, valuable inspiration, words of advice and
unstinted support throughout the project.
I express my profound thanks and gratitude to all the faculty
members and nonteaching staff, Department of Mechanical
Engineering, Don Bosco Institute of Technology, Bangalore, for
their guidance and need full help in carrying out the project.
ABSTRACT
Aluminium and its alloys have continued to maintain their mark
as the matrix material most in demand for the development of Metal
Matrix Composites (MMCs). This is primarily due to the broad
spectrum of unique properties it offers at relatively low
processing cost. Some of the attractive property combinations of Al
based matrix composites are: high specific stiffness and strength,
better high temperature properties (in comparison with its
monolithic alloy), thermal conductivity, and low thermal expansion.
The multifunctional nature of Al matrix composites has seen its
application in aerospace technology, electronic heat sinks, solar
panel substrates and antenna reflectors, automotive drive shaft
fins, and explosion engine components.
Aluminium matrix composites with multiple reinforcements (hybrid
MMCs) are finding increased applications because of improved
mechanical and tribological properties and hence are better
substitutes for single reinforced composites. Aluminum Silicon
alloys are widely used in automotive applications. The present
study focuses on the influence of addition of zirconium silicate
(ZrSio4) particulates as a second reinforcement and study the
influence on the mechanical properties of aluminium matrix
composites reinforced with silicon carbide (SiC) particulates. The
method employed for the development of castings was Stir casting.
Experiments have been conducted under laboratory condition to
assess the mechanical properties such as Fracture Toughness,
Tensile, Compression and Hardness of the aluminium, zirconium
silicate and silicon carbide composite. The Single Edge Notch Bend
specimen is used in the present study to determine the Fracture
Toughness of the fabricated composite. The composites were tested
for the presence of reinforcement particles using metallurgical
microscope.
TABLE OF CONTENTS
CHAPTER 1
INTRODUTIONPage No
1.1Preamble1
1.2Statement of the Problem1
1.3Aim and Objective of the Project2
1.4 Methodology3
CHAPTER 2
LITERATURE REVIEW4-10
CHAPTER 3
INTRODUCTION TO COMPOSITE
3.1Background11
3.2Composites12
3.2.1 Composite Definition13
3.2.2 Need for Developing Composite materials14
3.2.3 Characteristics of composites14
3.3Classification of Composite16
3.3.1 The Matrix Material16
3.3.2 The Reinforcing Material16
3.3.3 Classification (According to the Type of
Reinforcement)17
3.3.3.1 Particulate Composites18
A. Non-Metallic in Metallic Composites18
B. Metallic in Non-Metallic Composites18
C. Non-Metallic in Metallic Composites19
3.3.3.2 Fibrous Composites19
3.3.3.3Laminated Composites19
3.4Applications of Composites19
3.5Fabrication techniques of MMCs20
3.5.1 Solid Phase Fabrication Method20
3.5.2 Diffusion Bonding21
3.5.3 Powder Metallurgy Technique21
3.5.4 Liquid Phase Fabrication Techniques21
3.5.5 Liquid Metal Infiltration22
3.5.6 Squeeze Casting22
3.5.7 Spray Co-deposition Method23
3.5.8 Stir Casting23
3.5.9 Compo Casting23
3.6Metal matrix composites24
3.7Examples27
3.7.1 Advantages and Disadvantages of MMCs27
3.8Scope of Present Investigation28
CHAPTER 4
SELECTION OF MATERIALS
4.1Matrix Material: Al 35629
4.1.1 Chemical Composition and Mechanical Properties
of Matrix Material Al35629
4.2Reinforcement Material (silicon carbide)30
4.2.1 Physical Properties of SiC31
4.2.2 Applications of SiC32
4.3Reinforcement Material (Zirconium Silicate)32
4.3.1 Properties of Zirconium silicate33
4.3.2 Applications of ZrSiO433
CHAPTER 5
FABRICATION OF COMPOSITES
5.1Stir Casting35
5.2Steps Involved in Stir Casting Method37
5.3 Composition of matrix and reinforcement39
CHAPTER 6
EXPERIMENTAL DETAILS
6.1Fracture Toughness40
6.2Specimen dimensions as per ASTM standards41
6.3Test for Fracture toughness41
6.4Tensile test43
6.5Hardness test46
6.6Compression test48
6.7 Microstructure48
6.8Etching49
6.9Optical metallurgical microscope50
6.10 Finite element analysis51
CHAPTER 7
RESULTS AND DISCUSSIONS
7.1Fracture toughness results53
7.2Comparison of the experimental and FEA results54
7.3Tensile test results55
7.4Hardness test results57
7.5Compression test results59
7.6 Microstructure60
CHAPTER 8
CONCLUSION63
CHAPTER 9
SCOPE FOR FUTURE WORK
REFERENCES
64
65-67
LIST OF TABLES
Table NoDescriptionPage No
4.1Chemical composition of Al35630
4.2Mechanical properties of matrix material Al35630
4.3Properties of Zircon sand33
5.1Different wt% ratios of matrix metal and reinforcement39
6.1ASTM codes for mechanical test and sample dimensions41
7.1Variation of Fracture toughness with different wt%
reinforcements53
7.2Comparison of the experimental and FEA results54
7.3Tensile properties of the MMC55
7.4Variation of hardness with different wt% reinforcement57
7.5Compression strength for different wt% reinforcement59
LIST OF FIGURES
Fig NoDescriptionPage No
1.1Project Methodology3
3.1Classification of composites (based upon the matrix
materials)16
3.2Classification of composites (based upon the reinforcing
materials)16
3.3 Schematic Presentation of Three Shapes of Metal Matrix
Composite
Materials18
4.1Ingot Structure of Al 35629
4.2Reinforcement Material (SiC)31
4.3Reinforcement material (ZrSiO4)32
5.1Flow chart of fabrication of Composite34
5.2Split type mould box36
5.3Pre heating the mould box36
5.4Electric furnace37
5.5Molten Metal in Furnace38
5.6Formation of Vortex38
5.7Pre heating of reinforcement38
5.8Poured molten metal in mould box38
5.9Cast Aluminium Composites38
6.1SENB specimen41
6.2Fracture toughness specimens42
6.3Dimension of Tensile Specimen43
6.4Specimens for Tensile test45
6.5Universal testing machine45
6.6Hardness test specimens47
6.7Brinell hardness testing machine47
6.8Compression test specimens48
6.9Polishing machine49
6.10Optical Metallurgical microscope50
6.11SENB specimen model51
6.12FE mesh model51
6.13Stress distribution from finite element simulations52
7.1Variation of Fracture toughness with different wt%
reinforcement54
7.2 Variation of tensile strength and yield strength with
different wt%
Reinforcement56
7.3Hardness value for different wt% reinforcement58
7.4Compression strength for different wt% reinforcement59
7.5Microstructure of Al356+0%SiC+8%ZrSiO460
7.6Microstructure of Al356+6%SiC+2%ZrSiO460
7.7Microstructure of Al356+2%SiC+6%ZrSiO461
7.8Microstructure of Al356+4%SiC+4%ZrSiO461
7.9Microstructure of Al356+8%SiC+0%ZrSiO462
Experimental Determination and Analysis of Fracture Toughness of
MMC
CHAPTER 1
INTRODUTION
1.1 Preamble
New and high performance particle reinforced metal matrix
composites (PRMMC) are expected to satisfy many requirements for a
wide range of performance-driven, and price sensitive, applications
in aerospace, automobiles, bicycles, golf clubs, and in other
structural applications. In general, these materials exhibit higher
strength and stiffness, in addition to isotropic behavior at a
lower density, when compared to the un-reinforced matrix material.
PRMMC benefits from the ceramics ability to withstand high velocity
impacts, and the high toughness of the metal matrix, which helps in
preventing total shattering. This contribution leads to an
excellent balance between cost and mechanical properties, which are
appealing for many applications.
The recognition of the potential weight savings that can be
achieved by using the advanced composites, which in turn means
reduced cost and greater efficiency, was responsible for this
growth in the technology of reinforcements, matrices and
fabrication of composites. If the first two decades saw the
improvements in the fabrication method, systematic study of
properties and fracture mechanics was at the focal point in the
60s. Since then there has been an ever-increasing demand for new,
strong, stiff and yet light-weight materials in fields such as
aerospace, transportation, automobile and construction sectors.
These materials have low specific gravity that makes their
properties particularly superior in strength and modulus to many
traditional engineering materials such as metals. As a result of
intensive studies into the fundamental nature of materials and
better understanding of their structure property relationship, it
has become possible to develop new composite materials with
improved physical and mechanical properties.
1.2 Statement of the Problem
Aluminium and its alloys have continued to maintain their mark
as the matrix material most in demand for the development of Metal
Matrix Composites (MMCs). This is primarily due to the broad
spectrum of unique properties it offers at relatively low
processing cost. Some of the attractive property combinations of Al
based matrix composites are: high specific stiffness and strength,
better high temperature properties (in comparison with its
monolithic alloy), thermal conductivity, and low thermal
expansion.
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 1
Experimental Determination and Analysis of Fracture Toughness of
MMC
The project is associated with the study of Fracture Toughness
and mechanical properties of Aluminium, Zirconium Silicate and
Silicon Carbide Metal Matrix Composite (MMC). Here we have used the
Aluminium alloy of grade 356 with addition of varying weight
percentage composition of Zirconium Silicate and Silicon Carbide
particles by stir casting technique.
Finite element (FE) simulations for the proposed SENB geometry
was carried out using ANSYS software package (v12) to investigate
stress distribution around the notch and to validate the
experimental results.
The mechanical properties were tested under laboratory
conditions. The change in physical and mechanical properties was
taken in to consideration. For the achievement of the above, an
experimental set up was prepared to facilitate the preparation of
the required specimen. The experiments were carried out to study
the effect of variation of the percentage composition to predict
the mechanical properties as well as to measure the micro
hardness.
1.3 Aim and Objective of the Project
The aim of the project is to synthesize and characterize hybrid
metal matrix composite by stir casting technique and to
experimentally evaluate the fracture toughness and mechanical
properties of the composite. Then finite element analysis is
carried out to validate the obtained results. The objectives of the
project are listed below.
1. Preparation of composite casting by liquid metallurgy
route.
2. Preparation of specimen to required dimensions for the
various tests.
3. The micro structural observations to evaluate the quality of
the castings i.e., base alloy with Silicon Carbide and Zirconium
Silicate (Al356+Sic+ZrSio4).
4. Tests are conducted to evaluate the Fracture toughness and
mechanical properties such as tensile, hardness and
compression.
5. Finite element (FE) simulation to validate the results.
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 2
Experimental Determination and Analysis of Fracture Toughness of
MMC
1.4 Methodology
The methodology of the project in presented in figure 1.1
Literature review
Identification of the
problem
Development of MetalCasting and curingmatrix composites
Tensile
Fracture ToughnessTestingCompression
Hardness
Microstructure
FE analysis
Results and discussions
Conclusion
Fig 1.1 Project Methodology
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 3
Experimental Determination and Analysis of Fracture Toughness of
MMC
CHAPTER 2
LITERATURE REVIEW
J.Jenix Rino, Dr.D.Sivalingappa, Halesh Koti, V.Daniel Jebin[1]
The present study deals with the investigation of the mechanical
behaviour of Aluminium6063 alloy composites reinforced by Zircon
sand(ZrSiO4) and Alumina(Al2O3) particles were taken in to account
for investigating the properties such as density tensile strength
and hardness of the composites synthesized by Stir casting
technique. The mechanical properties evaluation reveals variations
in hardness and the tensile strength values with the composite
combinations. From the experimental studies, the optimum volume
fraction of hybrid reinforcement in Al 6063 alloy on the basis of
microstructure and mechanical properties it is found that the (4+4)
wt% combination.
The unique tailor ability of the composite materials for the
specific requirements makes these materials more popular in a
variety of applications such as aerospace, automotive (pistons,
cylinder liners, bearings), and structural components, resulting in
savings of material and energy. Discontinuous reinforced aluminum
metal matrix composites (DRAMMCs) are a class of composite
materials having desirable properties like low density, high
specific stiffness, high specific strength, controlled co-efficient
of thermal expansion, increased fatigue resistance and superior
dimensional stability at elevated temperatures etc. The properties
and behavior of various Al alloys and their composites are much
explored in terms of microstructure, mechanical properties, loading
conditions and applications.
K.K. Alaneme, A.O. Aluko [2] The tensile and fracture behavior
of as-cast and age-hardened aluminium (6063), silicon carbide
particulate composites produced, using borax additive and a two
step stir casting method, was investigated. Al (6063), SiCp
composites having 3, 6, 9, and 12 volume percent of SiC were
produced, and sample representatives of each composition were
subjected to age-hardening treatment at 1800 C for 3 hours. Tensile
and Circumferential Notched Tensile (CNT) specimens were utilized
for tension testing to evaluate, respectively, the tensile
properties and fracture toughness of the composites. Experimental
results show that the ageing treatment resulted in little
improvement in the tensile strength of the composites. The tensile
strength and yield strength increased to almost the same magnitude
with an increase in SiC volume percent
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 4
Experimental Determination and Analysis of Fracture Toughness of
MMC
for both as-cast and age-hardened conditions. The increase was,
however, more significant for the 9 and 12 volume percent SiC
reinforcement. The strain to fracture was less sensitive to volume
percent SiC reinforcement and ageing treatment, with values less
than 12% strain to fracture observed in all cases.
Aluminium and its alloys have continued to maintain their mark
as the matrix material most in demand for the development of Metal
Matrix Composites (MMCs). This is primarily due to the broad
spectrum of unique properties it offers at relatively low
processing cost. Some of the attractive property combinations of Al
based matrix composites are: high specific stiffness and strength,
better high temperature properties (in comparison with its
monolithic alloy), thermal conductivity, and low thermal expansion.
The multifunctional nature of Al matrix composites has seen its
application in aerospace technology, electronic heat sinks, solar
panelsubstrates and antenna reflectors, automotive drive shaft
fins, and explosion engine components, among others.
Mohan Vanarotti, SA Kori, BR Sridhar, Shrishail B.Padasalgi [3]
Aluminum alloy and silicon carbide metal matrix composites are
finding applications in aerospace, automobile and general
engineering industries owing to their favourable microstructure and
improved mechanical behavior. Aluminium alloy A356 and silicon
carbide composites were obtained by stir casting technique. Silicon
carbide content in the alloy was fixed at 5 Weight % and 10 weight
% during the casting. Microstructure revealed a uniform
distribution of the silicon carbide throughout the matrix. Hardness
and tensile properties of the composite showed an improvement as
compared to the alloy without silicon carbide additions.The present
paper highlights the salient features of casting technique and
characterization of aluminum alloy A356 and silicon carbide metal
matrix composite.
J.E. Perez Ipina, A.A. Yawny, R. Stukeb, C. Gonzalez Oliver[4]
Metal matrix composites (MMC) are materials made from the
dispersion of a ceramic phase, typically SiC or Al203 fibers or
particles, in order to improve the mechanical and physical
properties of the matrix. In the particular situation of Aluminum
MMCs, both pure Al and alloys are employed. Continuous fibers
(Continuous metal matrix composites CMMC) as well as short fibers
and particles (Discontinuous Aluminum reinforced DAR) are employed.
The production and use of composite materials is under intensive
development because of the interesting physical and mechanical
properties that these materials present and also due to the
possibility to manipulate them by means of the variation of the
type and proportion of
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 5
Experimental Determination and Analysis of Fracture Toughness of
MMC
the reinforcement employed as well as the type of the metallic
matrix. Materials with designed mechanical (yield stress, elastic
modulus, etc) and physical (thermal expansion coefficient,
resistivity, thermal conductivity, etc) properties can be produced
in this way.
E.G. Okafor , V.S.Aigbodion [5] The as-cast microstructure and
properties of Al-4.5Cu/ZrSiO4 particulate composite synthesized via
squeezed casting route was studied, varying the percentage ZrSiO4
in the range of 5-25wt%. The result obtained revealed that addition
of ZrSiO4 reinforcements, increased the hardness value and apparent
porosity by 107.65 and 34.23% respectively and decrease impact
energy by 43.16 %. As the weight percent of ZrSiO4 increases in the
matrix alloy, the yield and ultimate tensile strength increased by
156.52 and 155.81% up to a maximum of 15% ZrSiO4 addition
respectively. The distribution of the brittle ZrSiO4 phase in the
ductile matrix alloy led to increase strength and hardness values.
These results had shown that, additions of ZrSiO4 particles to
Al-4.5Cu matrix alloy improved properties. From the result of the
investigation in this research work it could be concluded that
addition of ZrSiO4 particles using Al-4.5%Cu alloy increased both
the strength and hardness and an overall reduction in toughness and
density. Also, little increase in the apparent porosity of the
composite with percentage increase in ZrSiO4 addition was observed.
From the result, maximum service performance of the Al-
4.5Cu/ZrSiO4 particulate composite synthesis via squeeze casting
should not exceed 15% in order to develop balance in the necessary
properties. Pronounce increase in hardness value was observed by
reinforcing the matrix alloy with 5-25% zircon sand.
Al-4.5Cu/15%ZrSiO4 particulate composite could be appreciable in
automobile industries (brake drum, crankshafts, values and
suspension arms), recreational products (golf club shaft and head,
skating shoe, bicycle frames and base ball shaft) and in
construction company (truss structure).
Khalid Mahmood Ghauri1, Liaqat Ali [6] The present work was
mainly carried out to characterize the SiC/Al composite which was
produced by reinforcing the various proportions of SiC (5, 10, 15,
25 and 30%) in aluminum matrix using stir casting technique.
Mechanical properties of test specimens made from stir-casted
Aluminum-Silicon Carbide composites have been studied using
metallographic and mechanical testing techniques. However, beyond a
level of 25-30 percent SiC, the results are not very consistent,
and depend largely on the uniformity of distribution of SiC in the
aluminum matrix It was observed that as the volume fraction of SiC
in the composite is gradually
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 6
Experimental Determination and Analysis of Fracture Toughness of
MMC
increased, the hardness and toughness increase. The experimental
results showed that the composition of the composite for the
optimized properties ranges between 70 to 80 percent aluminum and
20 to 30 percent silicon carbide. It is quite evident that
deformation in metals is because of the movement of dislocations
and if we block these dislocations by some means the strength which
is resistance against the applied force of the material,
sufficiently increases. There are numerous ways to block these
dislocations like increasing the dislocation density, alloying and
making composite in such a way that the newly reinforcing phase
acts like a barrier against movement of these dislocations. In the
current research work it is evident that as we increase the amount
of silicon carbide in aluminum matrix, there is improvement in
hardness and the impact properties, these are sufficiently high in
the vicinity of mechanical mixture of 25% silicon carbide and 75%
aluminum.
G. Hemath Kumar, M. Sreenivasan [7] The present work reports on
mechanical properties and microstructure analysis of Al-SiC
particulate composites with different wt. % of SiC. Al-SiC
composite specimens with different weight % of SiC (viz. 5, 10, 15,
20, 25 and 30 wt. % of SiC) were fabricated through casting
process. The induction furnace and open furnace were used for
melting of Al-SiC particulate composites. The induction furnace
gives the advantage of self stirring action on the introduction of
SiC particles. Grinding and fine polishing was done using diamond
paste to prepare different samples for microscopic study. The
microstructure examination of the polished and carefully etched
Al-SiC composite specimens showed that the structure consists of a
network of silicon particles, which were formed in inter-dendritic
aluminum silicon eutectic composition. These SEM micrographs
clearly indicate that the SiC particulates are dispersed uniformly
in the Al matrix even at higher percentage such as 20 weight % SiC.
The SiC particulates were observed to be in irregular shape. The
Al-SiCp composites were fabricated using induction melting show
higher compressive strength values than those fabricated using open
furnace melting. Al-SiC composites fabricated using induction
melting exhibited better mechanical properties than the composites
processed in open furnace. Al-SiC composite poppet valve guides
with 5 to 30 wt. % of SiC were successfully fabricated by casting
process. These guides possess very good surface finish. The
compressive strength, density and hardness of Al- SiC composites
increase with increase in wt. percent of SiC particulates for all
the composites tested. The tensile strength decreases when the
amount of reinforcement content exceed to 30 wt. % of SiC.
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 7
Experimental Determination and Analysis of Fracture Toughness of
MMC
Mohammad M. Ranjbaran [8] This experimental investigation was
initiated to study the low-toughness fracture in Al 356-SiCp
(silicon carbide particles) with respect to the role of the various
elements of the microstructure and their probable contribution. The
fracture in this composite is studied experimentally, in terms of
fracture toughness testing. The low-toughness fracture is believed
to be an inherent property of this composite and is caused mainly
by the differential elastic and thermal properties of the two
constituents. These differentials degrade the matrix alloy near the
interface by its strain hardening capacity and by stress
intensification introduced by the SiC particle geometry.
Consequently, the matrix near the interface is subjected to high
localized damage leading to premature fracture. It is found that
the matrix alloy controls both flow properties and fracture in the
materials investigated. It is concluded that a higher toughness
composite requires a proper choice of constituent properties which
dominate the stress state at the interface. The measurement of
valid plane strain fracture toughness, (KIC) values for particulate
reinforced metal matrix composites is an important step in the
process of developing useful products from these materials and
increasing confidence in their properties and performance. The
value of the KIC characterizes the fracture resistance of a
material in the presence of a sharp crack under tensile
loading,
This study shows that the failure is initiated by micro void
nucleation at the different initiation sites. Void initiation is
more pronounced in the matrix near the interface. The micro cracks
can grow from these micro voids to absorb available strain energy.
Crack propagation occurs by linking these micro cracks locating the
crack path preferentially in the matrix adjacent to the interface.
This study shows that this material must have adequate wettability
with both Al and SiC to achieve good bonding. Moreover, the
proposed material must have high value of tensile ductility and a
low yield stress in order to accommodate the plastic strain
developed during processing and relax stress concentrations
introduced by particle geometry.
Shuyi Qin, Guoding Zhang [9] A structure-toughened SiC particle
reinforced 6061 aluminum alloy matrix composite
(SiCp-6061Al/6061Al) was designed and fabricated by vacuum
infiltration processing. Its fracture toughness KQ was tested by
three-point bending method and compared with a conventionally
stirring-cast SiCp/6061Al composite's in case of same particle size
and volume fraction. The fractography of the SiCp-6061Al/6061Al
composite was observed on a Cambridge Instrument S360 Scanning
Electron Microscopy (SEM). The results showed that
SiCp-6061Al/6061Al
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 8
Experimental Determination and Analysis of Fracture Toughness of
MMC
composite has higher fracture toughness KQ but lower yield
strength and a comparative elastic modulus. The crack opening
displacement (COD) vs load curve of the designed composite showed
that the fracture procedure of SiCp-6061Al/6061Al composite is by
three stages and the maximum load on it can maintain for a long
time. The deformation of the unreinforced 6061Al matrix and the
SiCp-6061Al/6061Al interface debonding toughen this composite
cooperatively. The complete fracture procedure of the designed
composite was schemed by a model. This kind of composite can avoid
abrupt failure occurring in most other conventional composite.
Vignesh. S, Sanjeev. C [10] In this paper, turning experiments
on machining of particle reinforced Hybrid Metal Matrix composite
(MMC) have been carried out. The reinforcement particles selected
are Silicon-Carbide of 10% by weight and Boron-Carbide of 5% by
weight respectively. Stir casting method is followed to prepare
cylindrical rods of specific length and diameter. Poly Crystalline
Diamond (PCD) insert of grade 1600 is used for turning operations.
Taguchis method of design of experiment is followed by using
orthogonal array L9. Three level machining parameters selected are
cutting speed, feed rate and depth of cut. The influence of these
parameters on machined surface quality is determined by measuring
the surface roughness of the workpiece by surface roughness tester.
The optimal cutting conditions are arrived as feed rate 0.1 mm/rev,
cutting speed as 70 m/min and depth of cut as 0.5mm. The S-N plot
is drawn to show the characteristics of each parameter with respect
to surface roughness. The results are validated by analysis of
variance method (ANOVA) and the percentage of contribution of feed,
speed and depth of cut are determined. Tool wear study also
performed for a duration of 20 minutes. Hybrid metal matrix
composites are economically cheaper in both raw materials and
method of fabrication. Due to the reinforcement of ceramic
materials, the machining of these metal matrix composites become
significantly more difficult than those of conventional materials.
The surface quality obtained in turning aluminium (Al 356) metal
matrix composites with reinforcements of ceramic particles with 10%
by weight of SiC and 5% by weight of
B4C under different cutting conditions with a PCD tool of 1600
grade, have been investigated using Taguchis orthogonal array (L9).
The following conclusions are drawn based on the experimental and
analytical results: 1. By using Taguchi method, the effect of
machining parameters on the surface quality (Ra) has been evaluated
and optimal machining conditions would be arrived to minimize the
surface roughness.
M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 9
Experimental Determination and Analysis of Fracture Toughness of
MMC
Dinesh Kumar Koli, Geeta Agnihotri [11] This paper reviews the
characterization of mechanical properties with production routes of
powder metallurgy and castings for aluminium matrix- Al2O3
composites. Reinforcing aluminium matrix with much smaller
particles, submicron or nano-sized range is one of the key factors
in producing high-performance composites, which yields improved
mechanical properties. Nearly 92% increase in the hardness and 57%
increase in the tensile strength were obtained in the
nano-composites as compared to the commercially pure aluminium.
Ultrasonic assisted casting and powder metallurgy methods are
becoming more common for the production of Al-Al2O3 composites.
Agglomeration of the reinforcing particles along with the
increasing volume percentage is still a challenging task in
composites materials manufacturing. There are exciting
opportunities for producing exceptionally strong, light weight,
wear resistant metal matrix composites with acceptable ductility by
solidification processing and powder metallurgy. In addition,
processing methods must be developed to synthesize these materials
in bulk, at lower cost, with little or no voids or defects, and
with improved ductility, possibly as a result of bimodal and
tri-modal microstructures. Metal matrix nanocomposites can lead to
significant savings in materials and energy and reduce pollution
through the use of ultra-strong materials that exhibit low friction
coefficients, high wear resistance, low coefficient of thermal
expansion and light weight.
Don-Hyun CHOI, Yong-Hwan KIM [12] Friction stir processing (FSP)
was used to incorporate SiC particles into the matrix of A356 Al
alloy to form composite material. Constant tool rotation speed of
1800 r/min and travel speed of 127 mm/min were used in this study.
The base metal (BM) shows the hypoeutectic AlSi dendrite structure.
The microstructure of the stir zone (SZ) is very different from
that of the BM. The eutectic Si and SiC particles are dispersed
homogeneously in primary Al solid solution. The hardness of the SZ
shows higher value than that of the BM because some defects are
remarkably reduced and the eutectic Si and SiC particles are
dispersed over the SZ. The composite material of A356 with SiC
particles was produced successfully by FSP. 2) In the SZ, the
homogeneous distribution of SiC particles as well as the
spherodization of Si needles and their spreading through the matrix
are the dominant reasons for improvement of properties in the SZ.
3) The mechanical properties of the SZ with SiC particles, compared
to the BM and SZ without SiC, were improved by the dispersed Si,
SiC particles and the homogeneous microstructure.
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CHAPTER 3
INTRODUCTION TO COMPOSITE
3.1 Background
New and high performance particle reinforced metal matrix
composites (PRMMC) are expected to satisfy many requirements for a
wide range of performance-driven, and price sensitive, applications
in aerospace, automobiles, bicycles, golf clubs, and in other
structural applications. A PRMMC consists of a uniform distribution
of strengthening ceramic particles embedded within a metal matrix.
In general, these materials exhibit higher strength and stiffness,
in addition to isotropic behavior at a lower density, when compared
to the un-reinforced matrix material. PRMMC benefits from the
ceramics ability to withstand high velocity impacts, and the high
toughness of the metal matrix, which helps in preventing total
shattering. This contribution leads to an excellent balance between
cost and mechanical properties, which are appealing for many
applications.
The recognition of the potential weight savings that can be
achieved by using the advanced composites, which in turn means
reduced cost and greater efficiency, was responsible for this
growth in the technology of reinforcements, matrices and
fabrication of composites. If the first two decades saw the
improvements in the fabrication method, systematic study of
properties and fracture mechanics was at the focal point in the
60s. Since then there has been an ever-increasing demand for new,
strong, stiff and yet light-weight materials in fields such as
aerospace, transportation, automobile and construction sectors.
Composite materials are emerging chiefly in response to
unprecedented demands from technology due to rapidly advancing
activities in aircrafts, aerospace and automotive industries. These
materials have low specific gravity that makes their properties
particularly superior in strength and modulus to many traditional
engineering materials such as metals. As a result of intensive
studies into the fundamental nature of materials and better
understanding of their structure property relationship, it has
become possible to develop new composite materials with improved
physical and mechanical properties. Based on information now in the
public domain, the following military applications for MMCs appear
attractive: high-temperature fighter aircraft engines and
structures; high-temperature missile structures; and spacecraft
structures. Testing of a National Aerospace Plane (NASP) prototype
is scheduled for the early to mid 1990s, which might be too early
to include MMCs. However, it may be possible to incorporate MMCs in
the structure or engines of the production vehicle.
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3.2 Composites
Three decades of intensive research have provided wealth of new
scientific knowledge on the intrinsic and extrinsic effects of
ceramic reinforcement to metals and their alloys. The successes of
these various researches have stimulated application of composite
in the design of many engineering and non engineering
component.
Further, the need of composite for lighter construction
materials and more seismic resistant structures has placed high
emphasis on the use of new and advanced materials that not only
decreases dead weight but also absorbs the shock & vibration
through tailored microstructures. Composites are now extensively
being used for rehabilitation/ strengthening of pre-existing
structures that have to be retrofitted to make them seismic
resistant, or to repair damage caused by seismic activity.
While composites have already proven their worth as
weight-saving materials, the current challenge is to make them cost
effective. The efforts to produce economically attractive composite
components have resulted in several innovative manufacturing
techniques currently being used in the composites 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 be an integrated effort in
design, material, process, tooling, quality assurance,
manufacturing, and even program management for composites to become
competitive with 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 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 uses and volume. The
increased volume has resulted in an expected reduction in costs.
High performance FRP can now be found in such diverse applications
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 cascades for engines, curved fairing
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and fillets, replacements for welded metallic parts, cylinders,
tubes, ducts, blade containment bands etc.
Unlike conventional materials (e.g., steel), the properties of
the composite material can be designed considering the structural
aspects. The design of a structural component using composites
involves both material and structural design. Composite properties
(e.g. stiffness, thermal expansion etc.) can be varied continuously
over a broad range of values under the control of the designer.
Careful selection of reinforcement type enables finished product
characteristics to be tailored to almost any specific engineering
requirement.
The production and use of composite materials is under intensive
development because of the interesting physical and mechanical
properties that these materials present and also due to the
possibility to manipulate them by means of the variation of the
type and proportion of the reinforcement employed as well as the
type of the metallic matrix. Materials with designed mechanical
(yield stress, elastic modulus, etc) and physical (thermal
expansion coefficient, resistivity, thermal conductivity, etc)
properties can be produced in this way.
3.2.1 Composite Definition
A Composite material is defined as a structural material created
synthetically or artificially by combining two or more materials
having dissimilar characteristics. The constituents are combined at
macroscopic level and are not soluble in each other. One
constituent is called as Matrix phase and the other is called
Reinforcing phase. Reinforcing phase is embedded in the matrix to
give the desired characteristics.
Generally, a composite material is composed of reinforcement
(fibers, particles, flakes, and/or fillers) embedded in a matrix
(polymers, metals, or ceramics). The matrix holds the reinforcement
to form the desired shape while the reinforcement improves the
overall mechanical properties of the matrix. When designed
properly, the new combined material exhibits better strength than
would each individual material. As defined by Jartiz, Composites
are multifunctional material systems that provide characteristics
not obtainable from any discrete material. They are cohesive
structures made by physically combining two or more compatible
materials, different in composition and characteristics and
sometimes in form.
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Kelly very clearly stresses that the composites should not be
regarded simple as a combination of two materials. In the broader
significance; the combination has its own distinctive properties.
In terms of strength or resistance to heat or some other desirable
quality, it is better than either of the components alone or
radically different from either of them.
3.2.2 Need for Developing Composite Materials
Composites with high specific stiffness and strength could be
used in applications in which saving weight is an important factor.
Included in this category are robots, high-speed machinery, and
high-speed rotating shafts for ships or land vehicles. Good wear
resistance, along with high specific strength, also favors MMC use
in automotive engine and brake parts. Tailorable coefficient of
thermal expansion and thermal conductivity make them good
candidates for lasers, precision machinery, and electronic
packaging. However, the current level of development effort appears
to be inadequate to bring about commercialization of any of these
in the next 5 years, with the possible exception of diesel engine
pistons.
The increasing demand for lightweight, inexpensive, energy
saving, stiff and strong materials in aircraft, space, defense and
automotive applications have stimulated steadily growing efforts to
develop composite materials. Lightweight composites are attracting
a great deal of attention due to the possibility of weight saving
in industrial applications. Automobile weight reduction can
directly translate into reduced fuel consumption. Reduction in the
weight of aircraft and marine vessels can lead to increased loading
capacity.
3.2.3 Characteristics of the Composite
Metal matrix composites are strong and tough and can be
plastically deformed easily. The crystalline structures make the
metals posses excellent properties like thermal and electrical
conductivity, high malleability and ductility. Dislocations are
critically important as they drastically reduce shear stress
required to the slip process and hence make the metals to deform
plastically.
Metals can be strengthened by a number of strengthening
mechanisms namely, strain hardening, grain boundary strengthening,
precipitation strengthening, strengthening due to phase
transformation and dispersoid strengthening. By intentional
addition of hard
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dispersoid particles in the matrix it is possible to increase
the strength as these particles retard the motion of the
dislocation.
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 by 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 interfacial area, which
plays an important role in determining the extent of the
interaction between the reinforcement and the matrix.
Concentration, 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. The orientation of the reinforcement affects
the isotropy of the system.
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3.3 CLASSIFICATION OF COMPOSITE
3.3.1 The Matrix Material
Matrix Material
Polymer MatrixMetal MatrixCeramic Matrix
Thermo-Light metalsCeramics
plastics& alloys(Al,Carbon
Thermo setsMg, Li & Ti)Glass
Refractory
metals
Fig 3.1 Classification of composites (based upon the matrix
materials)
3.3.2 The Reinforcing Material
Reinforcing Material
Particulate reinforcedFiber reinforcedStructural composites
Large particlesContinuous fibersLaminates
DispersoidsDiscontinuesSandwich
(short)panels
Aligned or
random
Fig 3.2 Classification of composites (based upon the reinforcing
materials)
The classification of composites based upon the Reinforcing
materials is as shown in figure. 1.2
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Systematic combinations of different constituents, conventional
monolithic materials have limitations in terms of achievable
combinations of strength, stiffness, coefficient of expansion, and
density engineered MMCs consisting of continuous or is continuous
fiber,
Whiskers or particle in a metal results in combination of very
high specific strength and specific modulus. Furthermore,
systematic design and syntheses procedures can be developed to
achieve unique combination of engineering properties such as high
elevated-temperature strengths, fatigue strength, damping
properties, electrical conductivity, thermal conductivity,
coefficient of thermal expansion.
A variety of methods for producing MMCs, including foundry
techniques, have recently become available. The potential advantage
of preparing these composite materials by foundry technique is
near-neat shape fabrication in a simple and cost-effective manner.
In addition, foundry processes lend themselves to the manufacture
of large number of complexly shaped components at higher production
rates, which is required by automotive and other consumer oriented
industries.
Structurally, as cast MMCs consist of continuous or
discontinuous fibers, whiskers, or particles in an alloy matrix
that solidifies the restricted spaces between the reinforcing
phases to form the bulk of the bulk of matrix. By carefully
controlling the relative amounts and distributions of the
ingredients constituting a composite and by controlling the
solidification conditions, MMCs can be imparted a tailoring set of
use full engineering properties that cannot be realized with
conventional monolithic materials. In addition, the solidification
microstructure of the matrix is refined and particles, indicating
the possibility of controlling macro segregation, and grain size in
the matrix. This represents an opportunity to develop new matrix
alloys.
3.3.3 Classification (According to the Type of
Reinforcement)
1. Particulate Composites (Compose of Particle in a
Composite)
2. Fibrous Composites (Consists of Fibers in Matrix)
3. Laminated Composites (Consists of Layers of Various
Materials)
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3.3.3.1 Particulate Composites
This consists of particulates of one or more material suspended
in a matrix of another material. The particle can be metallic or
non-metallic, as can the matrix following are the types. Fiber
composite materials can be further classified into continuous fiber
composite materials (multi- and monofilament) and short fibers or,
rather, whisker composite materials as shown in figure. 1.3.
A. Non-Metallic in Metallic Composites
The most common example in this case is concrete. Flakes of
non-metallic materials such as mica or glass can form an effective
composite material when suspended in a glass or plastic
respectively. Mica in glass composite is used in electrical
application because of good insulating and machining qualities.
`
Figure 3.3 Schematic Presentation of Three Shapes of Metal
Matrix Composite
Materials
B. Metallic in Non-Metallic Composites
The most common example is rocket propellants, which consists of
inorganic particles such as Al powder and per chlorate oxidizer in
a flexible organic binder such as polyurethane or Poly-sulphide
rubber. Metal flakes in a suspension are also common. Aluminium
paint is actually Aluminum flakes suspended in paint. Upon
application the flakes orient themselves parallel to the surface
giving good coverage. Similarly, similar flakes can be applied to
give good electrical conductivity. Cold solder is metal powder
suspended in thermosetting resin. The composite is strong, hard and
conducts heat and electricity. Inclusion of copper in an epoxy
resin increases the conductivity immensely.
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C. Non-Metallic in Metallic Composites
Non-metallic particles such as ceramic can be suspended in
metallic matrix. The resulting component is called cermets. Two
common classes of cermets are; oxide based and carbide based
composites.
1. Oxide Based Cermets can be either oxide particles in a metal
matrix or vice- versa. These basically used in tool and high
temperature application where erosion resistance is required. 2.
Carbide Cement has particles of carbide of tungsten, chromium, and
titanium in metal matrix, generally cobalt matrix. These are used
in dies, valves, turbine parts. 3. Cermets are also used as nuclear
reactor fuel element and control rods.
3.3.3.2 Fibrous Composites
A fiber is defined with respect to its length to diameter ratio
and its near crystal diameter. A whisker has essentially the same
near crystal size diameter as a fiber, but generally very short and
stubby, although the length to diameter ratio can be in hundreds.
Fibers and whiskers are of little use unless they are bounded
together to take the form of a structural element, which can take
loads.
3.3.3.3 Laminated Composites
Laminated composites consist of layers of at least two different
materials that are bonded together. They are of following
types:
Bimetals
Clad metals
Laminated glass
These are hybrid class of composite involving both fibrous
composite and laminate technique. A more common is laminated fiber
reinforced composite. Here layers of fiber-reinforced materials are
built up with the fiber direction of each layer typically oriented
in different direction to give stiffness and strength to fiber.
3.4 Applications of Composites
a. Aerospace and Space-craft applications.
b. Automotive
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c. Defense
d. Space hardware
e. Marine application
f. Electrical and electronic devices
3.5 Fabrication techniques of MMCs
There are several fabrication techniques available to
manufacture the MMC materials: there is no unique route in this
respect. Due to the choice of material and reinforcement and of the
types of reinforcement, the fabrication techniques can vary
considerably. The processing methods used to manufacture
particulate reinforced MMCs can be grouped as follows.1.
Solid-phase fabrication methods: diffusion bonding, hot rolling,
extrusion, drawing, explosive welding, PM route, pneumatic
impaction, etc.
2. Liquid-phase fabrication methods: liquid-metal infiltration,
squeeze casting, compo casting, pressure casting, spray co
deposition, stir casting etc.
3. Two phase (solid/liquid) processes: Which include Rheocasting
and Spray atomization.
Normally the liquid-phase fabrication method is more efficient
than the solid-phase fabrication method because solid-phase
processing requires a longer time. The matrix metal is used in
various forms in different fabrication methods. Generally powder is
used in pneumatic impaction and the powder metallurgy technique,
and a liquid matrix is used in liquid-metal infiltration, plasma
spray, spray casting, squeeze casting, pressure casting, gravity
casting, stir casting, investment casting, etc. A molecular form of
the matrix is used in electroforming; vapor deposition and metal
foils are used in diffusion bonding, rolling, extrusion, etc.
There are certain main manufacturing processes which are used
presently in laboratories as well as in industries are diffusion
bonding, the powder metallurgy route, liquid-metal infiltration,
squeeze casting, spray co-deposition, stir casting and compo
casting. Brief Description of these processes is given below.
3.5.1 Solid Phase Fabrication Method
There are several ways to fabricate MMC using solid-phase
materials but among them diffusion bonding and the powder
metallurgy route are used widely.M-Tech, MDE, Dept of Mech Engg.,
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3.5.2 Diffusion Bonding
This method is normally used to manufacture fiber reinforced MMC
with sheets or foils of matrix material.
Here primarily the metal or metal alloys in the form of sheets
and the reinforcement material in the form of fiber are chemically
surface treated for the effectiveness of interdiffusion. Then
fibers are placed on the metal foil in pre-determined orientation
and bonding takes place by press forming directly, as shown by the
dotted line. However sometimes the fibers are coated by plasma
spraying or ion plating for enhancing the bonding strength before
diffusion bonding, the solid line shows this. After bonding,
secondary machining work is carried out. The applied pressure and
temperature as well as their durations for diffusion bonding to
develop, vary with the composite systems. However, this is the most
expensive method of fabricating MMC materials.
3.5.3 Powder Metallurgy Technique
The PM technique is the most commonly used method for the
preparation of discontinuous reinforced MMCs. This technique is
used to manufacture MMCs using either particulates or whiskers as
the reinforcement materials. In general process the powders of
matrix materials and reinforcement are first blended and fed into a
mould of the desired shape. Pressure is then applied to further
compact the powder (cold pressing). In order to facilitate the
bonding between the powder particles, the compact is then heated to
a temperature that is below the melting point but sufficiently high
to develop significant solid-state diffusion (sintering). The
consolidated product is then used as a MMC material after some
secondary operation.
This method is popular because it is reliable compared with
other alternative methods, but it has also some demerits. The
blending step is a time consuming, expensive and potentially
dangerous operation. In addition, it is difficult to achieve an
even distribution of particulate throughout the product and the use
of powders requires a high level of cleanliness, otherwise
inclusions will be incorporated into the product with a deleterious
effect on fracture toughness, fatigue life, etc.
3.5.4 Liquid Phase Fabrication Techniques
Most of the MMCs are produced by this technique. In this
technique, the ceramic particles are incorporated into liquid metal
using various processes. The liquid composite
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slurry is subsequently cast into various shapes by conventional
casting techniques or cast into ingots for secondary processing.
The process has major advantage that the production costs of MMCs
are very low. The major difficulty in such processes is the
non-wettability of the particles by liquid aluminium and the
consequent rejection of the particles from the melt, non-uniform
distribution of particles due to their preferential segregation and
extensive interfacial reaction.
3.5.5 Liquid Metal Infiltration
This process can also be called fiber-tow infiltration. Fibers
tows can be infiltrated by passing through a bath of molten metal.
Usually the fibers must be coated in line to promote wetting. Once
the infiltrated wires are produced, they must be assembled into a
preform and given a secondary consolidation process to produce a
component. Secondary consolidation is generally accomplished
through diffusion bonding or hot molding in the two-phase liquid
and solid region.
In this technique, as the first step, FP yarn is made into a
handle able FP tape with a fugitive organic binder in a manner
similar to producing a resin matrix composite preparation. Fibre FP
tapes are then laid-up in the desired orientation, fiber volume
loading, and shape, and are then inserted into a casting mold of
steel or other suitable material. The fugitive organic binder is
burned away, and the mold is infiltrated with molten metal and
allowed to solidify. Metals such as Aluminium, magnesium, silver
and copper have been used as the matrix materials in this liquid
infiltration process because of their relatively lower melting
points. This method is desirable in producing relatively small-size
composite specimens having unidirectional properties.
3.5.6 Squeeze Casting
Squeeze casting is a one-step metal forming process in which a
metered quantity of liquid metal in a reusable die is subjected to
a rapid solidification under high pressures (50 to 100 MPa) to
produce close-tolerance, high-integrity finished shapes. The
fabrication process of MMC by squeeze casting, the preform of the
ceramic fiber is pre-heated to several hundred degrees centigrade
below the melting temperature of the matrix and then set into a
metal die. The Al or Mg alloy is heated to just above its melting
temperature and is then squeezed into the fiber preform by a
hydraulic press to form a mixture of fiber and molten metal.
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This process can be used for large scale manufacturing but it
requires careful control of the process variables, including the
fiber and liquid metal preheat temperature, the metal alloying
elements, external cooling, the melt quality, the tooling
temperature, the time lag between die closure and pressurization,
the pressure levels and duration and the plunger speed. Imperfect
control of these process variables results in various defects,
including freeze chocking, preform deformation, fiber degradation,
oxide inclusions and other common casting defects. However, in
practical use, squeeze casting is the most effective method of
constructing a machine parts with a complex shape in a short
time.
3.5.7 Spray Co-deposition Method
Spray co-deposition method is an economical method of producing
a particulate composite. The alloy to be sprayed is melted in a
crucible by induction heating. The crucible is pressurized and the
metal is ejected through a nozzle into an atomizer where, at the
same time, particles (reinforcement) are injected into the atomized
metal and deposited on a preheated substrate placed in the line of
flight. A solid deposit is built up on the collector. The deposited
strip, when cold, is moved from the substrate for subsequent
rolling. The shape of the final product depends on the atomizing
condition and the shape and the motion of the collector.
3.5.8 Stir Casting
This approach involves mechanical mixing of the reinforcement
particulate into a molten metal bath and transferred the mixture
directly to a shaped mould prior to complete solidification. In
this process, the crucial thing is to create good wetting between
the particulate reinforcement and the molten metal.
Micro structural in homogeneties can cause notably particle
agglomeration and sedimentation in the melt and subsequently during
solidification. In homogeneity 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. This process has
major advantage that the production costs of MMCs are very low.
3.5.9 Compo Casting
Other than PM, thermal spraying, diffusion bonding and
high-pressure squeeze casting, this is the most economical method
of fabricating a composite with discontinuous fibers
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(chopped fiber, whisker and particulate). This process is the
improved process of slush- or stir-casting.
The apparatus consists of an induction power supply (50 kW, 3000
Hz), a water-cooled vacuum chamber with its associated mechanical
and diffusion pumps and a crucible and mixing assembly for
agitation of the composites.
First, a metal alloy is placed in the system with the blade
assembly in place. Then the chamber is evacuated and the alloy is
superheated above its melting temperature and stirring is initiated
by the DC motor to homogenize the temperature. The induction power
is lowered gradually until the alloy is 40 to 50% solid, at which
point the nonmetallic particles are added to the slurry, However,
the temperature is raised during adding in such a way that the
total amount of solid, which consists of fibers and solid globules
of the slurry, does not exceed 50%. Stirring is continued until
interface interactions between the particulates and the matrix
promote wetting.
The melt is then superheated to above its liquid temperature and
bottom poured into the graphite mould by raising the blade
assembly. The melt containing the nonmetallic particles is then
transferred into the lower die-half of the press and the top die is
brought down to shape and solidify the Composite by applying the
pressure. This is using to make the composite of the highest values
of volume fractions of reinforcement.
Literature in general, suggests that MMCs will be less forgiving
in terms of processing practice than unreinforced alloys, but if
the appropriate practice is employed, useful combinations of
mechanical and physical properties can be obtained
3.6 Metal matrix composites
Metal matrix composites are materials with metals as the base
and distinct, typically ceramic phases added as reinforcements to
improve the properties. The reinforcements can be in the form of
fibers, whiskers and particulates. Properties of the metal matrix
composites can be tailored by varying the nature of constituents
and their volume fraction. They offer superior combination of
properties in such a manner that today no existing monolithic
material can rival and hence are increasingly being used in the
aerospace and automobile industries. The principal advantage MMCs
enjoy over other materials lies in the improved strength and
hardness on a unit weight basis.
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The family of materials classified as metal-matrix composites
(MMCs) comprises a very broad range of advanced composites of great
importance to both automobile, aerospace & defense
applications. However, the development and use of MMCs are still in
their infancy when compared to monolithic materials or even polymer
resin-matrix composite systems. The family of metal-matrix
composites is made up of many varieties of materials, which can be
categorized based on their matrix composition, fabrication process,
or reinforcement type.
Metal matrix composites have a metal matrix usually of lighter
metal such as (Al, Mg or Ti) or a super alloy (Ni based or Co based
super alloy). The reinforcement materials include Boron, Silicon
carbide, carbon, graphite, alumina, Boron carbide, Boron nitride or
metallic system like tungsten, beryllium or steel. The form of
reinforcement material can be either fiber or whisker or
particulate. Metals are reinforced either to increase certain
properties like elastic modulus and tensile strength or decrease
certain properties like coefficient of thermal expansion and
thermal conductivities.
In recent years, the development of metal matrix composite
(MMCs) has been receiving worldwide attention on account of their
superior strength and stiffness in addition to high wear resistance
and creep resistance comparison to their corresponding wrought
alloys. The ductile matrix permits the blunting of cracks and
stress concentrations by plastic deformation and provides a
material with improved fracture toughness.
Aluminium matrix composite (AMCs) have shown high mechanical
properties such as high strength, high stiffness, wear resistance
and good elevated temperature properties when compared to the
unreinforced matrix alloy, which has lead to the use of aluminium
matrix composite in the following; electronic heat sinks,
automotive drive shaft, ground vehicles brake rotors, jet fighters,
air craft firms, electronic instrument racks, satellite struts,
crankshafts, gear parts brake drum cylinder block and suspension
arms. New researches on metal matrix composite have focus on
particle reinforcement due to low cost of the ceramic reinforcement
and less complex fabrication technique. Stirring casting route has
been used successfully to synthesis metal matrix composite.
It has been proved that particle reinforced aluminum matrix
composites can improve considerably the strength and hardness of
aluminum and its alloys. However, at the same time, the plasticity
and ductility can substantially reduced. This will severely affect
the safety and reliability of components fabricated from Al matrix
composites (AMCs).
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In aluminum matrix composites system (AMCs), one of the
constituent is aluminum/ aluminum alloy, which forms percolating
network and is termed as matrix phase. The other constituent is
embedded in this aluminum/ aluminum alloy matrix and serves as
reinforcement, which is usually non-metallic and commonly ceramic
such as SiC and Al2O3. Properties of AMCs can be tailored by
varying the nature of constituents and their volume fraction.
Aluminum matrix composites system (AMCs) offer superior
combination of properties in such manner that today no existing
monolithic material can rival. Over the years, AMCs have been tried
and used in numerous structural, non-structural and functional
applications in different engineering sectors. Driving force for
the utilization of AMCs in these sectors include performance,
economic and environmental benefits. The key benefits of AMCs in
transportation sectors are lower fuel consumption, less noise and
lower airborne emissions.
AMCs can be classified into four types depending on the type of
reinforcement
1. Particle reinforced AMCs (PRAMCs)
2. Whiskers or short fiber reinforced AMCs (SFAMCs)
3. Continuous fiber reinforced AMCs (CFAMCs)
4. Mono filament-reinforced AMCs (MFAMCs)
New and high performance particle reinforced AMCs (PRAMCs) are
expected to satisfy many requirements for a wide range of
performance-driven, and price sensitive, applications in aerospace,
automobiles, bicycles, golf clubs, and in other structural
applications. A PRAMCs consists of a uniform distribution of
strengthening ceramic particles embedded within aluminum matrix. In
general, these materials exhibit higher strength and stiffness, in
addition to isotropic behavior at a lower density, when compared to
the unreinforced aluminum matrix. PRAMCs benefits from the
ceramic's ability to withstand high velocity impacts, and the high
toughness of the metal matrix, which helps in preventing total
shattering. This contribution leads to an excellent balance between
cost and mechanical properties, which are appealing for much
application; the main drawback of these materials is their low
ductility, which is caused by the nucleation, growth, and
coalescence of voids created by the ceramic reinforcement.
The main contribution to the strengthening of PRAMCs is particle
addition, which affects most of the mechanical properties of
PRAMCs. Several particle parameters, which are
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Experimental Determination and Analysis of Fracture Toughness of
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critical in determining the mechanical properties of PRAMCs,
include the volume fraction (vf), size, shape, and distribution of
reinforced particles within the metal matrix.
3.7 Examples
Continuous carbon fiber reinforced Al alloy likewise has been
used in most structures (vertical support structures) in the Hubble
telescope.
SiC continuous fiber reinforced Al alloy has been used vertical
section of advanced fighter aircrafts.
SiC continuous fiber reinforced Ti alloy has been used for
hypersonic aircraft.
Precision components of missile guidance system demand very high
dimensional stability i.e. geometries should not change with
temperature excursions during use. Al alloy with 20% SiC continuous
fiber satisfy this requirement.
Discontinuous SiC fibers 1 to 3mm in diameter and 50 to 200mm
long are mixed with Al powders consolidated by hot pressing and
then extruded or forged to the desired shape. With 20% SiC
whiskers, the tensile strength increased from 310Mpa to 480Mpa and
the tensile modulus can be increased from 69 to 115 Gpa.
Hybrid composites of 12% by volume fraction of alumina particles
(for high strength) and 9% volume fraction of graphite fibers (for
self lubrication) in alloys have been developed by Honda for Engine
blocks, connecting rods, piston rods etc., for
automobiles, which helps in reducing weight of automobile and
enhanced engine life. SiC coated on inter-metallic compound Ti3Al
fibers in Ti alloy matrix have been found to be very effective for
high temperatures resistance. These composites find
applications in compressor discs and blades in aero-engines.
A relatively new technique called rapid solidification rate
processing has been developed to obtain metallic glass ribbons
which can be effective reinforcing material in MMCs.
3.7.1 Advantages and Disadvantages of MMCs
Advantages
Very high specific strength and specific modulus. Low thermal
coefficient of thermal expansion. Retention of properties at higher
temperatures.
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Higher operating temperatures. Insensitivity to moisture.
Non-flammability.
Better capability to withstand compression and shear
loading.
Disadvantages
Higher densities as compared to PMCs.
MMCs demand higher processing temperatures. Processing methods
are expensive.
MMCs are expensive as compared to PMCs.
3.8 Scope of Present Investigation
Due to the advancement in the material technology to produce
desired materials from various industrial applications and fast
changing scenario in the production of lighter and stronger
materials, composite materials are gaining wide acceptance due to
their unusual characteristics of behavior with their high strength
to weight ratios. The most widely used material in these industries
is aluminum and their alloys because of their light weight
property.
To make these alloys of aluminum further versatile and flexible
for varieties of application, during which these materials is
expected to behave as expected and provide a long life under
different environments, the composites have emerged as the single
most material, which can provide a better service and better
quality.
Therefore in the present investigation, a study had been
conducted to evaluate the various mechanical properties such as
tensile, hardness, compression, microstructure and Fracture
Toughness of Al356 with Silicon Carbide (SiC) and Zirconium
silicate (ZrSio4) composite castings.
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Experimental Determination and Analysis of Fracture Toughness of
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CHAPTER 4
SELECTION OF MATERIALS
Cast Al356 is one of the most widely used commercial Al-Si-Mg
alloys in the aircraft and automotive industries due to its good
castability and the fact that it can be strengthened by artificial
aging. However, the mechanical properties of Al356 are
significantly affected by micro structural features such as
microporosity, intermetallics, eutectic silicon particles and heat
treatments. Aluminium Metal matrix composites (AMMC), where hard
ceramic particles are distributed in a relatively ductile matrix,
have widespread applications in aerospace, automobiles and other
engineering industries because of their excellent physical,
mechanical and tribological properties.
4.1 Matrix Material: Al 356
Fig 4.1 Ingot Structure of Al 356
4.1.1 Chemical Composition and Mechanical Properties of
Matrix
Material Al356
Chemical composition and mechanical properties of matrix
material Al356 is as shown in Table 4.1 and 4.2.
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ElementSiFeCuMnMgNiZnTiPbAluminium
Wt%7.5%0.2%0.25%0.35%0.45%0.1%0.35%0.2%0.1%Rem
Table 4.1 Chemical composition of Al356
Density(*1000 Kg/m3 )2.685
Poissons ratio0.33
Tensile Strength, Ultimate (MPa)228
Tensile Strength, Yield (MPa)165
Elongation (%)3.5%
Shear Strength (MPa)180
Thermal Conductivity (W/m-K)151
Melting Temperature5550C
Fatigue Strength(MPa)60
Table 4.2 Mechanical properties of matrix material Al356
4.2 Reinforcement Material (silicon carbide)
Silicon carbide (SiC), also known as carborundum, is a compound
of silicon and carbon with chemical formula SiC. It occurs in
nature as the extremely rare mineral moissanite. Grains of silicon
carbide can be bonded together by sintering to form very hard
ceramics which are widely used in applications requiring high
endurance, such as car brakes and ceramic plates in bulletproof
vests.
Silicon carbide, is a high-temperature structural material,
offering many advantages such as high melting temperatures, low
density, high elastic modulus and strength, and good resistance to
creep, oxidation and wear. These properties make SiC suitable for
use in applications such as gas turbines, piston engines and heat
exchangers, and where load-bearing components are required to
operate at temperatures up to 1500oC. Silicon carbide does not melt
at any known pressure. It is also highly inert chemically.
Silicon Carbide is the only chemical compound of carbon and
silicon. It was originally produced by a high temperature
electro-chemical reaction of sand and carbon. Silicon
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Experimental Determination and Analysis of Fracture Toughness of
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carbide is an excellent abrasive and has been produced and made
into grinding wheels and other abrasive products for over one
hundred years. Today the material has been developed into a high
quality technical grade ceramic with very good mechanical
properties. It is used in abrasives, refractories, ceramics, and
numerous high-performance applications. The material can also be
made an electrical conductor and has applications in resistance
heating, flame igniters and electronic components. Structural and
wear applications are constantly developing. The reinforcement
material (SiC) is as shown in figure. 4.2.
Fig 4.2 Reinforcement Material (SiC)
4.2.1 Physical Properties of Sic
Low density High strength
Low thermal expansion
High thermal conductivity High hardness
High elastic modulus
Excellent thermal shock resistance
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Experimental Determination and Analysis of Fracture Toughness of
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Superior chemical resistance.
4.2.2 Application of SiC
Fixed and moving turbine components Suction box covers
Seals, bearings Ball valve parts
Hot gas flow liners Heat exchanger
Semiconductor process equipment.
4.3 Reinforcement Material (Zirconium Silicate)
Zircon silicate is naturally occurring sand. Zirconium silicate
contains mainly zirconium oxide and silicon oxide with a minor
amount of potassium, gold and calcium oxide. It possesses
properties such as high temperatures up to 2400C, High density, Low
thermal conductivity (20% that of alumina), Chemical inertness,
Resistance to molten metals, Ionic electrical conduction, Wear
resistance, High fracture toughness and High hardness. This has
made it a good reinforce for the production of MMCs for engineering
applications.
Fig 4.3 Reinforcement material (ZrSiO4)
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Experimental Determination and Analysis of Fracture Toughness of
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4.3.1 Properties of Zirconium Silicate
Good strength
Corrosion resistant
Low thermal conductivity
Good thermal shock resistance Excellent Thermal resistance High
flexural strength
High hardness
4.3.2 Applications of ZrSiO4
Nuclear reactors.
Zirconium with Aluminium, iron and titanium are used in vacuum
tubes. Zirconium is used in satellites as reflective surface
agent.
Super conductive magnets.
Zirconium is used in optical glasses and for glass toughening.
Foundry/investment casting
PropertiesZircon Sand
M.P. (0C)2500
Limit of application (0C)1870
Hardness7.5
Density (g/cm3)4.5-4.70
Linear coefficient of expansion (m/m 0C)4.5
Fracture toughness (MPa-m1/2)5
Crystal structureTetragonal
Table 4.3 Properties of Zircon sand
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Experimental Determination and Analysis of Fracture Toughness of
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CHAPTER 5
FABRICATION OF COMPOSITES
Flow chart of composite fabrication is as shown in figure
5.1
Aluminium matirx (Al356)
Metal matrix composite
Al356-Sic-ZrSio4
Reinforcement
(Silicon Carbide+
Zirconium Silicate)
MatrixFurnace
Degassing+
Scum powder
Stir
ReinforcePouring
Casting
Fig 5.1 Flow chart of fabrication of Composite
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Experimental Determination and Analysis of Fracture Toughness of
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5.1 Stir Casting
The large ingots of matrix material were cut into small pieces
for accommodating into the crucible. Composites were produced by
Stir casting process. Melting was carried out in a cast iron
crucible in a resistance furnace. Degassing was carried out with
hexa-chloroethane tablets and Scum powder. Cut pieces of alloy A356
were heated at 840 C for 3 to 4 hours before melting, and before
mixing the SiC and ZrSio4 particles were preheated for 1 to 3 hours
to make their surfaces oxidized. Furnace temperature was first
raised above the liquidus to melt the alloy scraps completely and
was then cooled down just below the liquidus to keep the slurry in
a semi-solid state. At this stage the preheated SiC and ZrSio4
particles were added and mixed manually according to the required
proportions. Due to difficulties of mixing in semi solid state,
initially manual mixing was used for the synthesis of A356 and SiC
and ZrSio4. After this, the composite slurry was re-heated to a
fully liquid state and then automatic mechanical mixing was carried
out for about 5 minutes at an average stirring rate of 300 rpm. In
the final mixing processes, the furnace temperature was controlled
to be within 84010 C.
To ensure the homogeneity of the added reinforcement particles
through molten aluminum, electrical stirrer was inserted into the
crucible. Molten aluminum was stirred at (300 r.p.m.) to get
suitable vortex. Later reinforcement particles were added to molten
metal. This process was followed to modify reinforcement particles
distribution through the molten aluminum.
Due to the vortex effect, reinforcement particles were pulled
inside the molten metal and uniformly distributed. Molten aluminum
was stirred for (1- 5 min.) until the molten aluminum becomes
slurry. Later molten aluminum was poured into suitable cast iron
mould, which is preheated at 350C to prevent sudden cooling for
molten aluminum.
The pouring temperature was controlled to be around 820 C. A
preheated permanent cast iron mould with diameters in the range of
10 mm to 25 mm was used to prepare cast bars. Finally the super
heated melt was poured into the cast iron mould. The preheating
temperature 350 C for Cast Iron moulds was maintained for slower
cooling.
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Experimental Determination and Analysis of Fracture Toughness of
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5.2 Pre-heating of Mould Box
A mould is an assembly of two or more metal blocks. The mould
cavity holds the liquid material and essential acts as a negative
of the desired product. A permanent mould box which is prepared
according to required dimensions of the casting is used. Permanent
split type mould box is used for casting the composites in the
present study as shown in the figure 5.2. Pre heating of the mould
box is shown in figure 5.3. The mould box is tightened with the
help of screws and is checked for any gaps in the mould box. The
mould is then heated to a temperature of about 300-3500c to prevent
the sudden cooling of the molten aluminium.
Fig 5.2 Split type mould box
Fig 5.3 Pre heating the mould box
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Experimental Determination and Analysis of Fracture Toughness of
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5.2 Steps Involved in Stir Casting Method
Aluminum (Al356) 3kg was melted in the furnace to a temperature
of 850 degree centigrade as shown in figure 5.4.
Addition of scum powder. Formation of slag.
Slag removal.
After 10 mins titanium dioxide was added to remove the entrapped
gases (degasification) and Stirrer was introduced.
Stirrer was rotated at a speed of 0 to 300 rpm to create a
vortex in the liquid metal. Reinforcement material Sic and ZrSiO4
powder was added according to the
required proportions to molten metal in steps while
stirring.
After 15 mins molten metal is poured to the pre-heated mould and
left for solidification.
The mould box is opened and cast components are obtained.
Fig 5.4 Electric furnace
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Experimental Determination and Analysis of Fracture Toughness of
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Fig 5.5 Molten Metal in FurnaceFig 5.6 Formation of Vortex
Fig 5.7 Pre heating of reinforcementFig 5.8 Poured molten metal
in mould box
Fig 5.9 Cast Aluminium Composites
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Experimental Determination and Analysis of Fracture Toughness of
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5.3 Composition of matrix and reinforcement
SamplesAl356 (kg)Sic (%)ZrSio4 (%)
13-8
2362
3326
4344
538-
Table 5.1 Different wt% ratios of matrix metal and
reinforcement
The composition of the matrix metal and the reinforcement in
different wt% ratios is shown in the above table. The casting
samples with different wt% reinforcements were prepared
respectively as shown below.
Casting 1: Al356+0%SiC+8%ZrSiO4
Casting 2: Al356+6%SiC+2%ZrSiO4
Casting 3: Al356+2%SiC+6%ZrSiO4
Casting 4: Al356+4%SiC+4%ZrSiO4
Casting 5: Al356+8%SiC+0%ZrSiO4
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Experimental Determination and Analysis of Fracture Toughness of
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CHAPTER 6
EXPERIMENTAL DETAILS
6.1 Fracture Toughness
The measurement of valid plane strain fracture toughness, (KIC)
values for particulate reinforced metal matrix composites is an
important step in the process of developing useful products from
these materials and increasing confidence in their properties and
performance.
Fracture toughness is a material property that characterizes the
materials resistance to crack propagation when under load or
stress. In more precise terms, it refers to the resistance of a
preexisting crack to extend either under unstable (i.e., brittle
fracture) or by stable tearing means (i.e., ductile fracture).
Experimental methods for characterizing fracture toughness play a
critical role in applying fracture mechanics to integrity
assessment, fitness-for-service evaluation, and limit state
analyses for a wide variety of engineering structures. Fracture
toughness properties are frequently used as a basis for material
selection, material qualification programs, and quality assurance
for critical structures such as high-pressure gas and liquid
transmission pipelines, pressure vessels, nuclear reactor
components, petrochemical processing vessels, and aircraft.
Fracture toughness is a quantitative way of expressing a material's
resistance to brittle fracture when a crack is present. If a
material has much fracture toughness it will probably undergo
ductile fracture. Brittle fracture is very characteristic of
materials with less fracture toughness. Fracture mechanics, which
leads to the concept of fracture toughness, was broadly based on
the work of A. A. Griffith who, among other things, studied the
behavior of cracks in brittle materials.
The measurement procedure of fracture toughness is based on the
principle of linear-elastic fracture mechanics (LEFM) and contains
three main steps: generation of cracks in the test specimen,
measurement of the load at failure stress respectively, and crack
depth. In the case of ideally brittle materials, the fracture
toughness is independent of the crack extension. The crack growth
resistance increases with the increasing crack extension. Some
structural ceramics show an increase of fracture resistance with
crack extension under stable crack growth. The Single-Edge-Notched
Beam (SENB) method was developed as a simple and inexpensive
alternative, but the results can be influenced by the tip radius of
the sawed notch.
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Experimental Determination and Analysis of Fracture Toughness of
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6.2 Specimen dimensions as per ASTM standards
The samples were cut to the dimensions as per ASTM standards
ASTM C393-62 for Testing; ASTM standards are given in Table.
Sl. NoASTM CodeMechanical TestSampleSpan length
Dimensions(mm)
(mm)
1ASTM-D790Flexural127 x 13 x 665
Table 6.1 ASTM codes for mechanical test and sample
dimensions
6.3 Test for Fracture toughness
The Fracture toughness of the specimens were determined as per
ASTM-D790. The specimens (127 X 13 X 6 mm) were tested with a span
length of 65mm using three point bend setup with 10 ton capac