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UNIVERSITY OF NAIROBI DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING PROJECT REPORT TITLE: THERMAL SHOCK MEASUREMENT AND LIFETIME PREDICTION OF CERAMIC (CLAY) MATERIALS PROJECT CODE: JKM 03/2015 This project report is submitted in partial fulfilment of the requirement for the award of the degree of Bachelor of Science in Mechanical Engineering. Submitted by: KORIR WESLEY KIBET F18/1411/2010 KIBIRECH BENJAMIN KIBUNGEI F18/1434/2010 Project supervisor: PROF. J.K.MUSUVA April 2015 1

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UNIVERSITY OF NAIROBI

DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING

PROJECT REPORT TITLE:

THERMAL SHOCK MEASUREMENT AND LIFETIME PREDICTION OF CERAMIC (CLAY) MATERIALS

PROJECT CODE: JKM 03/2015

This project report is submitted in partial fulfilment of the requirement for the award of the degree of Bachelor of Science in Mechanical Engineering.

Submitted by:

KORIR WESLEY KIBET F18/1411/2010

KIBIRECH BENJAMIN KIBUNGEI F18/1434/2010

Project supervisor:

PROF. J.K.MUSUVA

April 2015

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DECLARATION The content of this document is our original work based on our own research and to the best of our knowledge it has not been presented elsewhere for academic purposes.

KORIR WESLEY KIBET F18/1411/2010

Signed…………………………………. Date: ……………………………

KIBIRECH BENJAMIN KIBUNGEI F18/1434/2010

Signed…………………………………... Date: ………………………………

This project is submitted as part of the Examiners Board requirement for the award of the degree of Bachelor of Science in Mechanical Engineering of the University of Nairobi.

Project supervisor:

PROF. J.K. MUSUVA

Signed …………………………………. Date: ……………………………….

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DEDICATION To our parents for their encouragement throughout our studies and the undying hope they instilled in our hearts

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ACKNOWLEDGEMENTS

First, we thank the Almighty God for guiding and giving us strength, peace of mind and grace that has brought us this far.

We would like to extend our sincere gratitude to the following;

Our project supervisor, Prof. J. K. Musuva: we are very grateful for the encouragement, guidance and assistance that he accorded us from the beginning of the project to its successful completion.

We would also like to thank The Chairman’s office (Dept. of Mechanical &Manufacturing Engineering) for the project funding.

Our sincere appreciation also extends to all the staff of Concrete Laboratory, Department of Civil and Construction Engineering (U.O.N) and Kenya Industrial Research and Development Institute (KIRDI) ceramic laboratory staff who provided assistance at several occasions.

Our special thanks to the University of Nairobi Mechanical Engineering workshop staff. In this regard, we would like to single out Mr. Njue, Eng. Aduol, Mr. Macharia and Mr. Kimani for their unwavering assistance in the project.

We are grateful to all our families and friends who supported us all through this time to the successful completion of this project

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LIST OF SYMBOLS σf Average Modulus of Rapture

σt Thermal stress

σST Tensile strength

α Coefficient of thermal expansion

k thermal conductivity

Kc critical stress intensity factor

KIc plane strain fracture toughness

L span

M bending moment

m Weibull modulus

F fracture load

Pf probability of failure of the specimen

Ps probability of survival of the specimen

R radius of the specimen

T time

ΔT Thermal shock temperature range

a crack length

D diameter of the specimen

E elastic modulus

I second moment of area

K stress intensity factor

Y geometric constant

σ stress

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ABSTRACT Thermal fatigue and thermal shock properties of ceramic materials has been a major concern in industry. The susceptibility of these materials, particularly clay ceramics, to damage by thermal shock has possibly been recognized since Neolithic times. By far, the major part of the understanding of the important features involved has been accumulated during the past research work and experimentations on the effect of addition of silica and alumina on thermal shock properties of locally obtained clay ceramics in Kenya.

In this project, a major concern was to further conduct research on thermal shock properties of locally obtained clay products in Kenya.

The main objective was to determine the effects of addition of silica and alumina on the thermal shock properties and lifetime prediction of ceramic clay in Kenya and to obtain optimum ratio of silica to clay and alumina to clay with the best thermal shock resistance and highest thermal fatigue resistance.

The study involved collection of raw materials that includes locally available clay from Nyeri and Muranga counties, the preparation of these raw materials, mounting of extrusion machine, preparation of specimen that involved moulding, extrusion, drying and firing. Later, tests on the specimens were carried out and analyses were done.

The clays were soaked in water for four days while stirring regularly to mix properly, it was then sieved in to a drying basin and left to dry under the sun. After drying, the clays were milled into powder form and the sand sieved to obtain fine particles and also to remove impurities. The Plain clays were then mixed with sand and alumina in different proportions and mixed thoroughly. Thereafter, controlled amount of water was added to the mixtures and kneaded into moulds. Extrusion followed using motorized extruder to make 27 different groups of samples, each group having approximately 50 specimens. The specimens obtained were left to dry in controlled conditions for about 4 to 5 days after which they were fired to 1000°C to obtain its desired strength. The shocking procedure was done where the specimens were heated to temperatures of 400°C, 600°C and 800°C respectively and then dipped into a water bath at room temperature averaging about 24.3°C. Thermal fatigue testing was done by putting the specimens back to the furnace after quenching, for up to fifteen cycles and twenty cycles respectively. The fracture load of the specimens was then measured using a three point Hounsfield Tensometer, from which the strengths (modulus of rapture) and Weibull modulus were calculated.

Analysis from the graphs and tables of average fracture stress showed that clay from Muranga with 15% alumina content had the highest bend strength before any shocking was done. Generally, from discussions and conclusions it was found that in various percentage mixtures of Muranga clay with sand and alumina, 55% alumina content in Muranga clay had the lowest

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percentage reduction in strength of 60.42% {table 6.7(d)} after twenty shocks hence had the best thermal fatigue properties

Nyeri clay mixed with sand was the weakest in terms of the flexural strength but was found to be the most thermal shock resistant compared to other specimens with an average percentage reduction in strength of 41.8% {table 6.7(a)}.Specimens made of 55% sand and 45% Nyeri clay were the most resistance to thermal shock with an average reduction in strength of 56.2% after 20 shocks from 800°C in water at 24.3°C.

From the analysis and conclusions, it was recommended that, based on strength and thermal shock resistance, Muranga clay with 15% alumina had the highest average strength after thermal shock {figure 6.3 (a)} and can be utilized for the lining of electric resistance furnaces because of its high modulus of rupture and good thermal shock resistance.

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PROJECT OBJECTIVES 1) To further investigate the effect of addition of silica and alumina on the thermal shock

properties of ceramic clay. 2) To carry out thermal shocking of the ceramic clay specimens for 15 and 20 cycles 3) To investigate the effect of mixing Nyeri clay which has better thermal fatigue resistance

with Muranga clay which has good flexural strength 4) To determine the optimum ratio of silica to clay and alumina to clay with the best thermal

shock resistance and highest thermal fatigue resistance. 5) To investigate and recommend the appropriate local clay ceramic for use in domestic

charcoal stoves.

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Table of contents

DECLARATION .......................................................................................................................... ii

DEDICATION ............................................................................................................................ iii

ACKNOWLEDGEMENTS ........................................................................................................... iv

LIST OF SYMBOLS ..................................................................................................................... v

ABSTRACT ................................................................................................................................ vi

PROJECT OBJECTIVES ............................................................................................................ viii

CHAPTER 1: INTRODUCTION .................................................................................................... 1

1.1 CERAMICS ................................................................................................................... 1

1.2 GENERAL PROPERTIES OF CERAMICS .......................................................................... 1

1.3 CLASSIFICATION OF CERAMICS .................................................................................... 1 1.3.1 Traditional ceramics ............................................................................................. 1 1.3.2 Engineering ceramics ........................................................................................... 1 1.3.3 Natural ceramics .................................................................................................. 2 1.3.4 Cement and concrete ........................................................................................... 2 1.3.5 Glasses................................................................................................................. 2

1.4 CRYSTALLINE MATERIALS IN CERAMICS ...................................................................... 2

1.5 BONDING IN CERAMICS .............................................................................................. 3

1.6 CERAMIC STRUCTURES ............................................................................................... 3

1.7 PROCESSING OF CERAMICS ......................................................................................... 4

CHAPTER 2: BACKGROUND ...................................................................................................... 7

2.1 RESEARCH DONE BY WATARE AND WANJOHI ............................................................. 8 2.1.1 NYERI CLAY AND SILICA ........................................................................................ 8 2.1.2 MURANG’A CLAY AND SILICA ............................................................................... 8 2.1.3 NYERI CLAY AND ALUMINA .................................................................................. 9 2.1.4 MURANG’A CLAY AND ALUMINA ....................................................................... 10 2.1.5 COMPARISON OF RESULTS OF PREVIOUS RESEARCHERS .................................... 10 2.1.6 COMBINED RESULTS OF PREVIOUS RESEARCHERS .............................................. 12 2.1.7 OBSERVATIONS .................................................................................................. 16

2.2 PROBLEM STATEMENT ............................................................................................. 17

CHAPTER 3: THEORY .............................................................................................................. 18

3.1 THERMAL SHOCK AND THERMAL SHOCK RESISTANCE ............................................... 18 3.1.1 THERMAL SHOCK ............................................................................................... 18

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3.1.3 FACTORS INFLUENCING THERMAL SHOCK BEHAVIOR ......................................... 19

3.2 MODULUS OF RUPTURE ............................................................................................ 20 3.2.1 THE FLEXURE TEST ............................................................................................. 20 3.2.2 FLEXURAL STRENGTH ......................................................................................... 22

3.3 FRACTURE STRENGTH AND WEIBULL STATISTICS ...................................................... 22 3.3.1 STRESS CONCENTRATION ................................................................................... 22 3.3.2 FRACTURE TOUGHNESS ..................................................................................... 23 3.3.3 THE WEIBULL STATISTICS ................................................................................... 23

3.4 POROSITY ................................................................................................................. 24

CHAPTER 4: MATERIALS AND APPARATUS ............................................................................ 25

CHAPTER 5: METHODOLOGY ................................................................................................. 26

5.1 COLLECTION OF RAW MATERIALS ............................................................................. 26

5.2 PREPARATION OF RAW MATERIALS .......................................................................... 26

5.3 PREPARATION OF SPECIMEN .................................................................................... 26 5.3.1 MIXING OF CLAYS WITH SAND AND ALUMINA ................................................... 26 5.3.2 MIXING OF NYERI CLAY AND MURANGA CLAY ................................................... 27 5.3.3 NUMBER OF SPECIMENS REQUIRED ................................................................... 27

5.4 EXTRUSION PROCESS ................................................................................................ 27

5.5 DRYING AND FIRING ................................................................................................. 28

5.6 THERMAL SHOCK AND THERMAL FATIGUE TEST ....................................................... 28 5.6.1 THERMAL SHOCK ............................................................................................... 28 5.6.2 FLEXURE TEST .................................................................................................... 28

5.7 ASSUMPTIONS DURING PROJECT UNDERTAKING ...................................................... 28

CHAPTER 6: RESULTS .............................................................................................................. 30

6.1 MIXTURE OF MURANGA CLAY AND SAND ................................................................. 30

6.2 NYERI CLAY WITH SAND MIXTURE ............................................................................ 34

6.3 MIXTURE OF MURANGA CLAY AND ALUMINA .......................................................... 38

6.4 NYERI ALUMINA MIXTURE ........................................................................................ 42

6.5 RESULTS OF MIXTURE MURANGA CLAY AND NYERI CLAY .......................................... 46

6.6 RESULTS OF PREVIOUS RESEARCHERS COMBINED WITH OURS .................................. 47 6.6.1 THERMAL SHOCKING: OUR RESULTS COMBINED WITH RESULTS OF WATARE AND WANJOHI & CHERONO AND MOSIRIA ........................................................................ 47 6.6.2 THERMAL FATIGUE: OUR RESULTS COMBINED WITH RESULTS OF WATARE AND WANJOHI ................................................................................................................... 54 6.7 PERCENTAGE REDUCTION IN AVERAGE STRENGTH AFTER SHOCKING .................... 66

6.8 TABLES OF WEIBULL MODULUS ................................................................................ 68

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6.9 QUANTITY OF MAJOR COMPONENTS OF MATERIALS USED IN PREPARATION OF SPECIMEN ...................................................................................................................... 70

CHAPTER 7: OBSERVATIONS AND DISCUSSION OF RESULTS ................................................. 72

7.1 OBSERVATIONS ........................................................................................................ 72

7.2 DISCUSSION .............................................................................................................. 74 7.2.1 THERMAL SHOCK ............................................................................................... 74 7.2.2 THERMAL FATIGUE ............................................................................................ 75

7.3 COMPARISON BETWEEN MURANGA CLAY AND NYERI CLAY ..................................... 77

7.4 COMPARISON OF OUR RESULTS WITH PREVIOUS RESEARCHERS ............................... 77

7.5 COMPARISON OF MIXTURES TESTED WITH THOSE OF PREVIOUS RESEARCHERS ....... 79

7.6 WEIBULL MODULUS .................................................................................................. 80

CHAPTER 8: CONCLUSIONS .................................................................................................... 81

8.1 THERMAL SHOCK ............................................................................................... 81 8.1.1 THE AVERAGE BREAKING STRENGTH .................................................................. 81 8.1.2 VARIATION IN STRENGTH ................................................................................... 81 8.1.3 DEGREE OF DAMAGE ......................................................................................... 81

8.2 THERMAL FATIGUE ................................................................................................... 81 8.2.1 AVERAGE BREAKING STRENGTH ......................................................................... 81 8.2.2 REDUCTION IN THE AVERAGE BREAKING STRENGTH .......................................... 81 8.2.3 DEGREE OF DAMAGE ......................................................................................... 82

8.3 APPLICATION TO INDUSTRY ...................................................................................... 82

CHAPTER 10: CHALLENGES AND RECOMMENDATIONS ....................................................... 83

10.1 CHALLENGES ........................................................................................................... 83

10.2 RECOMMENDATIONS ............................................................................................. 83

LIST OF REFERENCES .............................................................................................................. 84

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CHAPTER 1: INTRODUCTION

1.1 CERAMICS

Ceramic are inorganic, nonmetallic crystalline materials which consist of nonmetallic and metallic elements bonded together primarily by ionic and/or covalent bonds. They include a broad range of silicates, metal oxides and combinations of silicates and metal oxides. Elements such as carbon, boron and silicon carbides, borides and nitrides are often considered as ceramics. Ceramics are processed and used at high temperatures.

1.2 GENERAL PROPERTIES OF CERAMICS

Ceramics are the hardest of all solids. Most ceramics are intrinsically hard. Ionic or covalent bonds present an enormous lattice resistance to dislocation motion.

Ceramics are brittle at low temperatures; this is because of the presence of independent slip planes and the lack of plane systems. Dislocations cannot move easily in the structure. Ceramics have high melting point, they are poor conductors of heat and electricity and often transparent. Ceramics and glasses do not demonstrate any plastic deformation in a typical test.

Ceramics are relatively weak in tension but relatively strong in compression. The compressive strength of ceramics is about fifteen times larger than the tensile strength. Fracture strength of ceramics is very low compared to that of metals.

Ceramics creep just like metals when they are subjected to temperatures above 13Tm, where Tm is

the melting temperature in the Kelvin scale.

1.3 CLASSIFICATION OF CERAMICS Ceramics can be classified into the following broad groups;

1.3.1 Traditional ceramics All are made from clays; clays have their origin in the mechanical and chemical disintegration of rock, they are complex aluminum silicates containing attached water molecules which are formed in the wet plastic state and then dried and fired. After firing they consist of crystalline phases (mostly silicates) held together by a glassy phase based on silica. The glassy phase forms and melts when the clay is fired and spreads around the surface of the inert but strong crystalline phases bonding them together. They include porcelain, chinaware and earthenware.

1.3.2 Engineering ceramics These include oxides, nitrides, carbides, borides and silicates. Diamond (the synthetic variety) is also included in this group. Such materials are widely used in engineering in such items as furnace components, combustion tubes. Harder ceramics like dense alumina, silicon carbide,

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silicon nitride, sialons, and cubic zirconia are used to make cutting tools, dies, wear resistant surfaces bearings, medical implants, engine and turbine parts, and armor.

1.3.3 Natural ceramics Stone is a natural ceramic. It is the oldest of all construction materials and the most durable. Stone used in a load bearing capacity behaves like any other ceramic and criteria used in design with stone are the same. Ice is another natural ceramic, it is unique and forms a substantially large part of the earth surface. The mechanical properties are of primary importance in some major engineering problems notably ice breaking and construction of offshore and gas rigs in the Polar Regions.

1.3.4 Cement and concrete Cement is a mixture of a combination of lime, silica and alumina which sets when mixed with water. Concrete is sand and stone (aggregate) held together by cement. Both are used in construction on an enormous scale equaled only by steel brick and wood.

1.3.5 Glasses These include various types of glasses and glass ceramics. All are based on silica (SiO2) which is derived from sand. Additives are added to reduce the melting point or give other special properties. Glasses are used in enormous quantities, as much as 80% of the surface area of a modern office block can be glass, it is also in a vast range of products from car windscreens to jam jars.

1.4 CRYSTALLINE MATERIALS IN CERAMICS Generally, bonding in ceramics is stronger than that in metals and gives the material high melting temperatures.

Ceramics have metallic and non-metallic phases i.e. have elements with ionic and covalent bonds. The most common types of unit cell in these materials include

a) BCC (Body Centered Cubic) b) HCP (Hexagonal Close Packed)

In ceramics both arrangement of atoms and the type of bonding result in the brittle nature and hence brittle fracture rather than slip. However, the regular arrangements of atoms determine the path of fracture while the plane of fracture, cleavage, is closely related to the makeup of the planes of atoms. Non-equilibrium structures are prevalent in ceramics because the more complex crystal structures are difficult to nucleate and to grow from the melt.

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1.5 BONDING IN CERAMICS The major types of bonding in ceramics include ionic and covalent bonds.

Ionic bond refers to bonding between metallic and non-metallic elements. The atom of the metallic element donates its outer (valence) electrons to the nonmetallic atom to form a cation and an anion respectively. The electrostatic force of attraction between these ions creates the bond. The force of attraction depends on how electropositive the metallic ion is and how electronegative the nonmetallic ion is. The negative ions are clustered around the positive ions.

Covalent bond is a type of bonding in which the valence electrons are shared, with the shared electrons remaining equidistant between the atoms. For example in diamond each carbon atom is attached to four other carbon atoms forming a very strong covalent bond

Ceramics may have bonding that is partly ionic and partly covalent, such as in silica, aSiO2; the bonding is divided between ionic and covalent depending on the position of bonding electrons relative to the ions.

In addition to the ionic and covalent bond, van der Waals forces of attraction are active in bonding but are smaller in magnitude. Ionic and covalent bonds are stronger in plane of a layer of clay and weaker van der Waals forces hold adjacent layers together.

Unit cells-Cubic, Hexagonal and tetragonal types of unit cells are most prevalent in ceramic structures. Their characteristics are illustrated in table 1.1.

Table 1.1 characteristics of unit cells prevalent in ceramics

System Axes Axial angles Cubic a1=a2=a3 All angles= 90° Hexagonal a1=a2=a3≠c Angles= 90° and 120° Tetragonal a1=a2≠c All angles =90°

1.6 CERAMIC STRUCTURES Ceramics have a structure at the atomic scale; it can be a crystalline structure or an amorphous structure like for glass. The nature of chemical bond in ceramics controls the crystal structure. This structure at a larger scale consists of the shape and arrangement of its grains and/or phases with varying degrees of porosity. The extent of porosity influences the properties of the ceramic.

The most frequently occurring chemical formulas can be generally represented as: AX, AX2, A2X, ABX3, A2X3 and AB2X3. In each case, A and B are metallic while X a nonmetallic; hence in ceramics, certain groups of crystal structures are known by their naming, such as sodium chloride, calcium fluoride and cesium chloride.

The particles of the key crystal structures forming a unit cell are important because it represent a large structural group. Example FeO, MgO and CaO have same crystal structure as sodium

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chloride. Ceramic structures also include the corundum structure, the spinel structure, the diamond structure, and the silica and silicate structures.

1.7 PROCESSING OF CERAMICS Ceramics cannot be cast because they have very high melting temperatures and normally dissociate chemically before melting. They cannot be shaped by plastic deformation because of its high hardness and brittle nature.

The starting material in fabrication of ceramics is normally a paste composed of small solid particles, water and often a binder. Paste is easily shaped at room temperature. In general, the manufacture of ceramics bodies or shapes consists of the following steps:

1. Batching and preparation of powders-Raw materials for ceramics must be in powder form in order to be prepared for forming and mixing. The powders are prepared into states that are compatible with the forming process.

2. Forming processes-The principal forming processes in ceramics are: a) Slip casting- Slip casting include drain casting and solid casting. In this forming

process, low-viscosity slurry is poured into a porous plaster mold that draws the water from the slurry that is in the contact with the wall of plaster mold.

b) Tape casting-In this process ceramic slurries containing substantial amount of binders and plasticizers are poured into thin layers on either a glass plate or an impervious polymer film and allowed to dry. This process produces thin, flexible ceramic tapes. It is widely used for production of ceramic substrates in the electronic industry.

c) Plastic forming processes-This process is normally used in production of clay-based ceramics due to their highly plastic nature. It is also applied to advanced ceramic bodies. The principal processes in plastic forming are: Extrusion-Stiff plastic ceramic mass is forced through a rigid die to produce a column of uniform section.

d) Injection molding- It involves heating of granular ceramic-binder mix until it becomes plastic (soft) then forcing the plastic mix into a mold cavity where it cools and re-solidifies to produce the desired shape of a ceramic body.

e) Plastic pressing and jiggering-These are batch processes, whereby in plastic pressing, a pre-form or extruded slug is pressed under relatively low pressure to conform to the shape of the die. The die material may be an impervious metal or porous plaster mold. In jiggering process, extruded slug is placed on a revolving plaster form and a template tool is brought in contact with the slug. The slug is pressed onto the plaster mold while the template cuts the excess body. This leaves the shape with its surface conforming to the shape of the template tool.

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f) Dry pressing-This is the process whereby nearly dry, free flowing powders are consolidated into a predetermined shape of a metal die under uni-axial high pressure. It is sometimes called dust-pressing or uniaxial compaction. Compaction force is generally applied in the vertical direction by oppositely acting mechanical or hydraulic rams.

3. Densification- This process involves drying, firing and sintering of ceramic molds;

I. Drying and firing-

Drying removes water held between the particles of the wet ceramic (shrinkage/pore water), this is the water that accounts for the plasticity of clays, exit of the shrinkage water causes the water particles to come together (shrinkage). This increases the attraction forces between the solid particles leading to much higher dry strength compared to wet strength.

Dried “green” ware is fired in a furnace to densify it into a hard body. The aim of firing is to convert a molded or dried clay particle (green) into a permanent product possessing required strength, durability and a better appearance. During firing, the following changes occur in the clay body.

a) 110 OC-260 OC. Remaining traces of hygroscopic moisture are removed; there is little further change until the next stage.

b) 425 OC- 600 OC. Clay minerals break down to silica and alumina, the chemically combined water is liberated according to the reaction:

Al2O3.2SiO2 .2H2O → Al2O3 +2SiO2 +2H2O

Clay loses its ability to form plastic dough with water, it cannot be remolded again. There is little change in strength and porosity.

c) 800 OC- 900 OC. All gas forming reactions are completed at this stage. Any organic matter left is burned up followed by oxidation of iron pyrites (FeS2).

d) 900 OC-1000 OC. Fusion and vitrification begins, firing shrinkage commences due to decreasing porosity. Vitrification occurs as a result of liquid formation that fills up the pores, when cooled the liquid solidifies to a vitreous glassy matrix that cements the inert particles together. Porosity is decreased and strength increases by a large factor.

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e) 1400 OC. Full vitrification is achieved due to more liquid formed. Further increase in temperature will lead to fusion. If proportion of liquid is so high the specimen softens and may collapse.

f) Cooling. There is sudden shrinkage of cristobalite , a crystalline form of silica, as it cools past 220⁰C is found in all clay bodies, so care must be taken to cool the oven slowly as it moves through this critical temperature. Otherwise, specimens will develop cracks.

II. Sintering

This is the final stage in which the grains of ceramics fuse together. Molecules diffuse at high temperatures and the decrease of surface energy are two phenomena that take place during sintering. Surface diffusion is more rampant than bulk diffusion. Molecules that diffuse to the contact area fill the crevice between the two grains and reduce the surface area, and therefore the total surface energy.

Sintering involves:

Hot isostatic pressing- ceramic is encapsulated in a glass skin and subjected to high pressure gas during the process. The particles thus are pressed together.

Liquid phase sintering- this is where sintering aid that liquefy at the sintering temperature is added to the powder and penetrates the pores.

Allowance must be made for shrinkage in the design of ceramics since sintering involves shrinking of materials.

III. Shaping and surface finishing- This is done by machining process which involves the use of abrasives that includes grinding, lapping, horning, polishing, use of abrasive fluid jet cutting and ultrasonic machining.

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CHAPTER 2: BACKGROUND

Thermal shock properties of materials have been a major concern in ceramic materials and a lot of research has been done to investigate on how this property can be improved.

The susceptibility of materials, particularly clay ceramics, to damage by thermal shock has possibly been realized since pre-colonial times. By far, the major part of understanding of the important features involved have been accumulated during the past research work and experimentations on the effect of addition of alumina and silica on the thermal shock properties of locally obtained clay (from Nyeri and Muranga)

Research work and experimentations on this property have been carried out by a number of researchers namely; Songok J K and Suresh P J, Mbithi F and Florida S, Nzioki N J and Mogusu C O, Cherono S and Mosiria D B.

On their attempts of research and experimentations on locally obtained clay from Muranga and clay from Nyeri, the above groups of researchers used the following proportions of alumina to clay and silica to clay as detailed in the table below.

Table2. 1-composition of specimen tested by previous researchers

GROUP SILICA: CLAY PROPORTIONS (%)

ALUMINA: CLAY PROPORTIONS (%)

SONGOK & SURESH M 60:40, 70:30, 80:20 N 60:40, 70:30, 80:20

M 60:40, 70:30, 80:20 N 60:40, 70:30, 80:20

MBITHI & SIMIYU M 25:75 N 25:75

NOT APPLIED NOT APPLIED

NZIOKI & MOGUSU M 25:75, 33:67, 50:50 N 25:75, 33:67, 50:50

NOT APPLIED NOT APPLIED

CHERONO & MOSIRIA M 0:100, 20:80, 30:70, 40:60 N 15:85, 55:45, 65:35, 85:15

M 20:80, 30:70, 40:60 N 20:80, 30:70, 40:60

WATARE & WANJOHI M 10:90, 20:80, 40:60, 70:30 N 10:90, 20:80, 40:60, 70:30

M 10:90, 50:50, 70:30, 85:15 N 10:90, 50:50, 70:30, 85:15

KEY: M-Muranga Clay N- Nyeri Clay

The thermal shocking was done in water at room temperature, oil bath at room temperature, ice and air blast. The following are the results analysis of the past experiments and experimental analysis that have already been done by the five groups mentioned. This analysis will greatly

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influence the objectives and also how this project is intended to be conducted. The results of Watare and Wanjohi are analyzed in relation to the past projects done by Sheila and Mosiria, Nzioki and Mogusu and that of Songok and Suresh.

2.1 RESEARCH DONE BY WATARE AND WANJOHI 2.1.1 NYERI CLAY AND SILICA

Figure 2.1- graph of average strength against % sand for 5 & 10 shocks at 600⁰c

Figure 2.2- graph of average strength against % sand for 5 & 10 shocks at 800⁰c

2.1.2 MURANG’A CLAY AND SILICA Figure 2.3- graph of average strength against % sand for 5 & 10 shocks at 400⁰c

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Figure 2.4- graph of average strength against % sand for 5 & 10 shocks at 800⁰c

2.1.3 NYERI CLAY AND ALUMINA Figure 2.5- graph of average strength against % alumina for 5 & 10 shocks at 400⁰c

Figure 2.6- graph of average strength against % alumina for 5 & 10 shocks at 600⁰c

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2.1.4 MURANG’A CLAY AND ALUMINA

Figure 2.7- graph of average strength against % alumina for 5 & 10 shocks at 400⁰c.

Figure 2.8- graph of average strength against % alumina for 5 & 10 shocks at 600⁰c

2.1.5 COMPARISON OF RESULTS OF PREVIOUS RESEARCHERS

Figure 2.9 - combined graphs for average strength against % sand for unshocked specimens -Nyeri clay

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20

0 20 40 60 80 100STR

ENG

TH (M

PA)

% ALUMINA

average strength against % alumina for murang'a clay

5 SHOCKS

10 SHOCKS

05

101520

0 20 40 60 80 100STRE

NGT

H (M

PA)

% ALUMINA

average strength against % alumina for nyeri clay

SHOCKED IN WATER AT 23⁰CSHOCKED IN WATER AT 2⁰C

0

5

10

15

0 20 40 60 80 100

AVER

AGE

STRE

NGT

H

% SAND

average strength against % sand for nyeri clay

WATARE & WANJOHI

SHEILA & MOSIRIA

10

Figure 2.10- combined graphs for average strength against % alumina for unshocked specimens -Nyeri clay

Figure 2.11 - combined graphs for average strength against % sand & % alumina

0

10

20

30

0 20 40 60 80 100AVER

AGE

STR

ENG

TH

% ALUMINA

average strength against % alumina for nyeri clay

WATARE & WANJOHI

SHEILA & MOSIRIA

010203040

0 10 20 30 40 50 60 70 80AVER

AGE

STRE

NGT

H

% SAND

average strength against % sand for murang'a clay

WATARE & WANJOHI

SHEILA & MOSIRIA

11

2.1.6 COMBINED RESULTS OF PREVIOUS RESEARCHERS The tables and graphs in this section include results from Cherono & Mosiria {23} and Watare & Wanjohi {24}.

Table 2.1.1-percentage sand against average strength-Nyeri clay

% SAND AVERAGE STRENGTH (Mpa)

0 7.9801 10 10.5181 * 15 11.1778 20 11.7139 * 25 11.4400 40 12.9926 * 55 13.0110 60 7.5798 65 8.9673 70 8.8580 * 80 6.0784 85 4.7278

Key: *-results Watare and Wanjohi {24}

FIGURE 2.1.1- COMBINED GRAPH OF AVERAGE STRENGTH AGAINST % SAND

02468

10121416

0 20 40 60 80 100STRE

NGT

H (M

pa)

% SAND

COMBINED GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR NYERI CLAY- UNSHOCKED

KEY

Cherono and Mosiria {23}

Watare and Wanjohi {24}

12

TABLE 2.1.2-PERCENTAGE ALUMINA AND AVERAGE STRENGTH-NYERI CLAY

% ALUMINA AVERAGE STRENGTH (MPa)

0 7.9801

10 9.6092 *

20 10.6805

30 19.6989

40 20.6022

50 19.9573 *

60 15.9567

70 10.2662 *

85 6.30731 *

Key: *-results Watare and Wanjohi {24}

FIGURE 2.1.2- COMBINED GRAPH OF AVERAGE STRENGTH AGAINST % ALUMINA

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80 90

STRE

NGT

H (M

Pa)

% ALUMINA

COMBINED GRAPH OF AVERAGE STRENGTH AGAINST % ALUMINA FOR NYERI CLAY-UNSHOCKED

KEY

Cherono and Mosiria {23}

Watare and Wanjohi {24}

13

TABLE 2.1.3-PERCENTAGE SAND AGAINST AVERAGE STRENGTH-MURANG’A CLAY

Key: *-results Watare and Wanjohi {24}

FIGURE 2.1.3-PERCENTAGE SAND AGAINST AVERAGE STRENGTH-MURANG’A CLAY

05

101520253035

0 10 20 30 40 50 60 70 80

STRE

NGT

H (M

pa)

% SAND

COMBINED GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR MURANG'A CLAY- UNSHOCKED

KEY

Cherono and Mosiria {23}

Watare and Wanjohi {24}

% SAND AVERAGE STRENGTH (Mpa)

0 29.679

10 30.379 *

20 27.991 *

30 24.163

40 23.936

45 20.210 *

70 14.057 *

14

TABLE 2.1.4-PERCENTAGE ALUMINA AND AVERAGE STRENGTH-MURANG’A CLAY

Key: *-results Watare and Wanjohi {24}

FIGURE 2.1.4- COMBINED GRAPH OF AVERAGE STRENGTH AGAINST % ALUMINA

05

1015202530354045

0 10 20 30 40 50 60 70 80 90

STRE

NGT

H (M

Pa)

% ALUMINA

COMBINED GRAPH OF AVERAGE STRENGTH AGAINST % ALUMINA FOR MURANG'A CLAY-UNSHOCKED

KEY

Cherono and Mosiria {23}

Watare and Wanjohi {24}

% ALUMINA AVERAGE STRENGTH (MPa)

0 29.679

10 35.823 *

20 40.039

30 25.457

40 24.002

50 21.097 *

70 16.451 *

85 9.1747 *

15

2.1.7 OBSERVATIONS After a critical analysis of the previous work, the following observations were made;

Nyeri clay

I. For the sand clay mixture, the highest flexural strength was found for all thermal shock tests at sand to clay ratio ranging from 10%-50% sand.

II. For alumina clay proportions, maximum flexural strength for all thermal shock tests was found between alumina to clay ratio ranging from 30%-70% alumina.

III. Investigation of thermal fatigue has been done by using 5 and 10 shocking cycles

Muranga clay

I. Mixing this clay with sand increased the average strength after shocking up to a proportion of 30% sand 70% clay. Further increase in the proportion of sand causes a general decrease in strength.

II. For alumina clay mixture, the average strength generally increases with increase in percentage of alumina added up to 50% where the average strength generally decreases with increase in percentage of alumina addition.

III. Investigation of thermal fatigue has been done by using 5 and 10 shocking cycles

After making the above observations, we undertook to develop the research further by;

I. Investigating the life of ceramic clays made from the selected samples by shocking 15 and 20 times at temperatures of 400°C, 600°C and 800°C.

II. Investigating the effect of addition of Alumina and sand to the selected clays on thermal shock properties and life of the clay ceramic, the proportions selected are shown in table 2.2

III. Investigating the effect of mixing Muranga clay which has good flexural strength and

Nyeri clay which has good thermal shock properties.

16

2.2 PROBLEM STATEMENT

From the analysis of the available research documentations and results analysis of the work done in the past. We see it appropriate to carry out a further research in the strength of the locally available clay from Muranga and Nyeri County. From the observations, we conclude that the following proportions needed further research as we narrow down the ratios of alumina/sand to clay to a ultimate proportion for (%) alumina in clay and (%) sand in clay that will give a maximum thermal strength of the local clay and hence a high life ceramic product. In this project, the specimens will be shocked from temperatures of 400°C, 600°C and 800°C. Specimens will be shocked once to measure thermal shock resistance. The specimen will be shocked 15 and 20 times to predict its life under given shocking conditions. The alumina to clay ratios and sand to clay ratios will be in the following proportions presented in the table 2.2. Nyeri clay will be mixed with an equal amount of Muranga clay. Table2. 2 proportions of mixtures selected for research

LOCAL CLAY SAND: CLAY RATIO (%)

ALUMINA: CLAY RATIO (%)

MURANGA 5:95 15:85 25:75 35:65

5:95 15:85 35:65 45:55 55:45

NYERI 15:85 25:75 35:65 45:55 55:45

35:65 45:55 55:45 65:35

17

CHAPTER 3: THEORY

3.1 THERMAL SHOCK AND THERMAL SHOCK RESISTANCE 3.1.1 THERMAL SHOCK

A body is considered to be subjected to thermal shock when the temperature of its surroundings is changed so rapidly that transient thermal gradients, and hence, stresses are set up within it. If the maximum stress developed exceeds that breaking stress of the material corresponding to the mode of stressing involved, then 'failure' occurs. The failure is manifested either as visible cracking and/or is depreciation in mechanical properties. If we consider a rod of length L, then its temperature rises from T0 to T1, its length will increase by αL(T1 − T0), where α is the coefficient of linear expansion. If the rod has been constrained, expansion cannot occur and thus we have a compressive strain α(T1 − T0),

Assuming Hooke’s law, then the compressive stress, σc, produced when a material is subjected to a sudden increase in temperature is given by,

σC = αE(T1 − T0) ………………………. (1)

For uniaxial stress

The assumption is that the hot material is in contact with one surface. The expansion on the surface layer parallel to the surface and also in a direction at right angles to the surface is constrained by the colder underlying layers , this results in a biaxial compressive stress given by;

σC = αE(T1−T0)1−𝑣𝑣 ............................................. (2)

For biaxial stress.

Where α is the coefficient of linear expansion, 𝑣𝑣 is the Poisson ratio, T0 and T1 are the initial and final temperatures respectively.

3.1.2 THERMAL SHOCK RESISTANCE

The resistance to thermal shock can be defined as the temperature change that gives fracture. Thus

𝑅𝑅 = 𝜎𝜎𝑓𝑓(1−𝑣𝑣)

𝛼𝛼𝛼𝛼 ………………………………… (3)

Where σf is the stress required to give failure.

18

The above equation applies when there is no heat flow from the body to the underlying layers. If conduction is taken into account, then the definition of thermal shock resistance can be modified. Use a resistance R1 as a better measure of thermal shock resistance;

𝑅𝑅1 =𝜎𝜎𝑓𝑓(1−𝑣𝑣)𝜆𝜆

𝛼𝛼𝛼𝛼 …………………………. (4)

Where ,𝜆𝜆 is the thermal conductivity.

Under conditions where the surface temperature changes at a constant rate, the surface stress is found to depend on thermal diffusivity D. thermal diffusivity is a measure of how fast heat is transmitted through a solid.

𝐷𝐷 = 𝜆𝜆𝜌𝜌𝜌𝜌

…………………………………………….. (5)

Where 𝜌𝜌 is the density of the material. And c, is the specific heat. Under such a situation a resistance R2 is used as a measure of thermal resistance.

𝑅𝑅2 = 𝜎𝜎𝑓𝑓(1−𝑣𝑣)𝐷𝐷

𝛼𝛼𝛼𝛼 …………………………. …..…. (6)

3.1.3 FACTORS INFLUENCING THERMAL SHOCK BEHAVIOR

Thermal shock resistance is a complex property which is dependent on a combination of three seemingly distinct features which are interwoven. These are;

1. Material characteristics, such as expansion coefficient, thermal conductivity, strength, Elasticity and plasticity, which are themselves influenced often significantly, by

a. Bulk textural features, notably porosity (both amount and nature) and grain size, and;

b. Surface textural features, notably the absence of more usually the presence of surface flaws.

2. Dimensional factors. The important features here are specimen size and shape. 3. Environmental conditions. Here the principal features are the temperature of the

surroundings, rate of change of temperature of the surroundings and the relevant heat transfer coefficients.

19

3.2 MODULUS OF RUPTURE The modulus of rupture is also referred to as the flexural strength, bend strength or fracture strength. It is a parameter which defines the ability of a material to resist deformation under loading.

3.2.1 THE FLEXURE TEST It is difficult to perform tensile tests on brittle materials like ceramics since most specimens tend to break prematurely when they are gripped by the testing machine. This is because of their sensitivity to surface defects and notches.

The preferred method of testing ceramics in tension is the transverse bending test which measures the modulus of rupture. The specimen may be of circular or rectangular cross section. Preparing specimens from brittle materials, such as ceramics is difficult because of the problems involved in shaping and machining them to proper dimensions. Furthermore, because of their sensitivity to surface defects and notches, clamping brittle materials for testing is difficult. Also, improper alignment of the test specimen may result in non uniform stress distribution along the cross-section of the specimen. A commonly used test method for brittle materials is the bend or flexure test.

This test is done using a Hounsfield Tensometer. The flexure test method measures behavior of materials subjected to simple beam loading. Maximum stress and maximum strain are calculated for increments of load. Results are plotted in stress-strain diagram. Flexural strength is defined as the maximum stress in the outermost fiber. This is calculated at the surface of the specimen on the convex or tension side. Flexural modulus is calculated from the slope of the stress versus deflection curve. If the curve has no linear region, a secant line is fitted to the curve to determine slope. A flexure test produces tensile stress in the convex side of the specimen and compression stress in the concave side. This creates an area of shear stress along the midline. To ensure the primary failure comes from tensile or compression stress the shear stress must be minimized. This is done by controlling the span (S) to depth (d) ratio; the length of the outer span divided by the height (depth) of the specimen. For most materials S/d=16 is acceptable. Some materials require S/d=32 to 64 to keep the shear stress low enough.

There are two test types; 3-point bending and 4-point bending. In a 3-point test the area of uniform stress is quite small and concentrated under the center loading point. In a 4-point test, the area of uniform stress exists between the inner span loading points (typically half the outer span length). When a 3-point flexure test is done on a brittle material like ceramic or concrete it is often called modulus of rupture (MOR).

The main advantage of a three point flexural test is the ease of the specimen preparation and testing. However, this method has also some disadvantages: the results of the testing method are sensitive to specimen and loading geometry and strain rate.

20

In a four point the area and volume under peak stress or near peak stress is higher than in a three point bend testing hence the probability of a large flaw in a four point bend test being exposed to a high stress is increased. Thus, the bend strength measured in a four point is lower than that measured using a three point bend test.

The two flexure tests are shown in figures 3.1 and 3.2

Figure 3. 2: the four point flexure test ,source www.substech.com

Figure 3. 1:the three point flexure test, source: www.substech.com

21

3.2.2 FLEXURAL STRENGTH

This can be calculated from the following expression

𝜎𝜎𝑓𝑓 = 𝑀𝑀𝑀𝑀𝐼𝐼

………………………………………………. (7)

Where, 𝜎𝜎𝑓𝑓 is the flexural strength, M is the bending moment, y is the distance from the neutral axis to the outermost fiber.

For a cylindrical cross section; y=R, M = FL4

,𝐼𝐼 = ⫪𝑅𝑅4

4 hence;

σf = FL⫪R3 ..................................................................... (8)

Where R, is the radius of the cylinder.

For a rectangular specimen of section b by d, the flexural strength is given by

𝜎𝜎𝑓𝑓 = 3𝐹𝐹𝐹𝐹2𝑏𝑏𝑑𝑑2 ……………………………………………………… (9)

The tensile strength may be found from the modulus of rupture using the equation

𝜎𝜎𝑇𝑇𝑇𝑇 = 𝜎𝜎𝑟𝑟[2(𝑚𝑚+1)]2 ………………………………………. (10)

Where m is the Weibull modulus, 𝜎𝜎𝑟𝑟 is the rupture stress, 𝜎𝜎𝑇𝑇𝑇𝑇 is the tensile strength.

3.3 FRACTURE STRENGTH AND WEIBULL STATISTICS

3.3.1 STRESS CONCENTRATION The measured fracture strength of most ceramics are significantly lower than those predicted by theoretical calculation based on atomic bonding energies. This discrepancy is explained by the presence of microscopic flaws or cracks that always exist under normal conditions at the surface and within the interior of a body of material. These flaws amplify the applied stresses in their locale and are sometimes called stress raisers.

22

If it is assumed that a crack is similar to an elliptical hole and is oriented perpendicular to the applied stress, then the maximum stress at the crack tip may be approximated by;

𝜎𝜎𝑚𝑚 = 2𝜎𝜎0(𝑎𝑎 𝜌𝜌𝑡𝑡� )0.5 ……………………………………… (11)

Where; 𝜎𝜎0,is the magnitude of the nominal applied stress, 𝜌𝜌𝑡𝑡 is the radius of curvature of the crack tip. a, is the length of the surface crack or half the length of an internal crack. The stress concentration factor K is given by the ratio of 𝜎𝜎𝑚𝑚/𝜎𝜎0

𝐾𝐾 = 𝜎𝜎𝑚𝑚𝜎𝜎0

= 2(𝑎𝑎 𝜌𝜌𝑡𝑡� )0.5 …………………………………….. (12)

The critical stress 𝜎𝜎𝜌𝜌required for crack propagation is given by

𝜎𝜎𝜌𝜌 = (2𝛼𝛼𝛾𝛾𝑠𝑠𝜋𝜋𝑎𝑎

)0.5 ……………………………………………………………. (13)

Where, E is the modulus of elasticity, 𝛾𝛾𝑠𝑠is the free surface energy, a is the length of a surface crack or half the length of an internal crack.

3.3.2 FRACTURE TOUGHNESS

Fracture toughness is a property that is a measure of a materials resistance to fracture in the presence of a crack. It is given by;

𝐾𝐾𝜌𝜌 = 𝜎𝜎𝜌𝜌√𝜋𝜋𝑎𝑎 ………….………………………. (14)

where 𝐾𝐾𝜌𝜌 is the fracture toughness

When the specimen thickness is much bigger than the crack dimension, then we have plane strain fracture toughness, 𝐾𝐾𝐼𝐼𝐼𝐼 and is given by.

𝐾𝐾𝐼𝐼𝐼𝐼 = 𝜎𝜎√𝜋𝜋𝑎𝑎 ………………………………………….. (15)

3.3.3 THE WEIBULL STATISTICS

Ceramics show statistical variation in strength; this is a consequence of a sample or component containing multiple crack-like defects with a distribution of crack lengths and orientation. There is no single tensile strength of the material and generally only a distribution of strength.

When designing ceramic components for high performance applications, it is necessary to specify a survival probability, PS rather than acceptable stress, σ. This is calculated by the Weibull equation

𝑃𝑃𝑇𝑇(𝑣𝑣) = exp{−𝜎𝜎0𝑚𝑚𝑣𝑣0

1 ∫𝜎𝜎𝑚𝑚𝑑𝑑𝑣𝑣} ………………………. (16)

23

This can be simplified to

𝑃𝑃𝑇𝑇(𝑣𝑣) = exp{− 𝑉𝑉𝑉𝑉0

( 𝜎𝜎𝜎𝜎0

)𝑚𝑚 } ……………………………. (17)

Where, PS is the survival probability, σ is the applied stress, m is the Weibull Modulus, σ0 and V0 is the starting stress and volume (characteristic of the material), σ and V are the stress and volume at a particular instant

3.4 POROSITY A fired ceramic product shows porosity to a variable degree; porosity is a measure of the volumes of all pores present in a material. The pores may be open or closed. Accordingly two types of porosity can be distinguished; apparent porosity and true porosity. Apparent porosity is expressed as a percentage of the volume of the open pores with respect to the exterior volume of the material under consideration.

%P = W−DW−S

100 …………………………………….. (18)

Where; P = the apparent porosity D = Weight of dry solid S = Weight of the suspended solid in water after having been soaked in water so that all open pores in the body are completely filled with water W = Weight of the soaked body determined by weighing the soaked specimen from which excess water has been removed by dabbing with a damp cloth.

24

CHAPTER 4: MATERIALS AND APPARATUS

The following is a list of materials and apparatus used to perform the research.

1. Clay 2. Silica 3. Alumina 4. Extrusion machine 5. Drying bay 6. Electric furnace 7. Sieves 8. Electronic weighing machine 9. Hounsfield Tensometer 10. Vernier calipers and steel rule

11. Tongs and specimen holders 12. hydraulic lubricant (Tellus 68)

13. Surface plate and glass plate 14. Specimen racks 15. Sprit level

16. Water basins

Figure 4. 1 the extrusion machine

Figure 4. 2 the Hounsfield Tensometer

25

CHAPTER 5: METHODOLOGY

5.1 COLLECTION OF RAW MATERIALS The raw materials collected were clay from Maragua in Muranga county and Mukurweini in Nyeri county.

Sand was acquired from Maragua in Muranga County.

Kaolin powder was acquired from Kenya Industrial Research and Development Institute (KIRDI).

5.2 PREPARATION OF RAW MATERIALS The clays were first soaked in water for one week to break the large clay lumps into small particles. While soaked, the mixture was stirred periodically to further enhance mixing process. The final product of this was even slurry which was then sieved into a drying basin and left to dry in the sun. The filtrate took about three weeks to dry completely. The clay was then collected in bags and transported to KIRDI ceramics lab for milling into powder.

The sand was sieved to fineness using 250μm aperture MTI test sieves to remove impurities and large particles. This was to ensure proper mixing with clay particles.

5.3 PREPARATION OF SPECIMEN 5.3.1 MIXING OF CLAYS WITH SAND AND ALUMINA

The milled clays were mixed with sand and alumina respectively in the following proportions

(a) NYERI CLAY AND SAND

15% Sand and 85% Clay 25% Sand and 75% Clay

35% Sand and 65% Clay

45% Sand and 55% Clay 55% Sand and 45% Clay 75% Sand and 25% Clay (b) NYERI CLAY AND ALUMINA

10% Alumina and 90% Clay 30% Alumina and 70% Clay

35% Alumina and 65% Clay 45% Alumina and 55% Clay

26

65% Alumina and 35% Clay 80% Alumina and 20% Clay (c) MURANGA CLAY AND SAND

5% Sand and 95% Clay

15% Sand and 85% Clay 25% Sand and 75% Clay 35% Sand and 65% Clay 40% Sand and 60% Clay (d) MURANGA CLAY AND ALUMINA 5% Alumina and 95% Clay 15% Alumina and 85% Clay

35% Alumina and 65% Clay

45% Alumina and 55% Clay 55% Alumina and 45% Clay

5.3.2 MIXING OF NYERI CLAY AND MURANGA CLAY

The clays were mixed in the following proportions

25% Muranga Clay and 75% Nyeri Clay 50% Muranga Clay and 50% Nyeri Clay 75% Muranga Clay and 25% Nyeri Clay

5.3.3 NUMBER OF SPECIMENS REQUIRED

The above ratios amounts to 27 groups, each group was to undergo 10 different tests. For each test, averages of 5 specimens were used. At least 1350 specimen were made with the excess accounting for any damages.

3.6 kilograms of raw materials in powder form were mixed with controlled amount of water to produce 50 specimens for each group.

5.4 EXTRUSION PROCESS A modified meat mincing machine was used to produce long cylindrical clay specimen in plastic state. The machine is an auger type extruder which utilized screw motion to force a plastic mass of wet clay into a de-airing chamber to compact it and force out as much air as possible. The material was then passed through a 24.5mm cylindrical die resulting in long cylindrical wet clay. The clay was collected and passed through the extruder again to achieve more homogeneity in the plastic mass. During the final pass the clay was passed through a 13mm die and the

27

cylindrical specimen were sectioned to 12.5cm length. The specimen were received on a glass plate and guided with a straight rectangular perspex both of which were applied a hydraulic lubricant (Tellus 68). This lubricant aided in achieving a smooth specimen surface. It was appropriate because it would conveniently evaporate during firing. 27 groups of specimen were made, each group having at least 50 specimens.

5.5 DRYING AND FIRING The extruded specimen were rolled onto a flat plate and transported to a cold room where they were left to dry slowly, the specimen were turned regularly to minimize warping. It took each group about one week to dry completely.

The specimens were fired up to 1000oc, with increments of 100oc. an hour wait was allowed between each increment.

5.6 THERMAL SHOCK AND THERMAL FATIGUE TEST 5.6.1 THERMAL SHOCK

Specimens were set in various groups according to the type of test required and the group heated to the desired shocking temperature of either 400°C, 600°C and 800°C. After heating to this temperatures and maintained for about 20 minutes, the group of specimens were then removed from the furnace and quickly dipped in a bath of water at room temperature (22.3°C). One group of specimen was left unquenched, another quenched once while the rest quenched for 15 cycles and for 20 cycles.

Thermal shock test was achieved by shocking the specimens once while thermal fatigue test was achieved through the 15 cycles and 20 cycles quenching.

5.6.2 FLEXURE TEST The flexure test was carried out using the Hounsfield tensometer at the strength of materials laboratory in the Mechanical Engineering workshop. A three-point load technique was applied where the test specimen was supported at the ends and the load applied at the center. The load was applied and increased progressively until specimen breaks at fracture load. The load at fracture, F (N), the span length, L (mm) and the diameter, D (mm) for each specimen was recorded for calculation of flexural strength and hence further analysis.

5.7 ASSUMPTIONS DURING PROJECT UNDERTAKING • The methodology of production for all the clay samples extruded was the same, that is,

soaking, kneading, extrusion, firing all fired under the similar conditions.

• The loading rate during testing(using Hounsfield Tensometer) was uniform and pure bending load applied

• The time transfer of the specimen from furnace into the quenching water-bath during quenching was negligible

28

• All the specimens shocked between a particular pre-determined temperature experienced the same degree of thermal shock

• The specimen constituents were homogeneously mixed during molding and sufficiently de-aired to the same degree before extrusion was undertaken.

• The furnace used provided uniform heating of all the specimens at any particular time.

29

CHAPTER 6: RESULTS

The tables containing laboratory data and values of average strength, and graphs used to obtain the Weibull modulus are attached in the appendix A and appendix B

6.1 MIXTURE OF MURANGA CLAY AND SAND TABLE 6.1 (a) PERCENTAGE SAND AND AVERAGE STRENGTH FOR MURANGA CLAY AVERAGE STRENGTH (Mpa) % PERCENTAGE SAND

UNSHOCKED

SHOCKED ONCE AT 400°C

SHOCKED ONCE AT 600°C

SHOCKED ONCE AT 800°C

0 30.06467 24.3733 18.9073 13.2558 5 31.5689 26.42722 17.399 13.4532 15 32.3049 25.0272 14.933 12.6553 25 29.6965 22.9131 13.6031 12.344 35 24.241 20.3379 12.8889 11.679 40 21.358 19.1368 12.5912 10.516

FIGURE 6.1 (a) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR MURANGA CLAY

0

5

10

15

20

25

30

35

0 10 20 30 40 50

Ther

mal

Str

engt

h (M

pa)

Percentage silica (%)

UNSHOCKED

SHOCKED ONCE AT 400°CSHOCKED ONCE AT 600°CSHOCKED ONCE AT 800°C

30

TABLE 6.1 (b) PERCENTAGE SAND AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED 15 AND 20 TIMES AT 400°C

AVERAGE STRENGTH (MPa) % SILICA SHOCKED 15 TIMES SHOCKED 20 TIMES 0 5 10.3968 15 11.5908 8.5776 25 12.9368 9.0283 35 13.0495 7.4505 40 10.0834 7.0523

FIGURE 6.1 (b) A GRAPH OF PERCENTAGE SAND AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED AT 400°C

0

2

4

6

8

10

12

14

0 10 20 30 40 50

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SILICA (%)

SHOCKED 15 TIMES

SHOCKED 20 TIMES

31

TABLE 6.1 (c) PERCENTAGE SAND AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED 15 AND 20 TIMES AT 600°C

AVERAGE STRENGTH (MPa) PERCENTAGE SILICA SHOCKED 15 TIMES SHOCKED 20 TIMES 0 5 7.8019 15 10.3561 6.24808 25 10.9123 7.678 35 9.5125 7.2278 40 7.3141 6.1687

FIGURE 6.1 (c) A GRAPH OF PERCENTAGE SAND AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED AT 600°C

0

2

4

6

8

10

12

0 10 20 30 40 50

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SILICA (%)

SHOCKED 15 TIMES

SHOCKED 20 TIMES

32

TABLE 6.1 (d) ) PERCENTAGE SAND AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED 15 AND 20 TIMES AT 800°C

AVERAGE STRENGTH (MPa) PERCENTAGE SILICA SHOCKED 15 TIMES SHOCKED 20 TIMES 0 5 5.20349 15 6.11423 25 5.4133 4.9142 35 5.2197 4.7206 40 4.9088 4.3449

FIGURE 6.1 (d A GRAPH OF PERCENTAGE SAND AND AVERAGE STRENGTH FOR MURANGA CLAY 15 1ND 20 TIMES AT 800°C

0

1

2

3

4

5

6

7

0 10 20 30 40 50

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SILICA (%)

SHOCKED 15 TIMES

SHOCKED 20 TIMES

33

6.2 MIXTURE OF NYERI CLAY WITH SAND

TABLE 6.2 (a) NYERI CLAY WITH SILICA RATIOS

AVERAGE STRENGTH (MPa)

AVERAGE STRENGTH (MPa)

AVERAGE STRENGTH (MPa)

AVERAGE STRENGTH (MPa)

% SAND

UNSHOCKED SHOCKED ONCE AT 400

SHOCKED ONCE AT 600

SHOCKED ONCE AT 800

0 8.121 6.2091 6.0629 4.2458 15 11.4776 10.8709 9.1712 8.5275 25 13.7986 12.0297 10.4016 8.8513 35 14.5919 11.8424 9.1065 8.47 45 12.1684 8.851 8.4171 7.158 55 10.0461 7.6028 7.0944 4.7145 75 8.0405 5.2215 4.9899 4.2045 FIGURE 6.2 (a) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR NYERI CLAY

0

2

4

6

8

10

12

14

16

0 20 40 60 80

STRE

NG

TH (M

PA)

%SAND

UNSHOCKED

SHOCKED ONCE AT 400

SHOCKED ONCE AT 600

SHOCKED ONCE AT 800

34

TABLE 6.2 (b) AVERAGE STRENGTH AGAINST % SAND FOR NYERI CLAY

SHOCKED 15 AND 20 TIMES AT 400°C

AVERAGE STRENGTH (MPa) %SAND 15 SHOCKS 20 SHOCKS 0 4.3737 4.4289 15 5.43 4.8566 25 6.4489 5.8957 35 5.9613 5.7081 45 5.3732 4.8878 55 5.183 4.7123 75 4.5554 4.3462

FIGURE 6.2 (b) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR NYERI CLAY SHOCKED 15 AND 20 TIMES AT 400°C

0

1

2

3

4

5

6

7

0 20 40 60 80

STRE

NG

TH (M

PA)

%SAND

15 SHOCKS

20 SHOCKS

35

TABLE 6.2 (c) AVERAGE STRENGTH AGAINST % SAND FOR NYERI CLAY

SHOCKED 15 AND 20 TIMES AT 600°C

AVERAGE STRENGTH AGAINST % SAND FOR SHOCKING AT 600 DEGREES CELCIUS AVERAGE STRENGTH (MPa) %SAND 15 SHOCKS 20 SHOCKS 0 4.0954 3.8754 15 5.2059 4.9362 25 6.1704 5.6377 35 5.8842 4.9288 45 5.1002 4.6161 55 5.0129 4.2567 75 4.6938 4.0282 FIGURE 6.2 (c) A GRAPH OF AVERAGE STRENGTH AGAINST % SILICA SHOCKED AT 600°C

0

1

2

3

4

5

6

7

0 20 40 60 80

STRE

NG

TH (M

PA)

%SAND

15 SHOCKS

20 SHOCKS

36

TABLE 6.2 (d) NYERI CLAY WITH SILICA RATIOS SHOCKED 15 AND 20 TIMES AT 800°C

AVERAGE STRENGTH AGAINST % SAND FOR SHOCKING AT 800 DEGREES CELCIUS AVERAGE STRENGTH (MPa) %SAND 15 SHOCKS 20 SHOCKS 0 5.2979 4.9542 15 6.0076 5.7538 25 6.0709 5.8231 35 5.6611 5.5558 45 5.098 4.8052 55 4.8467 4.337 75 4.6238 4.0023

FIGURE 6.2 (d) A GRAPH OF AVERAGE STRENGTH AGAINST % SILICA SHOCKED AT 800°C

0

1

2

3

4

5

6

7

0 20 40 60 80

STRE

NG

TH (M

PA)

%SAND

15 SHOCKS

20 SHOCKS

37

6.3 MIXTURE OF MURANGA CLAY AND ALUMINA TABLE 6.3 (a) PERCENTAGE ALUMINA AND AVERAGE STRENGTH FOR MURANGA CLAY

AVERAGE STRENGTH (MPa)

PERCENTAGE ALUMINA

UNSHOCKED SHOCKED ONCE AT 400°C

SHOCKED ONCE AT 600°C

SHOCKED ONCE 800°C

0 30.0646 24.3732 18.9073 13.2558

5 34.1121 26.0348 19.2906 16.2011

15 37.5252 28.0348 21.3103 18.1127

35 33.703 22.1103 20.05718 15.9258

45 23.911 20.465 17.04696 15.4947

55 20.2863 18.6005 15.40906 12.9056

FIGURE 6.3 (a) A GRAPH OF PERCENTAGE ALUMINA AND AVERAGE STRENGTH FOR MURANGA CLAY

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA(%)

UNSHOCKED

SHOCKED ONCE AT 400°C

SHOCKED ONCE AT 600°C

SHOCKED ONCE 800°C

38

TABLE 6.3 (b PERCENTAGE ALUMINA AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED 15 AND 20 TIMES AT 400°C

AVERAGE STRENGTH (MPa)

PERCENTAGE ALUMINA SHOCKED 15 TIMES SHOCKED 20 TIMES

0

5 10.0676 9.08366

15 10.2416 8.8875

35 11.05494 8.8006

45 10.4139 8.7558

55 10.1748 8.70417

FIGURE 6.3 (b) A GRAPH OR PERCENTAGE ALUMINA AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED AT 400°C

0

2

4

6

8

10

12

0 10 20 30 40 50 60

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA(%)

SHOCKED 15 TIMES

SHOCKED 20 TIMES

39

TABLE 6.3 (c) PERCENTAGE ALUMINA AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED 15 AND 20 TIMES AT 600°C

AVERAGE STRENGTH (MPa)

PERCENTAGE ALUMINA SHOCKED 15 TIMES SHOCKED 20 TIMES

0

5 9.6644 8.8567

15 9.4175 8.638

35 9.7496 8.5507

45 9.4024 8.5465

55 9.3389 8.373

FIGURE 6.3:(c) A GRAPH OR PERCENTAGE ALUMINA AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED AT 600°C

0

2

4

6

8

10

12

0 10 20 30 40 50 60

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA (%)

SHOCKED 15 TIMES

SHOCKED 20 TIMES

40

TABLE 6.3 (d) PERCENTAGE ALUMINA AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED 15 AND 20 TIMES AT 800°C

FIGURE 6.3 (d) A GRAPH OR PERCENTAGE ALUMINA AND AVERAGE STRENGTH FOR MURANGA CLAY SHOCKED I5 AND 20 TIMES AT 800°C

0

2

4

6

8

10

12

0 10 20 30 40 50 60

Axis

Titl

e

Axis Title

SHOCKED 15 TIMES

SHOCKED 20 TIMES

AVERAGE STRENGTH (MPa)

PERCENTAGE ALUMINA SHOCKED 15 TIMES SHOCKED 20 TIMES

0

5 8.7324 7.79353

15 9.6228 8.7905

35 9.1976 8.4631

45 8.9746 8.2774

55 8.6558 8.0283

41

6.4 MIXTURE OF NYERI CLAY AND ALUMINA

TABLE 6.4 (a) AVERAGE STRENGTH AGAINST % ALUMINA IN NYERI CLAY

% ALUMINA

STRENGTH (MPa) STRENGTH (MPa)

STRENGTH (MPa)

STRENGTH (MPa)

UNSHOCKED SHOCKED ONCE AT 400

SHOCKED ONCE AT 600

SHOCKED ONCE AT 800

0 8.1422 6.2288 5.345 4.2593 10 12.3601 9.039 7.4238 7.2339 30 18.818 16.9409 14.1179 12.9194 35 19.6728 17.711 15.7297 14.0884 45 19.3727 16.2709 15.3887 13.4649 65 14.7446 12.1691 9.4154 8.0379 80 13.1673 6.9021 5.313 4.1146

FIGURE 6.4 (a) A GRAPH OF AVERAGE STRENGTH AGAINST % ALUMINA IN NYERI CLAY

0

5

10

15

20

25

0 20 40 60 80 100

STRE

NG

TH (M

PA)

% ALUMINA

UNSHOCKED

SHOCKED ONCE AT 400

SHOCKED ONCE AT 600

SHOCKED ONCE AT 800

42

TABLE 7.4 (b) NYERI CLAY WITH ALUMINA RATIOS SHOCKED 15 AND 20 TIMES AT 400°C

AVERAGE STRENGTH (MPa) % ALUMINA 15 SHOCKS 20 SHOCKS 0 4.3737 4.4289 10 5.1322 4.8601 30 7.1591 6.376 35 8.3392 7.6134 45 10.0762 9.2085 65 9.3351 8.7615 80 7.0568 6.7992

FIGURE 6.4 (b) A GRAPH OF AVERAGE STRENGTH AGAINST % ALUMINA IN NYERI CLAY SHOCKED AT 400°C

0

2

4

6

8

10

12

0 20 40 60 80 100

STRE

NG

TH (M

PA)

% ALUMINA

15 SHOCKS

20 SHOCKS

43

TABLE 7.4 (c) AVERAGE STRENGTH AGAINST % ALUMINA IN NYERI CLAY

SHOCKED 15 AND 20 TIMES AT 600°C

AVERAGE STRENGTH (MPa) % ALUMINA 15 SHOCKS 20 SHOCKS 0 4.0974 3.875 10 4.3722 4.1669 30 6.9091 6.4188 35 7.5309 7.002 45 8.6456 7.9195 65 5.2002 4.8313 80 4.3254 4.2203

FIGURE 6.4 (c) A GRAPH OF AVERAGE STRENGTH AGAINST % ALUMINA IN NYERI SHOCKED AT 600°C

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100

STRE

NG

TH (M

PA)

% ALUMINA

15 SHOCKS

20 SHOCKS

44

TABLE 6.4 (d AVERAGE STRENGTH AGAINST % ALUMINA IN NYERI CLAY

SHOCKED 15 AND 20 TIMES AT 800°C

AVERAGE STRENGTH (MPa) % ALUMINA 15 SHOCKS 20 SHOCKS 0 5.2979 4.9528 10 5.8312 5.6847 30 8.218 7.8934 35 8.6127 8.1012 45 8.7738 8.005 65 4.2791 4.2253 80 3.995 3.8485

FIGURE 6.4 (d) A GRAPH OF AVERAGE STRENGTH AGAINST % ALUMINA IN NYERI CLAY SHOCKED AT 800°C

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100

STRE

NG

TH (M

PA)

% ALUMINA

15 SHOCKS

20 SHOCKS

45

6.5 RESULTS OF MIXTURE MURANGA CLAY AND NYERI CLAY

TABLE 6.5 (a) AVERAGE STRENGTH AGAINST PROPORTION FOR MIXTURE MURANGA CLAY AND NYERI CLAY

AVERAGE STRENGTH (MPa) % MURANGA CLAY IN MIXTURE

UNSHOCKED SHOCKED ONCE AT 400 C

SHOCKED ONCE AT 600 C

SHOCKED ONCE AT 800 C

0 8.1163 6.2208 5.314 4.2593 25 14.3309 11.1032 8.5695 5.5321 50 22.5631 17.2786 13.3945 8.5226 75 28.5403 22.3623 17.4995 10.751 100 30.1351 24.3732 18.9073 13.2558

FIGURE 6.5 (a) AVERAGE STRENGTH AGAINST PROPORTION FOR MIXTURE MURANGA CLAY AND NYERI CLAY

0

5

10

15

20

25

30

35

0 20 40 60 80 100

AVER

AGE

STRE

NG

TH (

Mpa

)

% MURANGA CLAY IN MIXTURE

UNSHOCKEDSHOCKED ONCE AT 400 CSHOCKED ONCE AT 600 CSHOCKED ONCE AT 800 C

46

6.6 RESULTS OF PREVIOUS RESEARCHERS COMBINED WITH OURS

6.6.1 THERMAL SHOCKING: OUR RESULTS COMBINED WITH RESULTS OF WATARE AND WANJOHI & CHERONO AND MOSIRIA

TABLE 6.6 (a) AVERAGE STRENGTH AGAINST % OF SAND FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 400°C

Key: *-result by Watare and Wanjohi{24} **-result by Cherono and Mosiria{23}

AVERAGE STRENGTH (MPa) % SAND

MURANGA CLAY SHOCKED ONCE AT 600OC

NYERI CLAY SHOCKED ONCE AT 600OC

0 18.9073 6.2091 5 17.3399 10 17.4568* 8.8349* 15 14.933** 9.1712** 20 16.3059 8.8359 25 13.6061 10.4016 30 14.441 35 12.8889 9.1065 40 12.5912** 9.2235** 45 12.9028* 8.4171 55 7.0944 65 5.8623** 70 9.6224* 75 4.9899 85 3.2466*

Key: *-result by Watare and Wanjohi {24} **-result by Cherono and Mosiria {23}

47

FIGURE 6.6 (a) A GRAPH OF AVERAGE STRENGTH AGAINST % OF SAND FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 400°C

KEY

Plot of Korir and Kibirech results

Plot of Watare and Wanjohi results

Plot of Cherono and Mosiria results

Note: this key is applicable to all graphs in section 6.6 {figure 6.6 (a) through figure 6.6(f)}

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60 70 80 90

AVER

AGE

STRE

NG

TH (M

pa)

% SAND

MURANGA CLAY SHOCKED ONCE AT 600OC

NYERI CLAY SHOCKED ONCE AT 600OC

48

TABLE 6.6 (b) AVERAGE STRENGTH AGAINST % OF SAND FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 400°C

Key: *-result by Watare and Wanjohi {24} **-result by Cherono and Mosiria {23}

FIGURE 6.6 (b) A GRAPH OF AVERAGE STRENGTH AGAINST % OF SAND FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 600°C

COMBINED RESULTS WITH PREVIOUS RESEARCHERS (WATARE AND WANJOHI) AVERAGE STRENGTH (MPa) % SAND

MURANGA CLAY SHOCKED ONCE AT 400OC

NYERI CLAY SHOCKED ONCE AT 400OC

0 24.373 6.2091 5 26.0348 10 23.3793* 9.0688* 15 25.0272 10.8709 20 22.2349** 9.6197** 25 22.9232 12.0297 30 19.3602** 35 20.3379 11.8424 40 19.3602 ** 9.2186** 45 17.8143* 8.851 55 7.6028 65 6.1353** 70 13.2848* 5.6332* 75 5.2215 85 3.8945*

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90

AVER

AGE

STRE

NG

TH (M

pa)

% SAND

MURANGA CLAY SHOCKED ONCE AT 400OC

NYERI CLAY SHOCKED ONCE AT 400OC

49

TABLE 6.6 (c)-AVERAGE STRENGTH AGAINST % OF SAND FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 800°C

AVERAGE STRENGTH (MPa) % SAND MURANGA CLAY

SHOCKED ONCE AT 800OC NYERI CLAY SHOCKED ONCE AT 800OC

0 13.2558 4.2458 5 13.4532 10 12.4952* 7.6388* 15 12.6553 8.8513 20 11.2987** 8.6936** 25 12.344 8.8513 35 11.679 8.47 40 10.516** 8.237** 45 9.4273* 7.158 55 4.7145 65 5.4019** 70 7.9832* 5.3856* 75 4.2085 85 2.7173* Key: *-result by Watare and Wanjohi {24}

**-result by Cherono and Mosiria {23}

FIGURE 6.6 (c) A GRAPH OF AVERAGE STRENGTH AGAINST % OF SAND FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 800°C

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70 80 90

AVER

AGE

STRE

NG

TH (M

pa)

% SANDNYERI CLAY

50

TABLE 6.6 (d) AVERAGE STRENGTH AGAINST % OF ALUMINA FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 400°C

PERCENTAGE ALUMINA IN MURANGA AND NYERI CLAY COMBINED AGAINST AVERAGE STRENGTH SHOCKED ONCE AT 400°C AVERAGE STRENGTH (MPa) % ALUMINA MURANGA CLAY NYERI CLAY 0 24.37328 6.2288 5 26.03481 10 9.039* 15 28.03933 30 16.9409* 35 22.1103 17.711 45 20.465 16.2709 50 19.1385* 16.1828* 55 18.60055 65 12.1691 70 13.43916* 80 6.9021** 85 8.198187* 5.6863*

Key: *-result by Watare and Wanjohi {24} **-result by Cherono and Mosiria {23}

FIGURE 6.6 (d) A GRAPH OF AVERAGE STRENGTH AGAINST % OF ALUMINA FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 400°C

02468

1012141618202224262830

0 20 40 60 80 100

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA (%)

MURANGA CLAY

NYERI CLAY

51

TABLE 6.6 (e) AVERAGE STRENGTH AGAINST % OF ALUMINA FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 600°C

AVERAGE STRENGTH (MPa) % ALUMINA MURANGA CLAY NYERI CLAY 0 18.9073 5.345 5 19.2906* 10 7.4238 15 21.3103 30 14.1179 35 20.0371 15.7297 45 17.0469 15.3887 50 17.631* 15.7061* 55 16.4361 65 9.4154** 70 11.9762* 7.0756* 80 5.313** 85 7.432* 4.8842* Key: *-result by Watare and Wanjohi {24}

**-result by Cherono and Mosiria {23}

FIGURE 6.6 (e) A GRAPH OF AVERAGE STRENGTH AGAINST % OF ALUMINA FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 600°C

02468

1012141618202224

0 20 40 60 80 100

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA (%)

MURANGA CLAY

NYERI CLAY

52

TABLE 6.6 (f) AVERAGE STRENGTH AGAINST % OF ALUMINA FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 800°C

AVERAGE STRENGTH (MPa) % ALUMINA MURANGA CLAY NYERI CLAY 0 13.2558 4.2593 5 16.20111 10 7.2339* 15 18.1127 30 12.9194 35 15.9258 14.0884 45 15.4947 13.4649 50 55 12.9056 65 8.0379** 70 9.802* 5.955* 80 4.1146** 85 5.90773* 4.1598* Key: *-result by Watare and Wanjohi{24}

**-result by Cherono and Mosiria{23}

FIGURE 6.6 (f) A GRAPH OF AVERAGE STRENGTH AGAINST % OF ALUMINA FOR NYERI CLAY AND MURANGA CLAY SHOCKED ONCE AT 800°C

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60 70 80 90

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA (%)

MURANGA CLAY

NYERI CLAY

53

6.6.2 THERMAL FATIGUE: OUR RESULTS COMBINED WITH RESULTS OF WATARE AND WANJOHI

TABLE 6.6(g) A MIXTURE OF NYERI CLAY AND ALUMINA SHOCKED ONCE AT 400OC

NYERI CLAY AVERAGE STRENGTH (MPa) FOR SHOCKING AT 400 OC % ALUMINA 5 SHOCKS 10 SHOCKS 15 SHOCKS 20 SHOCKS 0 7.0111 5.6629 4.3737 4.4289 10 6.1197 4.4217 5.1322 4.8601 30 7.1571 6.376 35 8.3392 7.6134 45 10.0762 9.2085 50 15.2969 13.4463 65 9.3351 8.7615 75 8.1201 6.8457 80 7.0568 6.7992 85 4.2229 3.9454

FIGURE 6.6(g) A GRAPH OF AVERAGE STRENGTH AGAINST %ALUMINA FOR NYERI CLAY SHOCKED ONCE AT 400OC

0

2

4

6

8

10

12

14

16

18

0 20 40 60 80 100

AVER

AGE

STR

ENG

TH (M

pa)

% ALUMINA

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

54

TABLE 6.6(h) A MIXTURE OF NYERI CLAY AND ALUMINA SHOCKED ONCE AT 600OC

NYERI CLAY AVERAGE STRENGTH (MPa)

% ALUMINA 5 SHOCKS 10 SHOCKS 15 SHOCKS 20 SHOCKS 0 7.0111 5.6629 3.875 4.0974 10 5.5976 4.959 4.3722 4.1669 30 6.9091 6.4188 35 7.5309 7.002 45 8.6456 7.9195 50 13.9245 12.5491 65 5.2002 4.8323 75 6.4407 5.8564 80 4.3254 4.2203 85 4.0582 2.9852

FIGURE 6.6(h) A GRAPH OF AVERAGE STRENGTH AGAINST %ALUMINA FOR NYERI CLAY SHOCKED ONCE AT 600OC

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70 80 90

AVER

AGE

STR

ENG

TH (M

pa)

% ALUMINA

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

55

TABLE 6.6(i) A MIXTURE OF NYERI CLAY AND ALUMINA SHOCKED ONCE AT 800OC

NYERI CLAY

AVERAGE STRENGTH (MPa) FOR SHOCKING AT 800OC

% ALUMINA

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

0 7.0111 5.6629 5.2979 4.9542 10 4.8042 3.8264 5.8132 5.6847 30 8.218 7.8934 35 8.3834 8.1012 45 8.7738 8.005 50 12.4246 11.8024 65 4.2791 4.2253 75 4.2846 3.7499 80 3.995 3.8485 85 3.2687 3.0059

FIGURE 6.6(i) A GRAPH OF AVERAGE STRENGTH AGAINST %ALUMINA FOR NYERI CLAY SHOCKEDONCEAT800OC

0

2

4

6

8

10

12

14

0 20 40 60 80 100

AVER

AGE

STR

ENG

TH (M

pa)

% ALUMINA

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

56

TABLE 6.6(j) A MIXTURE OF MURANGA CLAY AND ALUMINA SHOCKED AT 400OC

AVERAGE STRENGTH (MPa) PERCENTAGE ALUMINA(%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

0 13.8046 12.7413 5 8.7324 7.7935 10 13.8626 13.0696 15 9.6228 8.7905 35 9.1976 8.4631 45 8.9746 8.2774 50 15.00947 13.2813 55 8.6558 8.0283 70 9.0404 7.4718 85 5.2023 4.83166

FIGURE 6.6(j) A GRAPH OF AVERAGE STRENGTH AGAINST %ALUMINA FOR MURANGA CLAY SHOCKED ONCE AT 400OC

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

57

TABLE 6.6(k) A MIXTURE OF MURANGA CLAY AND ALUMINA SHOCKED AT 600OC

AVERAGE STRENGTH(MPa) PERCENTAGE ALUMINA (%) 5 SHOCKS 10 SHOCKS 15 SHOCKS 20 SHOCKS 0 13.8046 12.7413 5 9.6644 8.8567 10 15.72097 14.86216 15 9.4175 8.638 35 9.7496 8.5507 45 9.4024 8.5465 50 16.16741 15.49302 55 9.3389 8.373 70 10.2143 9.8406 85 6.8997 6.029394

FIGURE 6.6(k) A GRAPH OF AVERAGE STRENGTH AGAINST %ALUMINA FOR MURANGA CLAY SHOCKED ONCE AT 600OC

0

2

4

6

8

10

12

14

16

18

0 20 40 60 80 100

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

58

TABLE 6.6(l) A MIXTURE OF NYERI CLAY AND ALUMINA SHOCKED AT 800OC

AVERAGE STRENGTH(MPa) PERCENTAGE ALUMINA (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

0 13.8046 12.7413 5 10.0676 9.08366 10 20.7076 18.18693 15 10.2416 8.8875 35 11.05494 8.8006 45 10.4139 8.7558 50 18.45102 17.42183 55 10.1748 8.70417 70 12.56595 10.46809 85 7.45436 6.142944

FIGURE 6.6(l) A GRAPH OF AVERAGE STRENGTH AGAINST %ALUMINA FOR MURANGA CLAY SHOCKED ONCE AT 800OC

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80 90

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE ALUMINA (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

59

TABLE 6.6(m) A MIXTURE OF MURANGA CLAY AND SAND SHOCKED AT 800OC

AVERAGE STRENGTH (MPa) PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

5 5.20349 10 11.49393 10.52371 15 6.11423 20 10.56542 9.807954 25 5.4133 4.9142 35 5.2197 4.7206 40 7.882615 6.9559 4.9088 4.3449 70 7.025154 6.2379

FIGURE 6.6(m) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR MURANGA CLAY SHOCKED ONCE AT 800OC

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

60

TABLE 6.6(n) A MIXTURE OF MURANGA CLAY AND SAND SHOCKED AT 600OC

AVERAGE STRENGTH(MPa) PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

0 13.8046 12.7413 5 7.8019 10 16.29388 15.58397 15 10.3561 6.24808 20 15.93231 13.77032 25 10.9123 7.678 35 9.5125 7.2278 40 11.17331 10.4927 7.3141 6.1687 70 8.2926 7.68024

FIGURE 6.6(n) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR MURANGA CLAY SHOCKED ONCE AT 600OC

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60 70 80

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

61

TABLE 6.6(o) A MIXTURE MURANGA CLAY AND SAND SHOCKED AT 800OC

AVERAGE STRENGTH (MPa) PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

0 13.8046 12.7413 5 10.3968 10 22.05608 20.7311 15 11.5908 8.5776 20 20.99846 19.07334 25 12.9368 9.0283 35 13.0495 7.4505 40 17.0609 15.39665 10.0834 7.0523 70 11.97169 8.438071

FIGURE 6.6(o) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR MURANGA CLAY SHOCKED ONCE AT 800OC

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

62

TABLE 6.6(p) A MIXTURE NYERI CLAY AND SAND SHOCKED AT 400OC

AVERAGE STRENGTH (MPa) %SAND 5 SHOCKS 10 SHOCKS 15 SHOCKS 20 SHOCKS 0 7.0111 5.6629 4.3737 4.4289 10 6.268 5.5166 15 5.43 4.8566 20 8.8473 7.6108 25 6.4489 5.8957 35 5.9613 5.708 40 7.9652 6.0723 45 5.3732 4.8872 55 5.183 4.7123 70 5.115 4.9786 75 4.5554 4.3462

FIGURE 6.6(p) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR NYERI CLAY SHOCKED ONCE AT 400OC

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

63

TABLE 6.6(Q) A MIXTURE NYERI CLAY AND SAND SHOCKED AT 600OC

AVERAGE STRENGTH (MPa) FOR SHOCKING AT 600 OC %SAND 5 SHOCKS 10 SHOCKS 15 SHOCKS 20 SHOCKS 0 7.0111 5.6629 3.8754 4.0974 10 7.363 6.9265 15 5.2059 4.9362 20 7.8438 7.151 25 6.1704 5.6377 35 5.8842 4.9288 40 7.6025 5.929 45 5.1002 4.6161 55 5.0129 4.2567 70 4.495 3.6157 75 4.6938 4.0282

FIGURE 6.6(q) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR NYERI CLAY SHOCKED ONCE AT 600OC

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50 60 70 80

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

64

TABLE 6.6(r) A MIXTURE NYERI CLAY AND SAND SHOCKED AT 800OC

AVERAGE STRENGTH (MPa) %SAND 5 SHOCKS 10 SHOCKS 15 SHOCKS 20 SHOCKS 0 7.0111 5.6629 5.2979 4.9542 10 7.18 6.2723 15 5.8172 5.7538 20 7.802 7.55 25 6.0709 5.8231 35 5.6611 5.5558 40 7.5484 6.2757 45 5.098 4.8052 55 4.8467 4.337 70 4.4588 3.5632 75 4.6233 4.0023

FIGURE 6.6(r) A GRAPH OF AVERAGE STRENGTH AGAINST % SAND FOR NYERI CLAY SHOCKED ONCE AT 800OC

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50 60 70 80

AVER

AGE

STRE

NG

TH (M

pa)

PERCENTAGE SAND (%)

5 SHOCKS

10 SHOCKS

15 SHOCKS

20 SHOCKS

65

6.7 PERCENTAGE REDUCTION IN AVERAGE STRENGTH AFTER SHOCKING

TABLE 6.7 (a)-REDUCTION IN STRENGTH FOR NYERI CLAY AND SAND MIXTURE

PERCENTAGE REDUCTON IN STRENGTH FOR NYERI CLAY AFTER 20 SHOCKS GROUP AVERAGE STRENGTH (MPa) % REDUCTION IN STRENGTH % SAND

UNSHOCKED SHOCKED AT 400 °C

SHOCKED AT 600 °C

SHOCKED AT 800°C

SHOCKED AT 400 °C

SHOCKED AT 600°C

SHOCKED AT 800°C

0 8.121 4.4289 3.8754 4.9542 45.4636 52.2792 38.9952 15 11.4776 4.8566 4.9362 5.7538 57.6862 56.9927 49.8693 25 13.7986 5.8957 5.6377 5.8231 57.2732 59.1429 57.7993 35 14.5919 5.7081 4.9288 5.5558 60.8817 66.2223 61.9254 45 12.1684 4.8878 4.6161 4.8052 59.8320 62.0648 60.5108 55 10.0461 4.7123 4.2567 4.337 53.0932 57.6283 56.8290 75 8.0405 4.3462 4.0282 4.0023 45.9461 49.9011 50.2232

TABLE 6.7 (b)-REDUCTION IN STRENGTH FOR NYERI CLAY AND ALUMINA MIXTURE

PERCENTAGE REDUCTON IN STRENGTH FOR NYERI CLAY AFTER 20 SHOCKS GROUP AVERAGE STRENGTH (MPa) % REDUCTION IN STRENGTH % ALUMINA

UNSHOCKED SHOCKED AT 400°C

SHOCKED AT 600°C

SHOCKED AT 800°C

SHOCKED AT 400 °C

SHOCKED AT 600°C

SHOCKED AT 800°C

0 8.1422 4.4289 3.875 4.9528 45.6056 52.4084 39.1712 10 12.3601 4.8601 4.1669 5.6847 60.6791 66.2874 54.0076 30 18.818 6.376 6.4188 7.8934 66.1175 65.8901 58.0539 35 19.6728 7.6134 7.002 8.1012 61.2998 64.4077 58.8203 45 19.3727 9.2085 7.9195 8.005 52.4666 59.1203 58.6789 65 14.7446 8.7615 4.8313 4.2253 40.5782 67.2334 71.3434 80 13.1673 6.7992 4.2203 3.8485 48.3629 67.9486 70.772

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TABLE 6.7(c)-REDUCTIONS IN STRENGTH FOR MURANGA CLAY AND SAND MIXTURE

PERCENTAGE REDUCTON IN STRENGTH FOR NYERI CLAY AFTER 20 SHOCKS AVERAGE STRENGTH (MPa) % REDUCTION IN STRENGTH % SAND

UNSHOCKED

SHOCKED AT 400°C

SHOCKED AT 600°C

SHOCKED AT 800°C

SHOCKED AT 400°C

SHOCKED AT 600°C

SHOCKED AT 800°C

5 31.5689 0 0 0 0 0 0 15 32.3049 8.5776 6.2480 0 73.4477 80.65903 0 25 29.6965 9.0283 7.6780 4.9142 69.5981 74.14497 83.4519 35 24.241 7.4505 7.2278 4.7206 69.2645 70.18357 80.5263

45 21.358 7.05236 6.1686 4.3449 66.9802 71.11771 79.6568

TABLE 6.7(d)-REDUCTION IN STRENGTH FOR MURANGA CLAY AND ALUMINA MIXTURE

AVERAGE STRENGTH (MPa) % REDUCTION IN STRENGTH

% ALUMINA

UNSHOCKED

SHOCKED AT 400°C

SHOCKED AT 600°C

SHOCKED AT 800°C

SHOCKED AT 400°C

SHOCKED AT 600°C

SHOCKED AT 800°C

5 34.1121 9.0836 8.8566 7.7935 73.3713 74.0367 77.1531

15 37.5252 8.8875 8.6380 8.7905 76.3151 76.9806 76.5744

35 33.703 8.8006 8.5507 8.4631 73.8877 74.6290 74.8891

45 23.911 8.7558 8.5466 8.2774 63.3814 64.2565 65.3824

55 20.2863 8.7044 8.373 8.0283 57.0914 58.7258 60.4250

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6.8 TABLES OF WEIBULL MODULUS Values of the Weibull modulus were obtained from graphs of ln{ln(1/[1-pf])} against Ln(σ/σf) in Appendix B and tabulated in the tables 6.8 (a) through 6.8 (d)

TABLE 6.8 (a)- VALUES OF THE WEIBULL MODULUS FOR MURANGA CLAY AND SAND MIXTURE

WEIBULL MODULUS % SAND SHOCKED ONCE AT SHOCKED 15 TIMES AT SHOCKED 20 TIMES AT UNSHOCKD 400°C 600°C 800°C 400°C 600°C 800°C 400°C 600°C 800°C 0 -1.5371 22.65 -4.333 9.9691

5 -1.7057 -3.579 -8.1714 0.5027 -0.399 -1.150 -1.8001 15 16.991 8.194 -3.5852 -11.97 -5.553 -3.358 -2.309 1.097 -2.428 25 -0.5307 -33.93 -11.124 4.8515 -3.988 1.651 3.382 -0.97 -1.780 -3.12 35 -3.2713 -10.27 5.8089 8.5011 -1.268 -3.358 -2.246 2.036 4.164 0.621 40 -2.9706 10.92 0.5591 22.77 0.332 -0.013 1.904 -1.49 0.420 -2.458

TABLE 6.8 (b)- VALUES OF THE WEIBULL MODULUS FOR MURANGA CLAY AND ALUMINA MIXTURE

WEIBULL MODULUS

SHOCKED ONCE AT SHOCKED 15 TIMES AT SHOCKED 20 TIMES AT

% ALUMINA UNSHOCKED 400°C 600°C 800°C 400°C 600°C 800°C 400°C 600°C 800°C

0 -1.537 -22.652 -4.333 9.969

5 -9.702 -63.3 7.755 -2.622 -2.871 -0.891 0.500 2.228 2.178 0.835

15 4.897 -26.344 -1.5648 13.303 -0.585 -1.638 0.571 1.794 1.096 -0.803

35 -5.651 6.0506 4.36 4.369 -0.915 -0.607 -0.018 2.897 -0.144 -2.707

45 -3.147 -10.859 -5.624 -0.964 -3.728 0.750 5.852 -1.983 2.691 -0.489

55 -20.547 -5.045 -3.109 0.353 3.157 -3.033 3.244 2.324 1.290 -10.528

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TABLE 6.8 (c)-VALUES OF THE WEIBULL MODULUS FOR NYERI CLAY AND SAND MIXTURE

WEIBULL MODULUS

% SAND SHOCKED ONCE AT SHOCKED 15 TIMES AT SHOCKED 20 TIMES AT

% RATIOS UNSHOCKED 400°C 600°C 800°C 400°C 600°C 800°C 400°C 600°C 800°C

0 -0.437 -1.912 -2.724 -2.623 -1.973 -2.658 -1.167 1.486 -6.403 3.392

15 -3.753 5.744 -0.182 0.329 -2.083 -1.271 -2.287 -3.076 2.483 -1.2

25 -1.73 0.287 3.008 -0.013 -3.437 -0.711 0.267 -2.042 2.278 1.775

35 6.268 1.424 -0.644 0.013 -1.484 -2.259 6.138 -1.332 -6.035 0.723

45 8.828 2.473 3.634 -1.941 2.5387 -2.494 1.872 -2.617 1.037 1.3

55 24.47 1.909 1.187 2.186 -1.387 3.092 -2.783 -1.736 0.515 -0.425

75 -4.327 -1.7 -3.608 -1.072 3.534 -4.196 2.216 -2.311 2.189 -6.058

TABLE 6.8 (d)-VALUES OF THE WEIBULL MODULUS FOR NYERI CLAY AND ALUMINA MIXTURE

WEIBULL MODULUS

% ALUMINA SHOCKED ONCE AT SHOCKED 15 TIMES AT SHOCKED 20 TIMES AT

UNSHOCKED 400°C 600°C 800°C 400°C 600°C 800°C 400°C 600°C 800°C

0 -0.437 -1.912 -2.724 -2.623 -1.973 -2.658 -1.167 1.486 -6.403 3.392

10 7.405 1.08 -3.475 0.144 -5.259 2.014 2.668 -1.861 1.405 -1.2

30 -0.385 -9.681 6.364 1.061 -3.190 -1.842 1.703 -1.684 2.421 1.775

35 -1.976 3.576 0.832 -3.723 -1.401 -2.690 0.01 1.858 -1.593 0.723

45 9.125 -3.435 -0.985 2.395 -1.958 -0.912 -1.531 -1.669 -0.095 1.3

65 0.405 -3.475 -1.222 -1.076 2.633 0.789 -1.664 2.627 -0.571 -0.425

80 -0.145 0.741 -0.424 0.188 1.941 -0.130 -2.417 -0.228 -0.929 -6.058

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6.9 QUANTITY OF MAJOR COMPONENTS OF MATERIALS USED IN PREPARATION OF SPECIMEN

Full chemical analysis was carried out for Muranga clay, Nyeri clay and sand where the actual composition was found as shown in the appendix.

The composition of the major components were obtained for each group as follows

Sample calculation: Quantity of silica in mixture Group: mixture of 35% sand-65% Nyeri clay

0.35*59.6=20.86 %

0.65*52.08=33.852 %

Quantity of silica: 20.86+33.852=54.712%

The same calculation was done for the other mixtures and the results of the actual percentages of silica and alumina are a shown in tables 6.9 (a) t0 6.9 (d)

TABLE 6.9 (a)-NYERI CLAY AND SAND MIXTURE

NYERI CLAY

QUANTITY IN PERCENTAGE

% SAND Si02 Al2O3 LiO

0 52.08 27.2 15.22 15 53.208 26.4665 13.3405 25 53.96 25.9775 12.0875 35 54.712 25.4885 10.8345 45 55.464 24.9995 9.5815 55 56.216 24.5105 8.3285 75 57.712 23.5325 5.8225

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TABLE 6.9 (b)-NYERI CLAY AND ALUMINA MIXTURE

QUANTITY IN PERCENTAGE % ALUMINA Si02 Al2O3

10 50.732 29.11 30 48.086 32.93 35 47.362 33.885 45 46.014 35.795 65 43.318 39.615 80 41.296 42.48

TABLE 6.9 (c)-MURANGA CLAY AND SAND MIXTURE

MURANGA CLAY QUANTITY IN PERCENTAGE % SAND Si02 Al2O3 LiO Fe2O3 TiO2

0 52.18 18.79 13.16 9.6 4.92 5 52.456 18.966 12.7035 9.2545 4.704

15 53.208 19.318 11.7905 8.5635 4.272 25 53.96 19.67 10.8775 7.8725 3.84 35 54.712 20.022 9.9645 7.1815 3.408 40 55.088 20.198 9.508 6.836 3.192

TABLE 6.9 (d)-MURANGA CLAY AND SAND MIXTURE

QUANTITY IN PERCENTAGE % ALUMINA Si02 Al2O3

5 51.886 19.8405 15 51.298 21.9415 35 50.122 26.1435 45 49.534 28.2445 55 48.946 30.3455

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CHAPTER 7: OBSERVATIONS AND DISCUSSION OF RESULTS 7.1 OBSERVATIONS Table 7.1: observations

ACTIVITY OBSERVATIONS

COLLECTION OF MATERIALS

o Muranga clay was more sticky compared to Nyeri clay, Nyeri clay was whitish grey while Muranga clay was dark grey

o Nyeri clay had a large quantity of impurities compared to Muranga. These impurities included grass roots, sand particles

o The sand had almost fine particles and small quantity of impurities

PREPARATION OF SPECIMEN

o In the molding process, Muranga clay was highly sticky and most difficult to mould by hand. However, the stickiness reduced as the proportion of sand increased.

o While the Nyeri clay was moderately sticky and became less sticky with increasing sand proportion.

o The mixture of Muranga and Nyeri clays was relatively easy to mould compared to pure Muranga clay.

o There was no significant variation of stickiness with the addition of alumina to both Muranga clay and Nyeri clay.

EXTRUSION o During extrusion process, Nyeri clay mixture was easy to extrude compared to Muranga clay mixture which had to be pressed and the exertion force maintained to ensure constant extrusion process.

o The extrude obtained from Muranga clay had a smooth surface with minimum imperfections. This smoothness, however, decreased with the increase in sand content in the mixture.

o Nyeri clay extrudes had less smooth surface compared to Muranga clay mixture. The surface roughness became more pronounced with increasing sand content.

o Addition of alumina had no significant effects on the surface texture.

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DRYING o During drying process, specimens obtained from Muranga clay had a high tendency to bending, thus it was checked regularly while turning it over and over until it was dry.

FIRING AND SHOCKING PROCESS

o Muranga clay specimens turned its dark grey color to reddish brown for Muranga clay sand mixture to dark brown color for pure Muranga clay. The alumina Muranga clay mixture was noticed to turn grey with the content of alumina.

o There was a notable longitudinal fracture in some of pure Muranga clay specimens after firing to 1000°C

o The rest of the Muranga clay specimens with sand and alumina ratios were stable upto 1000°C firing temperature.

o There was a significant shrinkage noticed in pure Muranga clay, low alumina ratio and low sand ratio specimens after firing

o The whitish Nyeri clay, Nyeri clay sand mixture and Nyeri clay alumina mixture turned more whitish grey after firing to 1000°C. However, as compared to Muranga clay, there was no noticeable reduction in volume and also all the specimens were stable at maximum firing temperature.

o Muranga- Nyeri clay specimens were stable to maximum firing temperature. A slight shrinkage was noticed in high Muranga clay content.

o The strength of the specimens was greatly increased during firing and were more compacted.

SHOCKING PROCESS

o Pure Muranga clay did not survive shocking in water for upto 15 shocks with most specimens failing before 9 shocks.

o Muranga clay with 5% sand did not survive upto 20 shocks at 600°C and 800°C

o Most of the specimens failed by cracking longitudinally. o Nyeri clay specimens (pure Nyeri clay, alumina Nyeri clay

mixture and Nyeri clay sand mixture) exhibited a good thermal shock resistance and no single specimen failed during thermal shocking however its strength were greatly decreased.

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7.2 DISCUSSION 7.2.1 THERMAL SHOCK 7.2.1.1 NYERI CLAY Mixture of Nyeri clay and alumina

Unshocked Nyeri clay had the highest modulus of rupture (MOR) hence the highest flexural strength. It is observed that strength decreases with increase in shocking temperature {fig 6.4(a)}. The proportion of the mixture with the highest bend strength was that containing 40% alumina.

Mixture of Nyeri clay and sand The lowest strength was recorded with the sand made up 30% of the mixture {fig 6.2(a)}. The average MOR in this case did not deviate much from that of plain Nyeri specimen. As percentage of sand was increased from zero, the average strength rises with addition of sand significantly up to a maximum when the proportion of sand made up 32% of the mixture.

As the proportion of sand approached 32%, the variation of strength for shocking temperature of 600°C reduced. The reduction was more obvious for shocking temperature of 800 °C {graph 6.2(a), after 32 % the percentage drop in bend strength was sudden for shocking at 400°C. it can be said with certainty that as shocking temperature increased, proportion of sand within the range 20%-40% produced a fairly constant value of bend strength.

This property can be utilized by potters to make clay cooking stoves without the need for accurate weighing scales. ¼ to 2/5 proportion sand will produce nearly the same thermal shock resistance for the same conditions

7.2.1.2 MURANGA CLAY Mixture of Muranga clay and sand

Unshocked Muranga clay had the highest strength compared to unshocked specimen. The maximum bend strength is 32.3 MPa at 15% of sand {fig 6.1(a)}. The reason for this is that the amount of sand was optimum. During firing the sand enhances vitrification and formed a glasslike matrix which helped cancel the effect of varied stresses in the specimen.

For shocking at 400°C, the maximum bend stress occurs when the percentage of silica was 5%. For shocking at 800 °C, the average bend strength decreases with the increase content of sand. Also worth noting is that the percentages drop in strength with increase in silica content decreased greatly at this temperature. Addition of sand does not affect average strength after shocking that much.

Mixture of Muranga clay and alumina Maximum strength of unshocked specimen occurs at 15% alumina {fig 6.3(a)}; here the maximum bend strength is 38MPa. The maximum bend stress lay at this proportion of mixture for shocking at 400°C 600°C, 800°C. the average strength reduced with increase in shocking

74

temperature. This is due to the higher thermal stresses set up in the specimen with increase in shocking temperature. Clay stoves operate at temperature is below 800oc

Hence proportion containing 15% alumina provides the best thermal shock property for this mixture.

Comparison of the alumina-clay, silica-clay mixture Muranga-Alumina clay mixture has maximum bend strength of 38 MPa. While Muranga-silica clay mixture has a maximum bend strength of 32 MPa. The mixture containing Alumina gives a better result. From tables 6.9(a-d), it is observed that alumina contains 46.3 % sand and 38.3 % sand. When mixed with clay and fired, the silica and alumina in the contents react to form Mullite, which gives the specimen better thermal shock property. In addition alumina was in powder form; hence the smaller particles allowed proper mixing, less porosity, hence less crack initiation sites and more strength.

7.2.2 THERMAL FATIGUE Mixture of Muranga clay with sand

I. Shocking at 400 o C {fig 6.1(b)}-Data for shocking of pure Muranga clay for 15 times was not available because most of the specimen started failing after 4 shocks and after 9 shocks all of them had failed. This is due its low porosity {22} hence very high thermal stresses were induced during shocking. The outside layer suddenly contracts due to exposure to extreme temperature drop while the inside section remains hot

Addition of 5% improved the thermal shock of the mixture and bend strength of 10.4 MPa was achieved after 15 shocking cycles.

Strength increased upto a maximum of 13.2 MPa at 33% silica. Whereby further r increment of the sand proportion resulted in a sudden fall in strength.

On shocking for 20 times, Muranga clay containing 5% and 15% sand did not survive 20 shocks. Longitudinal cracks were observable on the surface after 16 shocks and by 19 shocking cycles all specimens belonging to this group had failed completely or had large longitudinal cracks, consequently, these specimens were not tested. Highest strength was recorded at 9MPa for 24% silica after 20 shocks

II. Shocking at 600°C {fig 6.1(c)}-pure Muranga clay failed before 15 shocks, as in the case of shocking at 400°C, clay mixture with 5%, 15% sand failed before 20 shocks.

Strength after shocking increased with silica content up to 24% whereby a general decrease is observed in the graph.

III. Shocking at 800 °C {fig 6.1(b)}-as in shocking at 600°C pure Muranga clay failed before 15 shocks.5%, 15% silica proportion of the mixture failed before 20 shocks.

75

The specimen failed during repeated shocking by splitting into approximately two halves through the longitudinal section. Since Muranga clay had relatively less surface flaws, failure can be attributed to thermal stress build up at the outer layers of the specimen.

The surface layer contracted quickly while the inner layers remain in their initial condition because of low thermal conductivity of the material. This leads to development of cracks. The contraction of the surface layer was constrained by the hot inner layer inducing bi-axial state of compressive stress in the material.

Therefore, under the applied stress on intrinsic crack in the material, the cracks start to propagate when the stress concentrated at the crack tip exceeds the critical stress, as the length of the crack increases. The stress concentrated at the crack tip also increases, so that the crack can continue to propagate under a progressively reduced applied stress.

Crack propagation starts to occur when the decrease in stored energy associated with its extension equals the increase in surface energy associated with the new surface. The crack is spontaneously propagated without limit resulting in complete failure, any excess energy being transformed into kinetic energy of the moving crack.

Mixture of Muranga clay and alumina

When shocking 15 times and 20 times for the three temperatures, it was observed that increase in the proportion of alumina did not affect the strength after shocking. Shocking for 15 times depicted higher temperature than shocking 20 times. This is expected because the amount of damage on the material would be more after 20 shocks. The highest strength after shocking is observed at 15% alumina content at 9.6 MPa {fig 6.3(d)}. This observation supports the result obtained by previous researchers at 20%alumina

Generally, Muranga clay had poor thermal fatigue property. It is worth noting that specimens failed by splitting longitudinally. For this clay the deterioration in mechanical properties is not adequately depicted by the three point bend test as observed in the graphs. During the experiment, it was noted that specimens with large longitudinal cracks still posted high failure load. This means that the load carrying capacity in the transverse direction was not affected by the thermal stresses.

Mixture of Nyeri clay and alumina The average bend strength after 15 shocks increases with increase in % alumina up to 42% alumina where strength after shocking decreases with increase in alumina content. Maximum bend strength when shocked 15 times is 10 MPa at 400°C {fig 6.4(b)}. For shocking 20 times, the same trend is observed but the average strengths are lower. When shocked at 600 °C and 800 °C, average strength increases with % alumina up to exactly 42% alumina, where the

76

average strength after shocking is highest. Also observed is that there is no much change of average strength from 15 cycles to 20 cycles.

This confirms the result of previous researchers and a proportion of 42% alumina can be reliably recommended for addition to Nyeri clay for the purpose of making cooking stoves.

Mixture of Nyeri clay and sand mixture Strength increased with increase in amount of sand up to a maximum at 26 % sand for 15 shocks where it started to decrease with addition of sand. This trend was also observed for shocking 20 times for the three temperature ranges. There was a slight reduction in average strength after 20 shocks {fig 6.2(b-d)}. The average strength increased with shocking temperature. This is due to increased compressive stresses due to a higher temperature gradient.

7.3 COMPARISON BETWEEN MURANGA CLAY AND NYERI CLAY As observed in fig 6.5(a), the average modulus of rupture for pure Nyeri clay is 8.1 MPa. As Muranga clay was added, the average bend strength increases in a nearly sigmoid trend as shown in the graph. Pure Muranga clay had the highest flexural strength.

Nzioki and Mogusu found that Nyeri clay has got lower fracture toughness than Muranga clay. Here the intention of this test was to see the relationship between Nyeri and Muranga clay mixture and to see how their strengths vary as they are mixed to various proportions. It can be observed from the graph that a 3:1 ratio Muranga to Nyeri clay predicts a good thermal resistance compared to 1:1 ratio. It also has a better flexural strength.

7.4 COMPARISON OF OUR RESULTS WITH PREVIOUS RESEARCHERS The authors compared Watare and Wanjohi results with theirs; this group was chosen since their result depicted a similar trend with those of Cherono and Mosiria.

Watare and Wanjohi shocked their specimen 5 times and 10 times at temperatures of 400, 600, and 800oc.

We extended their experiment by shocking specimen made from mixtures of Muranga clay, Nyeri clay, sand and alumina at proportions that has never been tried before. This improved the availability of data for the clay under study,

We also extended their experiment of thermal fatigue by shocking our specimen15 and 20 times

Graphs of average modulus of rupture were plotted against the percentage of sand and alumina. Later similar graphs were plotted again but this time including data from Watare and Wanjohi in

77

order to show variation of thermal strength with increase in the number of shocks and the variation of composition

Combined results for Muranga and sand mixture These are plots of average strength against % composition for 5, 10, 15 and 20 thermal shocks.

It was observed from fig 6.6(m-o), that generally the average strength reduced with as the number of shocks was increased.

Increase in percentage of sand increased the survivability of the specimen especially at 15 and 20 shocks. Sand introduced small pores into the material.

It is also observed that increase in shocking temperature generally reduced the average strength.

At high values of 800°C variation of the composition of sand did not change the average MOR significantly.

Combined results for Muranga and alumina mixture General observation for variation of modulus of rapture with variation of alumina content is similar to the case of silica.

Between at the proportions of 20% to 50% alumina, the combined graphs for thermal shocking for 5, 10, 15 and 20 times, depict that the variation of the alumina proportion within this region does not affect the average flexural strength {fig 6.6(j-l)}.

This is true for shocking at 400 °C, 600°C and 800 °C

40% is recommended to be added to Muranga clay. It has the highest strength after thermal shocking and a higher strength compared to that achieved by addition of silica

Combined results for Nyeri clay and sand mixture Generally average strength after shocking decreased with increase in the number of shocks the specimen was subjected to. From fig 6.6(g-i), it was observed that the drop in average strength after shocking was much lower compared to Muranga clay. This confirms the findings by previous researchers that Nyeri clay has the best thermal shock resistance relative to Muranga clay.

However the flexural strength was about 44% lower after shocking compared to Muranga clay. It was also observed that the addition of 25% sand provides the best average strength after shocking for Nyeri clay. To improve the flexural strength of Nyeri clay, we considered addition of Muranga clay. This has been dealt with in the previous section.

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Combined results for Nyeri clay and alumina mixture This mixture depicted a general trend similar to that of Nyeri clay and sand mixture, variation in average flexural strength after shocking decreases with increase in the number of shocks.

Almost similar stresses were achieved compared to Nyeri clay silica mixture, but the stresses were slightly higher.

7.5 COMPARISON OF MIXTURES TESTED WITH THOSE OF PREVIOUS RESEARCHERS Data from the last two researchers (Cherono and Mosiria and Watare and Wanjohi) were tabulated and plotted in one graph.

The data obtained in this project were not compared to Nzioki and Mogusu and Suresh and Songok since the composition and method of preparation of their specimen were different from that used in this project.

The data plotted represent the most proportions researched and the combined graphs provide sufficient plot of the characteristics of each clay sample {see figs 6.6(a-f)}. The following observations were made for the different categories.

I. Muranga clay and alumina mixture;

Maximum strength at 15% alumina thereafter strength starts to decrease with increase in the %alumina. This is because alumina has a lower Young’s modulus; addition of alumina lowers the Modulus of rupture for the whole mixture.

The composition with 15% Alumina represents the optimum percentage for formation of Mullite

II. Nyeri clay and alumina mixture

Pure Nyeri clay has the lowest strength. Increasing the concentration of alumina increases the average MOR up to a 35% alumina content (MOR=18MPa).

Further increase of alumina content decreases the average MOR steadily

III. Muranga clay and sand mixture

From the graphs unshocked Muranga clay has the highest strength. For the case of shocking at 400°C, strength generally decreases with increase in the percentage sand.

This observations hold for shocking at 600°C and 800°C

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IV. Nyeri clay and sand mixture

Strength increases with increases with the percentage of sand upto a maximum up to a maximum at 25% proportion then decreases steadily with increase with the proportion of sand. The above trend is similar to that for shocking once at 600°C and 800°C.

It can also be observed that the average flexural strength for Muranga clay is twice that of Nyeri clay.

7.6 WEIBULL MODULUS This is a dimensionless parameter of the Weibull distribution which describes the variation of average strength for brittle materials like ceramics. If measurements made on many small samples of the brittle material tested, and they show little variation in mechanical strength from sample to sample, the calculated Weibull modulus will be high, a single strength value will offer a good description of the average strength. If measurements show high variation then the Weibull modulus will be low. This reveals that flaws are clustered inconsistently and measured strength will be generally weal and variable.

Weibull modulus was obtained from the slopes of a graph of ln{ln(1/[1-pf])} against Ln(σ/σf).

Tables in section 6.8, shows values of Weibull modulus for the various groups tested.

For a mixture of Nyeri clay and sand, the values of Weibull modulus are generally low and keep reducing with increase in shocking temperature. This shows that the variation of strength for this mixture was high and the material had inconsistent flaws.

For the case of Nyeri clay and alumina mixture, the Weibull modulus was high for unshocked specimen. The Weibull modulus decreases generally with increase in shocking temperature. This is because shocking introduces internal cracks in the material. This is becomes more severe with increase shocking temperature.

For Muranga clay, it is observed generally a high value of Weibull modulus hence Muranga clay has a low variation in mechanical properties. This sample was found to have a low porosity and less surface flaws.

Muranga clay was observed to start vitrifying at temperatures of 900°C. the development of a glasslike matrix filled internal pores and reduced the crack initiation sites in the specimen.

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CHAPTER 8: CONCLUSIONS 8.1 THERMAL SHOCK

8.1.1 THE AVERAGE BREAKING STRENGTH The average breaking strength of unshocked pure Muranga clay was 56.4% higher than that of unshocked pure Nyeri clay. Addition of sand to Nyeri clay not only improves its thermal shock property but also increases its average breaking strength to a maximum at 32% sand. Addition of sand to Muranga clay increases the average bend strength up to a maximum at 15% sand Addition of alumina to Nyeri clay increased the flexural strength up to a maximum at 40 % alumina. The percentage drop in strength from unshocked condition to shocking at 800°C was 28.20%. When alumina was added to Muranga clay, it was found that 15% alumina achieved the highest breaking stress of 38.3 MPa. It was also noted that this was higher than that achieved by addition of sand which was 32.3 MPa.

8.1.2 VARIATION IN STRENGTH Form the tables of Weibull modulus, it was concluded Muranga clay has the least variation of strength. Nyeri clay had low Weibull modulus values hence highest variation of strength. Nyeri clay was more sensitive to addition of sand and alumina. Muranga clay showed the least variation of strength with addition of sand and alumina.

8.1.3 DEGREE OF DAMAGE Muranga clay was found to be highly sensitive to shocking at 800°C. some specimen started bending as noted in the discussion. Pure Muranga clay started forming cracks along the longitudinal section. Nyeri clay was not damaged after shocking the experiment, thus it was concluded that Nyeri clay was not sensitive to thermal shocking.

8.2 THERMAL FATIGUE Life predictions for the samples tested were done by shocking the specimen 15 times and 20 times in water at room temperature. The following conclusions were made

8.2.1 AVERAGE BREAKING STRENGTH With increase in the number of shocks, the average breaking strength decreases. The average breaking strength was least after 20 shocks for all specimens, the only exception being those which failed before 20 shocks.

8.2.2 REDUCTION IN THE AVERAGE BREAKING STRENGTH When Nyeri clay and alumina mixture was shocked, the highest flexural strength was achieved at a proportion of 40% alumina. The corresponding percentage reduction in

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bend strength was 55% for 15 shocks and 60% for 20 shocks respectively at 800 °C. Nyeri clay mixed with silica depicted a percentage reduction in strength of 60.27% and 61.64 % for 15 shocks and 20 shocks respectively. Muranga clay mixed with sand showed a 81% percentage reduction in strength for shocking 15 times at 800°C, for the above selected proportion (15% sand) data was not available for 20 shocks, all specimen had failed. Percentage reduction in strength reduced with increase in the proportion of sand. This was also true for alumina. Muranga clay with alumina exhibited a percentage reduction in strength of 74.35% for 15 shocks and 76.57% for 20 shocks at 800°C at a proportion of 15% alumina.

8.2.3 DEGREE OF DAMAGE Pure Muranga clay suffered a high degree of damage; in fact the specimens could not survive up to 10 shocks for the temperature conditions used. It was noted that all specimen that failed, did so by cracking along the longitudinal section. However, addition of silica and alumina decreased the severity of damage. Specimen with more 30 % silica and alumina survived up to 20 shocks, but had very low strength compared to unshocked pure Muranga clay. In this case, the difference in strength was 86.6%

8.3 APPLICATION TO INDUSTRY Nyeri clay with 32% silica with the best thermal shock and thermal fatigue resistance was identified to be suitable for use in manufacture of charcoal cooking stoves. Muranga alumina mixture with 15 % alumina is applicable to use in making of furnace linings due to its resistance to mechanical damage.

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CHAPTER 10: CHALLENGES AND RECOMMENDATIONS

10.1 CHALLENGES 1. There was unexpected injury to one of the researchers during setting and mounting of

the extrusion machine which extended the duration needed to complete the specimen preparation.

2. Unfavorable weather conditions prolonged the drying process of clay hence delayed the onset of specimen preparation and testing.

3. The number specimens that were tested were almost overwhelming. 4. Delayed funding to the project. 5. Interference of project specimen during the drying stage.

10.2 RECOMMENDATIONS

1. Further investigation be done on the effect of mixing Muranga clay and Nyeri clay on thermal shock and thermal fatigue.

2. A cooling room should be set up to facilitate uniform drying and avoid interference of specimen.

3. A better approach for investigating thermal shock damage in Muranga clay be considered to obtain its true characteristics.

4. The University should facilitate prompt funding of projects to minimize delays and enable early start.

5. The department to make arrangement to acquire a ball milling machine 6. The researchers should be introduced to the extrusion machine and guided through its

setting, mounting and operation.

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