matselection lecnotes2011

Materials Selection Lecture otes, 2010 Ass. Prof. Dr. Saad B. H. Farid

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MATERIALS SELECTION 2010

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What are the Main Responsibilities of Materials Engineer?

1- Selection of Proper Materials, I.e. study and classify materials properties to be able to decide the proper material to the specific application. The materials properties are mainly physical, mechanical, thermal, electrical and magnetic properties. The materials engineer is also look at the availability and cost,

2- Proper choice (selecting) of substitute (alternative) materials, E.g. looking for the locally available materials substitute,

3- Selection of the proper manufacturing process or processes, Thus, the materials engineer decides The "Technological Root" Technological Root = Selection of materials +Selection of Processes

4- Doing Research Activities to enhance materials performance,

5- Other activates like; a- Contributing and evaluating materials characterization

results, like X-ray diffraction, optical & electron microscopy and IR-spectroscopy,

b- Studying and composing "materials data sheets" before placing an order


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Materials Selection

References: 1- Engineering Materials Technology. B. Bolton 2- Lecture notes by professors of international universities 3- Engineering Materials, Properties and SelectionK. G. Budinski 4- Materials for Engineers and Technicians .R. A. Higgins 5- Handbook of Materials Selection M. Kutz

PPaarrtt II--IInnttrroodduuccttiioonn Classification of Materials:

Basically; they are of four types; Metals, Ceramics, Polymers and Composites.

A. Metals: Elements with a valence of 1, 2 or 3. They are crystalline solids composed of atoms held together by a matrix of electrons. The Electron Gas that surrounds the Lattice of atomic nuclei is responsible for most of the properties.

B. Ceramics: Inorganic, non-metallic crystalline compounds, usually oxides (SiO2, Al2O3, MgO, TiO2, BaO), Carbides (SiC), Nitrides (Si3N4), Borides (TiB2), Silicides (WSi2, MoSi2). Some literature includes glasses in the same category, however; glasses are amorphous (non-crystalline) compounds i.e. they possess short range order of atoms.

C. Polymers: High molecular weight organic substance made up of large number of repeat (monomer) units. Their properties are linked directly to their structure, which is dictated (related) mostly by intermolecular bonds.

D. Composite: A combination of two or more materials to achieve better properties than that of the original materials. These materials are usually composed of a Matrix and one or more of Filler material. Wood is a natural composite of cellulose fibers in a matrix of polymer called lignin. The primary objective of engineering composites is to increase strength to weight ratio. Composite material properties are not necessarily isotropic, i.e., directional properties can be synthesized according to the type of filler materials and the method of fabrication.


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Main types of Engineering Materials (details below)

The main categories of Engineering Materials are Metals and alloy, Ceramics and Glassed, Polymers and Composites.

Other types of Engineering Materials

1. Classes: Some literatures allocate a unique category for it.

2. Biomaterials: (really using previous 5): Including Bone substitution, Wide variety of Dental materials and else.

3. Liquids and Gases: play a major role in thermal, hydraulic and pneumatic systems. Used in heat transfer, materials flow, power/pressure transmission and lubrication. Have low electrical and thermal conductivity.

4. atural materials: e.g. Wood, Leather, Cotton/wool/silk, Bone. Budinski Classification of Materials:

A. Metals:

1. General properties: High electrical conductivity, high thermal conductivity, ductile and relatively high stiffness, toughness and strength. They are ready to machining, casting, forming, stamping and welding. Nevertheless, they are susceptible to corrosion.

Materials


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2. Further description: Engineering metals are generally Alloys. Alloys are metallic materials formed by mixing two or more elements, e.g.

i. Mild steel Fe + C ii. Stainless steel Fe + C + Cr + Mn etc.

C improves Strength Cr improves the corrosion resistance etc.

3. Classification: of metals and alloys: i. Ferrous: Plain carbon steel, Alloy steel, Cast iron,

ii. Nonferrous: Light Alloys (Al, Mg, Ti, Zn), Heavy Alloys (Cu, Pb, Ni), Refractory Metals (Mo, Ta, W), Precious metals (Au, Ag, Pt)

4. Applications: i. Electrical wiring

ii. Structures: buildings, bridges, etc. iii. Automobiles: body, chassis, springs, engine block, etc. iv. Airplanes: engine components, fuselage, landing gear assembly,

etc. v. Trains: rails, engine components, body, wheels

vi. Machine tools: drill bits, hammers, screwdrivers, saw blades, etc.

vii. Magnets viii Catalysts

5. Examples: i. Pure metal elements (Cu, Fe, Zn, Ag, etc.)

ii. Alloys (Cu-Sn=bronze, Cu-Zn=brass, Fe-C=steel, Pb-Sn=solder) iii. Intermetallic compounds (e.g. Ni3Al)

B. Ceramics:

1. General properties: Light weight, Hard, High strength, stronger in compression than tension, tend to be brittle, low electrical conductivity, High temperature resistance and corrosion resistance.

2. Further description: Ceramics also includes ferrites (ZnFe2O4), semiconductors (ZnO, TiO2, CuO, SiC, AlN, BN, C, Si, Ge, SiGe), piezoelectric and ferroelectric ceramic (BaTiO3, PZT=PbZrTiO3) and superconducting ceramics (YBa2Cu3O7).

3. Classification: of ceramics: i. Traditional Ceramics: Includes pottery, china, porcelain

productsetc, these products utilizes natural ceramic ores.


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ii. Technical (Advanced) Ceramics: Alumina, magnesia, Carbides, Nitrides, Borides, Silicides etc, they are synthetic materials, usually of better mechanical properties. Electronic ceramics falls in the same category.

iii. Glass, Glass Ceramic and Vitro Ceramic: Glasses are essentially vitreous (amorphous, non crystalline), Glass ceramics are mostly re-crystallized from glassy medium and, Vitro Ceramics have crystalline microstructure which are partially vitreous at the grain boundaries.

4. Applications: i. Electrical insulators

ii. Abrasives iii. Thermal insulation and coatings iv. Windows, television screens, optical fibers (glass) v. Chemical resistant applications

vi. Electrical devices: capacitors, varistors, transducers, etc. vii. Highways and roads (concrete)

viii. Biocompatible coatings (fusion to bone) ix. Self-lubricating bearings x. Magnetic materials (audio/video tapes, hard disks, etc.)

xi. Optical wave guides (fiber optics) xii. Night-vision (IR detectors)

5. Examples of Technical Ceramics:

i. Barium titanate (often mixed with strontium titanate) displays ferroelectricity, i.e. the mechanical, electrical, and thermal properties are coupled to one another and also history-dependent. It is widely used in electro-mechanical transducers, ceramic capacitors, and data storage elements.

ii. Bismuth strontium calcium copper oxide, a high-temperature superconductor

iii. Boron carbide (B4C), which is used in ceramic plates in some personnel, helicopter and tank armor.

iv. Silicon carbide (SiC), which is used as a susceptor in microwave furnaces, a commonly used abrasive, and as a refractory material.

v. Boron nitride is structurally iso-electronic to carbon and takes on similar physical forms: a graphite-like one used as a lubricant, and a diamond-like one used as an abrasive.


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vi. Ferrite (Fe3O4), which is ferrimagnetic and is used in the magnetic cores of electrical transformers and magnetic core memory.

vii. Lead zirconate titanate (PZT) is a well-known ferroelectric material.

viii. Magnesium diboride (MgB2), which is an unconventional superconductor.

ix. Sialons / Silicon Aluminium Oxynitrides, high strength, high thermal shock / chemical / wear resistance, low density ceramics used in non-ferrous molten metal handling, weld pins and the chemical industry.

x. Steatite (MgSiO3), used as an electrical insulator. xi. Silicon nitride (Si3N4), which is used as an abrasive

powder. xii. Uranium oxide (UO2), used as fuel in nuclear reactors.

xiii. Yttrium barium copper oxide (YBa2Cu3O7-x), another high temperature superconductor.

xiv. Zinc oxide (ZnO), which is a semiconductor, and used in the construction of varistors.

xv. Zirconium dioxide (zirconia), its high oxygen ion conductivity recommends it for use in fuel cells. The metastable cubic structure can impart (show) transformation toughening for mechanical applications; most ceramic knife blades are made of this material.

6. Semiconductors Applications and Examples: i. Computer CPUs

ii. Electrical components (transistors, diodes, etc.) iii. Solid-state lasers iv. Light-emitting diodes (LEDs) v. Flat panel displays

vi. Solar cells vii. Radiation detectors

viii. Microelectromechanical devices (MEMS) ix. Examples: Si, Ge, GaAs, and InSb

C. Polymers:

1. General properties: compared with metals, polymers have lower density, lower stiffness and tend to creep. They have higher thermal expansion and corrosion resistance. Furthermore, polymers have low electrical conductivity and low thermal conductivity. The prime weakness is that polymers do not withstand high temperatures.


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2. Further description: Polymers generally formed via a Polymerization Process, in which the polymer chain builds up from monomers with the aid of heat and/or chemical agents. The C-C bonds form the backbone of the polymer chain; when the chains grow very long, they get tangled (twisted) and loose their lattice order, thus, changing increasingly to the amorphous state. Consequently, polymers are semi-crystalline to some degree of crystallinity that can be measured by X-ray Diffraction.

3. Classification: according to their properties: i. Plastics: (Hard), they can be semi-crystalline or

amorphous (glassy). 1. Thermoplastics: Such as Polyethylene (PE)

and Polymethylmethacrylate (Acrylic and PMMA) are composed of linear polymer chains. They flow under shear when heated. They can be compression- or injection- molded.

2. Thermosets: Such as Polystyrene (PS) and Polyvinylchloride (PVC) are composed of branched polymer chains. They not flow when heated. The monomers are cured in a mold (RIM).

ii. Elastomers: (Soft) Rubbery cross-linked solids that will deform elastically under stress, e.g. natural rubber Thermoplastic elastomers are a special type of elastomer in which the cross-linking becomes reversible upon heating.

iii. Solutions: Viscosity modifiers, polymeric surfactants, lubricants.

4. Applications and Examples: i. Adhesives and glues

ii. Containers, e.g. for acid resistance applications iii. Moldable products (computer casings, telephone

handsets, disposable razors) iv. Clothing and upholstery material (vinyls, polyesters,

nylon) v. Water-resistant coatings (latex)

vi. Biodegradable products (corn-starch packing peanuts) vii. Biomaterials (organic/inorganic interfaces)

viii. Liquid crystals ix. Low-friction materials (Teflon) x. Synthetic oils and greases


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xi. Gaskets and O-rings (rubber) xii. Soaps and surfactants

D. Composites:

1. General properties: Low weight, high stiffness, brittle, low thermal conductivity and high fatigue resistance. Their properties can be tailored according to the component materials.

2. Further description:

3. Classification: i. Particulate composites (small particles embedded in a

different material): e.g. Cermets (Ceramic particle embedded in metal matrix) and Filled polymers.

ii. Laminate composites (golf club shafts, tennis rackets, Shield Glass)

iii. Fiber reinforced composites: e.g. Fiber glass (GFRP) and Carbon-fiber reinforced polymers (CFRP)

4. Applications: i. Sports equipment (golf club shafts, tennis rackets,

bicycle frames) ii. Aerospace materials

iii. Thermal insulation iv. Concrete v. "Smart" materials (sensing and responding)

vi. Brake materials

5. Examples: i. Fiberglass (glass fibers in a polymer)

ii. Space shuttle heat shields (interwoven ceramic fibers) iii. Paints (ceramic particles in latex) iv. Tank armor (ceramic particles in metal)


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Microstructure

The microstructure is a combination of grain boundaries

(grain/grain) and interfaces (pore/grain).

Grain size G and Grain Shape (Multi Phase Analysis)

G=1.56C//; where C is the measured total line length and / is the no. grains crossed that line;

Ref: Wurst J. C. and /elson J. A.; Lineal Intercept Technique for

Measuring Grain Size in Two Phase Polycrystalline Ceramics; J. Amer.

Ceram. Soc.; 55, 109, 1972.

Tip: Microstructure is Materials R&D Area

Microstructure Analysis Microscopic examination is an extremely useful tool in the study

and characterization of materials. Several important applications of microstructural examination are as follows:

1. To ensure that the associations between the properties and structure are properly understood,

2. To predict the properties of materials once these relationships have been established,

3. To design new materials with desired property combinations,

4. To determine whether or not a material has been correctly heat treated,

5. To determine the mode of mechanical fracture...etc.

grains bright space

pores dark space

grain boundaries, g/g

interfaces, pore/grain

Drawing Straight line to measure the grain size G


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Microscopy; E.g. optical microscopy instrument which aids (helps) in

investigations of the microstructural features of all material types. Grain size, shape and pore distribution are among the features of what is can be examined for the microstructure.

Pores; Defined as void spaces; they are distributed more or less frequently

through the material. The pores in a porous system may be interconnected (open pores), or non-interconnected (sealed pores). The voids are created by the escape of gases during the drying and firing. I.e. when bubbles of gas are frozen into the glassy matrix, or when open pores are sealed by melted material. Also, when the pore channels are separated to individual pores during mass transport as part of sintering process.

Example: corrosion of type 316 stainless steel shown on the upper half of the optical micrograph of polished and etched cross section of the sample. Individual and connected pores are shown in the corroded layer.


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anotechnology

What is ano Technology? Nano-technology is an advanced technology, which deals with the

synthesis of nano-particles, processing of the nano materials and their applications. Normally, if the particle sizes are in the 1-100 nm ranges, they are generally called nano-particles or materials. Tip: The sum of the radii of 700 Oxygen atoms is about 100 nm. Thus, quantum mechanics are more important in this field. Hydrogen and Vander wall's forces are more important than gravity. Anyway, the materials show different mechanical, electromagnetic and thermodynamic properties in the nano-range. The phrase "quantum dots" is usually issued to nanostructures. Quantum dots are nano-crystals of arbitrary diameter of about 10 to 105 atoms. Nevertheless, nano-particle is an aggregate of atoms bonded together with a radius between 1 and 100 nm. Why ano Technology?

In the materials world, particularly in ceramics, the trend is always to prepare finer powder for the ultimate processing and better sintering to achieve dense materials with dense fine-grained microstructure of the particulates with better and useful properties for various applications.

Exclusively; 1. The fineness can reach up to a molecular level (1 nm 100 nm), by

special processing techniques. More is the fineness; more is the surface area, which increases the reactivity of the material. Therefore, the densification or consolidation occurs very well at lower temperature than that of conventional ceramic systems, which is finally cost-effective.

2. Furthermore, better sintering improves the properties of materials like abrasion resistance, corrosion resistance, mechanical properties, electrical properties, optical properties, magnetic properties, and a host of other properties for various useful applications in diverse fields.

Some fields of applications, 1. Electronics: Thin Films, Electronic Devices, Electronic Memories 2. Electrical Ceramics: Insulators, Dielectrics, hard magnets 3. Photonics: photo-sensitivity in TiO2 etc. electromagnetic detectors,

solar cells 4. Bionics, Bio-Ceramics and Bio-Technology 5. Pharmacology, Medical Instrumentation, water purification and

other applications


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A short introduction in Ceramic ano-Technology

Ceramic materials are usually prepared by either solid-state reactions, or by precipitation from solution and subsequent decomposition. Solid-state reactions of ceramics involve the mixing of constituent metal oxides, carbonates, etc., and their repeated heating and grinding. To improve physical properties of the final product, there is an increasing demand to control the product particle size, stoichiometry, homogeneity and purity. Thus, alternate routes to the conventional methods of the synthesis of ceramic materials that give superior properties are devised. These new approaches are the soft chemical methods by which the ceramic materials are prepared at considerably low temperatures compared to solid-state reactions [1].

Soft chemical routes are now increasingly becoming important to prepare a variety of oxides including nanocrystalline oxide materials. It makes use of simple chemical reactions like coprecipitation, ion exchange, hydrolysis, acid leaching, and solgel. Sol-gel proves supreme success in the preparation of ceramic materials with control of stoichiometry, final particle size and the crystalline phase [2].

It is essential for the synthesis of ceramic products to prepare finer powder for the ultimate processing and better sintering; with the intention of achieving dense materials of fine-grained microstructure with better and useful properties for various applications. More is the fineness; more is the surface area, which increases the reactivity of the material. Therefore, the densification or consolidation occurs very well at lower temperature than that of conventional micrometer-particulate systems. Moreover, those dense microstructures improve the properties of materials like abrasion resistance, corrosion resistance, mechanical properties, electrical properties, optical properties and magnetic properties. Nano-technology is developed for these reasons. It is an advanced technology, which deals with the synthesis of nano-particles, processing of the nano materials and their applications. Normally, if the particle sizes are in the 1-100 nm ranges, they are generally called nano-particles or materials [3]. References: [1]: Patil K. C., Hegde M. S., Rattan T. and Aruna S. T., "Chemistry of Nanocrystalline Oxide Materials: Combustion Synthesis, Properties and Applications", chp.1, World Scientific Publishing Co., 2008 [2]: Chen Q. and Souter A. M., "Progress on Nanoceramics by Sol Gel Process", Key. Eng. Mat., Vol. 391, pp. 79-95, 2009 [3]: Bandyopadhyay, A. K., "Nano Materials", New Age International Publishers, Chp1, 2008


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Functionally graded materials 1. Functionally graded materials (FGM's) are a new generation of

engineered materials wherein the microstructural details are spatially varied through non-uniform distribution of the reinforcement phase(s), see the next figure.

2. The volume fraction of the reinforcement phase is increased gradually replacing the matrix phases.

3. FGM's is accomplished using reinforcements with different properties, sizes, and shapes.

4. The produced microstructure has varying thermal and mechanical properties continuously or discretely at the macroscopic scale.

5. Many techniques are involved in the fabrication of FGM's. E.g. mechanical mixing or powder deposition then compaction, tape casting and Electrophoretic Deposition.

6. Functionally graded materials are ideal for applications involving severe thermal gradients, e.g. thermal structures in advanced aircraft and aerospace engines to computer circuit boards.

7. This new concept of material's microstructure allows, for the first time, to fully integrate the material and structural considerations into the final design of structural components.

8. The important technological application is tailoring the response of structural components in a variety of applications involving

Phase B particles with phase A matrix

Phase A particles with phase B matrix

Transition zone


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uniform and gradient thermomechanical loading and biomaterials applications.

9. Many variables control the design of functionally graded microstructures. Therefore, Full management of the FGM's requires the development of modeling for their response to combined thermomechanical loads.

10. Other designs of FGM's

Examples of different types of functionally graded microstructures

11. Potential applications of FGM's are both diverse and numerous, e.g., FGM sensors and actuators, FGM metal-ceramic armor, FGM thin-walled rotating blades, FGM photo-detectors and FGM dental implant.

12. Examples of biomaterials application of FGM's

The cross-section of hydroxyl-apatite-glass-titanium functionally graded composite.

Medical application: FGM synthetic Bone which is

stronger at the joint Examples of Biomaterials applications of FGM's


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PPaarrtt IIII-- SSeelleeccttiioonn ooff PPrrooppeerrttiieess

In materials field, it is need to determine the required specification

of a material for a particular task. This is connected also to the correct processing, availability and cost.

Key Questions for Materials Selection:

I. What properties are required? II. What are the processing requirements and their implications for

the choice of material? III. What is the availability of materials? IV. What is the cost?

The following indicate the type of questions that are likely to-be considered in trying to arrive at answers to the above general questions. I. Materials Selection for Properties:

1. What mechanical properties are required? This means consideration of such properties as strength, stiffness, hardness, ductility, toughness, fatigue resistance, wear properties, etc. Coupled with this question is another one: Will the properties be required at low temperatures, about room temperature or high temperatures?

2. What chemical properties are required? This means considering the environment to which the material will be exposed and the possibility of corrosion.

3. What thermal properties are required? This means consideration of such properties as specific heat capacity, linear coefficient of expansion and thermal conductivity.

4. What electrical properties are required? For example, does the material need to be a good conductor of electricity or perhaps an insulator?

5. What magnetic properties are required? Does the material need to have soft or hard magnetic properties or perhaps be essentially non-magnetic?

6. What dimensional conditions are required? For example, does the material need to be capable of a good surface finish, have dimensional stability, be flat, have a particular size, etc.


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II. Materials Selection for Processing Parameters: 1. Are there any special processing requirements which will limit the

choice of material? For example, does the material have to be cast or perhaps extruded?

2. Are there any material treatment requirements? For example, does the material have to be annealed or perhaps solution hardened?

3. Are there any special tooling requirements? For example, does the hardness required of a material mean special cutting tools are required?

III. Materials Selection for Availability: 1. Is the material readily available?

Is it, for example, already in store, or perhaps quickly obtainable from normal suppliers?

2. Are there any ordering problems for that material? Is the material only available from special suppliers? Is there a minimum order quantity?

3. What form is the material usually supplied in? For example, is the material usually supplied in bars or perhaps sheet? This can affect the processes that can be used.

IV. Materials Selection for Cost:

1. What is the cost of the raw material? Could a cheaper material be used?

2. What quantity is required? What quantity of product is to be produced per week, per month, per year? What stocking policy should be adopted for the material?

3. What are the cost implications of the process requirements? Does the process require high initial expenses (costs)? Are the running costs high or low? Will expensive skilled labor be required?

4. What are the cost penalties for over specification? If the material is, for example stronger than is required, will this significantly increase the cost? If the product is manufactured to higher quality than is required, what will be the cost implications?


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Basic modes of materials Failure: Failure can be as a result of fracture for all materials, fatigue and creep failures occur for ductile materials.

Stress and Strain Figure 1 shows a material strip when an

applied mechanical force is applied. The applied force F over its cross-sectional area A, The applied force per unit area is F/A. The stress in units of Pascal is:

Stress [N/m2 or Pa] = force/area [1]

The area used in calculations of stress is generally the original area that existed before the application of the forces.

Strain= change in length/original length [2]

The behavior of materials subject to tensile and compressive forces can be described in terms of their stress-strain behavior. If gradually increasing tensile forces are applied to, say, a strip of mild steel, then initially when the forces are released the material springs back to its original shape. The material is said to be elastic.

Figure 2 shows the type of stress-strain graph which would be given by a sample of mild steel.

The term tensile strength is used for the maximum value of the stress that the material can withstand without breaking, the compressive strength being the maximum compressive stress the material can withstand without becoming crushed.

The point at which the strain increases without any increase in load is called the yield point. When the material did not show a noticeable yield point (e.g. rubber) the proof stress is defined as shown as shown in figure 3. The 0.2% proof stress is obtained by drawing a line parallel to the straight line part of the graph but starting at a strain of 02%.

Fig 1: Tension, Compression and Bending Stress

a

b

lower surface compressed

c upper surface stretched

Fig 2: stress strain graph for mild steel

Limit of proportionality

Sample breaks

Tensile strength

Yield point

Strain

Str

ess

0

Fig 3: 0.2% proof stress

Strain

Str

ess

0 0.2%

0.2%

proof

stress


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The point where this line cuts the stress-strain graph is termed the 0.2% yield stress. A similar line can be drawn for the 0.1% proof stress.

Modulus of elasticity (Young's modulus)

Modulus of elasticity E= stress/strain [3]

The units of the modulus are the same as those of stress, since strain has no units. With 1 GPa =109 Pa, typical values are about 200 GPa for steels and 70 GPa for aluminium alloys. For most engineering materials, the modulus of elasticity is the same in tension as in compression. Figure 4 shows the stress-strain graphs for a number of typical materials.

Figure 4: The stress-strain graphs for a number of typical materials; a: cast iron; b: glass; c: mild steel; d: polyethylene; e: rubber.


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a: Cast Iron: virtually all elastic behavior and small amount of plastic deformation. The limit of proportionality as about 280 MPa, the tensile strength about 300 MPa and the modulus of elasticity about 200 GPa.

b: Glass: The stress-grain graph for glass has a similar shape to that for cast iron, with virtually all elastic behavior and little plastic deformation. The limit of proportionality is about 250 MPa, the tensile strength about 260 MPa and the modulus of elasticity about 70 GPa.

c: Mild steel: The stress-strain graph for mild steel shows a straight-line portion followed by a considerable amount of plastic deformation. Much higher strains are possible compared with cast iron or glass. The limit or proportionality is about 240 MPa, the tensile strength about 400 MPa and the modulus of elasticity about 200 GPa.

d: Polyethylene: The stress-strain graph for polyethylene shows only a small region where elastic behavior occurs and a very large amount of plastic deformation possible. The limit of proportionality is about 8 MPa, the tensile strength about 11 MPa and the modulus of elasticity about 0.1 GPa.

e: Rubber: The tensile strength is about 25 MPa. Very large strains are possible and the material shows an elastic behavior to very high strains. The modulus of elasticity is not so useful a quantity for elastomers as it refers to only a very small portion of the stress-strain graph. A typical modulus would be about 30 MPa.

The percentage of elongation of a test piece after breaking is used as a measure of ductility:

Percentage elongation = final length-initial length 100% initial length

A reasonably ductile material, such as mild steel, will have a percentage elongation of about 20%, a brittle material such as a cast iron less than 1%. Thermoplastics tend to have percentage elongations of the order of 50 to 500%, thermosets of the order of 0.1 to 1%. Thermosets are brittle materials, thermoplastics generally not.

The stress-strain properties of plastics depend on the rate at which the strain is applied, unlike metals where the strain rate is not usually a significant factor. In addition, the stress-strain properties change significantly when there is a change in temperature. The modulus of elasticity and the tensile strength decreases with an increase in temperature.


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Selection for Static Strength

1. Static strength: is the ability to resist a short-term steady load at moderate temperatures without excessive deformations.

2. If a component is subject to a uni-axial stress, the yield stress (compressive or tensile) is commonly taken as a measure of the strength if the material is ductile and the tensile strength if it is brittle.

3. The design of the component also determines the final strength. Thus for bending, an I-beam is more efficient than a rectangular cross-section beam because the material in the beam is concentrated at the top and bottom surfaces where the stresses are at the highest values.

4. For most ductile-wrought metals, the mechanical properties in compression are sufficiently close to those in tension. Metals in the cast condition may be stronger in compression than in tension. Brittle materials, such as ceramics, are generally stronger in compression than in tension.

5. There are some materials where there is significant anisotropy, i.e. the properties depend on the direction in which it is measured. As an example are the wrought materials; where the elongated inclusions are orientated in the same direction. Other example is the composite materials which contains unidirectional fibers.

6. The mechanical properties of metals are very much affected by the treatment they undergo, like the heat treatment and working. Thus only a crude comparison can be given for alloys in terms of tensile strengths.

7. The properties of polymeric materials are very much affected by the additives mixed in with, thus, only a crude comparison of mechanical properties of different polymers is possible.

8. Special properties with thermoplastics in that, even at 20C they show significant creep; which increases as the temperature increases. Thus; their strengths are very much time dependent. Unreinforced thermoplastics have low strengths when compared with most metals; however, their low density means they have a favorable strength to weight ratio.

9. Ductile Fracture: In the case if a ductile material, there is a considerable amount of plastic deformation before failure occurs in the necked region as a result of excessive yielding. The fracture shows a typical cone and cup formation as a result of crack propagation.


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10. Brittle fracture: is the main mode of failure for glass and ceramics. With a brittle fracture the material fractures before plastic deformation has occurred.

Example; Table 1: Steel Selection

Tensile strength MPa HS steel code Description of steel 620-770 080M40

150M36 503M40

Medium carbon steel. hardened and tempered

Carbon - Mn steel, hardened and tempered 1% Ni steel, hardened and tempered

700-850 150M36 708M40 605M36

1.5% manganese steel. hardened and tempered

1% Cr-Mo steel, hardened and tempered 1.5% Mn-Mo steel, hardened and tempered

770-930 708M40 817M40

1% Cr-Mo steel, hardened and tempered

1.5% Ni-Cr-Mo steel, hardened and tempered 850-1000 630M40

709M40 817M40

1% Cr steel, hardened and tempered

1% Cr-Mo steel, hardened and tempered 1.5% Ni-Cr-Mo steel, hardened and tempered

930-1080 709M40 817M40 826M31

1.5% Cr-Mo steel. hardened and tempered 1.5% ;i-Cr-Mo steel, hardened and tempered

2.5% Ni-Cr-Mo steel, hardened and tempered 1000-1150 817M40

826M31 1% Ni-Cr-Mo steel, hardened and tempered 2.5% Ni-Cr-Mo steel, hardened and tempered

1080-1240 826M31 826M40

2.5% Ni-Cr-Mo steel. hardened and tempered 2.5% Ni-Cr-Mo steel, hardened and tempered

1150-1300 826 M40 2.5% Ni-Cr-Mo steel, hardened and tempered 1240-1400 826 M40 2.5% ;i-Cr-Mo steel, hardened and tempered

>1540 835M30 4% ;i-Cr-Mo steel, hardened and tempered

Stiffness

The stiffness of a material is the ability of a material to resist deflection (bending) when loaded.. When a strip of material is bent, one surface is stretched and the opposite face is compressed, as was illustrated in Figure 1. Thus, a stiff material is that give a small change in length when subject to tensile or compressive forces.

If a cantilever of length L is subjected to a point load F at its free end (Figure 5), then the deflection y at the free end is given by:

equation Cantilever3

3

EI

FLy = ,

with E is the tensile modulus and I the second moment of area of the beam cross-section with respect to the neutral axis.

Fig 5: Cantilever

L

F


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Selection for Stiffness

1. Thus, for a given shape and length of cantilever, the greater the tensile modulus results in the smaller the deflection. Similar relationships exist for other forms of beam. Hence we can state that the greater the tensile modulus the greater the stiffness.

2. The deflection of a beam is a function of both E and I. Therefore, for a given material, a beam can be made stiffer by increasing its second moment of area I.

Why a tube is more efficient than a solid rod?

3. The buckling of columns (when subject to compressive loads) is also related to the value of EI. The buckling for a column of length L takes place when the load F reaches the value;

4. equation sEuler'22

L

EIF

= ,

The bigger the value of EI the higher the load required to cause buckling.

5. Therefore, if the column is stiffer, the value of EI is higher. When the column is short and stubby (large x-section area), it is more likely to fail by crushing when the yield stress is exceeded. However, buckling is, more likely to be the failure mode if the column is slender (slimmer).

6. The tensile modulus of a metal is little affected by changes in its composition or heat treatment. However, the tensile modulus of composite materials is very much affected by changes in the orientation of the fillers and its relative amounts.

Table 2 shows typical tensile modulus values for materials at 20oC.

Table 2: Materials according to tensile modulus (GPa)

Tensile modulus Material Tensile modulus Material


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Creep

Creep: is the deformation of a material with the passage of time when the material is subject to a constant stress. When a material is exposed to a stress for a prolonged period of time, an instantaneous elastic strain takes place, followed by plastic deformation to failure.

Table 3: Temperature limitations of Materials

Temperature

limit C

Material

RT- 150

Few thermoplastics are recommended for prolonged use above about 100C. Glass filled nylon can be used up to 150C. The only metal which has limits within this range is lead.

150-400

1- Magnesium and Aluminium alloys can generally used up to 200C 2- Specific Aluminium alloys is used for pistons in engines 200-

250C 3- Wrought aluminium bronzes up to about 300C 4- Some cast aluminium bronzes can be used up to about 400C 5- Plain carbon and manganese-carbon steels are used in this

range

400-600

1- Plain carbon and manganese-carbon steels 400-450C 2- Low-alloy steels are used above 450C Carbon-0.5%Mo steel


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Selection for creep and temperature resistance

1. The creep resistance of a metal can be improved by incorporating (adding) a fine dispersion of particles to impede (resist) the movement of dislocations.

2. The Nimonic series of alloys, based on an 80/20 nickel-chromium alloy, have good creep resistance as a consequence of fine precipitates formed by the inclusion of small amounts of titanium, aluminium, carbon or other elements.

3. Creep increases as the temperature increases and is thus a major factor in determining the temperature at which materials can be used.

4. Another factor is due to the effect on the material of the surrounding atmosphere. This can result in surface attack and scaling (corrosion) which gradually reduces the cross-sectional area of the component and so its ability to carry loads.

5. The combined effects of the temperature increase and corrosion increase the creep rate. The Nimonic series of alloys have good resistance to such attack. Typically they can be used up to temperatures of the order of 900C.

6. For most metals creep is essentially a high-temperature effect.

7. The creep can be significant at room temperatures in the case with plastics. Generally thermosets have higher temperature resistance than thermoplastics.

8. The addition of suitable fillers and fibers can improve the temperature properties of thermoplastics. Table 3 indicates typical temperature limitations for a range of materials.

Fatigue

Fatigue: a failure, caused by the repeated stressing of a component. The situation of repeatedly stressed and unstressed, alternating stresses of compression and tension or the stress fluctuates about some value. The maximum stress is less than the fracture stress determined by a simple tensile test. The fatigue causes at least 80% of the failures in modern engineering components.


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Selection for fatigue resistance

1. Fatigue is the failure of a component when subject to fluctuating (alternating) loads. The cracks tend to start at some discontinuity in the material and grow until failure occurs.

2. The main factors affecting fatigue properties are stress concentrations caused by component design, corrosion, residual stresses, surface finish/treatment, temperature, the microstructure of the alloy and its heat treatment. Only to a limited extent does the choice of material determine the fatigue resistance of a component.

3. In general, for metals the endurance limit at about 107 to 108 cycles and lies between about a third and a half of the static tensile strength. For steels the fatigue limit is typically between 0.4 and 0.5 that of the static strength.

4. Inclusions in the steel, such as sulphur or lead improve machinability; but reduce the fatigue limit.

5. The fatigue limit for aluminium alloys is about 0.3 to 0.4 that of the static strength, for copper alloys about 0.4 to 0.5.

6. Fatigue effects with polymers are complicated by the fact that the alternating loading results in the polymer becoming heated. This causes the elastic modulus to decrease and at high enough frequencies the failure occurs. Thus, fatigue in polymers is very much frequency dependent.

Toughness

1. The materials in many products may contain cracks, sharp corners, or other changes in shape that can readily generate cracks. The materials failure under loads is a result of crack propagation (crack growing and running). A tough material can resists failure as a result of crack propagation.

2. Toughness can be defined as the resistance offered (obtainable) by a material to fracture. A tough material is resistant to crack propagation.

3. Toughness is the work needed to propagate a crack through a material, a tough material requiring more energy than a less tough one.

4. The area under the stress-strain graph up to the breaking point is a measure of the energy required to break unit volume of the material and so for a crack to propagate. A large area is given by a


- 26

material with a large yield stress and high ductility. Such materials can be considered to be tough.

5. An alternative way of considering toughness is the ability of a material to withstand shock loads. A measure of this ability to withstand suddenly applied forces is obtained by Impact tests, such as the Charpy and Izod.

Thus, a measure of toughness is given by two main measurements:

a. The resistance of a material to impact Loading which is measured in the Charpy or Izod tests by the amount of energy needed to fracture a test piece, the higher the energy the more ductile a material is.

b. The resistance of a material to the propagation of an existing crack in a fracture toughness test, this being specified by the plain strain fracture toughness K1C. The lower its value is the less tough the material. Table 4 gives typical values of the plane strain fracture toughness at 20C.

6. In Charpy and Izod tests, a test piece is stroked by an impulsive or shock load (like a hammer impact) and the energy needed to break it is measured. A brittle material will require less energy than a ductile material. The results of such tests are often used as a measure of the brittleness of materials.

Selection for Toughness to be revised

1. Ceramics:

a. Powder technology proves an effective tool for tailor ceramic properties. The toughness of Alumina, for example, is highly increased by including 2-5% stabilized Zirconia.

b. Traditional ceramics, like porcelains, can be made more tough by carefully adjusting their composition, additives and sintering schedule.

2. Alloys:

a. Within a given type of metal alloy there is an inverse relationship between yield stress and toughness, the higher the yield stress the lower the toughness. Thus, if the yield strength of low alloy is increased by metallurgical means (e.g. quenching and tempering of steels), then, the toughness decreases.

b. Steels become less tough with increasing carbon content and larger grain size.


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3. Polymers:

a. The toughness of plastics is improved by incorporating (adding, including) rubber or another tougher polymer, copolymerization, or incorporating tough fibers.

b. For example, styrene-acrylonitrile (SAN) is brittle and far from tough. It can, however, be toughened with the rubber polybutadiene to give the tougher acrylonitrile-butadiene-styrene (ABS).

4. Composites: numerous examples for toughening materials utilizing the root of composites. Examples are GFRP, CFRP, Ceramic particulate reinforced metal and multilayer ceramic reinforced ceramic or metal.

Table 4: Plane strain fracture toughness at 20C (K1C) K1C M; m

-3/2 Material


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5. Because wear is a surface effect, surface treatments and coatings play an important role in improving wear resistance. Lubrication can be considered to be a way of keeping surfaces apart and so reducing wear.

Selection for wear resistance

1. Because wear is a surface effect, surface treatments and coatings play an important role in improving wear resistance. Lubrication can be considered to be a way of keeping surfaces apart and so reducing wear.

2. Mild steels have poor wear resistance. However, increasing the carbon content increases the wear resistance.

3. Surface harden-able carbon or low-alloy steels enable wear resistance to be improved as a result of surface treatments such as carburizing, cyaniding or carbo-nitriding.

4. Even better wear resistance is provided by nitriding medium-carbon chromium or chromium-aluminium steels, or by surface hardening high-carbon high- chromium steels.

5. Grey cast iron has good wear resistance for many applications. Better wear resistance is, however, provided by white irons.

6. Among non-ferrous alloys, beryllium coppers and cobalt-base alloys, such as Stellite, offer particularly good wear resistance.

Selection of bearing materials

1. Metallic materials for use as bearing surfaces need to be hard and wear resistant, with a low coefficient of friction, but at the same time sufficiently tough.

2. Generally these requirements are met by the use of a soft, but tough, alloy in which hard particles are embedded.

3. Bearing materials can be classified, into main, categories: Whitemetals, Copper-base alloys, Aluminium-base alloys, Non-metallic bearing materials and Metal-non-metallic bearing materials.

4. A bearing material is a compromise between the opposing requirements of softness and high strength. One way of achieving strength with a relatively soft bearing material is to use the soft material as a lining on a steel backing, e.g. whitemetals, aluminium or copper-base alloys as thin layer on a steel backing.

5. Plastics when bonded to a steel backing can be used at higher speeds than plastics alone, because the steel is able to dissipate heat better than the plastic and also the thinner the layer of plastic the smaller the amount by which it will expand.


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6. There is a wide range of bearing materials including Whitemetals, Copper-base alloys and Aluminium-base alloys, Non-metallic bearing materials and Metal-non-metallic bearing materials. See tables 5, 6 and 7. Table 5: composition and properties of bearing materials

Alloy and composition % Manufacturing process

0.1% proof stress MPa

Strength MPa

Hardness HV

Whitemetal: tin base 89.2 Sn, 7.5 Sb, 3.3 Cu

Cast on steel strip Centrifugal casting on steel

65 39

76 70

27 31

Whitemetals: lead base 1- 84 Pb, 10 Sb ,6 Sn 2- 79.5 Pb, 10 Sb, 10 Sn,

0.5 Cu

Cast on steel strip Centrifugal casting on steel

30 60

42 73

16 25

Copper-base alloys 1- 90 Cu, l0 Sn, 0.5 P 2- 75 Cu, 20 Pb, 5 Sn 3- 80 Cu, 10 Pb, 10 Sn 4- 73.5 Cu, 22 Pb, 4.5 Sn

Cast on steel Cast on steel Sintered on steel Sintered on steel

233 124 249 81

420 233 303 121

120 70

120 46

Aluminium-base alloys 1- 92 Al, 6Sn ,1 Cu, 1 Ni 2- 89.7 Al, 6 Sn, 1.5 Cu,

1.4 Ni, 0.9Mg, .0.5 Si

Cast on steel Cast on steel

50 83

140 207

45 78

Table 6 shows the properties of commonly used polymeric bearing materials sliding against steel.

Material Max. load pressure MPa Max. Temp. C Max. speed m/s Phenolics 30 150 0.5 Nylon 10 100 0.1 Acetal 10 100 0.1 PTFE 6 260 0.5 Nylon with graphite filler 7 100 0.1 Acetal with PTFE filler 10 100 0.1 PTFE with silicon filler 10 260 0.5 Phenolic with PTFE filler 30 150 0.5

Table 7: shows comparison of bearing materials

Material Hardness BH Yield stress

MPa Strength

MPa Fatigue strength

MPa Elastic

modulus GPa Tin-base whitemetal 17-25 30-65 70-120 25-35 51-53

Lead-base whitemetal 15-20 20-60 40-110 22-30 29 Copper-lead 20-40 40-60 50-90 40-50 75

Phosphor bronze 70-150 130-230 280-420 90-120 80-95 Leaded tin bronze 50-80 80-150 160-300 80 95 Aluminium-base 70-75 50-90 140-210 130-170 73


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Polymers 5-20 -- 20-80 5-40 1-10 Graphite-impregnated metals Load pressures up to about 40 MPa and operating temp. up to 500C

PTFE-impregnated metals Load pressures up to about 40 MPa and operating temp. up to 500C

Material Density Mg/rn3

Thermal conductivity W m-1 K-1

Relative corrosion Resistance*

Relative wear Resistance*

Relative cost

Tin-base whitemetal 7.3-7.7 50 5 2 7 Lead-base whitemetal 9.6-10 24 4 3 1

Copper-lead 9.3-9.5 42 3 5 1.5 Phosphor bronze 8.8 42 2 5 2 Leaded tin bronze 8.8 42 2 3 2 Aluminium-base 2.9 160 3 2 1.5

Polymers 1.0-1.3 0.1 5 5 0.3 Graphite-impregnated metals It utilizes the advantages of load-bearing and temperature of metals

with the low coeff. of friction and soft properties of the non-metals PTFE-impregnated metals * The larger numbers the better the resistance.

Chemical properties: Attack of materials

Attack of materials by the environment in which they are situated can be a major problem. The rusting of iron in air is an obvious example of such an attack. Corrosion can often be much reduced by the selection of appropriate materials. Selection for corrosion resistance

1. Tables are available giving the comparative resistance to attack of materials in various environments, e.g. in aerated water, in salt water, too strong acids, strong alkalis, organic solvents and ultraviolet radiation.

2. The amount of pollution (like smokes from automobiles and factories) in the atmosphere can also affect the corrosion rate.

3. For metals subject to atmospheric corrosion the most significant factor in determining the chance of corrosive attack is whether there is an aqueous electrolyte present. This could be provided by condensation of moisture occurring as a result of the climatic conditions.

4. Metals: a. For metals immersed in water, the corrosion depends on the

substances that are dissolved or suspended in the water. b. Carbon steels and low-alloy steel have poor resistance. Painting

provides a protective coating of the surface, can reduce such corrosion.

c. The addition of 4-6% chromium to steel can markedly improve its corrosion resistance.

d. Aluminium and Copper in air develops an oxide layer on its surface which then protects the substrate from further attack.


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e. Nickel, Titanium and their alloys have excellent resistance to corrosion in industrial air, fresh and sea water.

5. Polymers: a. While some polymers are highly resistant to chemical attack,

others are liable to stain, craze, soften, swell or dissolve completely. For example, nylon shows little degradation with weak acids but attacked by strong acids; it is resistant to alkalis and organic solvents.

b. In general, plastics have excellent corrosion resistance and there is a wide use of plastic containers and pipes for the transmission of water and other chemicals.

c. Plastics do not corrode in the same way as metals. Polymers can deteriorate (degrade) as a result of exposure to UV radiation e.g. that in the rays from the sun; which can cause a breakdown of the bonds in the polymer molecular chains and result in surface cracking. For this reason, plastics often have an UV inhibitor mixed with the polymer when the material is produced.

d. Polymer bonds also affected by heat and mechanical stress. To reduce such effects, specific additives are used as fillers in the formulation of a plastic.

6. Glass and Ceramics: a. Most ceramic materials show excellent resistance to chemical

attack. Glasses are exceedingly stable and resistant to attack, hence the widespread use of glass containers.

b. Enamels, made of silicate and borosilicate glasses, are widely used as coatings to protect steels and cast irons from corrosive attack.

Table 8: gives a rough indication of the corrosion resistance of materials to different environments.

Corrosion resistance Material Aerated water

High resistance All ceramics, glasses, lead alloys, alloy steels, titanium alloys, nickel alloys, PTFE, polypropylene, nylon, epoxies, polystyrene, PVC

Medium resistance Aluminium alloys, polythene, polyesters Low resistance Carbon steels

Salt water

High resistance All ceramics, glasses, lead alloys, stainless steels, titanium alloys, nickel alloys, copper alloys, PTFE, polypropylene, nylon, epoxies, polystyrene, PVC, polythene

Medium resistance Aluminium alloys, polyesters Low resistance Low-alloy steels, carbon steels UV radiation

High resistance All ceramics, glasses, all alloys Medium resistance Epoxies, polyesters, polypropylene, polystyrene, HD


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polyethylene, polymers with UV inhibitor Low resistance Nylon, PVC, many elastomers

Strong acids

High resistance Glasses, alumina, silicon carbide, silica, PTFE, PVC, polythene, epoxies, elastomers, lead alloys, titanium alloys, nickel alloys, stainless steels

Medium resistance Magnesium oxide, aluminium alloys Low resistance Carbon steels, polystyrene, polyurethane, nylon, polyesters Strong alkalis

High resistance Alumina, nickel alloys, steels, titanium alloys, nylon, polythene, polystyrene, PTFE, PVC, polypropylene, epoxies

Medium resistance Silicon carbide, copper alloys, zinc alloys, elastomers, polyesters

Low resistance Glasses, aluminium alloys Organic solvents High resistance All ceramics, glasses, all alloys, PTFE, polypropylene

Medium resistance Polythene, nylon, epoxies Low resistance Polystyrene, PVC, polyesters, ABS. most elastomers

Table 9: Thermal properties

Thermal properties that are generally of interest in the selection of materials include how much a material will expand for a particular change in temperature; how much the temperature of a piece of material will change when there is a heat input into it, and how good a conductor of heat it is. Figure 6 illustrates thermal conductivity.

1. The linear expansivity or coefficient of linear expansion is a

measure of the amount by which a length of material expands when the temperature increases. It has the unit of K-1;

1

= K

etemperaturlengthoriginal

length

2. The term heat capacity is used for the amount of heat needed to raise the temperature of an object by l K. Thus if 300 J is needed to raise the temperature of a block of material by 1 K, then its heat capacity is 300 J/K. The specific heat capacity c is the amount of heat needed per kilogram of material to raise the temperature by 1 K, and has the unit of J kg-1 K-1;

Fig. 6: Thermal conductivity

Heat Heat

Distance

Temperature gradient

Tem

per

atu

re


- 33

11

= KkgJ

etemperaturmass

heatofamountc

3. The thermal conductivity of a material is a measure of the ability of a material to conduct heat. In other words; the thermal conductivity of a material is a measure of the rate at which heat is transferred through the material. It is defined as the quantity of heat flow per second divided by the temperature gradient T/L (Figure 6), and has the unit of Wm-1K-1;

11sec/ = KmWgradiantetemperatur

ondheatofquantity

Selection for Thermal properties

1. In general, metals have smaller specific heat capacities than plastics, e.g. copper has a specific heat capacity of about 340 J kg-1

K-1 while for polythene is about 1800 J kg-1 K-1. Ceramics has generally the smallest heat capacity e.g. for Alumina is about 8 J kg-1 K-1

2. Metals tend to be good conductors, e.g. copper has a thermal conductivity of about 400 Wm-1K-1. Plastics have thermal conductivities of the order 0.3 Wm-1K-1 or less. Ceramics have Thermal conductivities ranges from about 1 to 40.

3. Ceramics have low coefficients of expansion; metals have higher values and polymers even higher. Table 11 gives typical values for 20C. E.g. Alumina 20-40, copper 370 and polythene 0.25-0.35.

4. Very low thermal conductivities occur with foamed plastics, i.e. those containing bubbles of air. For example, foamed polymer polystyrene, known as expanded polystyrene and widely used for thermal insulation, has a thermal conductivity of about 0.02-0.03 Wm-1K-1.

5. Glass and ceramics has very low thermal conductivities, especially in foamed forms (foamed glass and foamed ceramics), Typical Values is less than 0.1 Wm-1K-1.

6. Example: The heating element for an electric fire is wound on an electrical insulator. What thermal considerations will affect the choice of insulator material?

a. The insulator will need to have a low heat capacity so that little heat is used to raise the material to temperature.

b. This means using a material with as low a density, and hence low mass, and low specific heat capacity as possible.

c. It also will need to be able to withstand the high temperatures without deformation or melting.

d. A ceramic is indicated.


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Table 10 gives typical values of the linear expansivity, the specific heat capacity and the thermal conductivity for metals, polymers and ceramics. Table 10: Typical values for Thermal properties

Material Linear expansivity

10-6K-1 Specific heat

capacity J kg-1 K-1 Thermal conductivity

W m-1 K-1 Metals Aluminium 24 920 230 Copper 18 385 380 Mild steel 11 480 54 Polymers Polyvinyl chloride 70-80 840-1200 0.1-0.2 Polyethylene 100-200 1900-2300 0.3-0.5 Epoxy cast resin 45-65 1000 0.1-0.2 Ceramics Alumina 8 750 38 Fused silica 0.5 800 2 Glass 8 800 1

Table 11: Thermal properties for materials Material [10-6K-1] c [J kg

-1 K-1] [W m-1 K-1] Metals Aluminium 24 0.90 220-230 Aluminium alloys 20-24 0.84 120-200 Copper 17 0.39 370 Copper alloys 16-20 0.39 30-160 Iron 12 0.44 81

carbon steels 10-l5 0.48 47 cast irons 10-11 0.27-0.46 44-53 alloy steels 12 0.51 13-48 stainless steel 11-l6 0.51 16-26

Magnesium 25 1.02 156 Magnesium alloys 25-27 ----- 80-140 Nickel 13 0.44 92 Nickel alloys 10-l9 0.48-0.50 11-30 Tin 23 0.23 67 Tin alloys 22-24 ----- 53-67 Titanium 8 0.52 22 Titanium alloys 8-9 ----- 5-12 Zinc 40 0.54 116 Zinc alloys 25-35 ----- 107-116 Polymers Thermoplastics 40-300 0.8-2.0 0.1-0.4

ABS 80-100 1.5 0.13-0.20 nylon 6 80-100 1.6 0.17-0.21 polythene 110-200 1.9-2.3 0.25-0.35


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polypropylene 100-120 1.9 0.16 polystyrene 60-80 1.2 0.12-0.13 PVC 50-250 1.1-1.7 0.l2-0.15

Thermosets 10-60 1.0-2.0 0.1-0.4 epoxy 60 1.1 0.17 phenol formaldehyde 30-40 1.6-1.8 0.13-0.25

Elastomers 50-250 l.3-l.8 0.1-0.3 natural rubber 22 1.9 0.18 neoprene 24 1.7 0.21 cellular polymers ----- ----- 0.02-0.04

Ceramics Alumina 8-9 0.7 20-40 Bonded carbides 4-6 0.2-1.0 40-120 Glasses 3-9 0.5-0.7 0.5-2 Composites Concrete 7-14 3.3 0.1-2 Wood 1.7 0.1-0.2

across grain 35-60 Along grain 3-6

Electrical properties: conductivity Ohm's law: V (dc volts) = I (dc amperes) R (resistance in ohm's ) ; R is the resistance of a length L of a material of cross-sectional area A. The electrical resistivity is a measure of the electrical resistance of a material, being defined by:

L

RA= m

Selection for electrical properties

1. In general, metals are good electrical conductors with low resistivities. The metals in common use in engineering which have the highest electrical conductivities are silver, copper and aluminium.

2. In each metal, the conductivity is highest when the material is of the 1- highest purity and in the 2- fully annealed condition.

3. High purity and annealed metals has lower strength, however, a compromise has to be reached.

4. An e1ectrical insulator such as a ceramic will have a very high resistivity, typically of the order of 1010 m or higher.

5. An electrical conductor such as copper will have a very low resistivity, typically of the order of l0-8 m.

6. The term semiconductor is used for those materials which have resistivities roughly halfway between conductors and insulators, i.e., of the order of 102 m.


- 36

Table 12 shows typical values of resistivity and conductivity at about 20C for insulators, semiconductors and conductors.

Table 12: Typical values of resistivity and conductivity at 20C for some

materials Material Resistivity m Conductivity -1m-1 Insulators

Acrylic (a polymer) >1014


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Cu, >99.90 %, electrolytic 1.71 Platinum, 10.6 >99.95%, oxygen free 1.71 -10% indium alloy 25. -1% cadmium wire 2.2 -10% rubidium alloy 43. -15% zinc alloy 4.7 Silver, 1.59 -20% zinc alloy 5.4 -10% copper alloy 2. -2% nickel alloy 5.0 -15% cadmium alloy 4.9 -6% nickel alloy 9.9 Steel, stainless 56.

Gold 2.35 17% cobalt 28. Iron, 99.99% pure 9.7 Tungsten 5.65 Iron, -0.65% C (carbon steel) 18.

Table 14: Resistivities for insulators Material Resistivity m Ceramic: alumina 109-1012

porcelain 1010-1012 Diamond 1010-1011

Glass: soda lime 109-1011

Pyrex 1012 Elastomer: butyl 1015

Natural rubber 1013-1015

polyurethane 1016 Mica 1011-1015

Paper (dry) 1010 Polymer: acrylic 1012-1014

cellulose acetate 108-1012 melamine 1010

polyamide (nylon) 1010-1013

polypropylene 1013-1015

polythene: high density 1014-1015

low density 1014-1018

polyvinyl chloride: rigid 1012-1014

flexible 109-1013

Electrical properties: dielectrics

1. If a potential difference V is applied between two conducting plates then;

Q (charge in coulombs) =C (capacitance in farads F) V

2. The factors determining the value of the capacitance are the plate area A, the separation d of the plates and the medium between them, i.e.

C = A / d; where is a factor, called the absolute permittivity. The permittivity is related to the medium between the plates. Its unit is Farad per meter F/m.

3. C = ro A / d ; where o is called the permittivity of free space and has a value of 8.85 10-12 F/m. r is called the relative


- 38

permittivity. It has no units and it is a material's property. The relative permittivity is 1 for vacuum.

4. The material between the conducting plates is called the dielectric and the relative permittivity is the dielectric constant which can be defined as a measure of the ability of a material to store charge

5. If the potential difference is too high or the thickness of the dielectric is too small, the dielectric breaks down and the electrical charge can move through it. The dielectric strength is a measure of the highest voltage that an insulating material can withstand without electrical breakdown. It is defined as:

Dielectric strength [V/m] = Breakdown voltage / Insulator thickness

6. Example: An electrical capacitor is to be made with a sheet of polythene of thickness 0.1 mm between the capacitor plates. What is the greatest voltage that can be connected between the capacitor plates if there is not to be electrical breakdown? The dielectric strength is 4 107 V/m.

Answer: The dielectric strength is defined as the breakdown voltage divided by the insulator thickness, hence: Break down voltage = dielectric strength thickness = 4 107 0.1 10-3 = 4000V

7. As you will learn next year, when an alternating current is applied to a dielectric, the current leads the voltage by 90o - . In addition, due to the polarization of materials molecules; a fraction of the energy is lost and termed as the loss factor which is given by:

loss factor = tan : is termed as the dielectric loss angle. tan = 1/RC; =2 frequency of alternating current Hz power loss = V2/R = V2C tan

8. Example: A 0.1 F capacitor has a dielectric with a loss factor of 0.003. What will be the power loss when an alternating voltage of 240 V, 50 Hz is connected across it?

Answer: The power loss in the parallel resistor, resulting from the dielectric loss is given by: since tan = 1/RC, =2 50 Hz: power loss = V2/R = V2C tan =2402 2 50 0.l 10-6 0.003 =5.4 10-3 W

9. Ferroelectricity is a spontaneous electric polarization of a material that can be reversed by the application of an external electric field.


- 39

10. The generation of a surface charge in response to the application of an external stress to a material is called piezoelectricity. A change in the spontaneous polarization of a material in response to a change in temperature is called pyroelectricity.

11. All ferroelectrics are required by symmetry considerations to be also piezoelectric and pyroelectric.

Selection for dielectric properties

1. The dielectric constant is a measure of the ability of a material to store charge; its a property for polar ceramic and polymeric materials. The dielectric strength is a measure of the highest voltage that the polar ceramic and polymeric materials can resist before electrical breakdown.

2. The loss factor is the fraction of the energy lost during alternate current transport; loss factor = tan, is the dielectric loss angle.

3. The Impurities generally reduces the dielectric strength, decreases the dielectric constant and increases the loss factor.

4. The optimum values of the dielectric properties depend on the chemical and structural formations.

5. The Real values of the dielectric properties depend, to a large extent, on the microstructural properties, namely, the grain boundaries and interfaces, i.e. the grain pore phase size and distributions.

6. Dielectric materials can be Ferroelectric, that is, their structure symmetry in non-centro-symmetric and exhibits spontaneous electric polarization, e.g. BaTiO3 and PZT=PbZrTiO3

Table 14 shows some typical values of dielectric constant, dielectric strength and loss factor tan. Material Relative permittivity Dielectric strength

106 V/m Loss factor

tan at 106 Hz 50 Hz 106 Hz

Alumina 9.0 6.5 6 0.0002-0.01 Glass (Pyrex) 4.3 4. 14 0.01-0.02 Mica 7.0 7. 40 0.001 Polyethylene 2.3 2.3 20 0.0002-0.0005 Polystyrene 2.3 2.3 20 0.0001-0.001 Titanium dioxide 100. 6 0.0002-0.005 Barium titanate 3000. 12 0.0001-0.02


- 40

Magnetic properties

1. Permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter .

2. In SI units, permeability is measured in the henry per meter Hm-1, or newton per ampere squared NA-2. The constant value o is known as the magnetic constant or the permeability of free space, and has the exact (defined) value o = 410

7 NA2.

3. Relative permeability r is the ratio of the permeability of a specific medium to the permeability of free space o. r= / o

4. Diamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect. Consequently, diamagnetism is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field. It is generally a quite weak effect in most materials, although superconductors exhibit a strong effect. Diamagnets are materials with a magnetic permeability r < 1.

5. Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field. The total magnetization will drop to zero when the applied field is removed. Therefore, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields; hence have a relative magnetic permeability r > 1.

Selection for magnetic properties

The magnetic materials are selected, for a specific application, according to their a-magnetic properties or b-magnetization category as shown below: a- Classification of magnetic materials: (selection for properties);

1. Diamagnetic materials: These have relative permeabilities slightly below 1. Copper is an example of such a material.

2. Paramagnetic materials: These have relative permeabilities slightly greater than 1. Aluminium is an example of such a material.

3. Ferromagnetic and ferrimagnetic materials: These have relative permeabilities considerably greater than 1. The relative permeability for a ferromagnetic or ferrimagnetic material is not constant, depending on the size of magnetizing field used.


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a. Ferromagnetic materials being metals like iron, cobalt and nickel.

b. Ferrimagnetic materials being ceramics like iron oxide Fe3O4 and nickel ferrite NiFe2O3.

c. For iron the relative permeability is typically about 2000 to 10 000, though special steels can have values of the order of 60 000 to 90 000.

b- Classification of magnetic materials: (selection for magnetization);

1. Soft magnetic materials: it is very easily demagnetized and little energy dissipated in magnetizing it.

a. Soft magnetic materials are used for transformers. b. A typical soft magnetic material used for a transformer core

is an iron-3% silicon alloy.

2. Hard magnetic materials: it is difficult to demagnetize and needs high energy for magnetization.

a. Hard magnetic materials are used for such applications as permanent magnets.

b. The main materials used for permanent magnets are the iron-cobalt-nickel- aluminium alloys, ferrites and rare earth alloys

Example: Which of the following applications (a) a compass needle, (b) the core of an electromagnet, requires a soft and which a hard magnetic material?

Answer: a. A hard magnetic material is required since the compass

needle is required to be a permanent magnet. b. A soft magnetic material is required since the electromagnet

is required to loose its magnetism when the energizing current is switched off.

Table 15: Relative permeability for selected mediums

Medium /o Medium /o -metal* 20,000 Aluminum 1.000022 Permalloy** 8000 Air 1.00000037 Electrical steel 4000 Vacuum 1 ferrite Ni Zn 16-640 Hydrogen 1.0000000 ferrite Mn Zn >640 Sapphire .99999976 Steel 700 Copper .999994 Nickel 100-600 Water .999992 Platinum 1.000265 Superconductors 0

*-metal is very high magnetic permeability alloy 75% Ni, 15% Fe, plus Cu and Mo

** Permalloy 80% Ni and 20% Fe, a high magnetic permeability alloy


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Comparing materials Table 16 summarizes the properties of metals, polymers and ceramics indicating the typical range of values likely to be encountered at about 20C. This table is a good summery to start with. You can then construct your own tables as you start decorating your office.

Table 16: The range of properties Property Metals Polymers Ceramics

Density g/cm3 Medium-high 2-16

Low 1-2

Generally medium 2-4

Melting point C Medium-high 200-3500

Low 70-200

High 2000-4000

Thermal conductivity High Low Medium-low Thermal expansion Medium High Low Specific heat capacity Low Medium High Electrical conductivity High Very low Very low

Optical properties Opaque Some transparent, some opaque

Some transparent, some opaque

Tensile strength MPa Medium-high 100-2500

Generally low 30-80

Generally low 10-400

Compressive strength MPa Medium-high, as tensile

Generally low*, as tensile

High 1000-5000

Tensile modulus GPa Medium-high 40-400

Low* 0.1-4

High 150-450

Toughness Good Some good, some poor* Poor Hardness Medium Low High Wear resistance Medium Low-moderate High Resistance to corrosion Medium-poor Good-medium Good *Polymers are widely used with fillers, such as fibers and particles, to change their

properties, in particular making them stiffer, stronger and tougher.

Example: Which type of material, metal, polymer or ceramic would be the most likely to give materials with each of the following properties:

(a) High density, (b) High melting point, (c) High electrical conductivity, (d) Low specific heat capacity, (e) Low tensile modulus of elasticity, Answer:

(a) Metals contain the materials with the highest densities, (b) The highest melting points are given by the ceramics, (c) The highest electrical conductivities are given by the metals, (d) The lowest specific heat capacities are given by the metals, (e) Polymers give the materials with the lowest tensile modulus of

elasticity


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Problems: 1. Suggest the key property or properties required of a material and the

types of materials that might be used in the following situations: i- Pipes used in the distribution of hot water.

ii- A component where strength is required at temperatures in the region of 700C

iii- A component which is required to be stiff iv- A component where strength is required in a marine environment v- A light-load bearing material

vi- A container to hold acids vii- A component to be used with direct stresses of about 1000 MPa

viii- A component subject to impact loading ix- A component subject to cyclic loading x- A transformer core

2. Define ceramic materials and compare it with metallic materials, 3. What is the strength of a composite primary depending on? 4. Draw the stress-strain curve (in detail) for three hypothetical materials:

(a) Brittle, (b) Ductile, (c) Elastomer, give example for each category. 5. On what basis the materials is selected for their Thermal Properties 6. Electrical properties 7. Dielectric properties 8. Magnetic properties 9. Wear resistance

10. As bearing materials 11. Address the necessary questions before making a decision about

selection of materials, explain one of them. 12. How you select materials for: (with full examples)

a. Limiting the corrosion resistance in various environments b. Limiting the creep according to Temperature limitations of

materials 13. Give full examples for the following:

a. Temperature limitations of materials (to limit the creep) b. Corrosion resistance in various environments

14. In considering selection of materials for properties, what are the key questions that need to be addressed?

15. Distinguish between Ferroelectricity, Piezoelectricity and Pyroelectricity

16. Distinguish between Diamagnetism and Paramagnetism 17. Compare Ferromagnetic and ferrimagnetic materials 18. Compare Soft magnetic and Hard magnetic materials


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PPaarrtt IIIIII-- SSeelleeccttiioonn ooff PPrroocceesssseess * dont confuse with the "processing parameters" page 16 2.1 Introduction

This section is a consideration of the characteristics of the various types of processes which determine the types of products that can be produced.

In making a decision about the manufacturing process to be used for a product there are a number of questions that have to be answered: 1. What is the material?

The type of material to be used influences the choice of processing method. For example, for casting of high melting point material, the process must be either sand casting or investment casting.

2. What is the shape? The shape of the product is generally a very important factor in determining which type of process. For example, a product in the form of a tube could be produced by centrifugal casting, drawing or extrusion but not generally by other methods

3. What is the kind of detail is involved?

Is the product to have holes, threads, inserts, hollow sections, fine detail, etc.? Thus, forging could not be used if there was a requirement for hollow sections.

4. What dimensional accuracy and tolerances are required? High accuracy would rule out sand casting, though investment casting might well be suitable.

5. Are any finishing processes to be used? Is the process used to give the product its final finished state or will there have to be an extra finishing process? For example, planing will not produce as smooth a surface as grinding.

6. What quantities are involved? Some processes are economic for small quantities; others are economic for large quantities or continuous production. For example, open die forging could be economic for small numbers where, closed die forging are economic for large numbers.


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2.2 Surface finish

1. Roughness is defined as the irregularities in the surface texture which are inherent in the production process but excluding waviness and errors of form; see figure (8-a).

-a- -b-

Figure 8: a- the terms roughness, waviness and error of form. b- measuring roughness

2. A sand cast product will have a surface finish which is much rougher than one which has been die cast.

3. Roughness takes the form of a series of peaks and valleys which may vary in both height and spacing and is a characteristic of the process used.

4. Waviness may arise from such factors as machine or work deflections, vibrations, heat treatment or warping strains. Roughness and waviness may be cause departures of the surface from the true geometrical form.

One measure of roughness is the arithmetical mean deviation,

denoted by the symbol Ra defined as the arithmetical average of the variation of the profile above and below a reference line throughout the prescribed sampling length. The reference line may be the centre line, this being a line chosen so that the sums of the areas contained between it and those parts of the surface profile which lie on either side of it are equal (Figure 8-b). Thus; for a sample length in millimeters and areas in square millimeters Ra is defined as:

1000+

=lengthsample

BareasofsumAareasofsumRa

Table 17 indicates the significance of Ra values in terms of the

surface texture. The degree of roughness that can be tolerated for a

Area between surfaces above center line equals area below

center line

A A

B B

Roughness

Waviness

Error of form


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component depends on its use. Thus, for example, precision sliding surfaces will require Ra values of the order of 0.2 to 0.8 m with more general sliding surfaces 0.8 to 3 m. Gear teeth are likely to require Ra values of 0.4 to 1.6 m, friction surfaces such as clutch plates 0.4 to 1.5 m, mating surfaces 1.5 to 3 m.

Table 18 shows what is typically achievable with different

processes. Thus, sand casting produces a much rougher surface than die casting; hot rolling produces a rougher surface than cold rolling; sawing produces a much rougher surface than milling.

Table 17: Relation of Ra values to surface texture

Surface texture Roughness Ra m Very rough 50 Rough 25 Semi-rough 12.5 Medium 6.3 Semi-fine 3.2 Fine 1.6 Coarse-ground 0.8 Medium-ground 0.4 Fine-ground 0.2 Super-fine 0.1

Table 18: Roughness values for different processes Process Roughness Ra m Sand casting 25 - 12.5 Hot rolling 25 - 12.5 Sawing 25 - 3.2 Planing, shaping 25 - 0.8 Forging 12.5 - 3.2 Milling 6.3 - 0.8 Boring, turning 6.3 - 0.4 Investment casting 3.2 - 1.6 Extruding 3.2 - 0.8 Cold rolling 3.2 - 0.8 Drawing 3.2 - 0.8 Die casting 1.6 - 0.8 Grinding 1.6 - 0.1 Honing 0.8 - 0.1


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2.3 Metal-forming processes

The following is a discussion of the characteristics of the various processes used for metal forming and the types of products that can be obtained from them.

2.3.1 Casting of Metals

Casting can be used for components from masses of about 10-3 kg to I04 kg with wall thicknesses from about 0.5 mm to 1 m. Castings need to have rounded corners, no abrupt changes in section and gradual sloping surfaces. Casting is likely to be the optimum method in the circumstances listed below but not for components that are simple enough to be extruded or deep drawn. The casting process is selected when:

1. The part has a large internal cavity

There would be a considerable amount of metal to be removed if machining was used.

2. The part has a complex internal cavity Machining might be impossible; by casing, however, very complex internal cavities can be produced.

3. The part is made of a material which is difficult to machine The hardness of a material may make machining very difficult, e.g. white cast iron.

4. The metal used is expensive and so there is to be little waste Machining is likely to produce more waste than occurs with casting.

5. The directional properties of a material are to be minimized Metals subject to a manipulative process often have properties which differ in different directions.

6. The component has a complex shape Casting may be more economical than assembling a number of individual parts.

7. The tooling cost for making the moulds When many identical castings are required; the mould cost (used many times) will be spread over many items and make the process economic.

8. The mould type and cost for single product Where just a single product is required, the mould used must be as cheap as possible.


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Each casting method has important characteristics which determine its appropriateness in a particular situation. Table 19 illustrates some of the key differences between the casting methods. The following factors determine the type of casting process used:

1. Large heavy casting Sand casting can be used for very large castings.

2. Complex designs Sand casting is the most flexible method and can be used for very complex castings.

3. Thin walls Investment casting or pressure die casting can cope with walls as thin as 1mm. Sand casting cannot cope with such thin walls.

4. Good reproduction of detail Pressure die casting or investment casting gives good reproduction of detail), sand casting is being the worst.

5. Good surface finishes

Pressure die casting or investment casting gives the best finish, sand casting being the worst.

6. High melting point alloys Sand casting or investment casting can be used.

7. 10. Tooling cost a. This is highest with pressure die casting. b. Sand casting is cheapest. c. With large number production, the tooling costs for metal

moulds can be paid over a large number of castings, d. Whereas, the cost of the mould fo

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