Chap 5 Engineering Materials

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GENERAL ENGINEERING & APPLIED SCIENCES ENGINEERING MATERIALS CHAPTER 5

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PROPERTIES OF

MATERIALS

I. CLASSIFICATION OF MATERIALS

Materials are classified as:

Metallic

Ferrous Example: Steel, cast iron, wrought iron, malleable cast iron

Nonferrous Example: Copper, tin, zinc, aluminum, magnesium, and titanium

Nonmetallic

Example: Wood, stone, brick, cement, concrete, resins (plastics), rubber, leather, and ceramics

II. PROPERTES OF ENGINEERING MATERIALS

CHEMICAL PROPERTIES – are properties of a material that relate to its behavior in chemical reactions. Some common chemical properties of materials are:

Corrosion – is the partial or complete wearing away, dissolving, or softening of any substance by chemical or electrochemical reaction with its environment.

Alloying - is the process of mixing one metal to another or mixing a

metal with nonmetallic material

Composition – means the percentage of the various elements that make up the metal

Compound formation

Crystal Structure – is the definite, repeating arrangement of atoms

and molecules in a material


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GENERAL ENGINEERING AND APPLIED SCIENCES

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CHAPTER 5 Engineering Materials

PHYSICAL PROPERTIES – are those distinguishing qualities or characteristics that are used to describe a substance in the absence of external forces. Some common physical properties of a material are:

Specific Heat – is the amount of heat needed to raise the temperature of one gram of a substance by one degree, usually measured in joules per kilogram per Kelvin.

Thermal Conductivity – is the rate at which heat flows through a

material between points at different temperatures, measured in watts per meter per degree.

Thermal Expansion Coefficient is a thermodynamic property of a

substance, which relates the change in temperature to the change in material’s linear dimensions.

Density – is the mass per unit volume of a substance.

Refractive Index - is the ratio, which relates the speed of light co in

vacuum to the speed of light cm in the medium. Electrical Resistivity – is defined as the resistance

Specific Gravity - is the ratio of the density of the substance to the

density of some standard substance. The standard is usually water (@ 4oC) for liquids and solids, while for gasses, it is usually air.

Poisson’s Ratio – is the ratio of the lateral strain to the axial strain of the material.

Mechanical properties – are those properties which describe the behavior

of materials under the application of force. Some common mechanical properties of materials are:

Strength- is the ability of the material to withstand force, pressure, or stress.

Hardness – is usually defined as resistance to scratching or penetration.

Ductility – is the property of a material by virtue of which it may be plastically elongated.

Elasticity – is the property of a material to regain its original shape upon the removal of the external load.

Plasticity – is the property of the material by virtue of which permanent deformation can occur.


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GEAS GENERAL ENGINEERING & APPLIED SCIENCES GEAS GEAS

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III. STRESS AND STRAIN STRESS Stress is the measure of the strength of the agent that is causing a deformation.

=F

Aσ

2

2

stress, (N/m )force, (N)

area, (m )

==

=

Where

F

A

σ

STRAIN Strain is the unit deformation resulting from a stress

Δ=

L

Lε

strain (unitless)change in lengthoriginal length

=Δ =

=

Where

L

L

ε

HOOKE’S LAW The Hooke’s law states that, “within the elastic range of materials, stress is proportional to strain.”

STRESS – STRAIN DIAGRAM Proportional limit is the point on the stress-strain curve below which, the stress is proportional to strain. The elastic limit of a body is the smallest stress that will produce a permanent deformation of the body. Yield point is a point at which there is an appreciable elongation or yielding of the material without any corresponding increase of load. The ultimate strength is the highest ordinate on the stress-strain curve. Rapture strength is the stress at failure.


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IV. FACTOR OF SAFETY The Factor of Safety of a material is the ratio of allowable stress to working stress.

=allowable stress

Factor of safetyworking stress

Allowable stress is that stress below which it is known that failure will not take place. Working stress is the actual stress under which the material is expected to operate.

Factors to be considered in choosing the factor of safety to be employed

☞ Variation of material properties

☞ Quality of the manufacturing operations

☞ The uncertainties of computations of stress, magnitude, and stress distribution on the basis of assumption that have been necessary in the theory employed.

☞ Dangers of personal injury from failure of the part of undue financial loss

☞ The influence of uncertainties such as may be experienced in corrosion and unforeseen types of loading

☞ Extent of inspection V. SELECTION OF MATERIALS

Factors to be considered :

Strength requirement

Ease of fabrication

Cost of the material

Availability of the material


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ELASTICITY

Elasticity is the property of the material to regain its original shape upon the removal of the external load. YOUNG’S MODULUS OR TENSILE MODULUS OF ELASTICITY The Young’s Modulus or tensile modulus describes the length elasticity of the material. It is the ratio of the stress to strain within the limit of proportionality.

= =ΔFL

EA L

σε

tension modulus of elasticity (Young's Modulus)stressstrain

===

Where E

σε

SHEAR MODULUS OF ELASTICITY The shear modulus describes the shape elasticity of the material.

( )2 1

=+E

Gμ

shear modulus of elasticitytension modulus of elasticity (Young's Modulus)poisson's ratio

===

Where G

E

μ

BULK MODULUS OF ELASTICITY The bulk modulus describes the volume elasticity of the material.

( )( )3 1 2

Δ= =

Δ −V p E

Vβ

μ

bulk modulus of elasticitychange in pressurechange in volumeoriginal volumetensile modulus of elasticity (Young's Modulus)poisson's ratio

=Δ =Δ =

===

Where

P

V

V

E

β

μ


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PLASTICITY

Plasticity is the property of a material by virtue of which permanent deformation can occur.

MEASURES OF PLASTICITY

Ductility is that quality of a material by virtue of which it may be plastically elongated.

=ultimate failure strain

Ductilityyielding strain

Measures of Ductility:

Percent Elongation:

100%⎛ ⎞−

= ×⎜ ⎟⎝ ⎠

f o

o

L Lpercent elongation

L

Reduction in area:

100%⎛ ⎞−

= ×⎜ ⎟⎝ ⎠

o f

o

A Areduction in area

A

Malleability is the quality of a material by virtue of which it may be plastically compressed.

ORDER OF PROPERTIES FOR THE SIX MOST COMMONLY USED METALS WHEN IN THE

PURE STATE

Order of Malleability Order of Ductility

1. Copper 1. Copper 2. Aluminum 2. Iron 3. Tin 3. Aluminum 4. Lead 4. Zinc 5. Iron 5. Tin 6. Zinc 6. Lead


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ENGINEERING

MATERIALS

POLYMERS A Polymer is a substance consisting of large molecules that are made of many small, repeating units called monomers, or mers. A copolymer or interpolymer is a large molecule with two alternating mers. Homopolymers are polymers consisting of only one kind of repeating unit. Elastomers are polymers with plastic properties similar to rubber.

Some Common Polymeric Materials Elastomers Plastics Adhesives Fibers Asphalt Natural rubber

DEGREE OF POLYMERIZATION The Degree of Polymerization (DP), is the number of repeating units in one large molecule, typically several hundred to several thousand.

= polymer

mer

MWDP

MW

degree of polymerizationmolecular weight

==

where DP

mw

High Polymers are materials with a very high degree of polymerization. Telenomers or oligomers are polymers with degree of polymerization of less than 10 THERMOPLASTICS AND THERMOSETTING PLASTICS Thermoplastics are polymers, which can be repeatedly softened by heating and hardened by cooling. Thermosetting plastics, on the other hand, harden permanently after being heated once.


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Some Common thermosetting thermoplastics:

Thermosetting Thermoplastics

Epoxy Acetal Melamine Acrylic Natural rubber (polyisoprene) Polyamide (Nylon) Phenolic Polyvinyl chloride (PVC) Polyester (DAP) Polyester (PBT and PET) Silicone Polytetrafluoroethylene (Teflon)

GLASS Glass is defined as supercooled liquids, because they do not crystallize when cooled below their melting points. The basic ingredient of glass compositions is silica, derived from sand, flint, or quartz Properties of silica:

At room temperature, silica is in the form of quartz. At 875OC (1607OF), the structure changes to tridymite. At 1470OC (2678OF), the structure changes to cristobalite.

Vitrification is the process a liquid glass solidifies without molecular change when cooled below the melting point. Some common glasses:

Type of glass Characteristics Use Fused silica Thermal shock resistant Laboratory equipment Borosilicate (pyrex) Thermal shock resistant Cooking utensils Aluminasilicate Thermal shock resistant Thermometers Soda-lime silica Easy to form Plate, bulbs Lead-alkali Dielectric Capacitors


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CERAMICS Ceramics are compounds of metallic and non-metallic elements. Some common examples of ceramics are:

Glass Brick Portland cement Refractories Abrasives

Properties of Typical Ceramics:

High melting point High hardness High compressive strength High tensile strength (perfect crystals) Low ductility (brittleness) High shear resistance (low slip) Low electrical conductivity Low thermal conductivity High corrosion (acid) resistance Low coefficient of thermal expansion

WOOD A wood is a hard, tough substance that forms the trunks of trees, and that has been used for thousands of years as a fuel and as a material of construction.

Softwoods contains tube-like fibers (tracheids) oriented with the longitudinal axis (grain) and cemented together with lignin.

Hardwoods contains more complex structures in addition to longitudinal fibers Moisture Content:

−= wet oven dry

oven dry

W WMC

W

moisture contentwet weightoven dry weight

===

wet

oven dry

Where MC

W

W

Note:

Wood is considered green if its moisture content is above 19%. Wood is considered dry if its moisture content is between (12-15)%


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ABRASIVES

An abrasive is a hard material that can cut other materials. Abrasives are classified into two:

Natural abrasives Natural abrasives include emery (50-60% Al2O3, rest iron oxide), corundum, quartz, garnets, and diamonds.

Artificial abrasives.

Artificial abrasives include carbides and man-made aluminum oxide (Al2O3).

Familiar abrasives include sandpaper, steel wool, emery boards, and whetstones, but abrasives are also used in the forms of powders, grains, chips, and blocks.

Common uses for abrasives include grinding, polishing, buffing, honing, cutting, drilling, sharpening, and sanding.

CONCRETE

Concrete is a mixture of mineral aggregates locked in a solid structure by a binding mineral (that is, cement). Aggregates refer to the sand and rock particles that have been added to increase mass and volume of concrete. Sand that will pass through #4 sieve (openings less than 0.25 in or 0.6 cm) is known as fine aggregates. Any particles that are larger than this are known as course aggregate. Proportions of Concrete Mixture:

1:2:3

This means that, 1 – part of cement 2 – parts of fine aggregate 3 – part of courses aggregate


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CEMENT Cement is any material that hardens and becomes strongly adhesive after application in plastic form. The term cement is often used interchangeably with glue and adhesive . The most common cement is the Portland Cement, manufactured from lime, silica, and alumina. The Five Common Types of Portland Cement:

Normal Portland Cement This is general-purpose cement typically used in sidewalks, pavements, beams, columns, and culverts.

Modified Portland Cement

This cement has a moderate sulfate resistance and is generally used in hot weather in the construction of large concrete structures.

High-Early-Strength Portland Cement

This cement develops its strength quickly and is suitable for structures that must be put to early use or when long term protection against cold temperature is not feasible.

Low – Heat Portland Cement

For used in massive concrete structures such as gravity dams. This cement is required to minimize the curing heat.

Sulfate-Resistant Portland Cement

This type of cement is used when exposure to sulfates is expected.

METALLURGY

Metallurgy is the science and technology of metals, including the extraction of metals from ores, the preparation of metals for use, and the study of the relationship between structures and properties of metals Extractive metallurgy is the subject, which covers the refinement of pure metals from their ores.


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CLASSIFICATION OF METALLURGICAL PROCESSES

Chemical metallurgy is the branch of metallurgy which deals with the reduction of metals from their minerals and the refining and alloying of those metals.

Physical metallurgy is the branch of metallurgy which deals with the

nature, structure, and physical properties of metals and alloys.

Mechanical metallurgy is the branch of metallurgy which deals with the working and shaping of metals and alloys, such as casting, forging, rolling, and drawing.

ORES

An ore is a combination of minerals, which can be profitably mined. Those ores which contain a predominance of silica or silicate in the gangue are considered as acid ores, while those with a predominance of limestone and magnesia are considered as basic ores. A gangue is a secondary minerals consisting of impurities of an earthy nature such as rock, sand, and clay. Primary metals are metals produced directly from ore. Secondary metals are metals that are reclaimed from scrap. Properties of Some Metals and Alloys:

High thermal conductivity

High electrical conductivity

High chemical reactivity

High strength

High ductility

h density

Highly magnetic


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MATERIAL TESTING

HARDNESS TESTING BRINELL TEST

This hardness test is used primarily with iron and steel casting. In Brinell test a Brinell Hardness Number (BHN) is determine by pressing a hardened steel ball (10 mm in diameter) into the surface of a specimen with a force of 500 kg and 3000 kg for soft and hard materials, respectively.

( )2

2= = =

− −contacts s S d

P P PBHN

A Dt D d D dπ π

load , (kg)area in contactdiameter of the steel ball, (mm)depression diameter, (mm)depth, (mm)

=====

b

d

Where P

A

D

D

T

ROCKWELL TEST

A Rockwell test is a penetration test in which a diamond cone is used for hard materials and a hardened steel ball (1/16 inches in diameter) for soft materials. The Rockwell hardness is determined from the depth of penetration and is read directly from a dial.

1 2= −R C C t

1 2&

Rockness hardnessthickness or depth of penetrationconstants which depend on the scale

===

Where R

T

C C

VICKERS TEST Also a type of penetration test using a square pyramid as penetrator.

( )2

1.854=

m

PVhn

d

Vickers hardness numberloadmean diagonal

===

m

Where vhn

P

D

DB

P

t

DD


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MEYER TEST A Meyer Test is a form of penetration hardness test using a hardened

steel ball as penetrator.

2

4=

Pmhn

dπ

Meyer hardness numberloaddepression diameter

===

Where mhn

P

D

KNOOP TEST

A Knoop Test is a form of penetration hardness test using asymmetrical pyramid as penetrator.

2

14.2=

PK

L

Knoop hardness numberloadlong diagonal

===

Where K

P

L


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TEST - 5 1. The property of metals that allows them to be drawn into thin wires

beyond their elastic limit without being ruptured is called

A. ductility B. malleability* C. elasticity D. hardness 2. Interaction between the surfaces of two closely adjacent bodies which

causes them to cling together is known as

A. friction B. cohesion C. adhesion* D. viscosity 3. Solids which break above the elastic limit are called

A. brittle* B. ductile C. plastic D. malleable 4. The property of some elementary particles that causes them to exert

force on one another is known as

A. potential difference B. charge* C. specific change D. nucleon interaction 5. The property which permits the flow of current under the action of a

potential difference is called

A. resistance B. permeance C. impedence D. conductance*


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6. When a body is resistant to heat, it is called

A. thermoscopic B. thermotropic C. thermoduric* D. thermoplastic 7. The property of fluids by virtue of which they offer resistance to flow is

known as ‘

A. gummosity B. glutinosity C. viscidity D. viscosity* 8. The tendency of a body to return to its original size or shape after having

been deformed is called

A. elastance B. elasticity* C. elastivity D. anelastivity 9. The emission of light by a material because of its high temperature is

known as

A. incandescence* B. luminescence C. scintillation D. phosphorescence 10. Which of the following statements is correct concerning the passage of

white light into a glass prism?

A. The violet color travels faster than the red color B. The violet color travels slower than the red color* C. All the colors of white light travel with the same speed D. Greater the wavelength, slower the speed of color


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11. The property by virtue of which a body resists any attempt to change its state of rest or motion is called

A. torpidity

B. passivity C. inactivity D. inertia* 12. The property of an isolated conductor to store electric charge is

A. capacitance* B. conductance C. permeability D. accumulation 13. If the properties of a body are the same in all directions, it is called

A. isodynamic B. isotropic* C. isogonic D. isotopic 14. The property of an object that determines the direction of heat flow when

in contact with another object is called

A. calidity B. pyrexia C. caloric D. temperature* 15. The rate of flow of thermal energy through a material in the presence of a

temperature gradient is called

A. thermal capacity B. thermal conductivity* C. thermal radiation D. thermal convection 16. The property of some crystals of absorbing light to different extents,

thereby giving to the crystals different colors according to the direction of the incident light is known as

A. dichroism*

B. dichromatism C. diastrophism D. chromaticity


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17. Emission of radiations from a substance during illumination by radiations of higher frequency is called

A. illuminance

B. fluorescence* C. radioluminescence D. incandescence 18. If a material is feebly repelled by a magnet it is

A. diamagnetic* B. paramagnetic C. ferromagnetic D. ferromagnetic 19. The progressive decrease of a property as a result of repeated stress is

called

A. debility B. rigidity C. elastic deformation C. fatigue* 20. Property of some pure metals and their alloys at extremely low

temperatures of having negligible resistance to the flow of an electric current is called

A. supercharging

B. supercooling C. superfluidity D. superconductivity*

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Chap 5 Engineering Materials