Types of Engineering Materials Metals • Crystalline in structure • Opaque and lustrous • Good electric and thermal conductivity • Changing geometric structures permanently • High strength, stiffness and ductility Alloy: When two or more metals are combined together Examples: Copper, Steel, Aluminium, Gold, Brass, Bronze, Super alloys

Polymers • Long molecular chains • Backbone of carbon atoms • Light weight • Low electrical and thermal conductivity • Generally resistant to atmospheric attack Examples: Polyamide, Polyvinyl chloride (PVC), Polystyrene, ABS, rubber and nylon Applications: Carrier bags, plumbing pipes, hoses, tyres, domestic items

Ceramics • Combination of 1 or more metals with a non-metallic element (oxygen, nitrogen, carbon) • Formation of oxides, nitrides, borides, silicides and carbides • Brittle • Good thermal and insulating properties • High compressive strengths but poor “plasticity” (the quality of being easily shaped or moulded) Examples: Refractory, glass, concrete, silica, magnesium oxide and fireclay Applications: Glassware, electrical insulators, cutting tools and building bricks

Composites • Two or more materials uniquely combined • One material will serve as a matrix to hold the reinforcement

• Example: Cement holding steel bars • Fibrous composites (roof covers)

• Light and strong • Particulate composites (cermets)

• Hard and abrasive resistant • Usually made of Substrate and the resin

• Substrate: Fibreglass, carbon fibre, Kevlar • Resin - structural glue: Polyester resin, Vinylester resin, Epoxy

• Epoxy Resin is a transparent matrix when cured, used in aerospace industry

ENGINEERING MATERIALS

Properties of Materials Physical Properties • Not easily altered • Remain intact whereas mechanical properties can be change through heat treatment

• Density • Mass/Volume • Packing arrangement of atoms

• Opacity/Transparency • Colour

• Inherent reflected wavelength • Melting Point • Thermal Conductivity

• Copper has the highest (1340 KJ/mh°C) • Wood has low TC (1 KJ/mh°C)

• Electrical Conductivity • m/Q-mm² • Copper has EC of 64,000 m/Q-mm² • PVC has EC of 1014 m/Q-mm²

• Thermal Expansion • 1/℃ • Coefficient of Thermal expansion is ∝ melting point of a material • Higher melting point materials have less expansion

• However steel is an exception • (non/Ferro) Magnetism • Corrosion Resistance

• Resistance to surface deterioration primarily caused by oxygen, chemicals or other agents • Though, degradation in plastics can be caused by UV and moisture

Mechanical Properties

• Strength • kgf/mm² ( σ ) Stress • Load per unit area • Tensile, Compressive, Shear • Strain ε

• mm of deformation per mm of material length • Tensile Properties

• Stress/Strain (for ductile materials) “able to be deformed without losing toughness; pliable, not brittle” • Strain

• Change a material undergoes during elongation or contraction • Elasticity

• Recovery of material after being deformed/ stress is removed • Deformation of a good elastic material is not permanent • Deformation of Plastic (plastic deformation) is permanent

• Figure 4 - General Stress/Straight Graph (Right) • Point A - Proportional / Elastic limit • Point B - Upper Yield Stress Point • Point C - Lower Yield Stress Point • Point D - Maximum tensile stress • Point E - Fracturing point

Thermal Conductivity

Electrical Conductivity

• Figure 5 - Different Force/Extension graph for different materials (Right) • Apart from ductile material such as mild steel, many materials do

not exhibit a marked discontinuity called yield point

• Common Materials Terminology • Yield Strength ( σy )

• Stress level which large strains take place without further increase in stress

• Maximum stress before plastic flow (permanent deformation) sets in

• Proof stress • Force / Extension • Equivalent to yield strength

• Plasticity • Material’s ability to undergo permanent deformation without

rupture • Modulus of Elasticity (E)

• Measure of a stiffness of a material • Ration of stress to strain at elastic limit

• Tensile Strength • UTS • Maximum stress of a material without fracture • Dividing the max load to cross sectional area of material

under test • N(-mm²)

• Force / Area • Compression

• Extent to which material deforms under a compressive load prior to rupture • Bubble gum does not rupture, concrete will crack under heavy load

• Bending • Characterised by outside fibres of a beam placed in tension and the inside fibres in compression

• Torsion • Application of torque - “a force that tends to cause rotation” causing it to twist about it’s

longitudinal axis • Shear Strength

• Maximum load a material can withstand without rupture when sheared • Bubble gum has little shear strength

• Ductility • Ability to withstand plastic deformation without rupture. • Examples: Low carbon steels, aluminium, copper, gold, silver and nickel

• Malleability • Same as ductility however malleability is associated with compressive deformation while ductility

is tensile deformation “ Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically without fracture. In material science, ductility specifically refers to a material's ability to deform under tensile stress; this is often characterised by the material's ability to be stretched into a wire.

Malleability, a similar concept, refers to a material's ability to deform under compressive stress; this is often characterised by the material's ability to form a thin sheet by hammering or rolling. Ductility and malleability do not always correlate with each other; for instance, gold is both ductile and malleable, but lead is only malleable.”, https://sg.answers.yahoo.com/question/index?qid=20090303051823AAFRJmT, 2008

• Hardness • The ability to withstand penetration, scratching or

wear • Clay is not hard, glass is hard but has a limit since

it shatters • If the material is deformed when hit by a hammer,

then the hammer is said to be harder than the material.

• Different scales and tests to measure hardness but the most common are:

• Brinell i ) Hardened steel ball as the indentor ii ) Brinell hardness number (BHN)

• Vickers i ) Vickers hardness scale (VHS)

• Rockwell ( B&C Scales ) i ) Rockwell hardness scale (HRC) ii ) N&T Scales use smaller indent force

• Brittleness • Opposite of ductility • No relation to tensile strength • Associated with extreme high hardness • Examples: Hardened steels, cast irons and

ceramics • Impact Strength

• Toughness of a material • Depends on ductility and strength • Chirpy and Izod impact tests

• Chirpy test is when the specimen is struck when in a horizontal position (Struct in the middle of the span)

• Izod test is when the specimen is struck when in a vertical position (Struck above the notch)

• Pendulum swings and strikes the specimen. The lost energy makes the difference in height.

• Fatigue Strength • Vibrational stress can cause fractures even when

stress is less than determined tensile test • Fatigue failure/ cyclic stressing is when you bend

something repeatedly • Examples: Connecting rods, gears, crane hooks,

bolts and aircraft bodies. • Creep Resistance

• Long period of time may deform and fracture well below tensile stress than determined

• Usually accompanied by high temperatures (Creep)

• Example: Turbine blades and boiler tubes • Structural beams and milling machines