ENGINEERING STUDIES - MATERIALS

general MATERIAL info

METALS

- ↑ electrical & thermal conductivities

- Ductile

- Stiffness, toughness & strength

Engineering metals: alloys - metals are too weak in pure state.

E.g. pure iron = weak & soft, +dding carbon = tough steel.

Ferrous Alloys: >50% iron:

- Plain carbon steels

- Alloy steels

- Cast irons

Non-Ferrous Alloys: <50% iron:

- Light alloys of Al, Mg, Ti and Zn

- Heavy alloys of Cu, Pb, Ni ($$$)

- Heat resistant metals - molybdenum, tungsten

- Precious metals - gold, silver, platinum

POLYMERS & ELASTOMERS

- Organic materials that are ↑ modified = desirable properties (rubbers +

elastics)

- Thermo soft ening plastics: soften w/ heat (flexible + rel. soft)

- Thermo set ting plastics: do NOT soften w/ heat (rigid + hard)

Elastomers: structure allows large extensions that are reversible (RUBBER BANDS!!)

Polymers: ↓electrical & thermal conductivity - softer, less dense + ↑ corrosion

resistance than metals.

CERAMICS

- HARD + BRITTLE

- ↑compressive + ↓tensile strengths

- ↓low electrical + thermal conductivity

- Chemically inert - do not react w chemicals

E.g. glasses, cements, fired clays (pottery), electronic ceramics (semiconductors,

superconductors), engineering ceramics.

COMPOSITES

- Bonding 2+ materials to create a final material = combo of good properties of bonded

materials

E.g. glass reinforced polymer (GRP), reinforced concrete, asphalt, wood.

Generally = ↑specific strength (strength:mass)

CIVIL STRUCTURES

Testing of materials - specialised testing, x-ray & testing concrete:

Non-destructive testing:

X-ray Dye penetrant Ultrasonic

X-ray passed through object

and expose a photo film on

opp side

= expose cavities within

component

Dye placed on surface and

examined, after cleaning -

under UV light

= find small cracks in surface

↑ Frequency radio waves

sent by transmitter,

through components.

Reflected signals show

result

= determine cavities

Concrete/Destructive testing:

Tensile Test piece stretched - load and ext recorded

= determine tensile strength of materials

Compressive Test piece compressed - load and

deformation recorded

= determine compressive strength of material

Slump Wet concrete placed in mould. When mould is

removed, amount of deformation of shape is

measured.

= measures workability of concrete

Transverse = determine a material performance when

going through bending or shearing

Torsion = how well material copes with twisting forces

Crack Theory - crack formation & growth, failure due to cracking, repair/elimination of

failure due to cracking:

Crack Formation & Growth:

- Method of brittle failure.

- When crack forms, strain energy is retained but released from the area adj to the crack

Failure due to Cracking:

- More brittle = shorter critical crack length

- Critical crack length exceeded = failure inevitable if stress levels maintained

Repair/Elimination of Failure due to Cracking:

Repair

Metal: welding.

- Weld repairs crack but changes the microstructure = weaker material. Might need heat

treatment to solve this issue

Polymer: adhesives.

- If failure in thermoset + no adh available - replacement of material

- Thermoplastic material - polymer welding = strength close to parent material

Ceramic: glue/cement

- Repairs in stone + concrete = difficult.

- Replacement!!

Composite: replacement.

- Only efficient way

Elimination

Design w/o sharp corners: corners concentrate stress

Placing an interface for material: an area within a material, weaker than surrounding area that

runs perpendicular to growth of crack.

- When crack travels through material = blocked and never reach critical crack length

Ceramics - structure/prop, glass, cement, bricks:

Structure/properties:

- Hard, brittle, chemically inert, electrical & thermal INSULATION, durable

- Compressive strength

GLASS

- Non-crystalline ceramics

- 3 basic ingredients: silica, limestone, soda ash

- Soda-lime glass: acc for 90% glass - windows, bottles etc.

- Borosilicate glass: used for ovenware, telescopes

- Lead glasses: optical comp, radiation shielding

- Main properties: transparent, brittle, compressive strength

- Properties can be improved: thermal toughening, chemical toughening, laminating

CEMENT

- Bonding material

- Compressive strength

- Low toughness

- Easily casted

- Excellent workability

Composites - timber, concrete, asphalt, laminates, geotextiles:

TIMBER

- Organic material

- Structure: cellulose tubes bounded together by glue lignin (wood grain)

- Factors affecting strength: loading duration, moisture content, defects within grain

- Exposure to chemicals

- adversely affected by weather & susceptible to attacks from pests e.g. termites.

2 types of wood: hardwood (pored) & softwood (non-pored)

CONCRETE

- Compound of sand, gravel, cement and water

- Reinforced: steel bars embedded in concrete to + tensile strength

- Pre-stressed: conc poured over steel wires/cables that are placed in tension. After

conc hardened, tensile stress on cables released

- Post-tensioned/stressed: conc is cast w tubes running thru slab. After setting and

curing, wires pulled through slab and anchored to plates @ one end and tensioned @

the other. When comp stress gained & tension gained, cables are left in this state &

cement slurry injected to stop corrosion.

ASPHALT

- Aggregate, bitumen, and air voids

- Aggregate held together w bituminous binder

- +dding small amts of materials: rubber, alter asphalt properties

- Toughness

- Durability

- Resistance to moisture, heat etc. (weather resistance)

LAMINATES

- Materials sandwiched together

- Plywood: layers of timber w adhesive

- Laminated glass: 2 layers of glass with PVB polymer in middle - adds strength

- Fiberglass: glass fibres bonded w polymer resin

GEOTEXTILES

- Woven polymers/ceramic fibres

- Used to stabilise road base, geotextile is placed under asphalt - prevents potholes

Corrosion - corrosive env, dry, wet, stress corrosion:

Corrosive environments:

- Availability of oxygen to enable reactions to proceed

- Temperature

- Oxidation occurs when the metal loses electrons, and occurs at ANODE

- Reduction is the consumption of electrons, and occurs at CATHODE

OIL RIG = oxidation is loose, reduction is gain

Dry, wet and stress corrosion:

Dry Corrosion Wet Corrosion Stress Corrosion

Occurs through chemical

reactions with gases @ high

temps

E.g. furnaces

Occurs when material is in

contact with fluid/moisture

using an electrolyte

Uniform attack - when metal

placed within electrolyte and

some parts anodic/cathodic

Galvanic corrosion - when

dissimilar metals placed in

presence of corrosive env

When material subjected to

stress & cracks begin to form.

Material eventually degrades

due to fatigue

Protecting civil structures - painting the surface of the material OR galvanising - dipping metal

into molten zinc that covers the steel and protects it from corrosion by acting as a passive layer.

Recyclability of materials:

Steel:

- BOF (basic oxygen furnace) - 25% recycled steel

- EAF (electric arc furnace) - 100% recycled steel

Concrete:

- Recycled conc weaker than OG product

- Usually used as rubble

- Conc = crushed/broken down & re-used

Wood:

- Can be recycled for basic uses - furniture, pallets

- Dependent on type of wood

- Chips for garden mulch, playground covering

- Smaller chips to form wood composites

- Recycled as paper/cardboard

Asphalt:

- Limited use for recycled products

- Crushed and refined w other materials added to reproduce asphalt

Glass:

- Reused to produce glass again

PERSONAL & PUBLIC TRANSPORT

Testing of materials - hardness, impact:

Hardness Testing

Indenter forced into material - the harder it is to make a dent, the stronger the material

Brinell Hardness Test Vickers Hardness Test Rockwell Hardness Test

Tungsten carbide ball forced

into surface of material that

affect the surface finish

Small diamond indenter in

pyramid shape forced into

surface of material

= more accurate than BHT

Diamond pyramid and ball

indenters

= more convenient and least

impacted by surface

conditions

Impact Testing

Measure the energy needed to fracture specimen

Notched-bar Impact Test Izod Impact Test Charpy Impact Test

Simulate the response of a

material to sudden blow

Notch used to localise stress

conc

= reveals brittleness caused

by incorrect heat treatment,

faulty casting, and alloying,

and structural defects

Has a vertical test piece (I is

vertical - so is the specimen

used!!)

Has a horizontal test piece (H

for horizontal!!)

Visual testing

● Dye penetration

○ Dye or coloured liquid is placed on the surface of a component and excess is

wiped clean

○ Any cracks or imperfections on surface of component will be highlighted by the

dye remaining

○ Fast, simple, inexpensive

○ Used for small specimens and various materials

○ Difficult to detect small cracks

○ UV light is also used to help show up any imperfections

● Magnetic particle testing

○ Component is placed on a conducting rod, that produces a magnetic field about

the component

○ Fluorescent liquid of charged particles is sprayed over component

○ Fluorescent magnetic particles are drawn to the cracks by the conducting rod,

highlighting surface imperfections

Radiographic examination

● X-Rays

○ Favourable because a photo film is produced, for close analysis

○ Detects subsurface defects

○ Radiation is used to penetrate the item, with any voids allowing the rays to pass

through more easily, resulting in a dark area of film

○ Used on large objects

○ Longer/more expensive than visual testing

○ Exposure to radiation can be harmful to humans

● Gamma Rays

○ Effective when testing thick structures, i.e., steels

○ Can be used to examine joining methods, i.e., welds

○ Exposure to radiation can be harmful to humans

Ultrasonic testing

● Detects subsurface defects

● A probe transmits high frequency vibrations throughout the component as it passes over

the surface of a component

● Any imperfections within the component causes the vibration to be reflected without

travelling to the bottom

● Results are displayed on detection machine

Heat treatment of ferrous metals - annealing, normalising, hardening & tempering,

changes in macro/microstructure, changes in properties:

ANNEALING Annealing = heating & cooling of a metal to

produce SOFTEST STATE.

(i) relief of int stresses

(ii) produce UNIFORM GRAIN structure

(iii) to soften material 4 further

working/machining

Process Annealing: BELOW RED HEAT

- Heat steel btwn 550-650 degrees C

- Relieve INTERNAL stresses in

material

- Air cooled

- Complete recrystallisation of material

FERRITE crystallised to form equiaxed

grains, but pearlite grains remain elongated

Full Annealing: RED HEAT

- Heated above 923 degrees C

- Soaked

- Cooled within furnace - SLOW

process

- Produces softer steel

All grains changed to AUSTENITE

Oversoaking = large grains = weakened

structure

$$$

SUB-CRITICAL ANNEALING Soften metals w carbon content >0.3%

Soaking fore several hours 650-700 degrees

C

= cementite form as spheres bc of surface

tension – cooled slowly in a furnace

= material easy to machine

NORMALISING Heating ABOVE RED HEAT

(austenising) and then cooling in still air to RT

Higher temp than Pannealing = normalised

metal cools faster even though they are both

cooled in air.

- Smaller grains than Pannealing =

harder & stronger steel – finer grain

structure

- Refine grain structure (uniform +

equal in size), improve machinability

- Increase in UTS

- Decrease in ductility

HARDENING/TEMPERING Hardening:

- Heating steel w sufficient carbon to

red heat (austenising) and cooling

quickly (quenching)

- Extremely hard & brittle material =

rapid cooling = martensite – needs

further treatment to become tougher

Tempering:

- Heating hardened steel to

temperature below 723 degrees C,

soaking to remove INT stresses + to

allow structural changes to go to

equilibrium followed by cooling.

↑tempering heat = ↓tensile strength,

yield stress & hardness, ↑ductility.

Annealed → coarse grain structure → soft with moderate strength

Normalised → fine grain structure → higher strength

Hardening → stressed grain structure → hardness + brittleness

Tempering → very fine grain structure → toughness + hardness

Manufacturing process for ferrous metals – forging, rolling, casting, extrusion, powder

forming, welding :

FORGING Forging:

- Forming metals by compressive

forces = above recrystallisation

temp

- Produces grain flow in metal =

better mechanical prop than

produced by casting/machining.

= valves, bolts, gears, connecting rods.

Closed Die Forging:

- Squeezing of hot metal between 2

shaped dies and the excess metal

trimmed off in trimming die

Drop Forging:

- Form of closed die forging

- Upper die dropped onto lower die

Open Die Forging:

- When metal hammered/pressed by

a vertically moving tool onto a

stationary tool

ROLLING Rolling:

- Metals pressed into shape through

rollers. LIKE PASTA/DOUGH!!

Hot Rolling:

- Done to reduce ingots and billets to

required shape

- Unstressed finished product

- Easier to do than cold rolling

- Favourable directional grain flow

= smaller grain structure - refined

Cold Rolling:

- Cleaner, smoother and more

accurate finish

- Grain flow = harder, stronger less

ductile product

CASTING Casting: pouring molten metal into mould

for specific shape.

- Cheap

- Can affect physical prop of metal

depending on type of cast used

Sand Casting:

- Mould MADE FROM SAND

- High dimensional accuracy

+ve:

- inexpensive

- good for forming

tough alloys

- no directional prop

-ve:

- thin & projecting

sections = difficult

- POOR surface

finish

- defects:

cracking,

porosity, blow

holes, piping.

Shell Moulding:

- Making a copy of desired part and

surrounding it with special sand that

has thermosetting resin that

hardens when heated.

Investment casting:

- Special wax runs through accurate

metal mould to produce exact WAX

replica.

Die Casting:

- Metal forced into mould cavity

under pressure

Centrifugal Casting:

- Molten metal injected into

SPINNING mould

- Forces molten metal to stick to

interior of metal

EXTRUSION Extrusion: metal forced through die, so it

takes shape of the die as it passes

POWDER FORMING Powder Forming:

- Metal powder mixed with other

materials and poured into mould at

RT

- Mixture pressed into mould 4

desired shape

- Pressure = particles together

- Pressed item = sintered in

controlled atm furnace

- Heated 2 temp. Where atoms are

allowed 2 diffuse between grains,

producing uniform grain structure

- Used = form brake pads →

materials with different properties

mixed together to give superior final

product

- Difficult to produce certain shapes

WELDING Electric Arc Welding:

- High elec current produces an arc

btwn work & electrode that jumps

from electrode 2 work.

- ↑ temps that melt the electrode and

part of the work metal = VERY

strong joint

- Strong, convenient, LOW COST &

flexible method of joining ferrous &

non-ferrous metals

Shielding:

- To an inert gas shield that is

directed over the molten metal

during welding

MIG = Metal-Inert-Gas, for general

welding & welding aluminium

TIG = Tungsten-Inert-Gas, for welding

stainless steels, aluminium

PAW = Plasma-Arc-Welding, more

precision than TIG

Electrical Resistance Welding:

- Produces ↑temps to melt metals by

passing an elec current through

interface of joint that creates

resistance heating at joint,

Butt welding – 2 surfaces which butt up to

each other

Seam welding – line of welded material

btwn 2 sheets

Spot welding – spot btwn two Cu

electrodes welded on either side of the

metal

Gas Welding:

- ↑ temp flame to melt parent metal w

filler rod being used to provide extra

metal 4 joint

Solid State Welding:

- Bonding 2 metals in SOLID STATE

Pressure welding – ductile metal pressed

onto similar/dissimilar metal – cladding of

Al drink cans

Friction Welding – 2 spinning

surfaces/spinning tool over surfaces to be

joined = sufficient heat to form bond as

strong as parent metals

Explosive Welding – 2 surfaces forced

together by controlled explosive charge.

Manufacturing processes for non-ferrous metals: alloying, annealing, solid solution

hardening:

Aluminium Brass Bronze

- Non-corrosive

- Lightweight

- Good strength to

weight ratio

- Easily fabricated

- Very good electrical

conductivity

- Ductile

- Alloy of copper and

zinc

- Corrosion resistance

- Cannot spark

- Low coefficient of

friction

- Alloy of copper and tin

- Excellent corrosion

resistance→ from

oxidization

- Hard

- Brittle

Aluminium silicon:

- Good casting

properties

- More corrosive than

pure aluminium

Aluminium copper:

- High strength

- Good electrical

conductivity

- More corrosive than

pure aluminium

- Hard

Aluminium silicon-

magnesium:

- Medium strength

- Weldable

- Car doors

Annealing:

Annealing is used to relieve any internal stress in a cold worked alloy. This results in an

equiaxed grain structure.

Precipitation hardening:

- Step 1-Solution Treatment: The alloy is heated to 530 degrees until the β phase

dissolves to produce a homogenous sing phase alloy. It’s then quenched to room

temperature.

- Step 2-Aging: Over time the trapped β phase precipitates out on stress planes within the

quenched phase, thus restricting dislocations and strengthening the alloy

Ceramics and Glasses – as an insulation material, laminating & heat treatment of glass,

structure/prop:

Semiconductors

they are essentially poor conductors, but they will allow a current to flow past

N-type semiconductor-an excess of electrons

P-type semiconductor-a deficiency of electrons

Semiconductors are most important for the use of a p-n junction. This is where a layer of p-type

and a layer of n-type are butted against each other. What this does is form a one-way gate for

electricity to flow through.

Extra shit: This is accomplished due to the depletion zone. This is a gap between the positive

and negative charges of the semiconductor material. If the current flows in one direction, the

unlike particles attract, which reduces the depletion zone and allows current to flow.

Ceramics

Ceramics have a greater tolerance to heat than metallic alloys. This means, if used, engines will

not require a cooling system which is responsible for the loss of 20% of the heat energy the

motor uses. Ceramics may lead to improved thermal efficiency and better fuel efficiency.

Glass

HIGH SILICA GLASS Refined from borosilicate glass and is nearly

entirely silica.

- perfectly clear and are used in

situations that experience great heat.

- E.g., of these applications include

missile cones and space vehicle

windows.

SODA LIME GLASS - It will not recrystallise, water resistant

and is cost effective.

- used for window and plate glass,

bottles, tableware, electric light bulbs

and windscreens.

BOROSILICATE GLASS - 20% boron and silica.

- good chemical resistance and low

thermal expansion.

- used in electrical insulation, gauge

glasses for laboratory ware, and

domestic cooking and ovenware.

LEAD GLASSES - 40% lead.

- They have a high refractive index,

which makes them optically clearer

which means they are used

extensively for optical glass.

- used for thermometer tubes and

tableware.

Heat treatment

Tempered glass - Heat treatment of glass increases resistance to fracture by creating

compressive surface layer

Glass heated to around 650˚C

Subjected to air quench → rapidly cools surface

Cooling surface contracts → placed under compression

Laminated glass

Consists of a sandwich of two layers of glass and a polymer interlayer of PVB joined

under heat and pressure

Polymers – thermosoftening, textiles, extrusion, injection moulding, blow moulding,

structure/prop:

Basic structure consists of molecules composed of repeating atoms of the same element that

are joined together by chains.

- The basic unit of any polymer is the carbon atom - forms the backbone of the polymer

chain.

- Different polymers with different properties can be produced when replacing the

hydrogen atom with another element.

Thermoplastics - covalent bonds (atoms sharing same electrons and hence fusing them

together) form the polymer chains but only weak secondary bonds between the chains →elastic,

malleable

Thermosets - covalent bonds (atoms sharing same electrons and hence fusing them together)

form both the polymer chains and secondary bonds between the chains →rigid, strong, less

elastic

Applications – thermoplastics - In transportation systems, thermoplastics are mainly used as

interior components (dashboards, linings etc.)

Applications – thermosets - In transportation systems, thermosets are mainly used as interior

components and textiles, however in some modern transportation systems composite

thermosets (more than 2 substances combined) can be used to make exterior parts such as

body panels. Exterior components of boats → waterproof, rigid, buoyant, hard, tough

Engineering textiles

Thermosets act as binder for textiles → adds tensile and compressive strength and durability to

textile

Manufacturing processes for polymer component

- Injection moulding - plastic is heated from granular form and melted into resin form and

then injected through a die by way of a ram into a cavity or cast → usually

thermoplastics

- Extrusion - plastic is heated from granular form and melted into resin form and then

injected through a die by way of a ram onto a conveyer belt to cool

- Compression moulding – in granular form, plastic is placed in mould where heat and

pressure is applied to melt the plastic allowing it to flow within cavities → used for

thermosets

AERONAUTICAL ENGINEERING

Aluminium and its alloys used in Aircraft

- Aluminium and its alloys are desired in aircraft due to their low density

- The precipitation hardening of duralumin is an important alloy for aeronautical

engineering.

- Pure aluminium and duralumin can be rolled together, and pressure welded so that

duralumin can gain the corrosion resistance that aluminium offers.

Polymers

- Polymers are used as a solution to reduce the weight of the aircraft.

Composites

Carbon Fibre

- It is lightweight, has very high specific strength and a high modulus of elasticity.

- is also used for its resistance to cyclic stress that carbon fibre exhibits

- An example of its use may include the Boeing 737 and A330 Airbus for control surfaces

and wingtips

- its disadvantage is failure is sudden and often catastrophic.

Aramid Fibre

- Often referred as Kevlar, it is more impact resistant in comparison to carbon fibre.   It is

important in battle situations as shrapnel and debris has the potential to cause damage

to the aircraft.

Metal Matrix Composites

- Used to simply improve the property of a material for example, boron fibre aluminium is

used to improve tensile strength

- They are difficult to manufacture

- Capable of withstanding high temperatures

Adhesives

- Epoxy adhesives are used to join composite aircraft surfaces to the base structures.

Corrosion: Corrosion poses as a large problem in aircraft design as the frames and skin of the

aircraft already stressed, so weakening via corrosion is a major concern

- Composites offer resistance to electrochemical corrosion; UV light and the weather may

degrade them.

Pit and Crevice Corrosion:

- this is a concentration cell that occurs because of different oxygen levels at the top and

bottom of a crevice

- Moisture forms in these gaps each time the aircraft passes what is known as a dew

point, which is a particular combination of pressure, temperature and moisture content in

the air.

Aircraft are always having moisture of them because:

- Condensation on the airframe from temperature changes in atmosphere of Airborne

moisture accumulating rain

TELECOMMUNICATIONS ENGINEERING

Voltage, Current and Insulation

- A material resistivity must be assessed and checked before its use

- High resistivity = insulator, low resistivity=conductor

- Allowing a current with a certain potential difference is a method of assessing the

material, this and the use of Ohm’s Law, telecommunications engineers can find

information about length and cross-sectional area of a conductor.

- Multimeters and Cathode Ray Oscilloscopes are used to test electrical quantities.

Copper and its Alloys, used in Telecommunications

- Copper in terms of conductivity, is second only to silver but it’s much more cost effective.

Pure Copper

- Pure copper is an excellent conductor, but impurities may lower this trait. So, keeping

impurities low is essential.

Copper Cadmium Alloy

- Has good wear resistance and greater strength than pure copper which reduces the

number of support towers per kilometre of overhead cable.

- The microstructure of annealed Copper Cadmium is an equiaxed single-phase structure

where all the Cd (cadmium) is dissolved in the solid copper.

Semiconductors Diodes

- Used in devices as one-way components. Zener diodes that act as a one-way

component up to a certain voltage. Beyond this voltage, current begins to flow both

directions through the diode.

Light emitting Diodes (LEDs)

- These diodes give off light and are extensively used as indicators doe

telecommunication equipment.

- The current can only flow in one direction and a visible radiation is emitted.

- They offer greater reliability and less power consumption than conventional lamps.

Integrated circuits

- They are a complex miniaturised circuit containing various semiconductors and other

electronic components. Various types are available, each with a differing function.

Lasers

- These are a type of semiconductor where; by creating to pn junctions, and passing a

large forward bias current through them, the electrons hols recombine and give off

photons. These are used in CD players and signal transmitters for fibre optic systems.

Polymers

- Polymers find extensive use in telecommunications

- Used for insulation and casings for various devices.

- Manufacturers make extensive use of polymer casings for mobile and fixed telephones.

- Polymers offer insulation, shock resistance and also lend themselves to mechanised

production by injection moulding.

Fibre Optics

- They function around the concept of transmitting electronic information, encoded onto a

light beam.

- A fibre optic system consists of three parts: a device to convert electric current into light,

the cable to carry the light and a receiver to convert the light back into electric current.

- Fibre optic cables are lighter and less expensive than copper cables and are less

affected from interference.

-  The primary reason for their use is that it can carry hundreds of times more information

than can be carried on a copper wire.

Types and Applications

- Single mode- it means a more narrows cable diameter. It is more favoured for long

distance telecommunications as it has the highest bandwidth and suffers the least from

losses of light. It is also more costly to manufacture.

- Multimode – These cables have a larger inner core diameter. There are two types;

graded index and stepped index. With a larger diameter core, light waves can take

differing paths, which in turn means they take different times to reach their destination.

Graded index uses a refractive index which allows the cable to flow through the cable

instead of reflecting along it.