327431569 Engineering Studies Notes 1 PDF

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Michael McElroy

Year 12 Engineering Studies

Engineering HSC Syllabus Summary Civil Structures

Engineering mechanics and hydraulics 

Stress and Strain - Strain › The proportional change in length caused when a specimen is under load › › -

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e(extensionm) L(originallength m)

No units (ratio)

Shear stress › A measure of the internal reaction that occurs in response to an externally applied load

Stress (σ Pa ) =

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Strain ( ε )=

P(Load N ) 2 A ( Area m )

Engineering and working stress › Engineering stress – the original C.S.A is used to calculate the stress for every value of the applied force › Working stress – The actual or constantly changing C.S.A value is used to calculate the stress Yield stress, proof stress, toughness, Young’s Modulus, Hooke’s law, engineering applications › Yield stress- the stress where there is a marked increase in strain without an increase in stress. Yield stress is always greater than the elastic limit, but less that the UTS › Proof stress- used as a measure on materials that do not show a marked yield point. Usually a set amount of strain is given to the material, usually 15 or 2% and the amount of stress can be calculated › Toughness- indicated by the area under the curve in a stress/strain diagram. Ability of a material to absorb energy › Young’s Modulus- Measure of the stiffness of a material. Applies up to the elastic limit of a material. The gradient of the straight line in section of the graph indicates YM. › Hooke’s law- the amount of elastic deformation that a material can sustain in tension or compression before it undergoes permanent plastic deformation. Factor of safety ›

For ductile materials –

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For brittle materials -

FOS= FOS=

Yield stress max . allowable stress

UTS max . allowable stress

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Stress/strain diagram

Truss analysis - Method of joints

Year 12 Engineering Studies

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Year 12 Engineering Studies

Methos of sections

Bending stress induced by point loads only

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Concept of shear force and bending moment › Shear force A shear force causes one part of a material to slide past the adjacent part of the material

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Bending moment The bending moment is the amount of bending that occurs in a beam

Michael McElroy

Year 12 Engineering Studies

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Shear force and bending moment diagrams

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Concept of neutral axis and outer fibre stress › As a beam bends, the concave side will compress and set up compressive forces within the beam

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Year 12 Engineering Studies

The convex side of the beam will stretch, producing tensile forces In between, there exists a plane where the fibres in the beam are not subjected to tensile or compressive forces. This plane is called the neutral axis The fibres furthest away from the neutral axis will be subjected to maximum stress

Bending stress calculation (second moment of area given)

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To calculate the bending stress (

σ ) at any section of the

beam.

σ=

My I

Where:

σ

= bending stress (Pa)

M = max. bending moment (Nmm) Y = distance from neutral axis (mm) I = second moment of area of the cross section of the beam (mm4)

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Year 12 Engineering Studies

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Uniformly distributed loads - Unlike a point load, UDL is a load that is spread across a beam

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Crack theory - Crack formation and growth › Cracking begins with micro cracks (crack initiation phase) › Cracking then leads to crack propagation, that phase in which the crack grows in size under cyclic loading to ultimate part failure › Crack initiation can be long and develops from repetitive stresses at stress concentrations › Dependant on material and method of component manufacture › Surface cracks can be detected using visual inspection techniques such as magnetic particle add dye penetration tests › Sub surface cracks require ultrasonic or radiographic methods to be detected - Failure due to cracking › Cracking can occur at stresses below yield stress, known as fatigue › Fatigue fracture begins as small crack, that grows in size from repeated stress › As a crack expands, the load carrying cross-section of the component is reduced, with the result that the stress on this section is raised - Repair and/or elimination of failure due to cracking › For metallic materials: welding can repair crack. However doing this will repair crack but micro structural changes will appear around the weld and weaken the material, with weld being a point of stress concentration. Heat treating the material after welding will avoid this › For polymers: adhesives can be used to repair crack. This cannot be done for thermosets, instead it must be replaced

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Year 12 Engineering Studies

For ceramics: usually can’t be repaired – need to be replaced Prevention: designing item without sharp corners will reduce cracks from forming as stress can be concentrated at these points

Engineering mechanics and hydraulics 

Testing of materials - Compressive testing › Used to determine the compressive strength of materials › Test piece is compressed and load deformation is recorded - Transverse beam testing › Many materials used are not only in compression or tension at the same time. They can be exposed to bending stresses › Used to determine bending and shear in materials › Transverse beam testing involves placing a test piece between two › supports and then gradually applying a load - Concrete testing › The water/cement ratio in concrete effects the workability of the mix and also the final strength of the concrete › Slump test – measures the workability of concrete. Wet concrete is placed in a mould. When the mould is removed, the amount of deformation of the shape is measures and is used to describe the workability of the concrete › Compression test – compression testing of concrete is measured after 28 days. This is done to test the strength of the concrete.

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Ceramics - Structure property relationships, applications › Hard, brittle, chemically inert, electrical/thermal insulation, durable › Compressive strength - Glass › Non-crystalline ceramics › 3 basic ingredients are: silica, limestone, soda ash › Soda-lime glass: accounts for 90% of glass – windows, bottles etc. › Borosilicate glass: used for ovenware, telescopes › Lead glasses: optical components, radiation shielding › Main properties: transparent, brittle, compressive strength › Properties can be improved by: thermal toughening (air quenching), chemical toughening, laminating - Cement › Bonding material › Compressive strength › Low toughness › Easily casted › Excellent workability

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Composites - Timber

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Year 12 Engineering Studies

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Organic materials Structure: cellulose tubes bounded together by glue lignin (wood grain) › Factors affecting strength of timber: loading duration, moisture content, defects (within grain) › Exposure to chemicals Concrete (reinforced and pre-stressed) › Concrete is a compound of sand, gravel, cement and water › Reinforced concrete: steel bars imbedded in concrete to add tensile strength › Pre-stressed: concrete is poured over steel wires or cables that are placed in tension. After concrete is hardened, tensile stress on cables is released Asphalt › Consists of aggregate, bitumen and air voids › Aggregate held together with bituminous binder › Adding small amounts of materials, such as rubber, alter asphalt properties › Toughness › Durability › Resistance to moisture, heat etc. (weather resistant) Laminates › Consists of materials that are sandwiched together › Plywood: layers of timber with adhesive › Laminated glass: two layers of glass with PVB polymer in middle – adds strength › Fibre glass: glass fibres bonded with polymer resin

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Geotextiles › Woven polymers or ceramic fibres › Used to stabilise road base, geotextile is placed underneath asphalt – prevents potholes Corrosion - Corrosive environments › Availability of oxygen to enable reactions to proceed › Temperature - Dry corrosion, wet corrosion, stress corrosion › Dry corrosion – occurs through chemical reactions with gases, at high temperatures i.e. in furnaces › Wet corrosion – occurs when material is in contact with fluid or moisture › Stress corrosion – when a material is subjected to stress (i.e. cyclic loads) and cracks begin to form. The material will eventually degrade due to fatigue Recyclability of materials - Steel › B.O.F (basic oxygen furnace) – 25% recycled steel possible › E.A.F (electric arc furnace) – 100% recycled steel possible - Concrete › Recycled concrete weaker that original product › Usually used as rubble

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› Concrete is crushed/broken down and re-used Wood › Can be recycled for basic uses i.e. furniture, pallets etc. › Dependant on type of wood › Used as chips for garden mulch, playground covering › Smaller chips to form wood composites › Recycled as paper or cardboard Asphalt › Limited uses for recycled products › Usually crushed and refined with other materials added to reproduce asphalt again Glass › Can be reused to produce glass again

Personal and Public Transport

Engineering mechanics and hydraulics 

Static friction - Concept of friction and its use in engineering › Friction is the resistance to motion and efficiency › Friction always acts opposite to the direction in which the body moves › Static friction – frictional force present when two bodies are at rest › Limiting friction - frictional force present when two bodies are at the point of moving › Dynamic friction – frictional force while a body is moving - Coefficient of friction → amount of friction that materials develops between them (μ) › ›

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μ=

FF Frictinforce = R N Normal reaction

Normal force  Reaction force  Always perpendicular to supporting surface  Equal to, but opposite direction to weigh force  Balances out forces Friction force  Force that prevents movement  Force that is exerted between contacting surfaces  Always opposes direction of motion  Increases as applied force increases Angle of static friction (φ)  Resultant force (of friction force and normal force) makes with the normal  The angle that the resultant force makes with the normal reaction  tan φ=μ Angle of repose

Michael McElroy

Year 12 Engineering Studies

The angle when the angle of static friction will equal the inclination of the plane  When the gravitational force down an inclined slope equals the frictional force, i.e. the system is an equilibrium Energy, power › Potential energy  Stored energy within an object with the ability to do work  The PE is equal to the work done in lifting a body’s weight (mg) through a vertical height (h).  PE =mgh  Hydro electricity uses PE › Kinetic energy  Energy a body possess due to its motion  KE = Energy a body possess due to its motion 

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KE =

1 2

mv2

Work  W=∆KE or W=Fs or W=Fscosθ  When a force causes motion  Total work – multiple forces acting on a body Power  The rate at which work is done  P=w/t = Fs/t = Fv

Engineering materials 

Testing of materials

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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 ›

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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

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Year 12 Engineering Studies

Favourable because a photo film is produced, for close analysis Detects sub-surface 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

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Ultrasonic testing › Detects sub-surface 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

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Heat treatment of ferrous metals Heat treatment of steels › Heat treatment used to give steel to alter their properties. Heat treatment involves 3 main processes 1. Heating of metal to pre-determined temp. 2. Soaking (holding) of metal at that temp. until heat becomes uniform throughout 3. Cooling of metal at pre-determined rate such that is will cause formation of, or will maintain desirable structures within the metal Annealing › Process annealing Heat steel to temp. Between 550 and 650˚C Relieve internal stresses from within material Air cooled Complete recrystallisation of the metal › Full annealing Heated above 923˚C Soaked Cooled within furnace – slow process Produces softer steel Normalising › Heating to higher temps. than annealing Soaked Air cooled Produces fine grain structure – stronger material Increase in UTS, hardness Decrease in ductility Hardening and Tempering

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Hardening Hardening steel depends on carbon content Heat above 800˚C Soak Quench (cool very quickly) – water, oil, brine can be used Quenching causes stress to build up in steel – becomes extremely hard Quenching produces martensite – hard + brittle material that needs further treatment to increase toughness › Tempering Remove internal stresses from material that have been quenched (martensite) Retains hardness and replaces brittleness with toughness Heat steel between 200 - 600˚C Soak Cool in air Tempered martensite produced Structure property relationships › 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 Structure/property relationship in the material forming processes Forging › Shaping a metal through use of force › Can be done above recrystallisation temp. (hot forging) or below (cold forging) › Extrusion › Drop forging – technique that uses hydraulic pressure to operate a hammer that shapes metal o Dimensional accuracy not good o Grain flow/direction is major advantage – grain flow follows profile of part, no points of weakness. Contrasting to machined part where grain flow does not follow profile and provides points of weakness Rolling › Metal pressed into shape between rollers › Can be done as hot or cold rolling › Cold rolling o Compressed grains result in specific directional properties o High strength o Grains in material remain stressed o Increased harness › Hot rolling o Unstressed finished product o Easily performed than cold rolling o Favourable directional grain flow Casting › Pour molten metal into a mould to form a specific shape › Good dimensional properties → close to finished product › Cheap

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Can effect physical properties of metal – depending on material mould is made from › Sand casting o Mould made form sand o Chill grains form on surface of metal o Larger columnar grains form within metal → present weakness in metal – allows for metal to shear along the grain boundaries › Shell moulding o Form of sand moulding o High dimensional accuracy o Fine clean sand with thermosetting binder › Die casting o Metal forced into mould cavity under pressure o Excellent surface finish › Centrifugal casting o Molten metal injected into spinning mould o Centrifugal force forces molten metal to stick to interior of the mould Extrusion › Metal forced through die so it takes shape of the die it passes Powder forming › Metal power is mixed with other desired materials and put into mould in room temperature › Mixture is then pressed into mould to form desired shape › Pressure compacts particles together › Pressed item is sintered in controlled atmosphere furnace › Heated to temp. Where atoms are allows to diffuse between grains, producing uniform grain structure › Used to form brake pads → materials with different properties mixed together to give superior final product › Difficult to produce certain shapes Non-ferrous metals Aluminium › Non-corrosive › Lightweight › Good strength to weight ratio › Easily fabricated › Very good electrical conductivity › Ductile o Aluminium silicon Good casting properties More corrosive than pure aluminium o Aluminium copper High strength Good electrical conductivity More corrosive than pure aluminium Hard o Aluminium silicon-magnesium Medium strength Weldable Car doors

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Year 12 Engineering Studies

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Brass › Alloy of copper and zinc › Corrosion resistance › Cannot spark › Low coefficient of friction

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Bronze › Alloy of copper and tin › Excellent corrosion resistance→ from oxidization › Hard › Brittle Structure/property relationship Annealing, strengthening

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Ceramics and glasses Semi-conductors › Operates on the basis of the deficiency/surplus of electrons within a material › Semi conductors are a unique group of materials that, when subjected to a certain type of energy (i.e. thermal, electrical), can act either as conductors or insulators. The properties of these materials make them highly favourable in the electronics industry. Diodes - Form the basis of rectifiers, which allow the conversion of AC power to DC. Allows an electric current to pass in one direction, while blocking current in the opposite direction. Rectifiers are used to function battery charges in most transportation vehicles. - Used to extract modulation from radio signals in radio receivers that can be found in various transportation vehicles such as cars, trains, aeroplanes and even motorcycles. - Used to produce light in LED’s (light emitting diode) that can be found in the instrument panels and consoles of various transportation vehicles. Also used in some modern day headlamps on vehicles such as cars and motorcycles. Transistors - Enables the amplification of current or voltage, or acts as a switch. These qualities a...