Materials and Manufacturing

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Six Main Constituents of Steels

  • Ferrite
  • Austenite
  • Cementite
  • Pearlite
  • Bainite
  • Martensite
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Definition of Dispersion Strengthening of Metal Al

Dispersion strengthening is also called precipitation hardening orage hardening.  It is defined as increasing the strength of a material by forming more than one phaseIt relies on changes in solid solubility with temperature to produce fine particles of an impurityphase. The impurities play the same role as the particle substances in particle-reinforced composite materials. By proper control of the size, shape, amount and individual properties of the phases, excellent combination of properties can be obtained because the interphase boundary will interfere with the slip or movement of dislocations to inhibitdivertblunt and/or block the slip or movement of dislocations.

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Substitutional and Interstitial Alloys

1.When a molten metal is mixed with another substance, there are two mechanisms that can cause an alloy to form, called atom exchange and the interstitial mechanism. 2.The relative size of each element in the mix plays a primary role in determining which mechanism will occur.  3.When the atoms are relatively similar in size, the atom exchange method usually happens, where some of the atoms composing the metallic crystals are substituted with atoms of the other constituent.  This is called asubstitutional alloy. 4.With the interstitial mechanism, one atom is usually much smaller than the other, so cannot successfully replace an atom in the crystals of the base metal. The smaller atoms become trapped in the spaces between the atoms in the crystal matrix, called the interstices. This is referred to as an interstitial alloy.  5.Examples of substitutional alloys include bronze and brass, in which some of the copper atoms are substituted with either tin or zinc atoms.  6.Steel is an example of an interstitial alloy, because the very small carbon atoms fit into interstices of the iron matrix.  7.Stainless steel is an example of a combination of interstitial and substitutional alloys, because the carbon atoms fit into the interstices, but some of the iron atoms are replaced with nickel and chromium atoms. 8.Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other and impede dislocation motionso alloys are harder than the pure metal.

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alpha - Fe (Ferrite)

Solid solution of C in BCC Fe; Stable form of iron at room temperature; Maximum solubility of C is 0.0218 wt%; Magnetic at room temperature

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gamma - Iron (Austenite)

Solid solution of C in FCC Fe; The maximum solubility of C is 2.14 wt %; Unstable below eutectoid temperature (727 °C); Non-magnetic;

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Cementite

1.Iron carbide (Fe3Cis a chemical compound of an orthorhombic crystal structure.  2.It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more important in metallurgy. 3.It has 6.67 wt% C. 4.Cementite forms directly from the melt in the case of white cast iron.  5.In carbon steel, it either forms from austenite during cooling or from martensite during tempering.  6.An intimate mixture with ferrite, the other product of austenite, forms a lamellar structure called pearlite. 

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Pearlite

1.It is two-phasedlamellar (or layered) structure composed of alternating layers of ferrite (88.90 wt%) and cementite (11.10 wt%) that occurs in some steels and cast irons. 2.During slow cooling pearlite forms by a eutectoid reaction as austenite is cooled below 727°C (the eutectoid temperature).  3.laminated structure formed of alternate layers of ferrite and cementite.  4.It combines the hardness and strength of cementite with the ductility of ferrite and is the key to the wide range of the properties of steels.   5.The laminar structure also acts as a barrier to crack movement as in composites.  This gives it toughness. 6.Carbon steel with up to about 0.2 wt% C consists mostly of ferrite, where the amount of pearlite increases with increasing the carbon content is increased. 

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Eutectic Reaction

A eutectic reaction is a three-phase reaction, by which, on cooling, a liquid transforms into two solid phases at the same time. It is a phase reaction, but a special one. The eutectic solid is usually laminar in form

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Eutectoid Reaction

A eutectoid reaction is a three-phase reaction by which, on cooling, a solid transforms into two other solid phases at the same time. 

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The Lever Rule

The Lever Rule is a way in which to calculate the weight percentage of each phase present on a phase diagram in a two phase field at a given temperature and composition.

For example, there are two phases in plain carbon steels, namely ferrite and cemntite or ferrite and pearlite or pearlite and cementite. According to the lever rule, the wieght percentage of the first phase in a plain carbon steel is:

is the weight percentage of carbon in the first (α) phase 

b is the weight percentage of carbon in the second (β) phase

c is the weight percentage of carbon in the carbon steel

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Effects of an Increase in Carbon Content on the Pr

1.Increased hardness; 2.Increased strength and stiffness; 3.Improved heat treatment ability; 4.Improved abrasion resistance; 5.Reduced malleability; 6.Reduced machinability; 7.Reduced toughness; 8.Reduced weldability; 9.Reduced melting point.

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Eutectic Reactions in Cast Irons

White Cast Iron

Grey Cast Iron

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Grey Cast Irons

1.Grey cast irons have carbon contents varying between 2.5 and 4.0 wt% and silicon contents of vary between and 1.0 and 3.0 wt%, respectively.  2.Silicon is a graphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead of iron carbides. 3.The graphite exists in the form of flakes, which are normally surrounded by ferrite or pearlite matrix.  4.Because of these graphite flakes, a fractured surface takes on a grey appearance, hence it is named as grey cast irons. 5.Grey irons have comparatively low strength and ductility because the graphite has no appreciable strength, so they can be treated as voidsThe tips of the graphite flakes are sharp and pointed, and may serve as points of stress concentration when an external tensile stress is applied.  6.They have much higher strength and ductility under compressive loads.  7.They also have very good damping capacity and hence it is often used as the base for machine tool mountings. 8.They exhibit good thermal conductivity and thermal shock resistance.  9.Molten state of grey cast irons have a high fluidity at casting temperature, which permits casting pieces having intricate shapes; also, casting shrinkage is low.  10.Grey cast irons are among the least expensive of all metallic materials.

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Corrosion Resistant Mechanisms of Stainless Steels

It is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or stain 'less' than other types of steel.  The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film.  The sizes of chromium atoms and iron are similar, so they forms substitutional alloys.  Chromium atoms pack neatly together on the surface of the metal.  They react with oxygen and develop an extremely thin (1-5 nanometre thick), tight, strongly adherentchemically stable chromium oxide layer.   The key to the durability of the corrosion resistance of stainless steels is that if the film is damaged it will normally self-repair and self-healing. More chromium will diffuse to the surface and the chromium oxide will quickly form and re-cover the exposed surface, protecting the iron below from oxidative corrosion.  Stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low oxygen environment.  They are many grades of stainless steels which contain other elements, such as nickel, niobium, molybdenum and titanium to enhance their corrosion resistance and mechanical properties.

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Limitations of Plain Carbon Steels

1.There cannot be strengthening beyond certain value without significant loss in toughness and ductility. 2.Large sections cannot be made with a martensite structure throughout, and thus are not deep hardenable. 3.Rapid quench rates are necessary for full hardening in medium-carbon leads to shape distortion and cracking of heat-treated steels. 4.Plain-carbon steels have poor impact resistance at low temperatures. 5.Plain-carbon steels have poor corrosion resistance for engineering problems. 6.Plain-carbon steel oxidises readily at elevated temperatures.

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Alloy Steels

Alloy steels are the standard term referring to steels with other alloying elements added deliberately in addition to the carbon.  Common alloy elements include manganese (the most common one), nickel, chromium, molybdenum, vanadium, silicon, and boron.  Less common elements include aluminium, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium. They are used to overcome the limitations of plain carbon steels because they can improve a wide range of properties in alloy steels such as strength, hardness, toughness, wear resistance, corrosion resistance, hardenability, and hot hardness.  The alloy steels may require heat treating to achieve some of these improved properties.

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Five Groups of Stainless Steels

Ferritic stainless steels Austenitic stainless steels Martensitic stainless steels Duplex (ferritic-austenitic) stainless steels Precipitation-hardening stainless steels

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Methods of Producing Different Polymer Materials

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Addition Polymerization of Polyethylene

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Number Average and Weigh Average Molecular Weight

Number average molecular weight is a way of determining the molecular weight of a polymer. Polymer molecules come in different sizes, so the average molecular weight will depend on the method of averaging. The number average molecular weight is the common average of the molecular weights of the individual polymers. It is determined by measuring the molecular weight of n polymer molecules, summing the weights and dividing by N.

Weight average molecular weight is a is calculated by  the following equation, where Ni is the number of molecules of molecular weight Mi.

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Degree of Polymerisation and Molecular Weight Dist

Degree of Polymerisation is a measure of the number of repeating unit in a polymer.  It can be calculated by the number average molecular weight divided by the molecular weight of the monomer. 

Polydispersity Index (PDI) is a measure of the distribution of molecular weights in a given polymer sample. The PDI is defined as the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual molecular weights in a batch of polymers. The PDI has a value always greater than 1 but as the polymer chains approach uniform chain length, the PDI approaches 1.  

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Melt Flow Index (MFI)

1.MFI is a measure of the ease of flow of the melt of a thermoplastic.  2.It is defined as the weight of polymer in grams flowing in 10 minutes through a capillary of specific diameter and length by a pressure applied via prescribed alternative gravimetric weights. 3.The method is given in ASTM D1238 and ISO 1133. 4.The melt flow rate is an indirect measure of molecular weight, high melt flow rate corresponding to low molecular weight.  5.The melt flow rate is a measure of the ability of the material's melt to flow under pressure.  6.The melt flow rate is inversely proportional to the viscosity of the melt at the conditions of the test, though it should be born in mind that the viscosity for any such material depends on the applied force.  7.Ratios between two melt flow rate values for one material at different gravimetric weights is often used as a measure for the broadness of the molecular weight distribution. 8.Melt flow rate is very commonly used for many polymers. 9.Polyethylene is measured at 190°C and 2.16 Kg weight. 

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Thermoplastics

Abbreviation

Full Name

Micro-Structures

Polymerization Technique 

Semi-crystalline

Amorphous

Addition Polymerization 

Condensation Polymerization

PS

Polystyrene

 

P

P

 

LDPE

 Low density polyethylene

 P

 

 P

 

PP

 Polypropylene

 P

 

 P

 

HIPS

 High Impact Polystyrene

 

P 

P 

 

ABS

 Acrylonitrile butadiene styrene copolymer

 

 P

 P

 

PVC

 Polyvinylchloride

 

 P

 P

 

PC

Polycarbonate

 

 P

 

 P

PA

Polyamide

 P

 

 

 P

PET

Polyethylene terephthalate

 P

 

 

 P

PBT

Polybutylene terephthalate

 P

 

 

 P

PMMA

Polymethyl methacrylate

 

 P

 P

 

POM

 Polyoxymethylene

 P

 

 P

 

PTFE

 Polytetrafluoroethylene

 P

 

 P

 

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Different Types of Copolymers

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ABD Resin Phase Diagram

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Semi-crystalline VS Amorphous Thermoplastics

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% of Crystallinity of Semi-crystalline Polymer

pa is the density of the totally amorphous polymer;

pc is the density of the perfectly crystalline polymer;

ps is the density of the semi-crystalline polymer sample;

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Characteristic Temperatures of Polymers

Glass Transition Temperature (Tg): The temperature at which the amorphous polymer change from rubbery state to solid state or vice versa. Polymers have different specific heat capacity before and after glass transitions.   Elastomers should have a lower Tg than ambient temperature to retain the elasticity;

Melting Temperature (Tm): The temperature at which the solid crystalline will melt when being heated up. The melting temperature is a range due to the different sizes of crystal structures. It is an endothermic event.

Crystallization Temperature (Tc):  The temperature at which the liquid polymer chain will crystallize when being cooled down. The crystallization temperature is a range due to the different sizes of crystal structures. It is an exothermic event.

Curing Temperature TcuringThe temperature at which the thermoset resin and hardener start reacting and becomes permanent solid.  It is exothermic event.

Decomposition Temperature (Td): The hydrocarbon molecular chains are thermally broken. Oxygen will exacerbate the decomposition and it is referred to as oxidization.  It is an exothermic event.

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Crystallization VS Solidification VS Crosslinking

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Word Equations for Producing Different Polymer Mat

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Composites

Materials which consisted of continuous phase ( matrix)and fibrous dispersed phase (reinforcement) with pronounced improvements in their properties and performances.

Metallic Matrix Composites Ceramic Matrix Composites Polymeric Matrix Composites

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Factors determining and dictating the mechanical a

1.The nature of reinforcing materials (polymer, metal, ceramic) 2.The nature of matrix materials (polymer, natural, ceramic) 3.Interactions between reinforcing and matrix materials  4.The ratio of fibre to resin in the composites (volume fractions of reinforcing and matrix materials) 5.Orientation of fibre reinforcement 6.Size and dispersion of particle reinforcement  (concrete) 7.Architectures of reinforcements (short, long, continuous and woven structures).  

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Thermoplastics VS Thermosets

Features

Thermoplastics

Thermosets

Examples

PE, PP, PC and ABS

Unsaturated polyester, epoxy

Structure

Linear molecules

3-D network molecules

Material State

Solid power or granules

Liquid

Viscosity

High

Low

Processing Temperature

High

Low

Processing Pressure

High

Low

Processing Time

Short

Long

Chemical Reaction

No 

Yes

Changes on Heating

Soften and flow on heating

Hardening and curing on heating

Consolidation

By cooling

By chemical reaction

Reversibility

Yes

No

Recyclable

Yes

No

Reinforcements

Optional

Essential

Fibre Concentrations

Low

High

Manufacture

Injection and extrusion

Hand lay-up, vacuum bagging, RTM, compression moulding

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Comparisons of Thermoset Matrices

Matrices

Advantages 

Disadvantages

Unsaturated Polyesters

Simplest to synthesise; Most versatile; Economical; Widely used; Moderate mechanical priorities; Brittleness and big shrinkage; Hydrolysis degradation; Styrene emission; Limited range of working times;

Epoxies

Excellent mechanical properties; Dimensional stability; Chemical resistance,  Low shrinkage; Low water absorption; High cost; Corrosive handling; Critical mixing with exact procedure and amounts;  Difficult to apply as it is so viscous;

Vinyl Esters

Balanced property between unsaturated polyester and epoxy reins;  Improved toughness; Improved hydrolysis resistance; Styrene emission; Increased cost; High temperature post curing;

Phenolic Resins

Good acid resistance; Good electrical properties; High heat resistance; Superb fire resistance; Poor mechanical property; Brittleness;  Dark-brown colour;

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Comparison of Different Engineering Materials

Materials

Young’s Modulus (GPa)

Tensile Strength (MPa)

Density

(g/cm3)

Specific Young’s Modulus

(GPa/gcm-3)

Specific Tensile Strength

(MPa/gcm-3)

Mild Steel

200

1000

7.8

25.6

128

Aluminium

80

200

2.7

29.6

74

Polycarbonate

2

60

1.2

1.7

50

Polyethylene

0.5

20

0.95

0.5

21

Glass fibre/epoxy Composites

45

1700

1.92

23.4

885

Carbon fibre/epoxy Composites

150

2000

1.58

94.9

1267

Kevlar fibre/epoxy Composites

70

1200

1.38

50.7

870

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Natural Fibre

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Disadvantages of Natural Plant Fibre Reinforced Po

1.Low mechanical properties 2.Inconsistency in quality 3.Dark colour  4.Hydrophilic 5.Limited thermal stability

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Weight fraction to volume fraction conversion

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Rules of Mixtures

Composite stiffness can be predicted using a micro-mechanics approach termed as the rule of mixtures with following assumptions.

1.Fibres are uniformly distributed throughout the matrix.  2.Perfect bonding between fibres and matrix.  3.Matrix is free of voids.  4.Applied loads are either parallel or normal to the fibre direction.  5.Lamina is initially in a stress-free state (no residual stresses).  6.Fibre and matrix behave as linearly elastic materials. 

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Density Determination of Fibre Reinforced Composit

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The Rule of Mixtures in Iso-Strain Condition

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The Rule of Mixtures in Iso-Stress Condition

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Three Typical Laminates with Different Ply Orienta

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Equations in The Rule of Mixtures for Composites

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Sandwich Composites

sandwich composite is a special class of composite materials that is fabricated by attaching two thin but stiff skins to a lightweight but thick core. The core material is normally low strength material, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density.

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Sandwich Beam Loading Profile

1.The top surface skin is under the utmost compression load 2.The bottom surface skin is under the utmost tension load 3.The load is decreasing when moving towards the centre from the top and bottom surfaces 4.The centre has a neutral axis because the tension and compression cancelling each other out

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Advantages of Sandwich Composites against Laminate

1.Higher stiffness 2.Higher strength 3.Higher buoyancy 4.Lower thermal conductivity and greater insulation 5.Improved anti-buckling under compression 6.Better impact damage resistance and puncture-proof 7.Sound attenuation and vibration damping 8.Reduced labour in manufacturing 9.Reduced number of needed stiffeners 10.Large increase in thickness with insignificant weight penalty

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