MATERIALS AND HARDWARE

MODULE 06

Introduction to Materials

• How are Engineering materials classified?– Metals– Polymers– Ceramics– Glass– Composite– Wood

Metals

• Pure Metals– Elements which come from a particular area of the

periodic table.

• Alloys– Contain more than one element– Their properties can be changed by changing the

elements present in the alloy– Ex: Steel (Fe+C), Stainless Steel (Fe+C+Cr+Ni), Gold

Jewellery (Au+Cu)

Metals

• Have good electrical and thermal conductivity. • Many metals have high strength, stiffness and have good

ductility.• Some metals are magnetic (Fe, Co, Ni etc)• At extremely low temperatures, some metals and

intermetallic compounds become superconductors.• Some metals alloys such as those based on Al have low

densities and are used in Aerospace applications.• Many metal alloys have high fracture toughness.

Contd..

Polymers

• Polymer has a repeating structure, usually based on a Carbon backbone.

• Useful because they are light weight, corrosion resistant, easy to process at low temperatures, generally cheap.

• Low strength and high toughness.• Stregth is often improved using reinforced composites.• Poor conductors of electricity and heat/ good insulators.

Ceramics

• Often broadly defined as any inorganic nonmetallic material.

• They have high melting temperature, low density, high strength, stiffness, hardness, wear & corrosion resistance

• Good electrical and thermal insulators.• Superconductors at very low temperatures.• Ceramic and glasses have one major drawback

(Brittleness)

Glass

• Amorphous, inorganic, non metallic materials.

• High range of applications (from bottles to fibre optics).

• Very brittle

Composites

• Formed from two or more types of materials (concrete, GFRP, Plywood, MDF).

• Qualities are superior to those of individuals.

• High tensile strength and Young’s modulus.

Structure of Atoms, Atomic Theories, Atomic Bonding in Materials

Atomic Structure

All the materials are composed of very small particles called atoms. An atom consists of a center nucleus of positive charge around which small negatively

charged particles called electrons revolve in different paths or orbits.

Atomic Structures

Atomic Structure (Fundamental Concepts)

• Atom consists of a very small nucleus (Protons and neutrons and is encircled by moving electrons).

• Both electrons and protons are electrically charged; (-) for electrons and (+) for protons; neutrons are neutral.

• Each chemical element is characterized by the number of protons in the nucleus, or the atomic number (Z).

• The atomic mass (A) is the sum of masses of protons and neutrons within the nucleus.

• Some elements have two or more different atomic masses, which are called isotopes.

Types of Bonding

Bonding

Primary Secondary

Types of Bonds

S/No Material Dominant Bond (Who Controls Material) Other Bonds

123

MetalsCeramicsPolymers

MetalicCovalent/ IonicVan Der Walls/ Hydrogen

CovalentIonic/ CovalentCovalent

Ionic Bonding

• Found in compounds that composed of both metallic and nonmetallic elements; elements that are situated at the horizontal extremities of the periodic table.

• Atoms of a metallic element easily give up their valence electrons to the nonmetallic atoms.

• In the process all the atoms acquire stable or inert gas configurations and, an electrical charge.

• Sodium chloride (NaCl) is the classic ionic material.

Periodic Table

Ionic Bonds

Covalent Bonds

The Structure of Crystalline Solids

Fundamental Concepts

• A crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances.

• Each atom is bonded to its nearest-neighbor atoms. • All metals, many ceramic materials, certain polymers

form crystalline structures under normal solidification conditions.

• Some of the properties of crystalline solids depend on the crystal structure of the material.

FCC Structure

BCC Structure

Unit Cells

• The small groups of atoms of crystalline structures which form a repetitive pattern.

• The crystal structures are often subdivided into small repeat entities called unit cells.

Crystal Defects

• Point defects

• Line defects or dislocations

• Planar defects

• Bulk defects

Point Defects

• Defects related to a single atom

• Specific for crystalline materials

Line Defects or Dislocations

• Related to line of atoms.

• Specific for crystalline materials.

• There are two types of dislocations.

– Edge dislocation

– Screw dislocation

Edge Dislocation

Screw Dislocation

Edge Dislocation Vs Screw Dislocation

Planar Defects

• Two dimensional defects.• Plane of atoms is moved.• Grain boundary defects.

Bulk Defects

• Three dimensional/ volumetric defects.

• Common for all material types.

Mechanical Properties of Materials

Application of Forces and Deformations of Materials

Tensile Test

• Mechanical properties of a material are studied by performing a tensile test.

• Tensile test speciman

The speciman will be subjected to a progressively increasing tensile force until it fractures.

Tensile Test

• There are four important stages in the testing. – Original speciman– Deformed speciman– Necked speciman– Fractured speciman

• From the test, Force-Extension curve is obtained.

F

e

• Force-Extension curve is then converted to a Stress-Strain curve.

σ

ε

Stress – Strain Curve

Typical Engineering Stress – Strain Curve

Stress-Strain Diagram of Steel

Stress-Strain Diagram for Brittle Materials

Elasticity

• All materials show temporary deformation to a certain extent – Elastic deformation.

• This property of the material is known as Elasticity.

• Elasticity of most of the materials gives a straight line in the stress-strain diagram known as Linear Elastic Materials.

Elasticity Contd...

• Elasticity of some other materials does not give a straight line in the σ-ε diagram.

• They are known as non linear elastic materials (ex: rubber)

• Elasticity of materials is accounted to the stretching of atomic bonds.

Elastic Modulus

• For linear elastic materials

• E is known as Elastic Modulus or Young’s Modulus.

• E of steel is 2x1011 Pa

E ;

Plasticity

• The deformation becomes permanent beyond a certain stress levels in metals.

• It is known as plastic deformation and the property is known as plasticity.

• Plastic deformation begins at the yield stress.

Plastic Deformation• Plastic deformation in metals occurs due to a

phenomenon known as slip (relative displacement of atomic planes).

Yield Drop in Steel

• Question 1:– Why do you observe a yield drop in the stress-strain

curve of steel? Relate your answer with the theory of “Cottrell Atmosphere”.

Ductile and Brittle Materials

• Ductile Materials– Materials that exhibit plastic deformation (Most

metals are ductile)

• Brittle Materials– Materials that do not have plasticity (glass, cast iron)

Ductility

• Ability of a metal to undergo plastic deformation is defined as ductility.

• Plastic strain at fracture is a measure of ductility.

• Ductility of Cu is greater than that of steel.

pfε

Ductility Contd...

• Ductility can be measured by:

– Percentage elongation

– Percentage reduction in area

%100x

o

of

LLL

%100x

o

of

AAA

Malleability

• Ability of a material to undergo plastic deformation in compression.

• All ductile materials are malleable but malleable materials are not necessary to be ductile always.

• A malleable material is preferred in process such as forging, rolling & rivets heading (hammering).

Strength

• Ability of a material to withstand the applied stresses without failure is defined as strength (maximum stress that can be applied on a material).

• Strength of a brittle material is given by it’s fracture stress.

• Yield strength is taken as the strength for a ductile material.

Tensile Test for Brittle Materials

• It is almost impossible to do a tensile test on brittle materials, as they tend to break in the grips.

• Tensile strength of a brittle material is therefore calculated from its MOR (Modulus Of Rupture).

• Tensile strength x 1.3 = MOR

• MOR is determined by perfoming bending test.

Bending Test

P

L b

d

223bdPLMOR

Compression Test

Proof Stress

• Stress-Strain diagram of most metals does not give a clear cut yield point.

?

Proof Stress Contd...

• In such cases yield stress could not be found.

• As such, proof stress is found and used in place of the yield stress.

• Hence, 0.1% proof stress is found.

• 0.1% proof stress is the stress that is required to cause a plastic strain of 0.1% (ε=0.001).

Proof Stress Contd...

ε=0.002

Work Hardening

• Strength (and hardness) of a metal increases as a result of plastic deformation.

• This is known as work hardening or strain hardening.

• Effect of work hardening on strength is demonstrated by a tensile test.

YS1

YS2

A

OP

BF

Necking

• At the UTS a localized deformation begins in the speciman.

Engineering Diagram Vs True Diagram

Ferrous and Non Ferrous Alloys

Types of Metal Alloys

• Metal alloys are often grouped into two classes.– Ferrous– Nonferrous.

• Ferrous alloys, those in which iron is the principal constituent, include steels and cast irons.

• Non-ferrous alloys - all alloys that are not iron based.

Ferrous Alloys

• Those of which iron is the prime constituent.

• Produced in larger quantities than any other metal type.

• Especially important as engineering construction materials.

Ferrous Alloys

• Their widespread use is accounted for by three factors:– Iron-containing compounds exist in abundant quantities

within the earth’s crust.– Metallic iron and steel alloys may be produced using

relatively economical extraction, refining, alloying, and fabrication techniques.

– Ferrous alloys are extremely versatile, in that they may be tailored to have a wide range of mechanical and physical properties.

• The principal disadvantage of many ferrous alloys is their susceptibility to corrosion.

Classification Scheme for Various Ferrous Alloys

Fe – C Phase Diagram

75

Steels• Iron-Carbon alloys that may contain appreciable

concentrations of other alloying elements.• The mechanical properties are sensitive to the content

of carbon, which is normally less than 2.0 wt%. • More common steels are classified according to carbon

concentration (low, medium, and high carbon types). • Subclasses also exist within each group according to the

concentration of other alloying elements. • Plain carbon steels contain only residual concentrations

of impurities other than carbon and a little manganese. • For alloy steels, more alloying elements are intentionally

added in specific concentrations.

Stainless Steels

• Highly resistant to corrosion (rusting) in a variety of environments, especially the ambient atmosphere.

• Predominant alloying element is Cr (at least 11 wt% Cr).• Corrosion resistance may also be enhanced by Ni and

Mo additions.• A wide range of mechanical properties combined with

excellent corrosion resistance make stainless steels very versatile in their applicability.

Cast Irons

• C contents above 2.14 wt% (in practice most cast irons contain between 3.0 and 4.5 wt% C).

• Alloys within this composition range become completely liquid at temperatures between approximately 11500C and (21000F and ), which is lower than for steels.

• Thus, they are easily melted and amenable to casting. • Some cast irons are very brittle, and casting is the most

convenient fabrication technique.• The most common cast iron types are gray, nodular,

white, malleable, and compacted graphite.

Fe – C Phase Diagram

82

Gray Cast Iron

• C and Si contents vary between 2.5 and 4.0 wt% and 1.0 and 3.0 wt%, respectively.

• For most of these cast irons, the graphite exists in the form of flakes (similar to corn flakes)

• Because of these graphite flakes, a fractured surface takes on a gray appearance, hence its name.

• Mechanically, gray iron is comparatively weak and brittle in tension.

Gray Cast Iron

• The tips of the graphite flakes are sharp and pointed, and may serve as points of stress concentration when an external tensile stress is applied.

• Strength and ductility are much higher under compressive loads.

• Used for base structures for machines and heavy equipment that are exposed to vibrations.

• In addition, gray irons exhibit a high resistance to wear.

Gray Cast Iron

Ductile (or Nodular) Iron

• Adding a small amount of Mg and/or cerium to the gray iron before casting.

• Graphite still forms, but as nodules or sphere-like particles instead of flakes.

• Castings are stronger and much more ductile than gray iron.

• Ductile iron has mechanical characteristics approaching those of steel.

• Applications include valves, pump bodies, crankshafts, gears, and other automotive and machine components.

Ductile (or Nodular) Iron

Aluminium and Aluminium Alloys

Properties of Aluminium

• Light in mass• Soft and ductile• High resistance to corrosion• No coloured salts are formed to stain surfaces.• Good electrical and thermal conductivities • Nontoxic (used for cooking utensils)• Lose part of their strength at elevated temperatures.

Aluminium Alloys

Classify into two categories• Non heat treatable– Stength depends on the hardening effect of

elements such as Mn, Si, Fe and Mg.– Not subjected to heat treatment

• Heat treatable– Treatments include solution heat treatment,

quenching and precipitation or age hardening.

Wrought Aluminium and Aluminium Alloy Designation System

Aluminium 99% and greater 1xxx

Aluminium alloys grouped by major alloying elements

CopperManganeseSiliconMagnesiumMagnesium and SiliconZincOther elements

2xxx3xxx4xxx5xxx6xxx7xxx8xxx

• 2nd digit is usually zero.Non zero numbers are used to indicate some modification to the original alloy.– If 2nd digit is zero – Indicate original alloy– If 2nd digit is 1-9 – intentionally introduced impurities.

• In 1xxx last 2 digits indicate the purity of Al– If last 2 digits are zero – Al 99%– If last 2 digits are 12 – Al 99.12%

• For other types, last 2 digits indicate the particular alloy within the family.– Ex: 2024 means alloy number 24 within the Al-Cu system or

2xxx

Effect of Alloying Elements

• 1xxx Series (Al)– Excellent corrosion resistance– High thermal and electrical conductivity– Excellent workability

• 2xxx Series (Al+Cu)– Require solution heat treatment to obtain optimum properties.– Artificial aging to further increase the mechanical properties,

yield strength.

• 3xxx Series (Al+Mn)– Non heat treatable

Effect of Alloying Elements (Contd..)

• 4xxx Series (Al+Si)– Lowering of the melting point without producing brittleness.

• 5xxx Series (Al+Mg)– High strength non heat treatable alloy– Good welding characteristics– Good resistance to corrosion in marine atmosphere.

• 6xxx Series (Al+Si+Mg)– Good formability and corrosion resistance with medium

strength.

Effect of Alloying Elements (Contd...)

• 7xxx Series (Al+Zn)– When coupled with Mg and Cu results in heat

treatable alloys of very high strength.– 7075, 7050 and 7049 are the highest strength alloys

used in airframe structures and for highly stressed parts.

Age Hardening

• Strengthening a metal by introducing small particles of another phase which barriers dislocation motion.

• Maximum hardness is achieved if the properties can resist cutting by dislocations and are too close to permit by-passing of dislocations.

Cutting Through and By Passing of Dislocations

Cutting through: When precipitates are too small.

Bowing and by pass:

When precipitates are

too strong to be cut

and inter particle space

become large.

Age Hardening Process

Al-Li Alloys

• Developed recently for the aircraft, aerospace industries.• Have relatively low densities (2.5 - 2.6 g/cm3)• High specific moduli (elastic modulus/ specific gravity

ratios)• Excellent fatigue, low-temperature toughness properties.• May be precipitation hardened. • More costly to manufacture than conventional Al alloys as

a result of Lithium’s chemical reactivity.

Duralumin (Al-4%Cu)• 2017-T4 Alloy which is called as duralumin.• Hardened by natural aging.• Widely used in aircraft industry.

Mg and Mg Alloys

• Main properties

– High strength to weight ratio

– Excellent machinability

– Relatively low cost

Other Properties and Applications

• Produced in various forms including casting, sheet, plate, forgings, bar and rod.

• Both standard structural shapes and sections of special design are made by the extrusion process.

• Can be machined at higher speeds and at lower costs.• Cast Mg alloys• Tensile strength upto about 280 Mpa• Yield strength upto about 160 Mpa

• Wrought Mg alloys• Tensile strenghts upto about 360 Mpa• Yield strengths upto about 300 Mpa

• The Yield strength, tensile strength and hardness of Mg alloys decrease with Rising temperature.

Titanium Alloys• High strength to weight ratio.

– Capable of operating at temperatures from sub zero to 600°C.

• For aero-engines– Blades, shafts and casings from the front fan to the last stage of

the high pressure compressor.

• Airframes– Ti Alloys with strength up to 1200MPa– Landing gears and large wing beams.

Corrosion on Aircraft Materials

Corrosion on Aircraft Materials

• Materials are primarily used for their strength and tenacity. • They may readily suffer serious damage from corrosion unless effectively protected.• Rate of corrosion attack can be extremely rapid in certain environments. • Control and prevention of corrosion is important in the design of aircraft structure.

Why the Corrosion prevention on Aircraft is Important?

• Corrosion may extend over an entire metal surface.

• Corrosion may penetrate locally to form deep pits.

• Corrosion may follow the grain boundaries within the metal.

• The weakening effect of corrosive attack may be aggravated by stresses in the metal and result in premature failure of the component.

Anti Corrosive Protection

• During manufacture and assembly, a range of surface treatments are applied. • Heat treatment to refine grain structure.• Sacrificial coatings in the form of plating and cladding to retard the onset of corrosion. • Epoxy primers, special paint finishes and the use of barrier sealants to prevent the ingress of dirt and moisture between component parts.

Methods of Surface Protection

There are many different types of surface protection added to the basic structural materials and

hardware.

•Cladding•Anodising•Chromating•Exterior Cleaning•Surface Cleaning

Cladding

• Cu and Zn commonly alloyed with Al for high strength skin and component parts.

• These suffer extensively from corrosion.

• ‘Alclad’ (soft, highly corrosion-resistant, pure Al skin) rolled onto the face of each base alloy sheet, effectively sandwitching the alloy.

Galvanic series

Anodising

• Protects Al based alloys from corrosion (when cladding is impractical).

• An electrolytic treatment which coats the host metal with a film of oxide.

• This film is hard, waterproof, air-tight.• Permanently accept a coloured dye for

identification of some parts.

Bonding the Anodized surfaces

The film acts as an insulator. When bonding leads are to be attached to an anodised part, the surface treatment must be carefully removed before the bonding lead is

attached.

Chromating

• Chromate coatings are used to protect Magnesium-based alloys, zinc and its alloys.

• Components are immersed in a bath containing potassium bichromate and results in a yellowish coating on magnesium alloys.

Exterior Cleaning

• Special points are to be protected from cleaning materials and high pressure water sprays.

• Wheels, tyres & brake assemblies need to be covered to keep free of cleaning agents.

• Use only the cleaning agents & chemicals recommended by the manufacturer.

Surface Cleaning

• Most aircraft will be cleaned before starting the large inspections.

• To keep clean the aircraft always.

• Dirt, trapped moisture, solvents can cover up cracked or damaged components.

Structural Assembly Techniques

• The A/C integrity depends on the way the parts are attached together.

• The most common attachment is by the use of rivets or fasteners.

• Nuts and bolts, adhesive bonding techniques are also used.

Riveting

Rivets

Special Fasteners

Special Fasteners

Blind Fasteners

Solid Shank

Solid Shank Rivets

The vast majority of structure is held together with solid rivets.

Solid Shank Rivet Head Shapes

• Flat Head

• Round Head

• Universal Head

• Flush Rivets

Flat Head (AN442)

This is used in internal locations where it can be driven more easily than either a round or

universal head rivet.

Flat Head Rivet

Round Head (AN430)

This is used on internal structure where the thicker head is more suitable for automatic

riveting equipment.

Round Head Rivet

Universal Head (AN470 or MS20470)

• Most popular and may be used to replace any protruding-head rivet.

• It is streamlined on top but thick enough to provide strength without protruding too much into the airflow.

Universal Head Rivets

Flush Rivets (AN426 or MS20426)

• Used in where a smooth skin is important.

• Rivets with a different countersink angle, such as 90, 100 and 120 degrees can be found.

Flush Rivets

Rivet Head Types

Types of Alloy used for Solid Shank Rivets

• The marks are used to identify the rivet alloy.

• Head markings are necessary to identify the type of rivets removed from an aircraft.

• Marks are made on rivet heads.

Solid Rivet Identification

'A' Rivets

• Used for non-structural applications.

• Made from pure Al.

Solid Rivet Identification

Type “AD” Rivet

• A very popular type of a rivet.

• Has Cu and Mg added to the Al base metal.

• Heat treated during manufacture to make it strong, whilst still being soft enough to be formed easily.

Solid Rivet Identification

'D' and 'DD‘ Rivets

• Used when more strength than 'AD' rivets is required.

• They must be heat treated to make them softer before they can be formed.

• After forming they may be put into a refrigerator to maintain the softening effect.

Solid Rivet Identification

'B' rivet

• When riveting Mg alloy sheets, there must be no Cu in the rivet alloy.

• 'B' rivet, manufactured from 5056 alloy.

• This contains a large amount of Mg with a little Mn and Cr but no Cu.