1 UNIT 2: Fundamentals of Materials 1 Unit 2 Copyright © 2014. MDIS. All rights reserved. Modern...

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UNIT 2: Fundamentals of Materials 1

Unit 2 Copyright © 2014. MDIS. All rights reserved.

Modern Manufacturing

2

• Understand and explain the physical properties of materials.

• Understand and explain the main properties of importance in materials.

• Understand the structure of metals.• Understand the types of tests for determining the

mechanical behavior of materials.• Understand characteristics like hardness, fatigue, creep,

impact strength and residual stresses.

Objectives


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Density

• “the concentration of matter in a region of space”

• ρ = Mass / Volume its unit in SI is kg/m3

• less dense metal will be lighter than a denser metal. E.g.: Aluminium is lighter than steel

• Automotive sector makes great use of Aluminium -higher mileage

• Aviation industry needs very tough but very light metals- Titanium and its alloys


Commonly material Properties

Unit 2 Copyright © 2014. MDIS. All rights reserved. 4

http://www.materials.eng.cam.ac.uk/mpsite/physics/introduction/e-rho_generics.jpg

http://en.wikipedia.org/wiki/Thermal_conductivity



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Melting Point

melting and boiling points of ice / and water are 0Ċ and 100Ċ

cast iron will melt into liquid at 2150F (1176C); lead at 621F(327C)

one cannot use lead in the creation of a ladle for a steel furnace; cannot use steel as a soldering wire as it will simply not melt

Physical Properties


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Thermal Conductivity and Expansion

• A measure of the materials ability to rise in temperature as it is positioned next to an object of high temperature

• As the temperature of the source object rises, the thermal agitation of the materials molecules causes the energy to propagate through the material quickly for metals. This is called high thermal conductivity


Physical Properties

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• High conductivity- necessary to heat up and cool down components of an electronic circuit

• A refrigerator door is lined with insulating foam on the inside to keep the outside high temperatures from seeping heat into the cooled space within


Physical Properties


http://periodictable.com/Properties/A/ThermalConductivity.st.html

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Electrical Properties

• Resistance: It is a measure of the ability of a material to offer resistance to the flow of a current

• It is what causes resistance in a material

• Dielectric Strength: This property of a material determines how well a material can withstand the application of high voltage


Properties of Materials

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Magnetic Properties

• Diamagnetic (repulsed by magnetism)• Paramagnetic (attracted by magnetism) or• Ferromagnetic (become permanent magnets

themselves)• Electromagnets are created by using electric current in

coils to create magnets

CAT scanners are made of huge magnets that cause magnetic response in biological tissues which are then mapped back into images




Optical Properties

• Transmissivity - how well light can travel through a material

• Reflectance - how well a material reflects oncoming radiation

• Absorbance - how much of the oncoming radiation is captured by the material within itself


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Corrosion Resistance

• As we know Iron rusts very quickly in presence of humidity

• alloys of steel that do not rust even in the most corrosive environments



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Tensile strength

• The ability of a material to withstand tensile stress - Industry normally uses the elastic limit, but the Ultimate strength is also used in designing



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Torsion and Bending strength

• Torsion strength of a material is the maximum applicable torsion stress

• Bending is subjecting a member of a structure to a bending moment that induces tensile and compressive forces in its cross-section that may cause failure



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Hardness

• Property of a material that resists indentation on its surface

• Higher hardness materials are capable of wearing out the softer materials



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Fatigue

• Failure occurs in components that are subjected to cyclic variation of stresses- aircraft wing struts, pistons, crankshafts, etc. continuously suffer forces in opposing directions in a rhythmic pattern

• Can cause components to fail at a fraction of its real tensile strength



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Creep

• Creep failure happens in situations where forces involved cause less than the limiting stresses on components

• The premature deformation of materials at stress levels that are otherwise safe, but in elevated temperature conditions



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Impact strength

• The ability of a material to sustain a standard impact in the impact test- standard test is the Izod Impact test with a notched or without a notched specimen


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Deformation of Single Crystal

• Permanent deformation of a single crystal under a tensile load. (a) Deformation by slip. (b) Deformation by twinning


The Structure of Metals

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Crystal Defects


• Schematic illustration of types of defects in a single-crystal lattice: self-interstitial, vacancy, interstitial, and substitutional

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Crystal Slip


• Movement of an edge dislocation across the crystal lattice under a shear stress.

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Grain and Grain Boundaries


• Schematic illustration of the stages during the solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other.

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Grain and Grain Boundaries


• Plastic deformation of idealized (equiaxed) grains in a specimen subjected to compression (such as occurs in the forging or rolling of metals): (a) before deformation; and (b) after deformation.

Mechanical Behaviour and Testing of Properties

26Unit 2 Copyright © 2014. MDIS. All rights reserved.


Tension test

• The tension test is the most common method for determining the mechanical properties of materials, such as strength, ductility, toughness, elastic modulus, and strain hardening capability



Ductility

• An important behaviour observed during a tension test is ductility-the extent of plastic deformation that the material undergoes before fracture. There are two common measures of ductility

– The first is the total elongation of the specimen

– The second measure of ductility is the reduction of area



Compression

• Many operations in manufacturing, particularly processes such as forging, rolling, and extrusion, are performed with the workpiece subjected to compressive stresses

• The compression test, in which the specimen is subjected to a compressive load, gives information that is useful for estimating forces and power requirements in these processes



Torsion

• A workpiece may be subjected to shear strains such as in the punching of holes in sheet metals

• The test method generally used to determine properties of materials in shear is the torsion test

• In order to obtain an approximately uniform stress and strain distribution along the cross section, this test is usually performed on a thin tubular specimen



Bending

• A commonly used test method for brittle materials is the bend or flexure test, which usually involves a specimen that has a rectangular cross section and is supported



Hardness

• Hardness gives a general indication of the strength of the material and of its resistance to scratching and to wear

• Hardness is usually defined as resistance to permanent indentation; thus, steel is harder than aluminium, and aluminium is harder than lead

• Hardness, however, is not a fundamental property, because the resistance to indentation depends on the shape of the indenter and on the load applied



Hardness tests (Brinnel)

• This test involves pressing a steel or tungsten-carbide ball 10 mm in diameter against a surface, with a load of 500, 1500, or 3000 kg

• The Brinnel hardness number (I-IB) is defined as the ratio of the load to the curved surface area of the indentation

• The harder the material to be tested, the smaller the impression



Hardness test (Vickers)

• This test uses a pyramid-shaped diamond indenter and a load that ranges from 1 kg to 120 kg

• The Vickers hardness number is indicated by HV



• Hardness test (Knoop)

• This test uses a diamond indenter in the shape of an elongated pyramid with applied loads ranging generally from 25 g to 5 kg

• The Knoop hardness number is indicated by HK

• Because of the light loads that are applied, the Knoop test is a micro hardness test; therefore, it is suitable for very small or very thin specimens, and for brittle materials such as carbides, ceramics, and glass.



Hardness Scale

Conversions

Chart for converting various hardness scales. Note the limited range of most scales. Because of the many factors involved, these conversions are approximate.


Fatigue

• Various components in manufacturing equipment, such as tools, dies, gears, cams, shafts, and springs, are subjected to rapidly fluctuating (cyclic or periodic) loads, in addition to static loads

• Cyclic stresses may be caused by fluctuating mechanical loads, such as (a) on gear teeth or reciprocating sliders, (b) by rotating machine elements under constant bending stresses, as is commonly encountered by shafts, or (c) by thermal stresses, as when a die comes into repeated contact with hot workpieces and cools between successive contacts

• Under these conditions, the part fails at a stress level below that at which failure would occur under static loading



Creep

• Creep is the permanent elongation of a component under a static load maintained for a period of time.

• This phenomenon occurs in metals and certain non-metallic materials, such as thermoplastics and rubbers, and it can occur at any temperature; lead

• For metals and their alloys, creep of any significance occurs at elevated temperatures, beginning at about 200°C for aluminium alloys and at about 1500°C for refractory alloys

• The mechanism of creep at elevated temperature in metals is generally attributed to grain-boundary sliding



Impact

• In many manufacturing operations and machinery components, materials are subjected to impact, or dynamic loading-for example, in high-speed metalworking operations such as heading to make bolt heads, and in drop forging

• A typical impact test consists of placing a notched specimen in an impact tester and breaking the specimen with a swinging pendulum



Bending

• A commonly used test method for brittle materials is the bend or flexure test, which usually involves a specimen that has a rectangular cross section and is supported


1 UNIT 2: Fundamentals of Materials 1 Unit 2 Copyright © 2014. MDIS. All rights reserved. Modern...

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