Kuliah Materials Eng

8/7/2019 Kuliah Materials Eng

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Ch 1: Engineering materials

Dr. Zuhailawati Hussain

EBB 224

Design of MaterialsEngineering


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Classifications & Specifications of Metallic Materials

Major characteristics of metallic materials arecrystallinity, conductivity to heat and electricity andrelatively high strength & toughness.Classification: systematic arrangement or division of

materials into group on the basis of some commoncharacteristicGenerally classified as ferrous and nonferrous

Ferrous materials-iron as the base metal,

range from plain carbon (>98% Fe) to highalloy steel (


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Within each group of alloy, classification can be

made according(a) chemical composition, e.g. carbon content oralloys content in steels;(b) finished method, e.g. hot rolled or cold

rolled;(c) product form, e.g. bar, plate, sheet, tubing,structural shape;(d) method of production, e.g. cast, wrought

alloys.


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Designation: identification of each class by a

number, letter, symbol, name or a combination.Normally based on chemical composition ormechanical properties.Example : Table 2.1 designation systems for steel

System used by AISI & SAE: 4, or 5 digits whichdesignate the alloy composition.1st two digits indicate Alloy systemLast two or three digits nominal carbon content

in hundredths of a percent


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In most eng. application, selection of metallic is usuallybased on the following considerations:

1) Product shape: a) sheet, strip, plate, (b) bar, rod, wire,(c) tubes, (d) forging (e) casting2) Mechanical properties-tensile, fatigue, hardness,

creep,impact test3) Physical & chemical properties-specific gravity, thermal

& electrical conductivity, thermal expansion4) Metallurgical consideration-anisotrophy of properties,

hardenability of steel, grain size & consistency ofproperties

5) Processing castability-castability, formability,

machinability6) Sales appeal-color, luster7) Cost & availability


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Design and selection for metals

One of the major issues for structural componentsis deflection under service load.

A function of the applied forces and geometry,and also stiffness of material.

Stiffness of material is difficult to change,either shape or the material has to be changed iforder to achieve a large change in the stiffnessof a component.


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Load carrying capacity of component can be related

to the yield strength, fatigue strength or creepstrength depending on loading & service condition.

All are structure sensitive.Changed by changing chemical composition of the

alloy, method and condition of manufacturing, aswell as heat treatmentIncreasing the strength cause metal ductility &toughness to decrease which affects the

performance of component.


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Electrical & thermal conductivities

Thermal conductivity, KIs measure of the rate at which heat istransferred through a material

Al & Cu- Manufacture of component where

electrical conductivity is primaryrequirement

Corrosion resistance & specific gravity limits

the materials.


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Manufacturing consideration

Majority of metallic components are wrought or cast

Wrought m/str:usually stronger and more ductile than cast.

Available in many shapes & size tolerance

Hot worked products:

Tolerance are wider thus difficult for automaticmachining

Poor surface quality, esp. in sheet/wire drawing

Cold worked product:

Narrow toleranceResidual stress cause unpredictable size change duringmachining


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Weldability a function of material composition.

So structure involve welding of the componentsneed to consider. Also for other joining means.

Machinability:

Important if large amounts of material haveto be removed

improvement by heat treatment or alloyingelements

Economic aspects:material able to perform function at lowestcost

Plain carbon steel & cast iron are the leastexpensive


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C

lassifications of PolymersPolymer low density, good thermal & electricalinsulation, high resistance to most chemicals andability to take colours and opacities.

But unreinforced bulk polymer are mechanicallyweaker, lower elastic moduli & high thermalexpansion coefficients.

Improvement Reinforced variety of fibrous

materialsC

omposites (PMC

).

Design for polymer


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Advantages : ease of manufacturing & versatility.

Can manufacture into complicated shapes in onestep with little need for further processing orsurface treatment.

Versatility : ability to produce accuratecomponent, with excellent surface finish andattractive color, at low cost and high speed

Application: automotive, electrical & electronicproducts, household appliance, toys, container,packaging, textiles

Basic manufacturing processes for polymer partsare extrusion, molding, casting and forming ofsheet.


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Thermoset & thermoplastic

Differ in the degree of their inter-molecularbonding

Thermoplastic-litle cross bonding betweenpolymer, soften when heated & harden when

cooledThermoset-strong intermolecular bonding whichprevents fully cured materials from softeningwhen heated

Rubber are similar to plastic in structure and thedifference is largely based on the degree ofextensibility or stretching.


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Design consideration for polymer

Structural part/When the parts is to carry load

Should remember the strength and stiffnessof plastics vary with temperature.

Troom data cannot be used in design calculation

if the part will be used at other temp.Long term properties cannot be predicted from

short term prop. Eg. Creep behavior

Engineering plastics are britle (notched impactstrength < 5.4 J/cm)

Avoid stress raiser


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Classification of Ceramic Materials

Ceramics inorganic compounds of one or more metalswith a nonmetallic element. Eg Al2O3, SiC, Si2N3.Crystal structure of ceramic are complex

They accommodate more than one element ofwidely different atomic size.

The interatomic forces generally alternatebetween ionic & covalent which leave few freeelectrons

usually heat & electrical insulators.Strong ionic & covalent bonds give high hardness,stiffness & stability (thermal & hostile env.).

Design for ceramics


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Structure:

(1) Amorphous or glass-short range order, (2)crystalline (long range order) & (3) crystallinematerial bonded by glassy matrix.Clasiification:

Whitewares, glass, refractories, structural clayproducts & enamels.Characteristics:

Hard & brittleness,

low mechanical & thermal shockHigh melting pointsThermal conductivities between metal & polymer


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Design consideration for ceramicsBritle, low mechanical & thermal shock-need specialconsideration

Ratio between tensile strength, modulus of rupture &compressive strength ~ 1:2:10. In design, load ceramicparts in compression & avoid tensile loadingSensitive to stress concentration

Avoid stress raiser during design.Dimensional change take place during drying and firing,should be considerLarge flat surface can cause wrapping

Large changes in thickness of product can lead tononuniform drying and cracking.Dimensional tolerances should be generous to avoidmachining


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IntroductionA composite material can be broadly defined as anassembly two or more chemically distinctmaterial, having distinct interface between themand acting to produce desired set of properties

Composites MMC, PMC & CMC.

The composite constituent divided into two

Matrix

Structural constituent / reinforcement

Design for composite


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Properties / behavior depends on properties, size

& distribution, volume fraction & shape of theconstituents, & the nature and strength of bondbetween constituents.

Mostly developed to improve mechanical

properties i.e strength, stiffness, creepresistance & toughness.

Three type of composite

(1) Dispersion-strengthened,

(2) Reinforcement continuous & discontinuous(3) Laminated (consist more than 2 layersbonded together).


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Designing with composite

A composite materials usually are more expensive on

a cost.Used when weight saving is possible when therelevant specific property (property/density) of thecomposite is better than conventional material

E.g. specific strength (strength/density), specificelastic modulus ( elastic modulus/density)

Efficient use of composite can be achieved bytailoring the material for the application

E.g., to achieve max. strength in one direction in afibrous composite, the fibers should be wellaligned in that direction


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If composite is subjected to tensile loading,important design criterion is the tensile

strength in the loading directionUnder compression loading, failure by bucklingbecome important

Fatigue behavior:

Steel- show an endurance limit or a stressbelow which fatigue does not occur

Composite-fatigue at low stress level becausefibrous composites may have many crack, whichcan be growing simultaneously and propagatethrough the matrix

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