20140814 me235 mechanical properties

8/10/2019 20140814 ME235 Mechanical Properties

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Screw dislocation: how it looks?


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How does grain form?


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Mechanical Properties


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Why we need to know properties of materials?

To choose appropriate material(s) for the desired application


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How do we obtain properties of materials?

Through various testing/ characterization techniques


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Mechanical Properties

Strength Youngs modulus

Poissons ratio

Toughness and stiffness

Hardness

Brittle and ductile fracture

Fatigue

Creep


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Strength

Tensile, compressive, shear, bending, impact


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Tensile strength

Brittle solid Ductile solid


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Variation in specimen geometry during tensile test

Initial geometry: cylindrical rod shape (dog bone specimen)

(Dog bone specimen)


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Tensile properties of selected metals at room temperature


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Tensile strength

Polymer Rubber


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Youngs modulus and Poisson ratio


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Toughness

ability of a material to absorb energy up to fracture Area under tensile stress-strain curve

Fracture toughness: is a property indicative of a materials resistance

to fracture when a crackis present

Notch toughness: For dynamic (high strain rate) loading conditions

and when a notch(or point of stress concentration) is present,


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True strain-True stress

Correction factor


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Methods of hardness testing

Brinell hardness (BHN) (500-3000 kg) Rockwell hardness (HRX) X=A,B,C,D,E, F

Vickers hardness (VHN) (1-1000 g)

Knoop hardness (KHN) (1-1000 g)


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Methods of hardness testing


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Rockwell hardness types


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Rockwell hardness types


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Relation between Hardness and Tensile strength


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Hardness testing: some comments

Surface should be flat

Thickness of the specimen

10 X indentation depth


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Indentations of VH test


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Indentations of KH test


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Advantages of micro-hardness test

Measure the hardness of various phases in alloys


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Hardness of hair


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Poisson's Ratio

Ratio between lateral strainand linear strain For metals: Typically around 0.26-0.35

Foams or cellular solids have negative Poissons ratio

Honeycombs


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Shear modulus

Ratio between shear stress and shear strain E=2G(1+)

G- Shear modulus

- Poissons ratio


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Safe stress

Safe stress= Yield stress/ N N is factor of safety

Normally N is between1.2 to 4


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Ductile to Brittle Transition

The temperature at which material changes ductile to brittlefracture

How to get this data?

Impact studies of polymer is shown here


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DBTT of metals

FCC- do not have DBTT BCC- have DBTT


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DBTT

Titanic disaster


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Creep


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Fatigue

20140814 me235 mechanical properties

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