Mechanical Properties of Metals

Tensile

A1Monday, September 15, 2014 8:50 AM

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Elastic, and plastic, Video https://www.youtube.com/watch?v=n7LXYyohmgg

Graph of Force/elongation would depend on geometry (Length and cross sectional area). To eliminate this dependency, we define stress and strain.

Plastic deformation of single crystal; videoPlastic deformation of polycrystal: video

strain

Unit:N/m2 or PaLb(f)/in2 = PSI

No unit, but cm/cm or in/in,..

stress

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From: http://www.engineeringarchives.com/les_mom_stressstraindiagram.html

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If Sample is "unloaded" in the elastic region, it will return to its original length, L0.

If sample is unloaded in the plastic region, it will only recover Maximum dL corresponding to the elastic region

Note the definitions of Yield strength, Ultimate Tensile strength

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Permanent deformation

Process of unloading during plastic deformation, establishes a new yield strength point, higher than previous one.

Plastic deformation is permanent slip of a plane ovr another plane in a specific direction

Deformation of single crystal

Movies: slip in poly-crystals

More on slip later!

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What happens if loading/unloading repeatedly?

Yield Strength increases each time sample is unloaded and loaded again. As the result, sample becomes more brittle. This is the basic reason for cold working.

Stress strain test video (watch it yourself) : https://www.youtube.com/watch?v=D8U4G5kcpcM

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Samples are intentionally machined with smaller diameter in the middle to ensure necking (and eventual failure) AT THE CENTER.

True stress,

Engineering stress,

Engineering strain,

True Strain,

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Types of Y.S

Up to necking

After necking

n and K are experimental values

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Example: A 400 kg load is suspended by a wire(d=p1Cm,L=12m) made from pure Cu.

wire is stretched to 12.08m while the load is on. Is the wire

deformed permanently?.

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Use 69 Mpa for yield strength and o.5 cm for radius

This answer could be wrong!

Correct approach would be to compare strain, not stress.

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what is the final length when land is removed?

A steel rod (T.S =380 Mpa, Y.S =180 Mpa and E=207 Gpa) is being used in construction of suspension bridge. Each rod can experience 50,000N load. What is the minimum diameter of rods so the they would never stretch plastically?

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Previously we saw for the force between two atoms:

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Some definitions:

Effect of T: Later we will disuse D. B. T Temperature

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Some definitions:

Ductility =% elongation =% reduction in area =Toughness=Resilience =Modulus of resilience =Modulus of elasticity =Poisson's ratio = measure of homogeneity

Area under the area in the inelastic region is Called Modulus of resilience.

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Poisson's ratio

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Poisson's ratio

A sample with Y.S.=180 MPa and 20 cm diameter is used for a part that should never be plastically deformed. Using M=1.2, calculate working stress and maximum load.

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A coper wire (YS=69MPa and E=110GPa) is subjected to 400 tensile load. As a result, its length increases from 2 to 2.02 m. Is the wire permanently deformed?

If Do=2mm, what is its final diameter when load is re moved?

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LINKS:Pres-stressed concrete:https://www.youtube.com/watch?v=2QQsaPYssVU

Long Movie: https://www.youtube.com/watch?v=LsoU4SeqhNk

How it is made, concrete: https://www.youtube.com/watch?v=0ALY_VmxaOkDocumentary about concrete: https://www.youtube.com/watch?v=e6dxe4wpm5M

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