12554 Ch7-Mechanical Properties

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Chapter 11

MECHANICAL

PROPERTIES


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Important Mechanical Properties

from a Tensile Test

Young's Modulus: This is the slope of the linearportion of the stress-strain curve, it is usuallyspecific to each material; a constant, known value.

Yield Strength: This is the value of stress at the

yield point, calculated by plotting young's modulusat a specified percent of offset (usually offset =0.2%).

Ultimate Tensile Strength: This is the highest

value of stress on the stress-strain curve. Percent Elongation: This is the change in gauge

length divided by the original gauge length.


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Terminology

Load - The force applied to a material duringtesting.

Strain gage or Extensometer- A device used for

measuring change in length (strain).

Engineering stress - The applied load, or force,

divided by the original cross-sectional area of the

material.

Engineering strain - The amount that a material

deforms per unit length in a tensile test.


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F

bonds

stretch

return to

initial

1. Initial 2. Small load 3. Unload

Elastic means reversible.

Elastic Deformation


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1. Initial 2. Small load 3. Unload

Plastic means permanent.

F

linear

elasticlinear

elastic

plastic

Plastic Deformation (Metals)


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Typical stress-strain

behavior for a metal

showing elastic andplastic deformations,

the proportional limit P

and the yield strengthy, as determined

using the 0.002 strain

offset method(where there

is noticeable plastic deformation).

P is the gradual elastic

to plastic transition.


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Plastic Deformation (permanent)

From an atomic perspective, plasticdeformation corresponds to the breaking of

bonds with original atom neighbors and

then reforming bonds with new neighbors. After removal of the stress, the large

number of atoms that have relocated, do

not return to original position. Yield strength is a measure ofresistance

to plastic deformation.


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(c)2003 Brooks/Co le, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.

Localized deformation of a ductile material during a

tensile test produces a necked region.

The image shows necked region in a fractured sample


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Permanent Deformation

Permanent deformation for metals isaccomplished by means of a process called

slip, which involves the motion of

dislocations.

Most structures are designed to ensure that

only elastic deformation results when stress

is applied.

A structure that has plastically deformed, or

experienced a permanent change in shape,

may not be capable of functioning as

intended.


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tensile stress,

engineering strain,

y

p = 0.002

Yield Strength, y


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Stress-Strain Diagram

Strain ( ) (DL/Lo)4

1

2

3

5

Elastic

Region

Plastic

Region

Strain

Hardening Fracture

ultimatetensile

strength

Elastic region

slope =Youngs (elastic) modulus

yield strength

Plastic regionultimate tensile strength

strain hardening

fracture

necking

yieldstrength

UTS

y

E

E

12

y

E


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Stress-Strain Diagram (cont)

Elastic Region (Point 12)

- The material will return to its original shape

after the material is unloaded( like a rubber band).

- The stress is linearly proportional to the strain in

this region.

E : Stress(psi)

E : Elastic modulus(Youngs Modulus) (psi)

: Strain (in/in)

- Point 2 : Yield Strength : a point where permanent

deformation occurs. ( If it is passed, the material will

no longer return to its original length.)

E or


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Strain Hardening

- If the material is loaded again from Point 4, the

curve will follow back to Point 3 with the same

Elastic Modulus (slope).- The material now has a higher yield strength of

Point 4.

- Raising the yield strength by permanently strainingthe material is called Strain Hardening.



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Tensile Strength (Point 3)- The largest value of stress on the diagram is called

Tensile Strength(TS) or Ultimate Tensile Strength

(UTS)

- It is the maximum stress which the material can

support without breaking.

Fracture (Point 5)

- If the material is stretched beyond Point 3, the stressdecreases as necking and non-uniform deformation

occur.

- Fracture will finally occur at Point 5.



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(c)2003 Brooks/Co le, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.

The stress-strain curve for an aluminum alloy.


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After yielding, the stress necessary to

continue plastic deformation in metals

increases to a maximum point (M) andthen decreases to the eventual fracture

point (F).

All deformation up to the maximum

stress is uniform throughout the tensile

sample. However, at max stress, a small

constriction or neck begins to form.

Subsequent deformation will be

confined to this neck area.

Fracture strength corresponds to thestress at fracture.

Region between M and F:

Metals: occurs when noticeable necking starts.

Ceramics: occurs when crack propagation starts.

Polymers: occurs when polymer backbones are aligned and about to break.

Tensile Strength, TS


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Tensile stress, : Shearstress, t:

Ft

Aooriginal area

before loading

Stress has units: N/m2 or lb/in2

Engineering Stress


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Another ductility measure: 100% xAAAARo

fo

Ductility may be expressed as eitherpercent elongation (%

plastic strain at fracture) orpercent reduction in area.

%AR > %EL is possible if internal voids form in neck.

100% xl

ll

ELo

of

Ductility, %EL

Ductility is a measure of the

plastic deformation that hasbeen sustained at fracture:

A material that

suffers very

little plastic

deformation is

brittle.


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Toughness

Lower toughness: ceramics

Higher toughness: metals

Toughness is

the ability to

absorb

energy up tofracture (energyper unit volume of

material).

A toughmaterial has

strength and

ductility.

Approximated

by the area

under the

stress-strain

curve.


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Energy to break a unit volume of material

Approximate by the area under the stress-strain

curve.

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smaller toughness-

unreinforced

polymers

Engineering tensile strain,

Engineering

tensile

stress,

smaller toughness (ceramics)

larg er toughness(metals, PMCs)

Toughness


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Linear Elastic Properties

Modulus of Elasticity, E:

(Young's modulus)

Hooke's Law:

= E

Poisson's ratio:metals: n ~ 0.33

ceramics:n

~0.25polymers: n~0.40

Units:

E: [GPa] or [psi]

n: dimensionless

n x/y


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Engineering Strain

Strain is dimensionless.


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Stress and strain: These are size-independentmeasures of load and displacement, respectively.

Elastic behavior: This reversible behavior often

shows a linear relation between stress and strain.

To minimize deformation, select a material with a

large elastic modulus (E or G). Plastic behavior: This permanent deformation

behavior occurs when the tensile (or compressive)

uniaxial stress reaches y.

Toughness: The energy needed to break a unitvolume of material.

Ductility: The plastic strain at failure.

Summary

12554 Ch7-Mechanical Properties

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