power point nurulSIAP 2007

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PREPARED BY:NURUL AMIRA BT MD YUSOP

PPISMP SCIENCE SEM 3 (JULY 2008 INTAKE)

KKBI SCIENCE 2 (PHYSICS)


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Hooke's Law

Modulus of Elasticity, E:

H ' L w: For l ti t ri l , tr i li rlFor l ti t ri l , tr i li rlroportional to trainand i independent of ti e.proportional to trainand i independent of ti e.

W = EI

W

Linear-

elastic

E

I

F

Fsimpletensiontest


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The slope of this graph is called the spring

constant and is symbolized by the letter k

That is, the force vs. extension graph forms a straight, positively sloped line

that passes through the origin, like this:

The area under this graph of force vs.

extension is in Joules, units of energy. This

area is the energy stored in the spring. The

symbol for the energy stored in the spring

could be Us


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Modulus of elasticity E (Youngs

modulus)

In the elastic region,

ookes la : = EE represents the stiffiness of

the material, i.e. Its resistance

to elastic strain


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PLASTICDEFORMATION

Line OP: Young modulus

P : limit of proportionality: where the linear relationship between stress and

strain finishes.

E : elastic limit.

Below the elastic limit, the wire will return to its original shape. Above this limit, amaterial permanently stretched

Y : yield point A large increase in strain is seen for a small increase in stress.

UTS : ultimate tensile stress, the maximum stress that is applied to a wire

without its snapping. It is sometimes called the breaking stress. Notice that

beyond the UTS, the force required to snap the wire is less.

S : the point where the wire snaps or also called breaking point

ELASTIC

DEFORMATION


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Because this is a division of two measurements of lengths,S

train has no unitsand remains a ratio.

Strain

However, due to Hooke's Law, it can be calculated in another form;

Strain energy

is the extension per unit length when there is force applied on

object

Has no unit


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The units forStress are N m-2, otherwise known as Pascals (Pa)

Stress

Ratio of stress and strain (W Vs I)

E = stressstrain= Force / cross section_________change in length / original length

=

E =

The unit for E is Nm-2

O

AF

NN/

/

(

N

N

(A

Fo

is the force acting on a unit cross-section area


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YOUNGs MODULUS


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Stress and Strain

If we stretch a wire, the amount it stretches by depends on:1)its length-If we have a wire of the same material and the same diameter, the ONGER

wire will stretch more for a given load

Strain = extension (m) = e

original length (m) l

2)its diameter-Ifwe have two of the same material and length, it is clear that the thickerwire

will stretch less for a given load

the compression force per unit area, i.e. the pressure.

Stress= F/A

3)the material its made of.-elastic materials will stretch more


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Deformation of the material is the change in geometry

when stress is applied (in the form of force loading,gravitational field, acceleration, thermal expansion, etc.

ELASTIC DEFORMATION PLASTIC DEFORMATION

Temporarydeformation,isfullyrecovered whentheloadisremoved

permanent

deformation,isnotrecovered

Obeys Hookes Law DoesnotobeyHookes Law

Doesnotchangetheinternalstructureofmaterial

Changetheinternalstructureofmaterial

Brittlematerials: likeceramics,ball,spring,rubber

Plasticin,glass


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Elastic means reversible!

Elastic Deformation

1. Initial 2. Load 3. Unload

F

H

bondsstretch

return toinitial shape

F

H

Linear-elastic

Non-Linear-elastic

Return to the original shapeReturn to the original shapewhen the applied load iswhen the applied load isremoved.removed.


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Plastic means permanent!

Plastic Deformation1. Initial 2. Load 3. Unload

planes

still

sheared

F

Helastic + plastic

bonds

stretch

& planes

shear

Hplastic

F

H

linearelastic

linearelastic

HplasticHelastic

Could not return to theCould not return to theoriginal shape when theoriginal shape when theapplied load is removed.applied load is removed.


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Curve A shows a brittle material.

little strain for a high stress. (so,strong)

The fracture of a brittle material issudden and catastrophic, with little or no

plastic deformation.

crack under tension and the stress

increases around the cracks and cracks

propagate less under compression.

Curve B is a strong material-not ductile.

Steel wires stretch very little, and break

suddenly. There can be a lot of elastic

strain energy in a steel wire under

tension and it will whiplash if it breaks.

Curve C is a ductile material

Curve D is a plastic

material. Notice a very large strain

for a small stress.The material will not go back to its

original length.


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Rubber is a polymer which has special

mechanical properties, they are :

Its range of elasticity is great.

Its value of the Young modulus is about 104 times smaller than most

solids and increases as the temperature rises.

Its stress-strain graph is a little bit different compared to a metal

graph. Its stress-strain graph forunloading liesbeneath its loading.This can be shown in the graph below.


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Ductile material e.g. COPPER

In the elastic strain region the bonds between the copper atoms behave

elastically. When the limit proportionality is reached the bonds between

the copper atoms start to break


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Stress vs. Strain curve of a very a

typical brittle material

1. Ultimate Strength

2. Tensile strength.

Brittle materials such as CERAMIC and GLASS

do not have a yield point, and do not strain-harden which means that the ultimate strength

and breaking strength are the same.

Typical brittle materials do not show any plastic

deformation but fail while the deformation is

elastic.

One of the characteristics of a brittle failure is

that the two broken parts can be reassembled to

produce the same shape as the original

component.

BRITT E MATERIA


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