power point nurulSIAP 2007
Transcript of 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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