DEFORMATİON BEHAVİOUR OF POLYMERİC MATERIALS.pptx
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Transcript of DEFORMATİON BEHAVİOUR OF POLYMERİC MATERIALS.pptx
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Mechanical Behaviour Of
Materials
Deformation Behaviour of Ceramics and
Polymers
23.05.2013
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MECHANICAL AND DEFORMATION
BEHAVIOURS OF POLYMERIC
MATERIALS
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QUESTION 1
Explain how many type of polymeric materials
exist and what makes them different to each
other
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Types Of Polymers
Thermoplastic Elastomer Thermoset
Polymeric
Materials
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Types Of Polymers
Thermoplastics: Also known asa thermosofteningplastic, is a polymer thatbecomes pliable or moldable above a specifictemperature, and returns to a solid state upon
cooling. Most thermoplastics have ahigh molecular weight, whose chains associatethrough intermolecular forces; Thats mean thereis no crosslinking between chains, this property
allows thermoplastics to be remolded becausethe intermolecular interactions spontaneouslyreform upon cooling.
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Types Of Polymers
Thermosets: A type of plastic that is
permanently hardened by cooling.
Thermosets are heavily cross-linked.
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Types of Polymers
Elastomer: A formation of a thermoplastic or
thermoset that can stretch and then return to
its original shape without permanent
deformation. Elastomers are only slightly
cross-linked.
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Question 2
How many basic stress-strain curve can we see
for polymeric materials and explain what kind
of mechanical behaviours they will show.
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Stress- Strain Curves
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A: Glassy polymers (rigit region)
B: Amorf polymers (plastic region)
C: Elastomers
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Question 3
Expalin Maxwell, Voigt Models with your own
words. And scketch loading time graphic for
each model.
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Question 4
How many types polymeric structure can we
see and explain the deformation behaviour for
one of them.
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Polymeric structures
Amourphous: totally lacking positional order
on the molecular scale
Semicrystalline: containing both crystalline
and amorphous regions in the same sample
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Deformation behaviour of
Semicrystalline Polymers Firstly, the deformation is (visco-)elastic,which is caused by straining
the inter-lamellar amorphous domains
At yield, two lamellar deformation mechanisms occur, The firstmechanism is the chain slipthrough the lamellae, where after thesecond process of coarse slip of lamellae occurs
This intra-crystalline shear results in a fragmentation of the lamellae Subsequently, the lamellae disintegrate and their remains will be
transformed into a fibrillar phase coupled with the appearance ofvoids in the amorphous layers.
Upon further deformation, the fibrils stretch, causing the chains toalign and causing a lateral coalescation of voids, eventually resultingin a macroscopic crack due to breaking fibril
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Question 5
What is the shrink wrap? What is the material
of shrink wrap and what kind of polymer is
that? How can you explain the behaviour of
shrink wraps under heating via the changes of
microstructre
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Shrink wrap
Shrinkwrap, also shrinkwrap or shrinkfilm, is
a material made up of polymer plastic film.
When heat is applied, it shrinks tightly over
whatever it is covering. Heat can be applied
with a hand held heat gun (electric or gas) or
the product and film can pass through a heat
tunnel on a conveyor.
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Shrink Wrap
The most commonly used shrink wrap is polyolefin. It is available ina variety of thicknesses, clarities, strengths and shrink ratios. Thetwo primary films (polyethylene and polypropylen )can be eithercrosslinked, or non crosslinked. Other shrink films include PVC andseveral other compositions.
Polyethylene:
Polyproplene:
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Shrink Wrap
The molecules of a sheet or tube are randomly intertwinedlike a bowl of spaghetti. The molecules are coiled andtwisted and have no particular alignment. However when adraw force is imposed, the amorphous regions of the chainsare straightened and aligned to the direction of orientation.
By applying proper cooling, the molecules will be frozen inthis state until sufficient heat energy is applied to allow thechains to shrink back.
One can visualize this phenomenon by stretching a rubberband and dipping it into liquid nitrogen so as to freeze in
the stretched state. The band will remain in this state aslong as it is kept at sufficiently cold temperatures. However,when enough heat energy is applied, the rubber band willshrink back to its original relaxed state.
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