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2012

1a)

Ceramics are brittle at room temperature, no plastic deformation. Slide 34. Ionic bonding for crystal

structures for slip to occur planes of ions have to move and ions of like charge will repel each other

therefore slip is resisted so that is for crystal. So for ceramics with strong covalent bonding in order

for slip to occur these bonds have to be broken. Quite complex for crystal and all this will resist slip

and resist plastic deformation making them brittle.

For amorphous the atoms are placed in a random order so the way they are placed it resembles a

liquid and the resistance to flow is the determined by the viscosity and at room temperature

amorphous ceramics have such a high viscosity that deformation by slip is not possible. It will fracture

before plastic deformation.

b) The properties and strength of ceramics is highly dependent on the defects. These defects will act

as stress concentrators and will make the ceramic fail. For the fibre the volume of the material is going

to be much less and hence the probability of finding a defect will be less and hence they display betterstrength properties in particular tensile strength.

c)

I.

True

II.

True

III.

True

IV. False

V. False

VI. False

VII.

TrueVIII. False

IX. True

X. False

XI. False

XII. True

XIII. True

XIV. False

XV. False (Crack formation is facilitated)

2a) b)

Relaxation modulus, grab a specimen and strain it to a particular constant strain and we measure this

stress after a specific period of time cos of this relaxation phenomena. Slide 43. Relaxation is time

dependent relief of stress permitted by polymer molecule mobility when the strain or deformation of

a polymer is constant. Salient points are glass transition and melting temperatures. Important graph

for polymers. For amorphous polystyrene, E is high. Glassy region in this region the material will be

below its glass transition temperature. The modulus here doesnt change much with time. Its like the

modulus in metal but then comes a region where this relaxation modulus will come down by orders

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of magnitude so the rigidity will drop th is relaxation modulus by orders of magnitude. So here were

experiencing a drop of 4 orders of magnitude in ER in just a very small temperature variation and this

transition will be in a leathery state, following that we will get the rubbery plateau. In these latter

regions the material will display visco elastic behaviour, between that of elastic solid and a viscous

liquid. That is the rubbery plateau. If we keep heating further we see a transition where the relaxation

modulus will drop again even further quite steeply and in this part the deformation of the material

will be viscous in nature. All permanent like a liquid, we refer to the polymer as being a liquid. The

melting temperature is over here. Strictly speaking melting temperature we use it for crystalline

materials. It will mark the transition between crystalline to an amorphous liquid.

If we increase the crystallinity of this polymer, if we use a polystyrene with a higher degree of

crystallinity the drop will be much less, the higher the crystallinity the lower will be this drop which

we would observe at TG and then we would observe a plateau with a much higher stiffness than this

rubbery plateau. Why are we getting at a higher value? Cos in a crystalline material we have the chains

closer to each other and a higher interaction of secondary bonding making the chains more rigid.

Here we can also see, plastics are polymeric material which have sufficient rigidity to be used incomponents at room temperature. If a polymer is totally amorphous for it to be used as plastic, non-

cross linked. For it to be used as a plastic the TG of this particular polymer has to be above room

temperature. So that at RT it will behave in a rigid fashion. For a crystalline polymer the material can

still be considered a plastic if it is operated at a temperature higher than TG. TG affects the minority

phase the amorphous making it possible to be used above the glass transition temperature.

If we cross link the amorphous polystyrene with cross linking however we would observe an increase

in the rubbery plateau the relaxation modulus wont go down turn into a viscous liquid. If we keep

increasing the temp the chains will start to decompose. Cross linking is in amorphous mostly to inhibit

crystallization. If the cross linking is very high we might not observe a TG. For a car rubber tyre we

want it to work above the glass transition. We will be shifting curve B upwards.

Note on visco elastic: Depending on the time of stress applied it will behave either elastically short

time or like a viscous flow for long time stresses.

c) Much lower strength and stiffness, can have much higher ductility than metal and an important

thing mechanically the polymers are very sensitive to temperature variations close to room

temperature.

d) The effect of temperature and strain rates, processing and structural, molecular weight stronger,

crystallinity stronger, pre deformation by drawing : stronger but anisotropic. Heat treatment

increasing crystallinity and perfection.

3a)

Fibrillar bridges elongated along the deformation direction and micro voids will form in between and

increase the strength in that direction and in this process these bridges will get thinner and thinner

forming something resembling a crack however a lot of energy is absorbed hence increasing fracture

toughness.

b)

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The ease by which the chains can rearranged themselves in a repeated ordered structure. Rate of

cooling: slower more time for the rearrangement to happen. The amount of cross linking. It wont

allow the chains to arrange themselves inhibiting crystallinity. The size of the molecules. Molecular

configuration usually head to tail, this will favour crystallinity the large macro molecules the smaller

they are the easier it will be to arrange themselves and also depends on the complexity of the repeat

unit. The less complex, the easier it is for that material to crystallize. PE it is difficult not to crystallize.

Then we have the structure of the polymer: we can have a linear structure, which can be highly

crystalline. We can have a branched structure which is never highly crystalline. The branches interfere

with crystallinity. Cross linking and networking are amorphous. For linear and branched we would

have an effect on the stereoisomerism. Isotactic, Syndiotactic, Atactic (cannot crystallize). So the

cooling rate, molecular weight, complexity of the repeat unit, the structure & sterio. We have to have

an R group. For example in PTFE all the side groups are fluorine. But for example PVC we can cos the

way the chlorine is placed determines the crystallinity. Polar side groups also influence the

crystallinity. Nylon has side groups with net positive/negative charges these polar side group swill help

the repeat unit will help align in a regular fashion.

c)

I. Linear PE is highly crystalline. Symmetrical.

II. Linear Atactic PVS amorphous. Since its Atactic its random. R is placed randomly.

III. Network phenol amorphous cos we have a triafunctional repeat unit.

IV. Lightly branched -- low crystallinity

V. PVC amorphous.

4a)

Proportion by volume of the reinforcing phase of fibre and volume. Bonding of the matrix and

composite. For effective stress transfer we require good bonding. It will depend on the morphologyof the reinforcing phase, fibre particle and the shape of that fibre phase. IT will also depend on the

orientation and distribution of that reinforcing phase. Slide 7.

b) Minimum fibre length Lc slide 24 for a given fibre diameter allowing tensile failure of the fibre

before shear failure at the interface. Its the minimum length required in order to allow the fibre to

reach its fracture strength. IF the fibre is too short it will never reach its max fracture strength and it

will be ineffective. At this point, fracture strength of fibre and shear strength of the bonding. IF the

shear strength of the bonding is stronger than that of the matrix hence the weakest link. Either of the

bonding or of the matrix depending which is lowest. A good bonding between the fibre and the matrix

will enable a lower critical length. So that is the critical length. For a continuous aligned fibrous

composite it has to be 15 times the critical length. If the fibre length is equal to critical length as we

pull the composite in tension only the centre of the fibre will experience the fracture strength. IF we

start increasing the fibre length above a bigger portion of that fibre will start carrying what it can carry

as a maximum load and when the net of the fibre is 15 times or bigger than the parts toward the end

of the fibre will become insignificant.

c) slide 28

d) ISOSTRAIN situation slide 29

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