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Modelling of Mechanical Behavior of Materials - A > Polymers > Flashcards

Polymers Flashcards

1
Q

What is a Polymer?

A

consist on macromolecules. Large molecular chains which the atoms are held together by covalent bonds (bonds between the different chains are much weaker)

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2
Q

Mention the 3 different types of polymers

A

Thermoplastic, Elastomers (rubbers) and Thermoset (duromers)

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3
Q

What is a crosslink and the difference between the polymers

A 
Crosslink = crucial fix chains relative to each other and thus render it impossible to draw out a single molecular chain.
Thermplastic = NO crosslinks (have just weak links)
Elastomers = Small # of cross links.
Thermoset/duromers = High # of crosslinks
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4
Q

Mentions some characteristics of Thermoplastic

A
  • Molecules are not chemically joined together.
  • Have secondary bonds/intermolecular (van der waals)
  • With heat, the 2ndary bonds broken and molecules move.
  • Can be heat-softened , melted and reshaped as many times.
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5
Q

Mention some characteristics of Elastomers and thermosets

A

Are always completly amorphous because the chemical bonds make a regular arrangement of the molecular chains impossible.

  • chemically joined together by crosslinks. (rigid 3D network structure)
  • cannot be melted by application of heat.
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6
Q

Glass transition temperature

A

AT certain temperature, the curve of the specific volume has a kink and the specific volume grows more strongly with the temperature than before.

  • Thermal expansion
  • “the bonds melt when the Tg is reached”

thermal movement of the molecular chains is less restricted by intermolecular bonds above the Tg. THE MATERIALS IS HIGHLY VISCOELASTIC

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7
Q

Mention some characteristics of Thermoplastic with the Tg

A
  • Bonds melt at Tg
  • Thermal movement is less restricted.
  • Material become highly viscoelastic
  • above Tg material doesnot behave as liquid.
  • Molecules are entrangled and sliding in geometrically constrained.
  • behave as liquid, molecules have to able to separate by large distance
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8
Q

Mention some characteristics of Elastomers and Thermosets with the Tg

A
  • with localized sliding of the molecules being consequently easier.
  • Due to covalent bonds between the molecules it is not possible to pull single molecules out of the network.
  • never becomes liquid and always remain solid.
  • Thermoset exhibit lower creep & stress relaxation than thermoplastic poymers.
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9
Q

Define Viscoelastic with creep and stress relaxation

A
  • stress raised abruptly from 0 to a Stress.
  • Polymer answer with a time-dependent strain
  • Strain increase instantaneously to a value of Strain0. As in the case without time-dependent elastic behavior but the it further increases with time.
  • Load remove after a time T0, strain decrease instantaneously by the time-independent strain0 and then reduces slowly to zero (relaxation)
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10
Q

Draw the graphs of Strain and Stress dependent of time related with Creep and stress relaxation

A
  • The strain is kept constant.

- STress increase instantaneously, but then it decrease with time and approaches a constant value (stress relaxation)

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11
Q

Generalities of VISCOELASTIC

A
  • mechanical properties depending on the variable TIME.
  • if we have viscoelastic we have creep and relaxation of stress.
  • thermoplastic take in mind the viscoelastic
  • thermoset or elastomers neglect viscoelastic.
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12
Q

Define the 3 models of Viscoelastic

A
  • Kevin Voigt (parallel)
  • Maxwell Model (series)
  • Generalized MAxwell model (mix)
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13
Q

Main characteristics of Kevin Voigt

A
  • THE MODEL DESCRIBES PURELY VISCOELASTIC MATERIAL.
  • PREDICT CREEP.
  • elastic contribution of the deformation (without time dependace)
  • plastic properties of a polymer also strongly depend on time
  • polymers are viscoelastic
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14
Q

Main characteristics of MAxwell model

A
  • PREDICT RELAXATION

- linear behaviour is increasing and not as a curve.

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15
Q

Main characteristics of Generalized Maxwell model

A
  • Stifness and damping are parameters of temperature and material dependent.
  • small temperatures (Tg decrease) elastic behaviour dominates assentially linear-elastic
  • Temperature increase->viscoelastic
  • # of springs and dampers as you need
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16
Q

What is a ISOCHRONOUS CURVE

A
  • Is the retardation of experiments kept at constant stress after a fixed load time is measured.
  • Deformation becomes larger the longer the loading time is.
  • Viscoelastic effects occur at temperatures well below the Tg
17
Q

Mention some ELASTIC PROPERTIES of the Thermoplastics

A
  • Elastic behavior determined by intermolecular bonds.
  • Secondary bonds melt.
  • Due to geometrical constrain, elastic modulus is not Zero.
  • applying load bonds stretch after load dispates.
  • In E-T Log diagram -> STIFNESS strongly decrease at temperatures close to the Tg
  • *In E-T Log diagram divided in Energy elastic and Entropy elastic
18
Q

Draw the E-Tlog diagram for Thermoplastic

A

E-T diagram

19
Q

What is de difference between Energy elastic and Entropy elastic region in the E-TLog diagram

A

ENERGY = Energy needed to displace atomos from equilibrium position. On unloading, the atoms return to their original position which gas the lowest energy. [covalent bonds not contribute significatly to the elastic properties]

ENTROPY = Load removed, no force on straightened molecules. No reason why they should return to their initial position. Deformation below Tg, it is not smaller energy of the initial configuration that drives to return to this form.

20
Q

Mention about Semi-Crystaline Thermoplastic Elastic Properties

A
  • Semicrystalline thermoplastics show a different behaviour*
  • Elastic stifness is usually larger than that of amorphous
  • Yound modulus decrease. Is smaller because only the amorphous regions become entropy-elastic.
  • chain molecules extended over several crystalline and amorphous regions.
21
Q

Properties of the Plastic behaviour of Thermplastics

A
  • Chain molecules sliding past each other over large distances.
  • Strongly depends on the temperature because of the obstacles.
  • Lower than 80% of Tg (bonds between the molecules are so strong and the specific volume is so small that chain molecules cannot move by sliding)
22
Q

Which are the 2 main plastic behaviour deformation mechanicms of Plastic behavior in Thermoplastics

A
  • CRAZING

- SHEAR BANDS

23
Q

How can it describe a CRAZING mechanism

A
  • crazes initial surface defects
  • plastic deformation start in those regions.
  • stress state become triaxial and increase hydrostatic tension (cavities star to deform plastically)
  • Crazed form
24
Q

How can it describe a SHERA BANDS mechanism

A

-LArge and localised plastic deformation
-Compression mechanism
(if one shear bond cross other shera band will be fail)
-Non-symmetrical behavior between tension-compression
-Hydrostatic pressure increase , yield strength increase
-Start from Von Mises

**have deformation Above and CLOSE to Tg

25
Q

What happends when Shear bands mechanism are close to the Tg

A

Deformation continues more and more, chain molecules are drawn and straightened in parallel

26
Q

What happends when Shear bands mechanism are above to the Tg

A

T>Tg chain molecules can easily slide past each other because the strong increase in the specific volume and Melting of intermolecular bands increase.

27
Q

Mention some ELASTIC & PLASTIC PROPERTIES of the Thermoset and the E-Tlog diagram

A

if TTg - change considerability

  • Energy-entropy regime bonds do not infuence elastic properties Youngs modulus only increase slightly.
  • T>Tg additional bonds become important
  • Thermoset show only a small decrease of Young modulus with Temp caused by relaxation processes.

PLASTIC= thermoset are brittle with covalent bonds between the chain molecule breaking in brittle failure.

28
Q

Mention some ELASTIC properties of the ELASTOMER and the E-Tlog diagram

A

ARE entropy-elastic at Temp>Tg
covalent bonds increase the linking between molecules.

-Elastic strain become very large molecules are straightened during deformation but Crosslinks prevent the molecules from sliding past each other and this inhibit plastic deformation. (Hyperelasticity)

29
Q

What is HYPERELASTICITY?

A

Material model is

  • Non linear
  • Isotropic
  • Just valid for materials that have instantaneous elastic response up to large Strains (rubbers or solid propellant)
  • Are define with the Strain Enery Potential (W(e))
30
Q

What is the STRAIN ENERGY POTENTIAL and how can you define it?

A

W(e_strain) = defines the strain energy store in the material per unit of reference volume (volume at initial configuration) ad a function of the strain at that point in the material.

  • Need the Stretch Tensor (lambda)
  • Define Invariants I1 I2 I3
  • define Lambdas 1, 2 ,3
31
Q

Compressibility in Elastomers (hyperelasticity)

A

Rubbers have very little compressibility compare to shear flexibility. BUT there are applications with no highly confined (compressibility is not crucial)

But there are applicationswith High confined materials (o-rings or seals) so the compressibility is modeled correctly.

Modelling of Mechanical Behavior of Materials - A (10 decks)

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