Polymer and composite failure and durability Exam Questions

Illustrate the major mechanism which may affect the durability of polymers and composites,

making a clear distinction between chemical, physical, reversible and non-reversible effects

Chemical degradation is a set of irreversible degradation mechanisms, they are very complex and polymer

specific and they lead to chain scission. It is a molecular deterioration in which components of the long chain

backbone begin to separate and react with one another to change the properties of the polymer.

Chemical degradation can be caused by thermal degradation (rearrangement, cyclization, oxidation of the

chain which becomes unsaturated), oxidation, interactions with liquid and electromagnetic radiation.

Physical degradation is a set of reversible mechanisms with no permanent variation of the chemical

composition of the macromolecules.

Physical degradation can be caused by interactions with liquids, physical aging, environmental stress crazing

ESC (irreversible).

CHAIN SCISSION: bonds absorb energy and go through a

homolytic breakage (rupture of covalent bond) with

subsequent radicals generation, propagation and

termination. Results are changes in morphological,

mechanical and optical properties. Main causes are:

1. Thermal degradation= material heating induces

energy for the breakage of bonds. Effective T are quite high (150-400 °C) and that is why this

mechanism normally works only as initiator for further degradation mechanisms (such as thermo-

oxidation). It is possible to have: rearrangement (lowers MW like in PP or increase of MW like in PE

due to radical terminations that form crosslinks), cyclization (leads to colored degradation products,

like in PAN), elimination of molecules and an unsaturated residue which is readily oxidized (like in

PVC). 2. Electromagnetic radiation (UV, gamma-rays...): radiation energy

promotes chemical reactions as bond rupture (photo-degradation). Effects are: decreasing of MW

(only in some cases as PET, in others MW remains constant as in PC), strong coloring of material

(yellowing) and loss of chains ductility.

3. Oxidation (O2 and O3): it induces chemical reactions as radicals formation and disruption of the chain

(it is an autocatalytic process that can be thermal or photo initiated).

4. Interaction with aggressive liquids: Specific chemicals may also react with specific polymer chains in

several ways: hydrolysis (water with polyesters and polyamides), solvolysis (chain scission), swelling

(increases material permeability thus accelerating other degradation mechanisms like oxidation).

5. Combinations of these factors, temperature increases the velocity of all processes.

Figure 1:water and temperature

Figure 2: oxygen and UV light

Physical degradations

Usually these processes are reversible, apart from some exceptions as loss of plasticizers, scratching or other

long lasting interactions. We can say in general that the following causes lead to MATERIAL PERFORMANCE

ALTERATION:

1. Interaction with liquids (reversible): the interaction with affine liquid that act as plasticizer with

cohesive energy similar to the polymer one. It can induce complete dissolution of non-crosslinked

polymers if the amount of liquid is enough. But in general liquid can always induce swelling (without

dissolution) as in case of cross-linked polymers (increase of FREE VOLUME). The material results more

ductile (Tg is reduced), its σ is greatly reduced and can lead to premature plastic deformation. It is

y

usually quite reversible.

2. Environment stress cracking (ESC) (irreversible): First requirement for this kind of degradation is the

presence of a stress (it can be externally applied or it can be a residual stress from production

process). When exposed to certain environment, depending on the polymer-environment coupling

(less viscosity=worst effect), we can have a decrease of the maximum strain for environmental stress

cracking with respect to the one in air. The liquid should have the same cohesive energy of the

polymer and a low viscosity. So failure initiation and propagation will be faster at same applied stress

in contact with a certain environment than in air. It is also possible to have a brittle-ductile transition.

3. Physical ageing (reversible): when a material is rapidly cooled under Tg, it will present a higher

volume than the equilibrium one (due to high cooling rate, the material will present a higher fraction

of amorphous material that has not been able to organize, which correspond to a higher free

volume). Depending on the service temperature (which affects chain mobility and so ageing rate,

higher T will correspond to a faster aging), the material will decrease this excess of free volume

(reorganization of macromolecules) tending to the equilibrium value. This process leads to a ductile-

brittle transition (material becomes more strong and less tough at increasing aging times, smaller

time for fracture initiation and propagation). Material can be repristinated with a thermal cycle

(heating above Tg and rapid cooling) which is called rejuvenation cycle (lower yielding value, better

toughness).

Describe the two yielding mechanism for polymers, write the equations for the

corresponding design criteria and discuss how the different parameters may depend on

time and temperature

Shear yielding is a ductile and localized mechanism of yielding.

The material is induced to form bands of aligned

macromolecules in the direction of the shear stress (35°<θ<50°),

which are called shear bands (figure on the right both for uniaxial

tension and compression). Bands are created along both

directions and expand through all the material, the phenomena

involves a huge energy consumption. This mechanism causes a

material distortion (change in size and shape) but volume

remains constant, it changes optical properties of the molecules

involved due to their orientation (if we start from an amorphous

polymer, shear bands become white and opaque).

The criterion to describe this yielding mechanism is the “modified Von Mises” (because T<C in polymers). It

needs two materials characteristics (T and C) and 2 characteristics of the state of stress (I1: first dilatoric

invariant of state of stress, J2: second deviatoric invariant of state of stress). In the triaxial stress space the

criterion turns out to define a safe region which is a cylinder for metals (T=C) and an ellipsoid for polymers

(0,5<=T/C<=0,7). If we consider a biaxial state of stress (sigma3=0), the safe region becomes an ellipse which

for polymers is centered in the pure compression region (third quadrant).

The effect of temperature or time is the

same and consists in a decrease of the

safety area (it reduces yielding limits of

the material).

Modified von Mises can be represented

like

Crazing is a brittle an