Creep in Plastics: Does Strain Cause Creep?

[mikas101]

I was doing a project to test for creep in materials, and one of the things that I wonder would be the fact that the translucent plastic would start to turn opaque when it starts to strain, just like why plastics when bent starts to turn white where the bend is. Why is this so? I know creep is caused when there is an influence of stress on the material.

Can we say that strain causes creep? Or are they both totally unrelated?

 

[Mapes]

Hi mikas101, welcome to PF!

All creep is strain, but not all strain is creep (creep occurs over time, but some types of strain, like some elastic strain, occur near-instantaneously).

You may find the search term "stress whitening" to be useful.

 

[mikas101]

Thanks for the reply!

Anyway i read up regarding Stress Whitening, sources say that the plastic crystallites were broken into smaller pieces in the stress-whitened material than in the translucent areas.

So plastic starts turning white when its molecular bond is being broken? Such as during creeping?

 

[Mapes]

Yes, that seems to be the consensus in the literature.

I wonder if it's also possible that creep induces crystallization in some cases, producing crystallites with sizes on the order of the wavelength of light and therefore changing a transparent material into a translucent or opaque material. (This is just a speculation, though; I haven't found any reports that say this is or isn't possible.)

 

[Studiot]

OK I have uploaded a rather scruffy sketch to help explain the cause of crazing in plastic materials.

It is important to carry the following thought forward.
Plastic materials are either fully amorphous or semicrystalline.
The loss of transparancy occurs in the amorphous region. It is not directly to do with the crystalline regions or zones. It happens in tension zones under stress.

Plastic polymers are capable of forming crystals, but only very small ones because, during formation, in the time to align several long chain molecules it is likely that another long molecule will come along at some akward angle and entangle the building crystal. the entangled zone remains amorphous.
These small crystals are called crystallites or spherulites.

A semicrystalline material contains zones of amorphous materials where the (long) chains are randomly interwound like plates of spaghetti and zones of crystallites (my fig1).

Because they have different densities and other physical properties the two zones have different light transmission characteristics. There is therefore dispersion at the boundaries between them.
In unstressed material the crystallites are randomly distributed and oriented and so the effect on light transmission is homogeny but translucence rather than clarity. Rather like specks of dust suspended in a liquid.
So the objective in manufacture is to minimise the size of the crystallites, to minimise the dispersive boundaries.

To move on to sketch 2 when we start to stress (strain) the material. The crystallite structure is very much stronger than the amorphous structure so when we pull the material apart (tension), the chains in the amorphous zones straighten and align themselves along the axis of principal stress. Nothing much changes in the crystallite zones (also called lamellae).
Now, of course, the chains have varying amount of uncoiling available so there is strain incompatibility between nearby chains. This leads to microcracking and even void formation between the straightened chains.

It is this microcracking which provides the necessary edge surfaces to break up light transmission and display the crazing. (my sketch 3)

Between the microcracks the straightened chains are now quite strong as they now rely on true covalent bonds.

 

[mikas101]

Wow, thanks for the detailed response.

I was still wondering whether it could be that polypropylene also has this kind of reaction during creeping? I also know that when the load has been taken away from the material there will be recovery...

So does that mean the chains are starting to wind back up again?

 

[Studiot]

Well yes crazing can occur during both the short term response to stress and the long term one (creep).

However as previously stated, the crazing is a result of microcrack propagation (brittle response) under stress.
The phenomenon of creep is effectively the opposite of this as it is a plastic response to stress where cracks do not propagate much because the stress is being relieved by the plastic response.

The the main instance of crazing will occur during the short term response . Don't forget it can also occur during repeated stress (fatigue).

 

[mikas101]

So what you are trying to say is that creep doesn't necessarily cause the microcrack propagation because the plastic is being relieved by its natural occurrence to recover?

Is that why once when the plastic has turned opaque, it doesn't totally recover to become translucent as it once was? Once when microcracks occur?

What are some of the factors that would lead to the recovery of the plastic? I know that the difference in temperature fluctuates the rate of extension in a material...

 

[Studiot]

Recovery only occurs during an elastic response.

A material does not recover from plastic or creep strain or, of course, cracking (actual failure) even if local.