Blow-moulded Plastic Bottle Stress-Strain Curve û peculiarities 2

[guym]

Blow-moulded Plastic Bottle Stress-Strain Curve – peculiarities

Hello,

I am analysing a plastic PET/HDPE bottle’s capability of taking pressure. By grasping the bottle and putting very little force (with my fingers) on it, I can deflect the walls by 5-6 mm. Also by blowing into it at a pressure of about 0.1 bar, I can also deflect the walls by 1.8 mm (each side for a square 65 x 65mm PET bottle with ~1mm wall thickness).

Whilst I apply these small pressures, am I actually going up and down the stress-strain curve, or is the initial deflection of the bottle caused by some buckling/spare capacity in the way the bottle is moulded?

The stress strain curve for HDPE, shown at 1:32 here

http://www.youtube.com/watch?v=I28m4FZzqro

Shows, what you would expect from the stress strain curve of a test specimen, however it does not explain [Because the stress-strain part is fairly linear, but at what feels like very little stress, I can create a great strain (displacement/original dimension)], why I can get so much initial deflection from such a small force?

Any explanations / theories would be most welcome

Thanks

Guy

 

[Demon3]

During molding, the polymer chains are orientated so that the tensile modulus is increased. So, if you were to cut out a piece and measure it, you'd get a higher modulus than the quoted one for less oriented polymer.

When you squeeze the bottle you are testing the flexural modulus which was not increased because the orientation of the chains is in the wrong direction to improve that property.

However, if you were to measure the flexural modulus it would probably be correct compared to a literature value. It's just that the wall is so thin. That's why you can deflect it so much.

Chris DeArmitt PhD FRSC CChem

 

[CoryPad]

You are generating very small elastic strains (but large elastic deflections) because you have a relatively large, thin wall part. You are barely above the zero point of the stress-strain curve during the events you describe.

 

[guym]

Thanks for your replies.

Guy