Shear stress in a polymer under pressure

[polymengr]

I am trying to design a polymer(thermoplastic) ring under a uniformly loaded fluid pressure at high temperature for an extended period of time to resist deformation into the gap between the gland and piston. What are the contitutive equation I need to be using to predict geometric parameters for the rings.
Any direction to follow or reference to literature will be helpful.

 

[Demon3]

It depends whether you mean instantaneous deformation in which can it depends on modulus and can be easily calculated, or whether you mean slow flow (creep) of the polymer in which case it's more difficult to determine because creep values are not normally available so you'd have to measure them.

You also need to check whether the fluid interacts with the polymer, for example by plasticizing it.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[polymengr]

I am looking at both instantaneous and long term deformation. Can you please point me to the equations required to calculate both?

 

[patprimmer]

I am not aware of any equations.

The only data I ever saw was published by ICI when they existed.

It was all based on extensive test results, not calculation.

Bayer and Akzo may also have published some years ago, but such data has tended to be withdrawn as the marketing experts tended to decide that complex technical data scared people off.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[Demon3]

As a rule of thumb, to avoid creep don't exceed about 20-25% of the yield stress.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[dsheets05]

The calculation is real simple. See the attached file to find the equations and quick picture that represents what I think you are doing. If you have any additional questions please let me know!

 

[dsheets05]

Your file's link is:

http://files.engineering.com/getfil...-abaf-20b883d75af2&file=Backup_ring_calc..bmp

Forgot that!

 

[patprimmer]

datasheet

Your formula does not take time into account which is the only difficult aspect.

Regards
Pat
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[Pud]

What you really need is the isochronous stress/strain curves at your desired temperature for your chosen material. (aka "Creep curves")

These are nowadays much like unicorn droppings, but if you can contact the "white coat" department of your material supplier, you may get lucky!

The calculation is real simple

No, it is definitely NOT!! You can only work from empirical data.

Patprimmer summed it up in his post of 16th Aug 21:24.

As for the original query

What are the contitutive equation I need?

A: There ain't one!

You might get lucky....

H

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Harry

Isochronous was the term I was trying to remember.

I actually have them somewhere for most of the OLD ICI range.

Regards
Pat
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[Elabbasi]

Constitutive models for thermoplastics are very well developed now, and can capture their nonlinear rate-dependent viscoplastic behavior.

These constitutive models typically require several material parameters, which should be calibrated from experimental stress-strain curves at different loading rates, and at the temperature of interest (isochronous data is more convenient but not required). Unless your geometry is very simple, it's better to perform a finite element analysis using the calibrated constitutive models.

You can find more details and discussions on these material models in PolymerFEM (

http://www.polymerfem.com)

which is a specialized forum on modeling of polymers developed and maintained by one of my coworkers Dr. Jorgen Bergstrom.

Nagi Elabbasi
Veryst Engineering