How to design polymer?

[precast78]

Hello, I was assigned to help design a small plastic structure. My background is more in concrete. I modeled the structure using FEA. The structure will be buried and loaded with soil cover and vehicle load. My questions are the following:

1. What shall I use as my limiting factor? Shall I use the yield strength of the material? Do I add some kind of reduction factor to account for creep? Is there a design book I can follow?
2. When I design concrete we typically factor the load instead of Factor of Safety. The load factors are determined by AASHTO. From what I see so far, most people use unfactored load and use factor of safety to design plastic. How do you determine the Factor of Safety?

Thank you!

 

[Demon3]

Yes, one does apply a factor to allow for creep. Use the material at only 20-25% of it's yield strength to avoid creep. That factor works for a wide range of polymers.

Chris DeArmitt PhD
President - Phantom Plastics LLC

Trusted adviser to leading companies around the world

http://www.phantomplastics.com

 

[Compositepro]

I agree with Chris. Unreinforced plastics are not really structural materials. In your case the plastic structure will support the soil from collapsing so that it can form a stable arch. If much stress is placed on the plastic it will creep until the stress is back on the soil. Sort of like wood structures.

 

[Demon3]

Adding some glass fiber in the plastic will dramatically reduce creep.

Chris DeArmitt PhD
President - Phantom Plastics LLC

Trusted adviser to leading companies around the world

http://www.phantomplastics.com

 

[precast78]

Yes, it will have glass in the plastic. So what shall I design it to? 25% yield? What safety factor shall I add on the top of that?

 

[precast78]

Is there a manual or standard code on how to design structural plastic?

 

[D2S]

All thermoplastics creep, even GF filled ones. You need to know the load, time and temperature and then get a set of creep curves form the manufacturer. The question isn't whether it will creep, it's how much. How much deflection can you handle?

Amorphous polymers creep less than semi-crystalline ones in general. You want the thermoplastics to have a Tg above the highest use temperature as well. Then load it up with glass fiber or even better, carbon fiber (way more expensive).