Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

How to design polymer?

Status
Not open for further replies.

precast78

Structural
Aug 12, 2013
82
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!
 
Replies continue below

Recommended for you

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
 
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.
 
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
 
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?
 
Is there a manual or standard code on how to design structural plastic?
 
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).
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor