50 year modulus of PP and PET ? 3

[obrockmeier]

I am designing a part to be buried underground permanently and at this point I am considering PET and PP. As the part will remain buried, I assume a 50 year modulus would be more appropriate than the standard modulus, however, I cannot find any information on percentage decrease for PP and PET. PE seems to be readily available though ? Any comments would be greatly appreciated.

 

[Pud]

Hi,

I would assume the PE data is available due to the usage of PE for gas and water pipes.

As it is a lot cheaper than PET, and you are already considering PP why cannot you just use PE?

btw, I am assuming that the PE data will be for extrusion grade materials, which will be a different data set cf injection moulding grade materials.

Rgds

Harry

 

[RichGeoffroy]

obrockmeier:

The long-term modulus will change with time due to creep or stress relaxation depending on whether you are holding the stress or the strain constant.

As a rule of thumb the modulus will change to 80% of the original modulus after 1 hour, 67% after 10 hours, 51% after 100 hours (4 days), 39% after 1000 hours (6 weeks), 30% after 10,000 hours (1.1 years), 24% after 100,000 hours (11 years), and 21% of the original modulus after 50 years (438,000 hours).

This will work as a reasonable estimate of the modulus change with most polymer materials.



Rich Geoffroy
Polymer Services Group
polyserv@cox.net

 

[RichGeoffroy]

For a better discussion of the above post, refer to faq334-956 in the Polymer Engineering forum.



Rich Geoffroy
Polymer Services Group
polyserv@cox.net

 

[RvL04]

Nice FAQ, Rich!

Though I'd be very cautious extending such a rule of thumb to other materials. Once you get into the actual testing of stress relaxation or creep, more commonly elasomers are tested at constant strain (stress relaxation) and plastics at constant stress (creep). Also, whether your experiment is flexural, compressive, or tensile makes a huge difference on the normalized stresses / strains you mention. Finally, the specimen or part shape makes a huge difference (can the material 'flow' or not etc.). I must say I haven't read the Husted & Thompson paper you refer to in the FAQ, but I know for a fact that most plastics will not be at 100% of instantaneous value after 6 minutes, and the value of 80% appears to be a little high for various materials like PVC (somewhere around 65% after 1 hour is pretty realistic). There are lots of practical problems, obviously, with running these actual tests, but there are some low-cost solutions out there with various labs in the world that can run these tests in mechanical frames for long periods of time.

-Ron