permanent set from cyclic loading?

[brashear]

I have a LDPE part that is subjected to a periodic compressive stress varying between about 20% of its yield strength and 0. When the machine is not running, it is subjected to a constant compressive stress of about 1% of its yield. Is it reasonable to expect this part to have no permanent set after millions of cycles and a few years sitting there? How about with a different polymer?

 

[brashear]

It should be mentioned that the service temperature never exceeds 50C, and the 20% of yield stress is experienced momentarily, like an impact

 

[patprimmer]

It will be subject to creep or cold flow.

All thermoplastics are subject to flow or creep under load. PE is one of the most susceptible. Acetal is one of the least susceptible.

If you can use a cross linked material you will have much better elastic recovery.

PE can be obtained in cross linkable form as can polyurethane and many rubbers. The middle ground are the vulcanized rubber filled alloys, the most well known being Santoprene.

50 deg C is relatively hot for LDPE

Regards
Pat
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[unclesyd]

A little off topic.

patprimmer,

Strange thing happen on the way to the Forum. I was on the phone with an engineer at the local Santoprene Plant when I open ths thread and read your post. He said thanks for the recommendation.

 

[brashear]

what's a good resource to find out how to quantify the amount of creep based on stress, time, and possibly some other parameters?

 

[patprimmer]

I have only ever seen creep data in documents published by raw materials manufacturers. They where called isocronus stress strain curves.

Regards
Pat
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[unclesyd]

I don't have access to any O-ring catalogues or brochures now but at one time they published permanent set data for their elastomeric materials versus various fluids at test temperature.
As 90% of our products in any table we always ran compatibility tests which included a test for permanent set