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What should I use as a failure theory for polymers. Does the Von mises stress give reasonable results?
The Von Mises (deviatoric) stress may be a good stress measure to use, but the von Mises yield criterion in particular is often inappropriate for polymers. There are indeed many theories of (and contributors to) polymer yielding. Among factors influencing the "yield stress" are:
-Hydrostatic pressure (which is one way von Mises fails outright)
-Presence of or exposure to solvents
Unfortunately, things are not as straightforward here as with steel! Happily, there is much written about this subject, and I would recommend some reading.
If you have access to textbooks:
Sperling: "Introduction to Physical Polymer Science", Chapter 11 (I think his 4th ed. is due out soon, but that chapter will be little changed from 3rd)
Anything by Ward and Hadley, "Mechanical Prop's of Solid Polymers" being the more introductory.
If you can't find these, Nielsen and Landel is very good, but tougher to get through.
There are also several review articles in the pertinent journals. I can't think of specific ones offhand, but a search of Polymer, Mechanics of Materials, Macromolecules, or J. Appl. Polymer Sci. may yield fruit. Good luck!
The most appropriate choice of failure model will depend on the type of polymer that you are studying and the loading mode that is applied.
For example, for monotonic loading to failure of high density polyethylene (UHMWPE) the chain stretch failure conditions has been shown to work better than the Mises conditions. You can read more about this in the following paper (http://www.polymerfem.com/modules.php?name=Downloads&d_op=getit&lid=39).
Failure of polymers is not always simple to predict and can be strongly influenced by environmental exposure in addition to mechanical loads. I suggest that you check out the books recommended by SQ.
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