I have always wondered why Simulation allows the user to assign a Plastic material in a Static study. Isn't this in 99% of the cases just asking for trouble? Would it not be better if Simulation would warn the user or outright forbid it to be possible?
Are there any cases where using a Plastic can be justified for a Static run, if so I'd love to hear people's experiences with this!
Hi Kevin: I believe you are expressing concern for how plastic materials can behave - namely, the loading can cause deformation beyond linear-elastic behavior. I can think of a few cases where you might want to use a linear-elastic behavior.
In one case we use the linear-elastic run in pragmatic FEA as a method to transition to a nonlinear run. That is, (as we taught the course), we encouraged a philosophy of "...just getting the model to run...", and "...take baby-steps..", using simple linear-elastic behavior. The turn-around is relatively quick compared to nonlinear, and you can trouble-shoot things like meshing issues, examining behavior due to applied boundary conditions (is the model deforming as expected?), evaluating reaction forces at supports, and so-on. Conversely, I have seen cases where users plow directly into a nonlinear model and uncover scores of issues that are super-difficult to identify - and the turn-around can take a very long time to trouble-shoot since the nonlinear stepping algorithm can take a long time. Only after conducting a linear-elastic run would we encourage setting-up a nonlinear model since the user has presumably fixed any issues and observed some sense of reasonableness in the linear run.
For a second case I have seen some PVC materials exhibit linearity up to about 3 or 4% strain (esp. at low temperature applications), so a linear-elastic set-up might be appropriate if loading is not expected to cause strain greater than these values.
For a third case I have seen customers who purposely do not want the response of their polymer plastic to "get into" the nonlinear response region. For a given loading, they iterate on geometry, wall thickness, adding ribs and so-on and conduct linear-elastic runs to stay under what they consider their "not-to-exceed" strain (which is usually the "proportional limit" of the stress-strain curve, as you may already know). I have observed this design philosophy for some plastic housing assemblies of portable power tools, for example.
However, I appreciate your comment about a warning from the software. It might be a good idea for the software to post a note to the user that linear-elastic behavior is assumed, should they choose a relatively soft material. However, it might be hard to define a threshold for the notice to appear - a good thought.
Hope that helps a little. - Tony