On Thu, Oct 13, 2016 at 12:34 PM, Rossi, Simone <sro...@email.unc.edu>

> Dear David,
> thanks for your answer. In this case (nu=0.49995), first order elements
> typically lock, but second order elements typically do not lock.
> In fact many use second order lagrangian elements for nearly
> incompressible materials. I wanted to use this example just to show that
> second order elements are not inf-sup stable.
> But the results I get running Ex4 are not "bad": in my opinion, they are
> nonsense.
> I wonder if the differences come from a different way of handling the
> boundary conditions or from a bug in the assembly.
> Let me know if you have any insight.
> Thanks,
> Simone

Not sure why that would be the case, I guess you'll need to do more tests
to figure out what's happening. Feel free to reach out if you have any
specific questions. I doubt there's an issue with the BCs since they use
DirichletBoundary code which is widely used, but it wouldn't hurt to check
the assembly (I normally use 3D elasticity, and I'd say that this 2D
elasticity example has not been widely used so a bug is possible, or
alternatively maybe it's a plane strain vs. plane stress issue).


> On Oct 13, 2016, at 12:00, David Knezevic <david.kneze...@akselos.com>
> wrote:
> On Thu, Oct 13, 2016 at 11:57 AM, Rossi, Simone <sro...@email.unc.edu>
> wrote:
>> Dear all,
>> I’m playing around with the elasticity tests in the system of equations
>> examples (more specifically Ex4 and Ex5).
>> In particular I’m trying to set the poisson ratio to nu = 0.49995.
>> With this choice the solution I get using second order lagrangian
>> elements does not make any sense.
>> For first order elements the solution looks more reasonable, but still
>> different from what I get from FreeFEM++.
>> Does it depend on the enforcement of the Dirichlet boundary conditions?
>> Thanks,
>> Simone
> nu=0.49995 is almost incompressible. Normally people use special
> formulations for that type of problem, e.g. a mixed method to enforce
> almost incompressibility (similar to Stokes in fluids). That probably
> explains why you get bad results by naively using the simple formulation
> from ex4 and ex5.
> David
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