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 
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.

On Oct 13, 2016, at 12:00, David Knezevic 
<<>> wrote:

On Thu, Oct 13, 2016 at 11:57 AM, Rossi, Simone 
<<>> 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?

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.


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