syfi team mailing list archive

[kent-and@xxxxxxxxx]

> kent-and@xxxxxxxxx wrote:
>> The nonlinear hyper-elasticity demos do not seem to produce
>> the correct solution. I have been looking in particular on the Fung
>> stuff.
>>
>> Newton seems to converge with quadratic convergence whenever
>> it is close to the solution so I think the differentiation is correct.
>
> OK. I have been trying a few things over the past week (compiling forms
> with FFC instead) and here is what I see:
>
> 1. Nonlinear scalar Poisson works.
> 2. Linear elasticity posed as a special case of a nonlinear problem
> works. (Converges in one iteration).
> 3. Nonlinear elasticity (hyperelasticity with the St. Venant-Kirchhoff
> model) a. does not converge or b. the direct solvers complain about a
> singular matrix.
>
> I didn't think to look into the boundary conditions for 3 because I used
> the same setting as 2. I will look at them more closely.
>
> Harish

I don't think it is a problem with the bc. I tested some nonlinear variants
of Poisson that come from convex functionals. In these cases Newton
should work. Can we say the same about SVK or Fung or do we either
need a relaxation scheme or start close to the solution ?

Kent