Iex6 Benchmark Question

[Yaakov]

Dear FEAP Team,

in Benchmark Iex6 (Thermomechanical Problem),  This model use command "MIXed"  (Mixed u-p-theta model).

If I change "MIXed" to "DISP", then the results between the two vary significantly.

Please refer the attachment for details.

Can you please let me know if the results should be this way?

Kind Regards,
Yaakov

[Prof. R.L. Taylor]

Did you look at the output file to see what the material properties used were?  What does the stress behavior suggest?  You need to recognize this type of response so you understand why we use a MIXEd formulation.  Try changing the properties to see if you can make the DISPlacement behave better.

[Yaakov]

Dear Prof. Taylor,

Thanks very much for your reply, now I know the problem coming from shear locking (v>0,40). in this case I can use either "mixed" or "enhanced (2d , 4 nodes)" formulation (user manual 73).

Kind regards,
Yaakov

[Yaakov]

Dear Prof. Taylor,

may I continue this topic? I would like to ask, if the value of Poisson's ratio is 0,49,  can I use high order displacement element in the simulation (no locking effects)?  As I read articles by Miehe et al. they used some advanced elements in the fe-simulation, e.g. mixed/assumed enhanced/....

Many thanks for your help again.

Kind regards,
Yaakov

[Prof. R.L. Taylor]

Probably, if the "p" is high enough on the displacement formulation it might work.  Of course, also if you refine the mesh enough, it probably will eventually start to converge - may take a million elements thought.  Feap can to "p" = 2 and 3 for standard elements and I think p=4 for isogeometric elements.

[Yaakov]

Dear Prof. Taylor,

Thanks for your answer!

Recently, I read a article by Caseiro and co workers, namely isogeometric solid-shell elements (H2ANS). In this element, they use ANS Method to alleviate locking effects. I would like to ask, If IGA is a "locking free" method, why ANS method is still necessary in isogeometric solid-shell elements (H2ANS)?

Warm regards,
Yaakov

[Prof. R.L. Taylor]

IgA is not locking-free.  For example if you were to subdivide a linear know vector on a 2-d NURBS patch you would get an identical formulation to the 4-node isoparametric FEA formulation.  Thus, for low order IgA forms one has to do something.  In the feap  IgA module there are some mixed u-p-theta elements which try to eliminate locking.

I have not read the paper you cite but using ANS is another way to make the formulation avoid most locking situations.

[Yaakov]

Dear Prof. Taylor,

Thanks Very Much for Your Answer!

Now I undertstand that the argument "IgA is locking-free" is not correct.

The cited article is "On the Assumed Natural Strain method to alleviate locking in solid-shell NURBS-based finite elements" http://rdcu.be/GV2S

   by  J. F. Caseiro, R. A. F. Valente, A. Reali, J. Kiendl, F. Auricchio, R. J. Alves de Sousa

Many thanks for your support!

Warm regards,
Yaakov