10460
2017/03/08
Ubuntu 16.04.2 LTS
64-bit
master
91c59c7910436c44ede608e29d9a90a287121a11
Hi everybody,
I would like to develop some hyperelastic incompressible models with calculix in my laboratory, but before this, it is necessary to test calculix.
For this, I do some simple cases like tensile test (simple and bi-axial) and calculix result are always differents of analytic result (and COMSOL Multiphysics results) for Neo Hookean or Yeoh model and 3D problem.
So, I ask some questions at Guido Dhondt :
and he has replied me this following message :1) Do you know why?
2) What's happening exactly? Can you tell me how Calculix solve hyperelastic problem?
I know that Neo Hookean is not good to solve a bi-axial hyperelastic problem, but I do that to understand how calculix solve this problem and why even with Yeoh model our results are differents.
3) Is it a problem with a Cauchy tensor?
4) With hyperelasticity, is it better to use *DLOAD or *CLOAD?
I am moderately satisfied with this answer because even if Calculix calculates differently from the analytic, strain results must be the same (I have a noticeable difference of 20% !!!).the background of the hyperelastic implementation is described in detail in my
book (Wiley 2004, cf. website http://www.dhondt.de). It involves a straight
differentiation of the 2nd Piola-Kirchoff stress w.r.t. the Lagrange strain
tensor (CalculiX uses a total Lagrangian approach).
The stresses which are reported in the .frd-file are Cauchy stresses.
So I come to ask you:
1) In FreeCAD, are the true constraints displayed (in the sense of Cauchy in Eulerian) or those of Piola-Kirchoff (in Lagrangian)?
2) What is his Von Mises based on?
3) Do you have any idea of where such discrepancies could arise?
4) The difference may also be the interpretation of the force applied between CLoad and DLoad, which then seems the most correct?
I suppose DLOAD is better with large deformation, but it is more difficulte to converge.
Thanks