Dear Samyak, As Yves has pointed out, in general one needs to add the multiplier p for incompressibility to the trace of stress tensor from the material law. This is because material laws can in general give hydrostatic pressure even if the deformation is isochoric. Hence I would suggest you to check both contributions, however I do not expect this to explain the large positive/negative values that you mentioned.
Another thing to check is a possible incompatibility between the finite element spaces for the multiplier and the displacements. A simple rule is to choose a finite element space for the multiplier of one order lower than for the displacements. Best regards Kostas On Thu, Feb 9, 2017 at 1:35 PM, samyak jain <[email protected]> wrote: > Dear Kostas, > > My mistake. I didn't mean to say that it is zero. I said that it is not > possible to calculate that because of incompressibility and we only get > the deviatoric part and so it is zero if we try to calculate that. > > I understand that p should be the Hydrostatic pressure as I add it as > incompressible brick and then use the equilibrium with boundary conditions > to get the value of p but the strange part is that some of the values are > positive and some negative and also they are quite high. > > I have checked everything and it seems that everything else is correct. > > Anyway thanks for answering. > > Yours sincerely > Samyak > > On Feb 9, 2017 5:26 PM, "Konstantinos Poulios" <[email protected]> > wrote: > >> Dear Samyak, >> >> It is definitely not true that the hydrostatic term -1/3*Tr(sigma) >> should be zero in an incompressible material. If this is the case you are >> simply calculating the deviatoric part of sigma instead of sigma. In order >> to get sigma you need to add the term p*Id(3). Actually the hydrostatic >> term you are looking for is simply equal to p. >> >> Best regards >> Kostas >> >> >> >> On Thu, Feb 9, 2017 at 4:20 AM, samyak jain <[email protected]> >> wrote: >> >>> Dear getfem-users, >>> >>> I am currently trying to solve a contact problem between a hyperelastic >>> rubber and a rigid bosy and I need to calculate the pressure values on >>> either on the rubber. >>> >>> I am using Incompressible Mooney-Rivlin Hyperelastic law and if my model >>> I am adding also adding finite strain incompressibility brick. >>> >>> Now when I calculate Cauchy Stress from second piola kirchhoff stress, I >>> am getting the Hydrostatic term (-1/3Tr(sigma)) of the cauchy stress tensor >>> as zero which is what it should be as the material is incompressible. >>> >>> So, is there a way is getfem to calculate the hydrostatic pressure term >>> for such incompressible materials.I believe treating the material as >>> nearly incompressible (Poisson's ratio 0.499) is one way to solve it but I >>> don't know how it works or if it is implemented in the model. >>> >>> Could you guys please provide any help or suggestion to calculate the >>> hydrostatic pressure for such a case. >>> >>> Thanks a lot. >>> >>> Yours sincerely >>> Samyak >>> >>> _______________________________________________ >>> Getfem-users mailing list >>> [email protected] >>> https://mail.gna.org/listinfo/getfem-users >>> >>> >>
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