BikerXC
Bioengineer
- Aug 3, 2013
- 3
Hello everyone.
I have been reading this forum for long time to answer most questions I have had. Nevertheless, I haven't found the answer to the following problem:
I am modelling a contact problem consisting of two plates and a cell between them. The top plate is a cantilever beam modelled with shell elements (S4), the lower plate is a rigid body and the cell is modelled with C3D8 elements. Moreover, I am using an UEXPAN subroutine to simulate the cell contraction between those plates. The elastic modulus of the cell is much lower than the cantilever modulus, which could be a problem.
The cell part has double elements divided in two sets: the first one represents the passive behaviour of the cell and the second one is the active behaviour (UEXPAN). This way, I can change the passive behaviour easily (or that was what I was thinking). I have been trying to run three types of passive behaviours: Linear elastic, hyperelastic (Neo-Hookean) and porous elastic. This model works fine with models without contacts, but not with them.
Surpringsinly, the more stable contact model is the porous elastic, but when I use hyperelastic and linear elastic passive behaviour I get convergence problems.
The contact must allow some sliding (I have to define a friction coefficient) with no penetration between surfaces.
I have tried several interaction properties:
*Surface Interaction, name=Lagrange
1.,
*Friction, lagrange
15., %% Here I have a question: Has a friction coefficient above 1 any sense? Some error in abaqus said that could be up to 1E+3 but...
*Surface Behavior, no separation, direct
*Surface Interaction, name=Penalty
1.,
*Friction, slip tolerance=0.005
15.,
*Surface Behavior, no separation, direct
1e+30,
*Surface Interaction, name=Rough
1.,
*Friction, rough
*Surface Behavior, no separation, direct
With these contacts I am trying a step with the following configuration:
*Step, name=Step-1, nlgeom=YES, inc=100000, unsymm=YES
*Static
1e-12, 1., 1e-20, 1.
The material definition is as follows:
For the hyperelastic behaviour:
** Titanio
*Material, name=Cantilever
*Elastic
552960000., 0.22 %With this modulus I obtain a specific stiffness needed to my model.
*Material, name=Kact
*Elastic
10000., 0.45
*Expansion, type=ORTHO, user
*Material, name=Kpas
*Hyperelastic, neo hooke
500., 4850.
*Material, name=Rigid
For the elastic behaviour:
** Titanio
*Material, name=Cantilever
*Elastic
973209600., 0.22
*Material, name=Kact
*Elastic
10000., 0.45
*Expansion, type=ORTHO, user
*Material, name=Kpas
*Elastic
1000., 0.45
*Material, name=Rigid
The model is drawn in microns so the units are Pa (pN/um^2) and pN for forces to be consistent.
I have tried surface-to-surface and node-to-surface discretization method, but as far as I know, surface-to-surface, even though could be slower, it is more likely to get convergence. The slave surface is the cell, which has finer mesh than the master surface, which is the cantilever. I also have tried several mesh refinement without any change in the result. Since I have to manually duplicate the mesh of the cell, adaptative meshing is not an option.
I have a collection of warnings and errors of different kinds depending on how I define the contact. For example:
With linear elastic behaviour, surface-to-surface and node-to-surface discretization method, lagrange, penalty or rough interaction and contact control to stabilize the solution, I get element distortion in very few elements in the top side of the cell and the convergence is judged unlikely.
Using hyperelastic behaviour, with the same configuration as above I get numerical singularity in the cantilver, which is encastred in one side so I don't know how can it be possible because I have fixed all the D.O.F.
I have been working in this model for several weeks but I don't get point. Any help and point of view will be helpful and thankful.
Thanks in advance,
Aaron.
I have been reading this forum for long time to answer most questions I have had. Nevertheless, I haven't found the answer to the following problem:
I am modelling a contact problem consisting of two plates and a cell between them. The top plate is a cantilever beam modelled with shell elements (S4), the lower plate is a rigid body and the cell is modelled with C3D8 elements. Moreover, I am using an UEXPAN subroutine to simulate the cell contraction between those plates. The elastic modulus of the cell is much lower than the cantilever modulus, which could be a problem.
The cell part has double elements divided in two sets: the first one represents the passive behaviour of the cell and the second one is the active behaviour (UEXPAN). This way, I can change the passive behaviour easily (or that was what I was thinking). I have been trying to run three types of passive behaviours: Linear elastic, hyperelastic (Neo-Hookean) and porous elastic. This model works fine with models without contacts, but not with them.
Surpringsinly, the more stable contact model is the porous elastic, but when I use hyperelastic and linear elastic passive behaviour I get convergence problems.
The contact must allow some sliding (I have to define a friction coefficient) with no penetration between surfaces.
I have tried several interaction properties:
*Surface Interaction, name=Lagrange
1.,
*Friction, lagrange
15., %% Here I have a question: Has a friction coefficient above 1 any sense? Some error in abaqus said that could be up to 1E+3 but...
*Surface Behavior, no separation, direct
*Surface Interaction, name=Penalty
1.,
*Friction, slip tolerance=0.005
15.,
*Surface Behavior, no separation, direct
1e+30,
*Surface Interaction, name=Rough
1.,
*Friction, rough
*Surface Behavior, no separation, direct
With these contacts I am trying a step with the following configuration:
*Step, name=Step-1, nlgeom=YES, inc=100000, unsymm=YES
*Static
1e-12, 1., 1e-20, 1.
The material definition is as follows:
For the hyperelastic behaviour:
** Titanio
*Material, name=Cantilever
*Elastic
552960000., 0.22 %With this modulus I obtain a specific stiffness needed to my model.
*Material, name=Kact
*Elastic
10000., 0.45
*Expansion, type=ORTHO, user
*Material, name=Kpas
*Hyperelastic, neo hooke
500., 4850.
*Material, name=Rigid
For the elastic behaviour:
** Titanio
*Material, name=Cantilever
*Elastic
973209600., 0.22
*Material, name=Kact
*Elastic
10000., 0.45
*Expansion, type=ORTHO, user
*Material, name=Kpas
*Elastic
1000., 0.45
*Material, name=Rigid
The model is drawn in microns so the units are Pa (pN/um^2) and pN for forces to be consistent.
I have tried surface-to-surface and node-to-surface discretization method, but as far as I know, surface-to-surface, even though could be slower, it is more likely to get convergence. The slave surface is the cell, which has finer mesh than the master surface, which is the cantilever. I also have tried several mesh refinement without any change in the result. Since I have to manually duplicate the mesh of the cell, adaptative meshing is not an option.
I have a collection of warnings and errors of different kinds depending on how I define the contact. For example:
With linear elastic behaviour, surface-to-surface and node-to-surface discretization method, lagrange, penalty or rough interaction and contact control to stabilize the solution, I get element distortion in very few elements in the top side of the cell and the convergence is judged unlikely.
Using hyperelastic behaviour, with the same configuration as above I get numerical singularity in the cantilver, which is encastred in one side so I don't know how can it be possible because I have fixed all the D.O.F.
I have been working in this model for several weeks but I don't get point. Any help and point of view will be helpful and thankful.
Thanks in advance,
Aaron.