Learning Forum

[Fraser Shields]

 

 

Hi Peter, thanks for the response. I’ve simplified the model down to it’s symmetry planes and it seems to run when the solid and shell moduli are similar in magnitude – even without applying any pressure to the shell walls. However, in reallity the solid modulus is actually significantly smaller than the shell modulus – approx. 1 kPa compared to 300MPa (shell elements have orthotropic properties, 300MPa parallel to facesheet, 1 kPa normal to facesheet).  I have attempted to fit a Mooney-Rivlin hyperelastic model to some experimental data. However, this has not helped with convergence.

The command snippet is the following:

!   Commands inserted into this file will be executed just prior to the ANSYS SOLVE command.

!   These commands may supersede command settings set by Workbench.

 

!   Active UNIT system in Workbench when this object was created:  Metric (um, kg, uN, s, V, mA)

!   NOTE:  Any data that requires units (such as mass) is assumed to be in the consistent solver unit system.

!                See Solving Units in the help system for more information.

 

/PREP7

 

! Define an element type with the desired real constant (CPT216)

ET, 9991, CPT216

KEYOPT, 9991, 12, 1

!KEYOPT,9991,6,1

! Select the solid elements you want to modify (SOLID186)

ESEL, S, TYPE,, 1, 69

! Modify the selected elements to use the specified element type (9991)

EMODIF, ALL, TYPE, 9991

 

ESEL, NONE

 

 

TB,HYPER,99999,,,MOONEY

TBDATA,1,-0.0042, 0.0044, 0 !c1 , c2 = material constants characterizing the deviatoric deformation of the material, c2=incompressibility parameter

FPX=1e5 ! PERMEABILITY

ALPHA=1 ! BIOT coeffient

TB,PM,99999,,,PERM 

TBDATA,1,FPX,FPX,FPX

TB,PM,99999,,,BIOT ! BIOT COEFFICIENT

TBDATA,1,ALPHA

 

 

ESEL, NONE

 

 

! Define orthotropic elastic material for shell elements 

MP,EX,88888,300 ! YOUNG’S MODULUS

MP,EY,88888,0.001

MP,EZ,88888,0.001

MP,GXY,88888,150

MP,GXZ,88888,0.001

MP,GYZ,88888,0.001

MP,NUXY,88888,0 ! POISSON’S RATIO

MP,NUXZ,88888,0

MP,NUYZ,88888,0

 

ALLSEL, ALL

 

! Select the solid elements you want to modify and assign material 1

ESEL, S, TYPE,, 9991

MPCHG, 99999, ALL

 

ALLSEL, ALL

 

! Select the shell elements you want to modify and assign material 2

ESEL, S, TYPE,, 70, 189

MPCHG, 88888, ALL

 

ALLSEL, ALL

 

! impervious (fluid flow flux=0)

NSEL,S,LOC,Y,1000 ! select top surface nodes

NSEL,A,LOC,Y,0   ! select bottom surface nodes

SF,ALL,FFLX,0    ! Set surface force (fluid flow flux) to zero

ALLSEL, ALL

 

ESEL, NONE

 

 

! permeable (fluid pressure=0) on sides of cylinder to allow fluid flow

CSYS, 5  ! change coordinate system to cylindrical with cartesian y as axis of rotation

 

NSEL,S,LOC,X,3250  ! select nodes at radius=3250um

D,ALL,PRES,0  ! set fluid pressure to zero 

 

ALLSEL, ALL

 

 

!CMSEL,S,internal_free_draining,node

!D,ALL,PRES,0

 

 

CSYS, 0  ! change coordinate system back to global coordinates

 

 

ALLSEL, ALL

 

! Continue with the solution phase

/SOLU

 

 

Any ideas how I could get this model to run with such large differences in stiffness between the solid matrix and reinforcing shell elements?

 

Thanks in advance.

 

 

 

Here is a linke to the archive file - can't figure out how to attach it to the comment:

 

https://livemanchesterac-my.sharepoint.com/:u:/g/personal/fraser_shields_postgrad_manchester_ac_uk/Eeew5xpFBGJPhyT0oSj4tRYB3mcMUJdEk-aWrWNkDLMmoA?e=Xc1juw