How to give INTIAL CONDITIONS(Pore pressure) and void ratio?

[krishnagoud]

Hi all,

I am facing the problem with Pore pressure elements (CAX4P)

How can I give initial conditions, like pore pressure and void ratio to the above elements?

I think I have to give them in input file. but how??

Would you please explain PHYSICS of that element and input data?

I have gone through all help documentation but I could not find it....(In help I dont have search column where i could type something., I donnno why?.....I was searching each and every index....)


Please help me...........

Thanking you
Regards,
krishna

 

[mrgoldthorpe]

*INITIAL CONDITIONS, TYPE=PORE PRESSURE
NALL , <first> , <vcoord1> , <second> , <vcoord2>

where "NALL" is the appropriate node set (say "all" the nodes), and the values in <> are those required as described in the ABAQUS manual.

I'm afraid I can't answer your question about the physics of the element.

 

[krishnagoud]

Thank you for your reply.

would you please send me one input file which uses pore pressure elements so that I can understand not only pore pressure elements but also how to write other KEYWORDS.

Any way thanks again !

/Krishna

 

[krishnagoud]

Hi,

I am modelling a porous medium(cartilage at human knee) with pore pressure elements.
(poroelastic)
For this, I can define single permeability and void ratio in under MATERIAL KEYWORD.

But permeability (strain dependent permeability) is a function of void ratio with a mathematical equation, like k=k0*((1+e)/(1+e0))^M

K=permeability
K0=initila permability
e=void ratio
e0=initial void ratio
M=coefficient

How can I assign the above equation in ABAQUS.

Any help is appreciated.

Regards,
krishna

 

[mrgoldthorpe]

Read the manual on User Defined Field and State Dependent Variables, that is by including the following as part of your material definition:

*USER DEFINED FIELD
DEPVAR
1

as well as using a field variable to define the variation of permeability of your material. User subroutine "USDFLD" is used to do the calculation of the field variable.

http://scv.bu.edu/documentation/software-help/scientific-engineering/abaqusdocs/v6.7/index.html

 

[geotechniqa]

If all are constants except e , then calculate the function k(e) using for example excel and copy and past results in the table defining permeability in terms of void ratio.
Thanks.
Physics of pore pressure is that : the formulations of finite element of porous medium are represented by coupled equations with displacement and pore pressure unknown at each point. The medium is descritized in space using elements that can be,for example, bilinear in displacment and pore pressure.In this case you get at each node value for pore pressure and displacement.

 

[krishnagoud]

Thank you all for your replies

/Krishna

 

[krishnagoud]

Hi all,

I modeled knee joint as axisymmetric as beginning step.

My axisymmetric model consists of following.
1. Cartilage is modelled with CAX4PH (axisymmetric pore pressure elements)
2. Meniscus is also with the CAX4PH elements
3.Tibial bone with CAX4 elements.

Since contact between two deformable bodies create computational difficulties, I am using membrane with low stiffness between the cartilage and meniscus.

Membrane is tied to meniscus and modeled with MAX1 elements.

I got all the above information from one of the journal papers and I did the same.

But unfortunately I could not solve the problem. I need help in following aspects.

Q1:complete description of pore pressure elements?

Q2:Regarding solution methods as my model contains CAX4, MAX1 and CAX4PH elements ? (I tried with soil analysis and also with general stress analysis with default options)

Q3:Input parameters and complete key words for pore pressure elements including solution techniques?(initial condions, pore pressure and ratio were already given by using kewwords)

Q4hysical behaviour of the porous elements ?

Q5:void ratio: should it be lessthan 1 or more ? I have solid volume fraction of 0.75. is this void ratio or other way round ?

Q6:at free surfaces pore pressure is zero as fluid flows freely in and out, but what should be the boundary condition at contact of cartilage and meniscus as there is no fluid exchange occurs across the borders ?

Please find an attached input file by which i could not get the solution.

I have read all the documentation regarding porous and permeability. But it was not sufficient.

Any help would be greatly appreciated

Awaiting for your kind reply

Thanking you in advance !

Regards,
krishna