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# Pressure load definition in Abaqus Explicit when the surface is constantly changing

## Pressure load definition in Abaqus Explicit when the surface is constantly changing

(OP)
I am trying to simulate a simple model in Abaqus- an elastic spherical shell being sucked into a rigid tube. The diameter to the sphere is greater than that of the tube and I want to study the large deformation of the shell into the tube.
I am unable to define a load to simulate the suction pressure on the shell applied by the tube. I am using Pressure as the distributed load, which can only be defined for a particular surface. In this problem, the surface on which the suction pressure will be applied will constantly change as the shell enters into the tube. I would be grateful if anyone can help me defining such suction pressure.

How to write a user subroutine using VDLAOD

### RE: Pressure load definition in Abaqus Explicit when the surface is constantly changing

Actually this is quite similar to the micropipette aspiration problem, only in larger scale and with different material probaly. There are some articles describing such simulations. In these articles you can find different approaches to the problem of suction pressure application. But first of all it is highly advised to use axisymmetric model (if possible in your case). Also make sure that initially sphere is already in contact with pipe.
I can recommend the following articles:
- ”A Finite Element Study of Micropipette Aspiration of Single Cells: Effect of Compressibility” by A.J. Bidhendi and R.K. Korhonen
- ”Large Deformation Finite Element Analysis of Micropipette Aspiratipn to Determine the Mechanical Properties of the Chondrocyte” by F.P.T. Baaijens et al.

The second article describes simplified approach to the problem of load application (which I would also advise, at least for the beginning). They simply partition the face of the sphere and apply suction pressure to the small face which is initially already inside the pipe.

### RE: Pressure load definition in Abaqus Explicit when the surface is constantly changing

(OP)

I saw both the articles which you attached I found that in both the models they are using fluid to load the cell. I am doing 3D modeling. In which I am using hyperelastic incompressible solid. I don't want to use the fluid to load the cell because it will further complicate my analysis. Do you have any other ideas for the micropipette Aspiration of single-cell without using fluid in Abaqus Explicit?

### RE: Pressure load definition in Abaqus Explicit when the surface is constantly changing

Actually I was thinking about something like this (suction pressure applied to the red part of the sphere):

It should eliminate the need for variable loaded surface.

### RE: Pressure load definition in Abaqus Explicit when the surface is constantly changing

(OP)

Yes, it is great if you could tell me how to apply the load without using the fluid in Abaqus Explicit, as shown in the above figure.

### RE: Pressure load definition in Abaqus Explicit when the surface is constantly changing

Fluid is not necessary here, pressure applied directly to the sphere in the outward direction can pull it towards the tube (it should work well in vacuum too, just leave the motion of the sphere in this direction free). Since it is applied only on the small region of the sphere, there will be no problem with variable area of load application during the aspiration process. You can try with prescribed displacement too.

### RE: Pressure load definition in Abaqus Explicit when the surface is constantly changing

(OP)
Hello,

For the above problem, I am interested in solving by applying the pressure load on the backside of the spherical cell. I am planning to solve using VDLOAD user Subroutine. Can anyone help me with the easiest way of doing using VDLAOD?

Thanks

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