CSUR command

[BSFEAP]

Dear All,

I obtain the experimental data containing coordinates on the top and bottom surfaces of the specimen and corresponding displacements.  I am trying to prescribe them in FEAP with the help of cbou and csur, see input file ielastic. It is not working.

However, if I only control four corners of the top and bottom surfaces of the specimen, it works well, see input file ielastic1.

I am using the same format for two inputs, yet the first one has more nodes.

I can not figure out why? The mesh is not fine enough or FEAP can not implement this function?

Thanks a lot.

[Prof. R.L. Taylor]

I can not understand what you expect to get for the top and bottom displacements.  FEAP is getting values, however, the solve gives NaN's so it is diverging.  if you check the mesh with CHECk you will find that there are no x-displacement b.c. so it is singular.  If you optimize the profile feap can solve directly without using an iterative solution???

If all the displacements on top and bottom are the same you only need one SURFace patch covering the top and one for the bottom, or since all are at the same elevation you could use EBOU and EDIS. 

[BSFEAP]

Dear Prof. Taylor,

Yes, you are right. It becomes singular when I used coarse mesh on the top and the bottom mesh.  This problem can be overcome by using fine mesh.

Basically, I will prescribe the displacement BCs from the experiment on specific nodes.   With fine mesh, some results come out, but they are not perfect as I expect.  I have no idea whether FEAP works well in this way (so many blocks for SURF).

Displacements are not the same.  As the next step, I will give different proportional loads to each corder of the sub-surface, thus copy experimental data to our simulation.

Is it working? Thanks a lot.

[Prof. R.L. Taylor]

First, if you run the solution command CHECK, it will Tess how many BC are imposed in each DOF.  I got zero for the x_1 direction. This must be corrected, a fine mesh may solve due to round off but the solution could be wrong.

I do not understand hou you gave the displacements you wish to impose.  Are the on the entire surface top and bottom?  Are they smooth, constant, or varying a lot?