TAGGED: apdl, matlab, stiffnessmatrix


April 18, 2024 at 8:02 amC DehoBbp_participant
I performed a modal analysis and used APDL SMAT to extract the stiffness matrix. Then, in MATLAB, I applied a force toÂ just one node, but the results are significantly different from what I obtained from a static analysis. In ANSYS, excluding the fixed points, deformation occurred at all nodes. However, when performed in MATLAB, more than half of the nodes showed zero deformation. I used the governing equation F=Kx. 2Why might this be happening?
Size of stiffness matrix is number of nodes*3 (X, Y, and Z directions). So, does this mean the order is node1x, 1y, 1z, ... nx, ny, nz? The analysis was conducted with default settings.Â
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April 19, 2024 at 2:52 pmmrifeAnsys Employee
Hi C Deho
Please check the Ansys MAPDL help on the *SMAT command.Â MAPDL does reorder the nodes/elements from what we as users see in the FEM.Â And there is a mapping vector available that relates the FEM to Solving order.Â Â

April 21, 2024 at 12:22 pmC DehoBbp_participant
Hi mike
Do you mean that the node numbers seen in Workbench (Node ID) differ from those in the exported stiffness matrix?Â
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April 22, 2024 at 3:53 pmmrifeAnsys Employee
Hi C Deho
I've replied a few times to this and received an odd error each time...so sorry if there ends up being multiple copies of basically the same answer.
Anyway, the stiffness matrix does not use node IDs.Â Nor does the force vector  it uses the degree of freedom IDs.Â In any case yes, the IDs used to solve differ from those that WB Mechanical shows us for the mesh.Â Hence the need for the vector of ID mapping.

April 26, 2024 at 11:52 amC DehoBbp_participant
Hi mike
I extracted a mapping vector using NOD2BCS and the resulting figure appeared as shown. It seems to map excluding fixed points starting from [2220,1]. What I'm curious about is what the values on both sides mean. Just like in the figure, there are fixed nodes at both the 200th and 800th positions, but in the mapping vector, all the values seem to be gathered towards the end. Isn't NOD2BCS supposed to provide the relationship between node IDs and the degrees of freedom (DOF) used by the solver?
I'm using a student license, so I can't access education sources provided by Ansys. I've also searched the internet, but I can't find the information I'm looking for. It would be greatly appreciated if you could provide a more detailed explanation on how to match the DOF of the stiffness matrix with the node IDs for F=Kx calculation, though I know it might be a hassle.
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Regards,
Deho
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