1. Step - Linear Perturbation - Buckle
2. Apply boundary conditions and force. This will essentially influence the result. Force direction is very important, but magnitude doesn't matter. Choose the number of eigenvalues we need, generally 10 is enough.
3. Edit inp file. In the output region, we add
** OUTPUT REQUESTS *Node file, global=yes U
The result node will be written in the *.fil
4. Observe in the result which mode we need. Try to adjust the scaling factor to see what is a appropriate value for imposing the imperfection, see 1e-5 for example
5. Edit the analysis inp file, add
*IMPERFECTION, FILE=aaa, STEP=1, (NSET=NodeSet) 7, 2e-5
This can be put just before all the steps.
When the NSET is used, be careful of the boundary condition and contact constrain if multiple instances exist in the assembly
6. Run the analysis