Hello,

I have some basic questions regarding the SSD direct in Abaqus. I know how to implement it and i have done that in matlab. However, my questions only concern abaqus. My model is a simple composite shell model with a number of layers.

1. According to the theory of SSD no Displacement Boundary Conditions (BC) are needed in SSD analysis. However, when i run the analyses in the absence of BC in abaqus i get warnings of 18 negative eigenvalues the system have. Why would the system have 18 negative eigenvalues?

2. I run the simulation with structural damping. Is there any issue there that should be taken into account?

3. In the extraction of the FRF i only get the real part, not the imaginary part, despite the fact that i have a complex system given the structural damping.

I would appreciate a clear and concise guidance!
Best