ABAQUS Negative eigenvalues

[Mario96]

Hi Everyone! I'm currently working on a non linear static analysis of a composite boom and I'm interested in stresses and deformation when there's a certain load at tip of it. The composite boom has a cross sectional area of a sector of a hollow circle. I applied just an encastre at the root, while at the tip nodes there's the concentreted force; there's also a 0.1g gravity load. The simulation seems okay at the beginning, but then at the end of it, negative eigenvalues starts to appear and I get "Excessive distortion at a total of 13396 integration points in solid (continuum) elements" message; the solution does not converge at the end. I read online that this could mean local instabilities, but I would like to have some guidelines. Moreover when i check the deformed shape, there's a strange displacement along the z-axis. If I re-run the simulation, but with a boundary condition at the tip nodes on U3, everything seems to work right, with just 3 warnings about negative eigenvalues.
I attached the model, I would really appreaciate your help!

 

[FEA way]

Try running this simulation without NLGeom first and see how it behaves under applied load. By the way, make sure that the load definition is correct and that its magnitude is not too high - that's the likely reason for errors in this case.