explicit analysis critical time-step Ansys

hello,

 I am trying to solve a simple cantilever beam (2D)  in ansys.

I wish to find out the critical time-step that can be used to explicitly solve a transient problem. 

I started with modal analysis to determine the maximum natural frequency of the model. I then computed dt < 2/Wmax.

I use this dt with beta = 0.0 and gamma = 0.25 (Newmark parameters). However the solution is unstable.

I am including my apdl script with this post.

I'd really appreciate if someone could explain why the computed dt does not yield a stable solution in this case.

thanks!

! ---- apdl script attached ----!