The problem in solving nonlinear dynamic problem with Newmark-Beta & Newton-raphson method. How to solve the problem efficiently with a converged solution and with any time step.

[Lihe168]

I'm trying to get the nonlinear dynamic response of a beam with both of its ends simply supported and its middle point is excited by a cyclic load. The nonlinear here mainly refer to the geometic nonlinearity during the vibration process. I'm tring to solve this problem with Newmark-Beta method and Newton-Raphson method. However, the solution diverges after 24s, Besides, only the transverse displacement is correct, the axial displacement and rotational angle of the cross section is totally incorrect. I really don't know what's the problem with my code, so I hope someone can help me with it.
I simulate this problem with abaqus, the abaqus can get the right answer with any time step. However, my procedure diverged if the time step is set to 0.1s.
The following is my code and the result.Uploading Nonlinear-Simplysupported.rar…

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