Dear all,
I coded a 8 node solid-shell user element. I try to simply test it by a cubic subjected to the fully constrained bottom and uniform tensile load on the top. When I use one element to model the cubic, the user element can give the convergence result and agrees exactly with the FEAP-built-in solid element(8 nodes). The stiffness matrix outputted by the user and built-in element are the same.
However, when I mesh the cubic with (2*2*2)8 or even more elements, it always gives me the error which seems indicate the global stiffness matrix is singular. Since the DOFs at the bottom nodes are all restrained and the same mesh scheme with the FEAP-Built-in solid element works well, I think the problem is not about the boundary condition.
I coded the user element to write text ('isw=3 is called') to the screen when the isw=3 is activated. I found that the isw=3 has been activated just one time even if more than one element were used for the cubic. Thus, I suppose my code may have errors in the assembly process. But I didn't make any change in the program file in feap.
Are there anyone encountered this kind of issue before, any comment is welcome! Thank you!
