Hello all,

Could anyone help me with the four node quadrilateral element with one quadrature point for nonlinear implicit solid mechanics problem? To be more precise, I would like to know
  (1) the form of the internal force vector and
  (2) the form of the tangent matrix

of this special element.

I read the book of Prof. Belytschko already but unfortunately it didn't help much. Literature on this element is not clear to me. For example in some papers I could find (1) but not (2) and vice versa in other papers :).

Thanks a lot for your time.
Phu