2.4. Semi-Implicit Method Limitations

Keep in mind the following limitations when using the semi-implicit method.

  • This method is only available for static (ANTYPE,STATIC) and full transient (ANTYPE,TRANS) analysis types.

  • This method is available only for structural degrees of freedom. It does not support any element with non-structural degrees of freedom.

  • This method does not support the element formulations listed below.

    • Solid elements:

      • PLANE182 and PLANE183 with the generalized plane strain option (KEYOPT(3) = 5) or the axisymmetric with torsion option (KEYOPT(3) = 6).

      • SOLID185 with the enhanced strain formulation (KEYOPT(2) = 2 or 3).

      • Solid shell element SOLSH190.

      • Generalized axisymmetric SOLID272 and SOLID273 elements.

      • SOLID285 4-node tetrahedral element.

    • Shell elements:

      • SHELL181 with incompatible modes (KEYOPT(3) = 2).

    • Beam elements:

      • BEAM188 and BEAM189 with the warping degree of freedom (KEYOPT(1) = 1).

    • Pipe elements PIPE288 and PIPE289.

    • Elbow element ELBOW290.

    • Contact elements:

    • Target elements relaxation option (KEYOPT(11) = 1).

    • MPC184 joint elements with friction, stops, and/or locks.

  • The semi-implicit method does not support nonlinear mesh adaptivity (NLADAPTIVE) and manual rezoning if the mesh changes occur during the semi-implicit solution phase. These capabilities are supported if the mesh changes occur during the implicit solution phase.

  • During the semi-implicit solution phase, you cannot access the element matrix files Jobname.emat and Jobname.full, as the definitions of the matrices and what is stored in these files are different during the semi-implicit solution phase compared to the implicit solution phase.

  • Every node in the model must have a mass associated with it. Therefore, each node must belong to at least one element that has material density defined. For example, you cannot have only joint elements (MPC184) in the model. Each joint element must be connected to a structural element.

  • The semi-Implicit method ignores the constant term of constraint equations (both externally and internally generated) while enforcing constraints during the semi-implicit solution phase. This can lead to some incompatibility in the enforcement of the constraint equations. For internally generated constraint equations arising from contact element (CONTA172 through CONTA177) with KEYOPT(2) = 2, it is recommended you set KEYOPT(9) = 1 so that there won't be any constant term in the constraint equation.

  • The semi-implicit method requires that the material density remains constant throughout the solution. Defining density as a function of solution variables via TB or MP commands is not supported.

  • The semi-implicit method does not support restart from the first substep of the semi-implicit solution phase. You can restart from any substep of the implicit phase or from the second substep and beyond of the semi-implicit phase.

  • The semi-Implicit method does not support element birth and death during the semi-implicit solution phase.

  • The semi-Implicit method does not support the XFEM and SMART methods for crack-growth simulations.

  • During the semi-implicit solution phase, no stabilization damping is applied. More specifically, stabilization force from the following sources are not added: nonlinear stabilization (STABILIZE command), contact stabilization damping (KEYOPT(15) on contact elements CONTA172, CONTA174, CONTA175, and CONTA177), and viscous regularization (TB,CZM,,,,VREG).