Mechanical APDL Basic Analysis Guide

rotational stiffness connected perpendicularly to a beam or pipe element. There is no in-plane rota-

tional stiffness at the joint. A linear analysis ignores an in-plane moment applied at that joint.

• Buckling.When stress stiffening effects are negative (compressive) the structure weakens under
  load. If the structure weakens enough to effectively reduce the stiffness to zero or less, a singularity
  exists and the structure has buckled. The "NEGATIVE PIVOT VALUE - " message is generated.


• Zero Stiffness Matrix (on row or column). Both linear and nonlinear analyses ignore an applied load
  if the stiffness is exactly zero.


• Overconstraint. As an example, overconstraint can happen when a few joint elements are defined
  on the same node if the joint elements are not orthogonal to each other. (See Addressing Overcon-
  straint Issues During Modeling for additional examples.) Overconstraint can also happen when an
  excessive number of MPC bonded contact elements are defined at a juncture where multiple parts
  meet. There are ever-increasing cases of overconstraint due to increased usage of automatic model-
  creation tools.

When overconstraint occurs, the following phenomena often occur as well:

• Negative pivot or zero pivot values are present.


• For a nonlinear solution, the solution may converge to a (slightly) different solution each time
  the job is executed under the same conditions.

If the above conditions do not apply or do not help to identify the problem area, the following sugges-
tions may help determine which (if any) part of the model is unconstrained:

• Solve the system as a modal analysis, if applicable, and look for the presence of any eigenvectors associated
  with zero-value eigenvalues (an indication of rigid body motion). Plotting such eigenvectors may help
  determine the unconstrained portions of the model.


• Review the boundary conditions in the model (including any contact pair definitions) and add arbitrary
  boundary conditions until any such zero pivot value messages are eliminated.

5.10. Stopping Solution Aft er Matrix Assembly

You can terminate the solution process after the assembled global matrix file (.FULL file) has been
written by using WRFULL. By doing so, the equation solution process and the process of writing data
to the results file are skipped. This feature can then be used in conjunction with the HBMAT command
in /AUX2 to dump any of the assembled global matrices into a new file that is written in Harwell-Boeing
format. You can also use the PSMAT command in /AUX2 to copy the matrices to a postscript format
that can be viewed graphically.

The WRFULL command is only valid for linear static, full harmonic, and full transient analyses when the
sparse direct solver is selected.WRFULL is also valid for buckling and modal analyses when any mode
extraction method is selected. The command is not valid for nonlinear analyses.

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Solution