- Enter the SOLUTION processor using /SOLU (Main Menu> Solution), and indicat e that this is a restart
analysis by issuing ANTYPE,,VTREST (Main Menu> Solution> Restart). - Because you are re-running the analysis, you must reset the load steps and loads. If resuming a database
saved after the first load step of the initial run, you must delete the loads and redefine the loads from
the first load step. - Initiate the restart solution by issuing the SOLVE command. See Obtaining the Solution (p. 124) for details.
- Repeat steps 4, 5, and 6 for the additional load steps, if any.
5.9. Singular Matrices
A singular matrix exists in an analysis whenever an indeterminate or non-unique solution is possible.
A negative or zero equation solver pivot value may indicat e such a scenario. In some instances, it may
be desirable to continue the analysis, even though a negative or zero pivot value is encountered. You
can use the PIVCHECK command to specify whether or not to stop the analysis when this occurs.
The default behavior is to check for negative and zero pivot values (PIVCHECK,ON). With PIVCHECK
set to ON, certain analyses stop when a negative or zero pivot value is encountered. If PIVCHECK,OFF
is issued, the pivots are not checked. Use this command if you want your analysis to continue in spite
of a negative or zero pivot value.
Currently, the program only checks for negative and zero pivot values when the sparse or PCG solver
is used. If a negative or zero pivot value is encountered when using the sparse solver, the appropriate
message is displayed indicating the particular node and degree of freedom where the negative or zero
pivot value occurred. You can then review that part of the model to determine what caused the negative
or zero pivot value (see possible causes listed below).
Note that negative pivots corresponding to Lagrange multiplier based mixed u-P elements are not
checked or reported. Negative pivots arising from the u-P element formulation and related analyses are
expected and lead to correct solutions.
The following conditions may cause a singular matrix in the solution process:
- Insufficient constraints.
- Contact elements in a model. If the contact conditions are not properly defined, a portion of the
model may “break loose” or become separat ed before coming into contact and essentially be partially
unconstrained. In this situation, adding weak springs to the unconstrained bodies or activating contact
damping usually helps to prevent potential rigid body motions. - Nonlinear elements in a model (such as gaps, sliders, hinges, cables, etc.). A portion of the structure
may have collapsed or may have "broken loose" or become “too soft.” - Hourglass modes. Higher order elements (such as SOLID186) that use a reduced integration scheme
may produce hourglass modes when used in a coarse mesh.This can result in a zero pivot value. - Negative values of material properties, such as DENS or C, specified in a transient thermal analysis.
- Unconstrained joints.The element arrangements may cause singularities. For example, two horizontal
spar elements have an unconstrained degree of freedom in the vertical direction at the joint. A linear
analysis ignores a vertical load applied at that point. Also, consider a shell element with no in-plane
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