Mechanical APDL Basic Analysis Guide - Ansys

Chapter 5: Solution
lation step and go down a different path to perform inertia relief calculations. Or, you can stop at the Guyan
reduction step (matrix reduction) and go on to calculate reduced eigenvalues.
Some possible uses of the PSOLVE approach are listed below.
• You can use the results of an intermediate solution step as input to another software package or userwritten
program.
• If you are interested just in inertia relief calculations or some such intermediate result, the PSOLVE approach
is useful. See the Structural Analysis Guide for more information.
5.11. Singularities
A singularity exists in an analysis whenever an indeterminate or non-unique solution is possible. A negative
or zero equation solver pivot value will yield such a solution. 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 value for the PIVCHECK command is ON. With PIVCHECK set to ON, a linear static analysis (in
batch mode only) stops when a negative or zero pivot value is encountered. The message "NEGATIVE PIVOT
VALUE" or "PIVOTS SET TO ZERO" is displayed. If PIVCHECK is set to OFF, the pivots are not checked. Set
PIVCHECK to OFF if you want your batch mode linear static analysis to continue in spite of a zero or negative
pivot value. The PIVCHECK setting has no effect for nonlinear analyses, since a negative or zero pivot value
can occur for a valid analysis. When PIVCHECK is set to OFF, ANSYS automatically increases any pivot value
smaller than machine "zero" to a value between 10 and 100 times that machine's "zero" value. Machine
"zero" is a tiny number the machine uses to define "zero" within some tolerance. This value varies for different
computers (approx1E-15).
The following conditions may cause singularities in the solution process:
• Insufficient constraints.
• 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."
• 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 will have an unconstrained degree of freedom in the vertical direction at the joint. A
linear analysis would ignore a vertical load applied at that point. Also, consider a shell element with no
in-plane rotational stiffness connected perpendicularly to a beam or pipe element. There is no in-plane
rotational stiffness at the joint. A linear analysis would ignore 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 will be printed.
• Zero Stiffness Matrix (on row or column). Both linear and nonlinear analyses will ignore an applied load
if the stiffness is exactly zero.
5.12. Stopping Solution After 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.
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