Issue:
What do instability types reported while calculating a model mean?
Causes:
The analysis can display the following ‘instability’ messages:
type 1 (or 0 for sparse solver in versions older than v.21.0) - There is zero value element on the diagonal of stiffness matrix.
type 2 - There is zero value element on the diagonal of inverted stiffness matrix.
type 3 – The ratio between maximum and minimum values on the diagonal of inverted stiffness matrix is suspiciously high.
type N>3 – Reported only by multi-threaded solver, it corresponds to zeros on the diagonal of stiffness matrix; the number of zeros is equal to N-3. This type of instability was reported up to 2013 release. Starting from 2014 release, the multi-threaded solver reports instabilities analogously to other solvers.
Instability of structure
The first, second, and sometimes the third type are usually caused by mechanical instability of a structure. For example, a part of the structure is a mechanism, or the number, type, and positions of supports are not sufficient. These are modeling errors that should be corrected before accessing the results.
Problems with section profiles, short elements, stiffness of elements, releases
The third type may also be reported due to big differences in section profiles of some elements, or existence of abnormally short elements, or generally large differences in stiffness of elements (observed also when using RLINK elements). It may also be caused by too many releases or incorrect definition of releases.
Solvers
For skyline solver, and starting from v.21.0 for sparse solver, all of the above types can be reported.
For frontal method, only the first and third type can occur.
For sparse solver, in versions earlier than v.21.0, only the first type can occur.
If the message is displayed, try recalculating the example with skyline method. In the case of skyline and sparse solvers, the number of nodes and degree of freedom where the instability has been found is precisely indicated. Sparse solvers can also report instabilities during Gauss elimination. They correspond to type 3 instabilities.
The multi-threaded solver reports only instabilities of N>3 type. It does not give information related to node and degree of freedom where the instability was detected. Try changing the solver to Sparse or Skyline to obtain more precise detection of the source of instability.
Iterative solvers do not report instability messages and the instability of the model may result in slow convergence of analysis. In this case, thorough verification of the model prior to calculations is advised.
The solution method (solver) can be changed in: Job Preferences dialog box > Structure Analysis.
See Also:
Instability Types That May Occur During Structure Analysis
See our dedicated Webinar for more information:
What do instability types reported while calculating a model mean?
Causes:
The analysis can display the following ‘instability’ messages:
type 1 (or 0 for sparse solver in versions older than v.21.0) - There is zero value element on the diagonal of stiffness matrix.
type 2 - There is zero value element on the diagonal of inverted stiffness matrix.
type 3 – The ratio between maximum and minimum values on the diagonal of inverted stiffness matrix is suspiciously high.
type N>3 – Reported only by multi-threaded solver, it corresponds to zeros on the diagonal of stiffness matrix; the number of zeros is equal to N-3. This type of instability was reported up to 2013 release. Starting from 2014 release, the multi-threaded solver reports instabilities analogously to other solvers.
Instability of structure
The first, second, and sometimes the third type are usually caused by mechanical instability of a structure. For example, a part of the structure is a mechanism, or the number, type, and positions of supports are not sufficient. These are modeling errors that should be corrected before accessing the results.
Problems with section profiles, short elements, stiffness of elements, releases
The third type may also be reported due to big differences in section profiles of some elements, or existence of abnormally short elements, or generally large differences in stiffness of elements (observed also when using RLINK elements). It may also be caused by too many releases or incorrect definition of releases.
Solvers
For skyline solver, and starting from v.21.0 for sparse solver, all of the above types can be reported.
For frontal method, only the first and third type can occur.
For sparse solver, in versions earlier than v.21.0, only the first type can occur.
If the message is displayed, try recalculating the example with skyline method. In the case of skyline and sparse solvers, the number of nodes and degree of freedom where the instability has been found is precisely indicated. Sparse solvers can also report instabilities during Gauss elimination. They correspond to type 3 instabilities.
The multi-threaded solver reports only instabilities of N>3 type. It does not give information related to node and degree of freedom where the instability was detected. Try changing the solver to Sparse or Skyline to obtain more precise detection of the source of instability.
Iterative solvers do not report instability messages and the instability of the model may result in slow convergence of analysis. In this case, thorough verification of the model prior to calculations is advised.
The solution method (solver) can be changed in: Job Preferences dialog box > Structure Analysis.
See Also:
Instability Types That May Occur During Structure Analysis
See our dedicated Webinar for more information:
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