- Random vibration analysis
The deformed mesh due to the prior static or full transient analysis is used in the linear perturbation
analysis and in the downstream analysis. As such, the database used for the downstream analysis should
correspond to the deformed mesh (after the linear perturbation solve).
In all analyses listed above, the first load vector used is {Fperturbed}. If more loading cases are required,
a new {Fperturbed} load vector must be generat ed ( MODCONT). The program assumes purely linear
analyses and uses linear material properties, even for all nonlinear materials.
9.3. Considerations for Load Generation and Controls
Generally, structural loads can be divided into two categories: mechanical loads and non-mechanical
loads. Non-mechanical loads relevant to this procedure include thermal, swelling, initial stresses/strains,
and other initial conditions. In the base analysis, a combination of mechanical and non-mechanical
loads can be freely applied without any restrictions; however, for the linear perturbation analysis, only
mechanical loads (and thermal loads in the case of linear perturbation static analysis and linear perturb-
ation eigenvalue buckling analysis) are allowed.
It is good practice to delete all loads from the base analysis in the first phase of linear perturbation. By
default, the program deletes all loads from the restart step, except for displacement boundary conditions,
inertia loads, and non-mechanical loads (see the LoadControl argument on the PERTURB command).
Non-mechanical loads (including thermal loads) must remain unmodified so that a detailed nonlinear
snapshot for various solution matrices and element history variables can be regenerated in the first
phase of the linear perturbation procedure.
You can apply new mechanical loads in the second phase of the linear perturbation process. However,
new non-mechanical loads are not allowed except in a few cases. See Generating and Controlling Non-
mechanical Loads (p. 299) for more information.
NoteWhen performing a linear perturbation full harmonic analysis, the following loads can act as
harmonically varying loads:- Nonzero displacements specified in the base analysis (D command).
- MPC bonded contact with initial penetration, gap, or offset. The effect of initial penetration,
gap, or offset can be excluded by setting KEYOPT(9) = 1 on the contact elements in the
base analysis. - Pretension loads (SLOAD command) specified in the base analysis.
- External constraint equations (CE command) with nonzero constant terms specified in the
base analysis.
In a linear perturbation modal analysis, the total loads obtained from the second phase of the analysis
may be used in a downstream analysis following the linear perturbation modal analysis. If a downstream
analysis is not needed, then this load generation step can be ignored.
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