A partial-solution procedure (PSOLVE) is available for performing a mode frequency analysis based on prior linear or nonlinear static or full transient analyses. The procedure can be used when the base analysis is a small-deflection analysis, and is similar to a prestressed modal analysis.
The partial-solution procedure has limitations, however, when compared to the linear perturbation procedure. Therefore, it is better to use the linear perturbation procedure instead. The following table outlines the advantages of the linear perturbation procedure:
Table 4.1: Linear Perturbation vs. Partial-Solution Procedures
| Linear Perturbation Procedure | Partial-Solution Procedure |
|---|---|
| A modal analysis can be done at any time point during the prior analysis as long as the multiframe restart file is made available. | A modal analysis can be done only at the last substep of the last load step of the prior analysis. |
| The stress expansion pass uses linear material properties and is always allowed. | The stress expansion pass is done assuming the material property is linear for the entire model. If it contains hyperelasticity or another nonlinear material that does not have an easy segregation from the linear material property in the constitutive law, the stress expansion pass is not allowed. |
| All nonlinear effects, including history dependent and large rotation effects, are taken into consideration and handled consistently, guaranteeing correct results for the linear perturbation analysis. | The stress expansion pass does not have any effects of the previous nonlinear analysis; any nonlinear history-dependent properties and large rotation effects are lost. |
| The nodal coordinate update is done automatically at the beginning of the second phase of the solution; the stress expansion is done based on the updated geometry. | The nodal coordinate update for the original mesh is done by an external UPCOORD command (and is optional). |
| All prestress effects are included automatically (independent of PSTRES, OMEGA, or CMOMEGA command settings) in the phases of a linear perturbation analysis. Also, even though the base analysis is linear, the prestress effects are still taken into account in the subsequent linear perturbation analysis. | All prestress and spin-softening effects are invoked by the commands PSTRES, OMEGA, and CMOMEGA at the earlier stage of the analysis, as well as in the PSOLVE phase. |
| If the base analysis includes geometric nonlinearity (NLGEOM,ON), the solution accuracy from a QR damped (MODOPT,QRDAMP) linear perturbation analysis is greatly improved. The linear perturbation architecture enables this improvement on the QRDAMP eigensolver. | For the QRDAMP eigensolver, the solution may be less accurate compared to the UNSYM eigensolver. |
| The EMATWRITE command is unnecessary. | Requires knowing ahead of time that a prestressed modal analysis is needed and requires the use of the EMATWRITE command to force writing of the .emat file. |
| Eigenvalue buckling analysis is supported. The base analysis can a be linear or nonlinear, and can be a static or full transient analysis. | Eigenvalue buckling analysis is not supported |
| Full harmonic analysis is supported, including stress/strain calculations during the harmonic substeps. The contact status from the base analysis is frozen and maintained in the full harmonic phase of the analysis. | Full harmonic analysis is not fully supported. |