9.4. Considerations for Perturbed Stiffness Matrix Generation

The first phase of a linear perturbation analysis depends heavily on the multiframe restart procedure; it also inherits the limitation of the multiframe restart (see Multiframe Restart Limitations in the Basic Analysis Guide). These limitations include the following:

  • The Jobname.Rnnn file does not save the EKILL and EALIVE commands. If EKILL or EALIVE commands are required in the first phase of the linear perturbation analysis, you must reissue these commands before the SOLVE,ELFORM command is executed.

  • The Jobname.rdb file saves only the database information available at the first substep of the first load step. If you input other information after the first load step and need that information for the restart, you must input this information in the first phase of the linear perturbation analysis. This situation often occurs when APDL parameters are used. You must use PARSAV to save the parameters during the initial run and use PARRES to restore them in the restart. The situation also occurs when you want to change element real constants values; reissue the R command in the first phase of the linear perturbation analysis.

    This limitation also applies to all other commands which modify element material properties or real constants from the base analysis in the second or later load step(s).

  • All loading and boundary conditions from the base analysis are stored in the Jobname.ldhi file; therefore, upon the first phase of a linear perturbation analysis, changing the boundary conditions is not allowed. At the end of the first phase (after the SOLVE,ELFORM operation is completed), the program automatically manipulates the boundary conditions according to the specification of the LoadControl key on the PERTURB command; however, any type of load manipulation is allowed in the second phase of the linear perturbation analysis so that the desired perturbation loads can be easily obtained.

Before execution of the SOLVE,ELFORM command, the settings for regeneration of prior solution matrices with additional or specific requirements should be made. These specific requirements can be met by using the following commands (these are optionally needed):

CMROTATE -- Used to specify the rotational velocity of an element component about a user-defined rotational axis. It is needed in brake squeal analysis in conjunction with the QR damped eigensolver (MODOPT,QRDAMP).
QRDOPT -- Used by QR damped eigensolver for reuse of symmetric eigenmodes from the Block Lanczos eigensolver (LANB).
CNKMOD -- Used for modifying contact status from the base analysis locally for each contact pair. The ContKey parameter on the PERTURB command can modify the contact status globally.
MP -- Used for modifying contact frictional coefficient, MU.
TBMODIF -- Used for modifying frictional coefficients only (similar to MP command usage).
EALIVE, EKILL, ESTIF -- These are used for reproducing the element birth/dead status at the point of the multiframe restart. They may not be used to change element status from the restarted point.
CORIOLIS -- Used for including Coriolis effects in the modal analysis when the base analysis is static.