Mechanical APDL Structural Analysis Guide

• Use the unsymmetric stiffness matrix option (NROPT,UNSYM).


• Specify the restart control points needed for the linear perturbation analysis (RESCONTROL command).


• Create components to be used with the CMROTATE command.
  The initial contact condition will be established and a prestressed matrix will be generat ed in the end
  of this step under external loading.

2. Perform the first phase of the linear perturbation analysis:

• Specify a restart point (load step number and substep number) using the ANTYPE command (for ex-
  ample,ANTYPE,STATIC,RESTART,LDSTEP,SUBSTEP,PERTURB).


• Specify the type of linear perturbation analysis as modal (PERTURB,MODAL command).


• Issue the CMROTATE command. The contact stiffness matrix is based only on the contact status at
  the restart point. The sliding direction no longer follows the nodal displacements; instead, it is defined
  through the CMROTATE command.


• Issue the RMODIF command if you need to modify the contact stiffness values (FKN and FKT ) from
  the base analysis. You can also issue the CNKMOD command to change the units of the contact stiffness
  from FORCE/LENGTH^3 (in the base static analysis) to FORCE/LENGTH.


• Issue SOLVE,ELFORM to regenerate the element stiffness matrices, which are generally unsymmetric.

3. Perform a QR damped or unsymmetric modal analysis (second phase of the linear perturbation analysis):

• Specify the QR damped or unsymmetric mode extraction method (MODOPT,QRDAMP or UNSYM).


• If you are planning to perform multiple solutions and want to reuse the Block Lanczos eigensolution
  from the previous load steps, issue QRDOPT,ON (QRDAMP only).


• Issue the SOLVE command.

4. Expand the modes and postprocess the results from Jobname.RSTP.


5. To perform friction sensitivity studies by reusing the normal modes from the Block Lanczos solution
   (QRDAMP only), repeat steps 2, 3, and 4 by changing the coefficient of friction at step 2 and reissuing
   the QRDOPT,ON command at step 3.

The difference between this procedure and the procedure described in Full Nonlinear Perturbed Modal
Analysis (p. 54) is that here the CMROTATE command is used in the first phase of the linear perturbation
analysis, while by the other procedure the CMROTATE command is invoked in the nonlinear base
analysis. The procedure described here is computationally less expensive compared to the Full Nonlinear
Perturbed Modal Analysis (p. 54).

The following example illustrat es the partial nonlinear perturbed modal analysis method for brake squeal
analysis:

!

! Create the brake model and apply force normal to contact surface to

! simulate contact pressure between brake pad and disc.

!

...

!

finish

!

/solu

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Modal Analysis