Mechanical APDL Structural Analysis Guide

Linear perturba-

tion modal solu-

tion

Establish forced

sliding contact first

(CMROTATE com-

mand).

Full nonlinear N/A

solution

Partial perturbed

modal analysis

Force frictional

sliding (CMRO-

Linear non- N/A N/A N/A

prestressed modal

analysis TATE command)

and perform a lin-

ear modal solution

3.7.3.1. Full Nonlinear Perturbed Modal Analysis

A full nonlinear perturbed modal analysis is the most accurat e method to model brake squeal.The fol-
lowing solution steps are required:

1. Perform a nonlinear static contact analysis to establish initial contact conditions:
   
   
   • Activate large-deflection effects (NLGEOM,ON); (optional).
   
   
   
   

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


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


• Create the element component to apply the rotational velocity (CMROTATE command) during the
  first load step.

2. Perform a forced frictional sliding contact analysis as an additional load step.

This step is needed if you want to model steady-state frictional sliding between a brake pad and

the associated rotating disc (brake rotor) with different velocities. In this case, the sliding direction

no longer follows the nodal displacements; instead, it is predefined through the CMROTATE com-

mand. This command defines the velocities on the contact and target nodes of the element com-

ponent which are used to determine the sliding direction for the rest of analysis. The rotating element

component (CM command) that is specified on the CMROTATE command should include only the

contact elements or only the target elements that are on the brake rotor.

For isotropic friction defined via the MP command, if the friction coefficient is modified after the

first load step (that is, a new constant value specified via MP) and the ramping option is set (KBC,0),

the friction coefficient used for the friction calculation at each substep is linearly interpolated from

the value of the previous load step to the value of the current load steps.

3. 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 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.
  
  
  
  

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Modal Analysis