Inserting a New Frequency Step
Inserting a frequency step within the Simulation History indicates that an eigenvalue extraction procedure should be used for the computation of the system natural frequencies and corresponding mode shapes. If the frequency step is the first step in the analysis, initial conditions (for example, temperature) form the base state for the analysis. Otherwise, the base state is the current state at the end of the previous general static step. In a frequency step you can apply a clamp boundary condition or a displacement boundary condition with selected degrees of freedom set to zero; all other boundary conditions and all loads are ignored. Frequency steps are available only in a Nonlinear Structural case. They can be defined in new cases or in cases that contain other general static or static linear perturbation steps. Frequency steps cannot be defined in an Explicit Dynamics case or in a Thermal case. See Step Sequence Restrictions for more information.
When you configure a frequency step, you must choose one of the following eigenvalue extraction methods:
Lanczos eigenvalue extraction is the default method because it has general capabilities. However, this method is usually slower than the AMS eigensolver. See Lanczos eigensolver in Natural frequency extraction in the Abaqus Analysis Guide, and Eigenvalue extraction in the Abaqus Theory Guide for more information.
Automatic multi-level substructuring (AMS) eigenvalue extraction is faster than the Lanczos method, particularly when you require a large number of eigenmodes for a system with many degrees of freedom. However, the AMS method has several limitations. See Automatic multi-level substructuring (AMS) eigensolver in Natural frequency extraction in the Abaqus Analysis Guide for more information.
When you select the AMS method, Abaqus automatically specifies values for the three cut-off frequency multipliers. You cannot edit these values. They are:
, the cutoff frequency for substructure eigenproblems, is defined as a multiplier of the maximum frequency of interest. This multiplier is set to 5.
, the first cutoff frequency for a reduced eigenproblem, is defined as a multiplier of the maximum frequency of interest. This multiplier is set to 1.7. 
, the second cutoff frequency for a reduced eigenproblem, is defined as a multiplier of the maximum frequency of interest. This multiplier is set to 1.1.
This task shows you how to insert a new Frequency step in a Nonlinear Structural case.
Select Start>Analysis & Simulation>Nonlinear Structural Analysis from the menu bar to enter the Nonlinear Structural Analysis workbench.
From the New Analysis Case dialog box, select Nonlinear Structural Case.
To create an additional Frequency step, do either of the following:
To append a new Frequency step to the end of the Simulation History, click the Frequency Step icon
.Tip: Alternatively, you can select Insert>Frequency Step from the menu bar.
To insert a new Frequency step between two existing steps in the Simulation History, right-click the step object in the specification tree after which you want to create the new step, then select Insert Step Below>Frequency Step from the menu that appears.
You can change the step identifier by editing the Step name field. This name will be used in the specification tree.
Enter a description for the step in the Step description field.
From the Eigensolver options, select either Lanczos or AMS.
Choose the Number of modes requested:
Choose All to request all modes within the specified frequency range.
Choose Value to enter a value for the number of modes requested. Enter the value in the field provided.
Enter the minimum and maximum frequencies of interest (Hz). The two values specify a frequency range within which Abaqus/Standard will calculate eigenvalues. Unless you request all modes, you must specify the maximum frequency value. Otherwise, there is no minimum or maximum frequency, and Abaqus calculates the first ten modes.
If desired, toggle on Frequency shift and enter a value for the shift point (Hz2). The eigenvalues closest to this point will be extracted. See Frequency shift in Natural frequency extraction in the Abaqus Analysis Guide for more information.
Toggle on Use SIM Architecture to activate the high performance, mode-based linear dynamic analysis capability. If the eigenmodes that are extracted in this step will be used for subsequent mode-based or subspace-based steady-state dynamic procedures, the SIM-based capability offers improved efficiency for large models with minimal output requests. See Using the SIM architecture for modal superposition dynamic analyses in About dynamic analysis procedures in the Abaqus Analysis Guide for more information.
Choose how you want to normalize eigenvectors:
Choose Displacement to normalize the eigenvectors so that the largest displacement, rotation, or acoustic pressure entry in each vector is unity. Displacement is the default option for the Lanczos eigensolver.
Choose Mass to normalize the eigenvectors with respect to the structure's mass matrix (the eigenvectors are scaled so that the generalized mass for each vector is unity). Mass is the default option for the AMS eigensolver.
Toggle on Residual modes to request that Abaqus/Standard compute residual modes based on the static response of the model to a nominal (or unit) load.
If you toggle on this option, specify the residual mode regions and the degrees of freedom (DOF) for which you want residual modes calculated. Abaqus/Standard computes one residual mode for every requested degree of freedom.
See Obtaining residual modes for use in mode-based procedures in Natural frequency extraction in the Abaqus Analysis Guide for more information.
Click OK when you have finished defining the step.
The Frequency objects set contains a default Field Output Request object in a Field Output Requests objects set. See Requesting Results for more information. You cannot request history output for a frequency step. However, several whole model history output variables are generated by default:
Eigenfrequency
Eigenvalue
Effective mass
Generalized mass
Participation factor