The following sections describe how to apply prestress effects in harmonic analyses:
A prestressed harmonic analysis calculates the dynamic response of a prestressed structure, such as a violin string. The prestress influences the stiffness of the structure through the stress-stiffening (or possibly a nonlinear tangent) matrix contribution. Response to harmonically varying loads is computed using this effective stiffness of the structure. The output stresses from this analysis, therefore, reflect the effect of prestress on the stiffness of the structure. The contribution of the stresses from the static (or transient) prestress is not included in the output as the harmonic response stress components are assumed to be at zero mean.
The linear perturbation harmonic procedure is the preferred method for performing a prestressed full, frequency-sweep, or mode-superposition harmonic analysis. Because the linear perturbation analysis accommodates both linearly and nonlinearly prestressed cases, the prestressed effects of the structure from the previous base analysis (linear or nonlinear, static or full transient) are included.
Only linear behavior is valid in a harmonic analysis. Nonlinear elements (if any) will be treated as linear elements based on their status at the end of the base analysis. An exception exists for contact elements, as their status can be changed during the first phase of the linear perturbation analysis. For more information, see First Phase of the Linear Perturbation Analysis.
If thermal body forces are present in the static prestress analysis, they are automatically disabled during the linear perturbation harmonic analysis (see THEXPAND).
For more information about the full and frequency-sweep harmonic procedures, see Harmonic Analysis Based on Linear Perturbation in the Theory Reference and Second Phase - Harmonic Analysis.
To include prestress effects in a mode-superposition analysis, you must first perform a prestressed modal analysis. Once prestressed modal analysis results are available, proceed as for any other mode-superposition analysis.
You must perform a linear perturbation modal
analysis prior to doing the downstream prestressed mode-superposition analysis. Both
Elcalc and MSUPkey must be
set to YES on the MXPAND command during the linear perturbation
modal analysis phases so that the downstream stress expansion pass can produce the
consistent solution to the linear or nonlinear base (static or full transient)
analysis. The prestressed nonlinear element history (saved variables) is accessible
only in the first and second phases of the linear perturbation. The downstream
analysis can only reuse this nonlinear information contained in the
Jobname.mode file which is generated in the linear
perturbation.
For more information about the mode-superposition procedure, see Downstream Analysis Using the Solution of a Linear Perturbation Analysis in the Mechanical APDL Theory Reference or Downstream Analysis Following the Linear Perturbation Analysis.
For a linear pertubation (prestressed) harmonic example, see Example 9.7: Linear Perturbation (Prestressed) Harmonic Analysis.
As an alternative to the linear perturbation full harmonic procedure, the legacy procedure based on the PSTRES command can be used, as described in the following section. The linear perturbation full harmonic method provides more options for controlling the contact status compared to the PSTRES procedure.
The procedure described here for doing a prestressed full-harmonic analysis is essentially the same as that for any other full-harmonic analysis except that you first need to prestress the structure by doing a static analysis:
Build the model and obtain a static solution with prestress effects turned on (PSTRES,ON). The procedure to obtain the static solution is explained in Structural Static Analysis.
Reenter SOLUTION and obtain the full-harmonic solution, also with prestress effects turned on (reissue PSTRES,ON). Files Jobname.emat (if created) and Jobname.esav from the static analysis must be available. Hence, if another analysis is performed between the static and prestressed harmonic analyses, the static analysis will need to be rerun.
Because only linear behavior is valid in a harmonic analysis, nonlinear elements (if any) will be treated as linear elements. If you include contact elements, for example, their stiffnesses are calculated based on their initial status and are never changed. For a prestressed harmonic analysis, the program assumes that the initial status of the contact elements is the status at the completion of the static prestress analysis.
If thermal body forces were present during the static prestress analysis, these thermal body forces must not be deleted during the full-harmonic analysis or else the thermal prestress will vanish. By default, thermal loads are included in the full-harmonic analysis as sinusoidally time-varying temperature loads (THEXPAND,ON). If you do not wish to include harmonic temperature loads, THEXPAND,OFF can be used to omit them.