GUI Path Command
Option
Main Menu> Solution> Analysis Type> ExpansionPass EXPASS
Expansion Pass On/Off
Main Menu> Solution> Load Step Opts> Expansion-Pass> Single Expand> Expand Modes
MXPAND No. of Modes to Expand
GUI Path Command
Option
Main Menu> Solution> Load Step Opts> Expansion-Pass> Single Expand> Expand Modes
MXPAND Freq. Range for Expansion
Main Menu> Solution> Load Step Opts> Expansion-Pass> Single Expand> Expand Modes
MXPAND Stress Calc. On/Off
Each of these options is explained in detail below.
Expansion Pass On/Off [EXPASS]
Choose ON.
Number of Modes to Expand [MXPAND, NMODE]
Specify the number. Remember that only expanded modes can be reviewed in the postprocessor.
Default is no modes expanded.
Frequency Range for Expansion [MXPAND,, FREQB, FREQE]
This is another way to control the number of modes expanded. If you specify a frequency range, only modes within that range are expanded.
Stress Calculations On/Off [MXPAND,,,, Elcalc]
Choose ON only if you plan to do a subsequent spectrum analysis and are interested in stresses or forces to do the spectrum. "Stresses" from a modal analysis do not represent actual stresses in the structure, but give you an idea of the relative stress distributions for each mode. Default is no stresses calculated.
3. Specify load step options. The only options valid in a modal expansion pass are output controls:
• Printed output
Use this option to include any results data (expanded mode shapes, stresses, and forces) on the printed output file (Jobname.OUT).
Command(s): OUTPR
GUI: Main Menu> Solution> Load Step Opts> Output Ctrls> Solu Printout
• Database and results file output
Use this option to control the data on the results file (Jobname.RST). The FREQ field on OUTRES can be only ALL or NONE; that is, the data are written for all modes or no modes. For example, you cannot write information for every other mode.
Command(s): OUTRES
GUI: Main Menu> Solution> Load Step Opts> Output Ctrls> DB/Results File 4. Start expansion pass calculations.
The output consists of expanded mode shapes and, if requested, relative stress distributions for each mode.
Command(s): SOLVE
GUI: Main Menu> Solution> Solve> Current LS
5. Repeat steps 2, 3, and 4 for additional modes to be expanded (in different frequency ranges, for example).
Each expansion pass is stored as a separate load step on the results file.
Section 3.6: Expand the Modes
Caution: Spectrum analyses expect all expanded modes to be in one load step. In the single point response spectrum (SPOPT,SPRS) and Dynamic Design analysis method (SPOPT,DDAM), the modal expansion can be performed after the spectrum analysis, based on the significance factor SIGNIF on the MXPAND command. If you want to perform modal expansion after the spectrum analysis, choose NO for mode expansion (MXPAND) on the dialog box for the modal analysis options (MODOPT).
6. Leave SOLUTION. You can now review results in the postprocessor.
Command(s): FINISH
GUI: Close the Solution menu.
Note — The expansion pass has been presented here as a separate step. However, if you include the MXPAND command in the modal solution step, the program not only extracts the eigenvalues and ei-genvectors, but also expands the specified mode shapes.
3.7. Review the Results
Results from a modal analysis (that is, the modal expansion pass) are written to the structural results file, Job-name.RST. Results consist of:
• Natural frequencies
• Expanded mode shapes
• Relative stress and force distributions (if requested).
You can review these results in POST1 [/POST1], the general postprocessor. Some typical postprocessing oper-ations for a modal analysis are described below. For a complete description of all postprocessing functions, see Chapter 4, “An Overview of Postprocessing” in the ANSYS Basic Analysis Guide.
3.7.1. Points to Remember
• If you want to review results in POST1, the database must contain the same model for which the solution was calculated.
• The results file (Jobname.RST) must be available.
3.7.2. Reviewing Results Data
1. Read in results data from the appropriate substep. Each mode is stored on the results file as a separate substep. If you expand six modes, for instance, your results file will have one load step consisting of six substeps.
Command(s): SET,SBSTEP
GUI: Main Menu> General Postproc> Read Results> substep
2. Perform any desired POST1 operations. Typical modal analysis POST1 operations are explained below:
3.7.3. Option: Listing All Frequencies
You may want to list the frequencies of all modes expanded. A sample output from this command is shown below.
***** INDEX OF DATA SETS ON RESULTS FILE *****
SET TIME/FREQ LOAD STEP SUBSTEP CUMULATIVE 1 22.973 1 1 1 2 40.476 1 2 2
3 78.082 1 3 3 4 188.34 1 4 4
Command(s): SET,LIST
GUI: Main Menu> General Postproc> List Results
3.7.4. Option: Display Deformed Shape
Command(s): PLDISP
GUI: Main Menu> General Postproc> Plot Results> Deformed Shape Use the KUND field on PLDISP to overlay the nondeformed shape on the display.
3.7.5. Option: List Master DOF
Command(s): MLIST,ALL
GUI: Main Menu> Solution> Master DOFs> User Selected> List All
Note — To display the master DOFs graphically, plot the nodes (Utility Menu> Plot> Nodes or command NLIST).
3.7.6. Option: Line Element Results
Command(s): ETABLE
GUI: Main Menu> General Postproc> Element Table> Define Table
For line elements, such as beams, spars, and pipes, use the ETABLE command to access derived data (stresses, strains, and so on). Results data are identified by a combination of a label and a sequence number or component name on the ETABLE command. See the ETABLE discussion in The General Postprocessor (POST1) in the ANSYS Basic Analysis Guide for details.
3.7.7. Option: Contour Displays
Command(s): PLNSOL or PLESOL
GUI: Main Menu> General Postproc> Plot Results> Contour Plot> Nodal Solu or Element Solu Use these options to contour almost any result item, such as stresses (SX, SY, SZ...), strains (EPELX, EPELY, EPELZ...), and displacements (UX, UY, UZ...).
The KUND field on PLNSOL and PLESOL gives you the option of overlaying the nondeformed shape on the display.
You can also contour element table data and line element data:
Command(s): PLETAB, PLLS
GUI: Main Menu> General Postproc> Element Table> Plot Element Table Main Menu> General Postproc> Plot Results> Contour Plot> Line Elem Res
Caution: Derived data, such as stresses and strains, are averaged at the nodes by the PLNSOL command.
This averaging results in "smeared" values at nodes where elements of different materials, different shell thicknesses, or other discontinuities meet. To avoid the smearing effect, use selecting (described in Chapter 7, “Selecting and Components” in the ANSYS Basic Analysis Guide) to select elements of the same material, same shell thickness, and so on before issuing PLNSOL.
3.7.8. Option: Tabular Listings
Command(s): PRNSOL (nodal results) PRESOL (element-by-element results) PRRSOL (reaction data), and so on NSORT, ESORT
GUI: Main Menu> General Postproc> List Results> solution option Main Menu> General Postproc> List Results> Sorted Listing> Sort Nodes
Section 3.7: Review the Results
Main Menu> General Postproc> List Results> Sorted Listing> Sort Elems Use the NSORT and ESORT commands to sort the data before listing them.
3.7.9. Other Capabilities
Many other postprocessing functions - mapping results onto a path, load case combinations, and so on - are available in POST1. See The General Postprocessor (POST1) in the ANSYS Basic Analysis Guide for details.
See the ANSYS Commands Reference for a discussion of the ANTYPE, MODOPT, M, TOTAL, EXPASS, MXPAND, SET, and PLDISP commands.
3.8. A Sample Modal Analysis (GUI Method)
In this example, you perform a modal analysis on the wing of a model plane to demonstrate the wing's modal degrees of freedom.
3.8.1. Problem Description
This is a modal analysis of a wing of a model plane. The wing is of uniform configuration along its length, and its cross-sectional area is defined to be a straight line and a spline, as shown. It is held fixed to the body on one end and hangs freely at the other. The objective of the problem is to demonstrate the wing's modal degrees of freedom.
3.8.2. Problem Specifications
The dimensions of the wing are shown in the problem sketch. The wing is made of low density polyethylene with the following values:
Young's modulus = 38x103 psi Poisson's ratio = .3
Density = 8.3e-5 lb-sec2/in4