教程5.13g加密所属

1、SECTION 7PRE-STIFFENED NORMAL MODESTABLE OF CONTENTS PagePRE-STIFFENED NORMAL MODES7-3CASE STUDY: PRE-STIFFENED NORMAL MODES USING SOL 106 METHOD 7-7SETTING UP PRE-STIFFENED NORMAL MODES IN MSC.PATRAN7-9COMPARING THE RESULTS7-13USING THE STATSUB METHOD7-15COMPARING THE RESULTS7-17WORKSHOP 12a, 12b,

2、12c7-18PRE-STIFFENED NORMAL MODESSection 2 looked at Normal Modes analysis of unloaded structures. In this section, Normal Modes Analysis of structures which have a static pre-load applied to them will be considered.Typical real-world examples are:Thin walled pressure vessels, where the pressurizati

3、on dominates the natural frequencies, such as rocket fuel tanks and satellite oxygen tanks.Structures subject to significant dead weight loading, such as suspension bridge cables and oil tankers.Tensioning of strings in musical instruments to achieve required frequencies.Structures subject to centri

4、fugal loading such as jet engine turbine and fan blades.PRE-STIFFENED NORMAL MODES (Cont.)The objective of applying a static load to a structure prior to carry out a Normal Modes Analysis is to allow the Stiffness Matrix to be updated.There are three terms that can influence the Stiffness Matrix.Mat

5、erial Nonlinearity.Geometric Stiffness due to changes in the structural shape (these can be small displacement theory or large displacement theory).Follower Force stiffness terms as the loading changes its line of action relative to the displacements (again, small displacement theory or large displa

6、cement theory).Three basic approaches are taken in Nastran.Run Sol103 as a Cold Start and introduce Stiffness updates for the Normal Modes analysis via STATSUB, to point to the Static Loading. (This permits only Geometric Stiffness and Follower Forces Stiffness small displacement theory updates.)Run

7、 Sol106 as a Cold Start and use the stiffness at the final load step for the Normal Modes analysis. (This can permit full Material and Large Displacement Geometric and Follower terms to be included if required.)Run Sol106 as a Cold Start and restart into Sol103. This was the “old way” in Nastran pri

8、or to v2001. This is not mended for inexperienced users as many parameters need to be controlled. Full nonlinear effect can be introduced.Please see Page 190 of the Nastran 2001 Release Guide for a summary of options available.PRE-STIFFENED NORMAL MODES (Cont.) mendations:If the displacements are sm

9、all and no significant nonlinearity is present, then use the SOL103 method.If nonlinear or large displacement are important, use the SOL106 method.PATRAN Supports the SOL106 method directly including multiple subcases.The SOL103 method is not supported directly in PATRAN so the .bdf will have to be

10、manually edited.A case study of each method is now shown.PRE-STIFFENED NORMAL MODES (Cont.)CASE STUDY: PRE-STIFFENED NORMAL MODES USING SOL 106 METHODThe Normal Modes of a simple thin walled cylinder with closed ends are considered under two conditions:No pre-load.Internal Pressurization of 100 psi.

11、tldGeometry of the Cylinder:The detail of the setup of condition (a) is not shown.For Case (b), define a static loading in the normal way and then set up Sol106.Cylinder Wall Thickness0.01625 inchesCap Thickness0.25 inchesLength (l)10 inchesDiameter (d)2 inchesElastic Modulus1.0E7 psiMaterial Densit

12、y0.101 lbf/in3Poisson Ratio0.33CASE STUDY: PRE-STIFFENED NORMAL MODES USING SOL 106 METHODSETTING UP PRE-STIFFENED NORMAL MODES IN MSC.PATRANThe pressure loading is set up as a Load Case called pre-stiffened. 100 psi internal pressure is applied to all elements.Request a Sol106 nonlinear solution ty

13、pe. The solution parameters permit us to include large displacement and follower force stiffness terms if so desired. Nonlinear material effects will be included if present.SETTING UP PRE-STIFFENED NORMAL MODES IN MSC.PATRAN (Cont.)Pick the subcase which includes the pressure loading. Then select Su

14、bcase Parameters and check the Normal Modes box to allow the Normal modes analysis to be defined. Notice that the submenu is identical to the standard Normal Modes form.SETTING UP PRE-STIFFENED NORMAL MODES IN MSC.PATRAN (Cont.)Looking at the Nastran .bdf file output this is seen: NLPARM and METHOD

15、are called out in Case Control.Static Preload is called out via LOAD = 2 and points to the pressure loading.LGDISP invokes the large displacement if selected.PARAM NMLOOP 1 forces the calculation of the Normal Modes using the updated stiffness matrix. NLPARM reflects the settings of the Nonlinear So

16、lution strategy menu chosen in the Subcase Parameter Form. EIGRL reflects the Normal Modes analysis parameters requested.SETTING UP PRE-STIFFENED NORMAL MODES IN MSC.PATRAN (Cont.)COMPARING THE RESULTSEigenvalue table from the Normal Modes analysisEigenvalue table from the Pre-Stiffened Normal Modes

17、 analysisThe Results of Pre-Stiffened analysis are compared with a previous Normal Modes analysis.First Mode of Normal Modes analysisFirst Mode of Pre-Stiffened Normal Modes analysisCOMPARING THE RESULTS (Cont.)Investigation of the first modes of the two analysis shows that the presence of the press

18、urization has also changed the mode shape from 3 lobes to 2 lobes.USING THE STATSUB METHODModify the previous SOL106 bdf file to perform a SOL103 pre-stiffened normal modes analysisChange the Solution Type to Sol 103, Normal Modes analysisCreate a second subcase for Normal Modes analysis.Use STATSUB

19、 to refer to the static analysis case.Delete NLPARM referenceUSING THE STATSUB METHOD (Cont.)The modified bdf file that will be submited to Nastran for pre-stiffened normal modes analysis.COMPARING THE RESULTSThe Normal Modes using the 2 methods are shown:Pre-Stiffened Normal Modes analysis using STATSUB method.Pre-Stiffened Normal Modes