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FULLY STRESSED DESIGN in MSC.Nastran

FULLY STRESSED DESIGN in MSC.Nastran. Presented by Erwin H. Johnson Project Manager MSC.Software 3rd MSC.Software Worldwide Aerospace Users Conference Toulouse, FRANCE April 8-10, 2002. AGENDA. Introduction Theory Requirements Implementation Examples Concluding Remarks.

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FULLY STRESSED DESIGN in MSC.Nastran

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  1. FULLY STRESSED DESIGN in MSC.Nastran Presented by Erwin H. Johnson Project Manager MSC.Software 3rd MSC.Software Worldwide Aerospace Users Conference Toulouse, FRANCE April 8-10, 2002

  2. AGENDA • Introduction • Theory • Requirements • Implementation • Examples • Concluding Remarks

  3. ACKNOWLEDGMENTS • The EA-3B Preliminary Design Model was provided by Mr. Kris Wadolkowski, Vice President, Aerostructures, Inc., San Diego, CA. • Mr. Dan Barker and Mr. Michael Love of Lockheed-Martin Aeronautics provided important guidance during the development of the design requirements.

  4. DESIGN SENSITIVITY & OPTIMIZATION ENHANCEMENTS IN THE 2001 RELEASE • Discrete Variables • Fully Stressed Design • Enhanced text interface • Support of FREQ3/4/5 • Random Analysis Support • Complex Eigenvalue Support • External Response - DRESP3

  5. DS&O RELATED ACTIVITES FOR THE MSC.Nastran 2002 RELEASE • Performance Enhancements • Eigenvector Sensitivity/Optimization • Dynamic Response Enhancements • Miscellaneous Enhancements • Updated User’s Guide

  6. INTRODUCTION • Fully Stressed Design (FSD) has been implemented in the 2001 Release of MSC.Nastran • Produces a design where each design variable is at its limit under at least one load case • Provides a rapid means of performing initial sizing of aerospace vehicles • Allows for the design of a virtually unlimited number of element sizes • FSD is a well known design technique that has long been implemented in codes such as FASTOP, LAGRANGEandASTROS

  7. BACKGROUND for FSD in MSC.Nastran • MSC.Software has been aware of FSD but has not previously implemented the technique because: • MSC.Software has concentrated on more general Mathematical Programming (MP) methods • FSD lacks a theoretical underpinning • There are several motivations for implementing the technique • FSD is fast • FSD can handle many thousands of design variables, something our MP methods cannot do • Numerous client requests

  8. FSD THEORY

  9. FSD REQUIREMENTS • Applicable for Static and Static Aeroelastic Analyses • Supports multiple load cases and multiple boundary conditions • Supports composite materials • Allowable limits on Stress and/or Strain • Limits can be imposed on design variables and property values • Design Properties - Areas of rods - Thicknesses of plates (PSHELL and PSHEAR) - Thicknesses of composite layers

  10. FSD LIMITATIONS • Bar and Beam Cross Sections cannot be designed • Ply Orientation is not an available design variable • If an element is constrained, but there are no design properties associated with the element, the constraint is ignored. • If a property is designed, but there are no constraints associated with the associated elements, the property is held invariant. • Shape design variables are not supported. Material and Connectivity Properties are not supported. • None of these limitations apply for Math Programming design tasks.

  11. FSD INPUT • The text interface developed for Math Programming is used for FSD • The DESSUB case control command identifies the constraints that are to be applied in each subcase • DESVAR and DVPREL1 entries define the designed properties • DRESP1 entries define the responses • DCONSTR entries define the constraints • Other Case Control Commands and Bulk Data entries are ignored • Two new parameters control the FSD algorithm: • FSDALP - The  relaxation parameter of the resizing algorithm (default = 0.9) • FSDMAX - Maximum number of FSD design cycles (default = 0)

  12. FSD RELATIONSHIP to MATH PROGRAMMING • FSD and Math Programming (MP) Design Cycles can be run sequentially • There are up to FSDMAX FSD design cycles followed by up to DESMAX MP design cycles • MP cycles can be skipped with DESMAX=0 • The FSD result is often an excellent starting point for an MP design task • All design model user inputs are honored in trailing MP design cycles • Additional ANALYSIS types (e.g. FLUTTER) can be included • DVGRID, DVPREL2, DVMRELi, DVCRELi, DRESP2 and DRESP3 entries are honored

  13. FSD OUTPUT • Output is very similar to that from standard MP jobs • Since there is no approximate model, there is no output from the • approximate model. Only results from exact analyses are printed • The SUMMARY OF THE DESIGN CYCLE HISTORY looks a little • different: • NUMBER OF FINITE ELEMENT ANALYSES COMPLETED 10 • NUMBER OF FULLY STRESSED DESIGN CYCLES COMPLETED 5 • NUMBER OF OPTIMIZATIONS W.R.T. APPROXIMATE MODELS 4 • OBJECTIVE AND MAXIMUM CONSTRAINT HISTORY • -------------------------------------------------------------------------------- OBJECTIVE FROM OBJECTIVE FROM FRACTIONAL ERROR MAXIMUM VALUE • CYCLE APPROXIMATE EXACT OF OF • NUMBER OPTIMIZATION ANALYSIS APPROXIMATION CONSTRAINT • --------------------------------------------------------------------- • INITIAL 4.828427E+00 -3.234952E-01 • 1 FSD 2.668171E+00 N/A 4.203515E-02 • . . . . . • 3 FSD 2.541077E+00 N/A 6.268603E-02 • 6 2.709053E+00 2.709045E+00 2.640250E-06 3.502930E-04

  14. Preprocessing DESCYCLE = 0 Analysis Initial Analysis Y Print Initial Design Y Print Input/Output of Design DESCYCLE >FSDMAX MP Y Hard Convergence MP DESCYCLE = DESCYCLE +1 REDESIGN Y MP Soft Convergence ALGORITHM FLOW CHART

  15. PRELIMINARY DESIGN MODEL EXAMPLE • General loads model of a US Navy EA-3B aircraft • Results shown here have no bearing on the actual structure • Model was supplied by

  16. DESIGN TASK FOR PRELIMINARY MODEL • Problem Statistics - 339 GRIDs 219 CBARs 295 CQUAD4s - 235 CRODs 69 CSHEARs 77 PBARs - 43 PRODs 3 PSHEARs 25 PSHELLS • 23 Static Load Cases - 23093 responses • Two Design Strategies - 1st Strategy - Existing PSHEARs, PSHELLs and PRODs were designed - 71 Design Variables • 2nd Strategy - Each CROD,CQUAD4 and CSHEAR Element was independently designed - 654 Design Variables

  17. RESULTS FOR PRELIMINARY MODEL

  18. MAXIMUM CONSTRAINT AS A FUNCTION OF DESIGN CYCLE 1st Design Strategy 2nd Design Strategy

  19. DESIGN VARIABLES AS A FUNCTION OF DESIGN CYCLE 1st Design Strategy (Design Appears Converged) 2nd Design Strategy (Not Yet Converged)

  20. CANTILEVERED PLATE EXAMPLE • Academic Problem to: • Test FSD with many design variables • Compare with Topology Optimization Results

  21. DESIGN TASK FOR CANTILEVERED MODEL • Symmetry has been used analyze half of the actual structure which has the load applied at the center of the tip face • 8000 PSHELL properties in the half-model • Each property is a design variable • Variables have an upper limit of 1.0 and a small lower limit • Limit applied on the von Mises stress in each element • Final design is a function of the allowable stress • Smaller allowables require more structure • Looking for a design concept, not a viable design

  22. CANTILEVERED PLATE RESULTS • Answers depend on stress limit - 10 KSI is shown • Result is a wishbone like structure • FSD is not a strong topology optimization option

  23. CONCLUDING REMARKS • Fully Stressed Design is available in the 2001 Release of MSC.Nastran • Enables rapid structural design of aerospace structures • User Interface borrows from SOL 200 interface with two additional user parameters • Possible future developments (with no current plans): • A specialized user interface to create the design –model • Extension to PBEAM, PBAR and/or PWELD properties • User feedback is solicited

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