FULLY STRESSED DESIGN in MSC.Nastran

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
DSO RELATED ACTIVITES FOR THE MSC.Nastran 2002
RELEASE

• Performance Enhancements
• Eigenvector Sensitivity/Optimization
• Dynamic Response Enhancements
• Miscellaneous Enhancements
• Updated Users 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,
  LAGRANGE and ASTROS

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 DESMAX0
• 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 4.828427E00
  -3.234952E-01
• 1 FSD 2.668171E00
  N/A 4.203515E-02
• . . . . .
• 3 FSD 2.541077E00
  N/A 6.268603E-02
• 6 2.709053E00 2.709045E00
  2.640250E-06 3.502930E-04

14
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