NASTRAN 101

OUTLINE

WHAT IS A FINITE ELEMENT 'SOLVER'?

  • NASTRAN is one of many available finite element analysis (FEA) solvers

    • Other solvers at Quartus: ABAQUS, ANSYS, LS-DYNA

  • What does a solver do? (the short answer)

    •  User provides input in the form of a text file

    • Solver reads the text file and performs analysis

    • The solver generates output files with analysis results

  • How does the user make the input text file?

    • Models are created and input files exported using FEA pre/post processing software

    • Various Pre/Post Processors available

      • FEMAP, PATRAN, and Hypermesh commonly used with NASTRAN

WHAT IS NASTRAN?

Nastran is a powerful finite element analysis program that is used widely in the aerospace and automotive industries

  • Industry standard finite element code originally developed for NASA by MSC (1960s)

  • Today there are many flavors (or versions) of Nastran MSC, NX, etc.

Nastran at Quartus

  • Primary program used for finite element analysis

  • Used extensively to perform static, buckling, and dynamic analyses of structures

  • Quartus has licenses for both NX/Nastran and MSC Nastran

    • Largely the same (basic functionality)

    • Some small differences and enhancements

WHAT UNITS DOES NASTRAN USE?

  • NASTRAN does not have a defined unit system

  • The user must be careful to maintain consistent units Units must be consistent such that units satisfy F= ma Examples for English and SI units are shown below:

Note: for English units (in, lbf, sec), the unit of mass is a ‘slinch’ (lbf-sec2/in), not a pound (lb). A slinch is the ‘inch version of a slug’. To convert from pounds to slinches you divide by the acceleration of gravity (386.1 in/sec^2)

WHAT IS AN INPUT FILE?

  • At the most basic level, it’s nothing more than a formatted text file

    • Defines the finite element model and all parameters necessary for analysis

  • Nastran input files are often referred to as ‘decks’

    • Origin of terminology comes from the time when the data was stored on actual punch cards and then fed into a machine that would read the ‘deck’ of cards.

  • File extensions vary

    • .dat usually used for input files

    • .blk or .bdf usually used for included files

  • Common text editors

    • EditPad, UltraEdit, EmEditor, Emacs

WHAT'S IN A NASTRAN INPUT DECK?

  • Every deck can have 5 main sections

    • Nastran statement

    • File management statements (FMS)

    • Executive control statements

    • Case Control commands

    • Bulk Data entries

  • The format and definition for all entries in the input deck can be found in the NASTRAN quick reference guides

    • Commonly referred to as “the NASTRAN bible”

NASTRAN STATEMENT

  • This section is optional

  • This section is usually used only on large jobs where modifications are needed to more effectively run the job

  • Used to change parameters for the solve

    • BUFFSIZE

    • DMP

    • Scratch file setup

FILE MANAGEMENT SECTION

  • This section is optional

  • File management section is used primarily for saving databases and setting up restart files

    • Restart a job from a previously analyzed job to reduce solve times

EXECUTIVE CONTROL SECTION

  • Executive control section is required for all runs

  • Includes:

    • DMAP control Section (optional)

    • ID (optional)

      • Identification for the Job

    • SOL (required)

      • What type of solution? (linear static, buckling, modes, etc.)

    • ECHO (optional)

      • Control whether the executive control section is output to file

    • Time (optional)

      • Set up max CPU time

    • DIAG (optional)

      • Options for diagnostic information

SOL – COMMON SOLUTION SEQUENCES

EXECUTIVE CONTROL – EXAMPLE INPUT DECK

Executive Control Section in the example deck:

This example deck performs a “normal modes” analysis.

SOL 103 = SOL SEMODES (either way will work)

CASE CONTROL SECTION

Case control section is required for all runs. Common features:

  • Selection of constraint set (SPC)

  • Selection of load set (LOAD)

  • Selection of eigenvalue extraction parameters (METHOD)

    • Used for buckling, modes, frequency response

  • Output requests

MAIN PARTS OF BULK DATA

  • Nodes

  • Elements

  • Coordinate Systems

  • Properties

  • Materials

  • Constraints

  • Analysis Parameters (PARAM, . . . )

BULK DATA: FORMAT

The bulk section is not order dependent. There are 3 options for format (can use each type within a single deck):

  • Tab delimited

  • Space delimited (default, short-field format = 8 spaces/field)

    • Decks written from FEMAP and Hypermesh are space delimited

  • Comma delimited

INPUT DECK NODE EXAMPLE

ELEMENT INFORMATION

5 major types of elements:

  • 1D Elements: Bars, Beams, Rods

  • 2D Elements: Plates, Laminates

  • 3D Elements: Solids

  • R-Type (rigids): RBE2, RBE3

  • Connector /Other Elements: Springs, Lumped Masses

1D ELEMENTS

Common element types: beams, bars, rods

DOF

  • Bars and Beams have axial, shear (2), bending (2), and torsion stiffness

    • Bars and beams are basically the same

      • Beams have more options

  • Rods only have axial and torsion stiffness

2D ELEMENTS

Common element types: plates, laminates, membranes

DOF

  • Plates and Laminates have in-plane (2), shear (in-plane and transverse), and bending stiffness

    • Stiffness is associated with attached nodes for DOFs T1, T2, T3, R1, and R2

      • No ‘drilling’ (R3) stiffness

  • Membrane elements only have in-plane (normal) stiffness

3D ELEMENTS

Common element types:

  • Solids

    • Shapes: bricks (CHEXA), wedges (CPENTA), tetrahedrons (CTETRA)


DOF

  • 3D element nodes have associated stiffness in 3 DOF (T1, T2, and T3)

R-TYPE

RBE2

  • Rigid element

  • Infinitely stiff

    • Adds stiffness to model

  • No mass


RBE3

  • Interpolation elements (constraint equations)

    • Used to ‘average’ the responses of a number of nodes

    • Does not add stiffness to model


Nodes on RBE's are either dependent or independent

  • Important to be aware of dependencies

    • Cannot apply boundary conditions to dependent nodes

    • Nodes cannot be dependent on more than 1 RBE

CONNECTOR / OTHER ELEMENTS

Common element types: Springs, Lumped Masses

DOF

  • Springs are normally used to connect coincident nodes

    • Connect elements

    • Recover forces

  • Two main types of spring elements

    • CELASi: connects only 1 DOF

      • Multiple elements are required to connect more than 1 DOF

    • CBUSH: can connect 1-6 DOF

      • Newer, more versatile spring element

  • Lumped masses are used to model mass and inertia at a node and have no stiffness

    • CONMi

INPUT DECK ELEMENT EXAMPLE

HINGING/PINNING

Common problem when elements with different DOF’s are connected

  • Plates to Solids

  • Beams and Bars to Plates or Solids

COORDINATE SYSTEMS

  • Coordinate systems are used to define node locations and output

    • Nodes can have different definition and output coordinate systems

  • Coordinate system zero is the default rectangular system located at (0,0,0)

  • Rectangular, cylindrical, and spherical coordinate systems can be used in Nastran

PROPERTIES

Properties define the characteristics of the elements

  • Plate thickness, beam cross-section, spring stiffness, etc.

  • Properties reference materials

    • Materials are defined on separate cards

Each element type has a different property

  • Some elements don’t use a property but instead input the information directly on the element card

EXAMPLE PROPERTY IN THE INPUT DECK

EXAMPLE MATERIAL IN THE INPUT DECK

NASTRAN FILES: COMMON OUTPUT FILES

  • .op2

    • Output2 File: binary file including results for FEMAP

    • Most commonly used file for output

  • .pch

    • Punch File: results in tabulated text format

  • .f06

    • Text file with results from analysis along with diagnostic messages

    • Can be read by FEMAP or processed by various custom programs

  • .f04

    • Text file containing run information; database file info, module execution summary, etc. (highly detailed log)

  • .log

    • Text file with general information; control file info, run time, licensing information, etc.

 
 
 
 
 
 
 

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