NASTRAN User Guide

OUTLINE

Introduction
File Structure
Executive Control
Bulk Data
Element Types
Element Examples
Output Files

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