How To Apply Acceleration In Sol 101

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.