Transcript Finite Element Concepts

Using Finite Element
ANSYS Online Manuals:
Operations Guide
Basic Analysis Procedures Guide
Getting Started w/ ANSYS
Organization of ANSYS Program
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The ANSYS program is organized into two basic levels:
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Begin level
Processor (or Routine) level
The Begin level acts as a gateway into and out of the ANSYS
program. It is also used for certain global program controls
such as changing the jobname.
At the Processor level , several processors are available.
Each processor is a set of functions that perform a specific
analysis task. For example, the general preprocessor (PREP7)
is where you build the model, the solution processor
(SOLUTION) is where you apply loads and obtain the
solution, and the general postprocessor (POST1) is where
you evaluate the results of a solution.
ANSYS File Types
File Type
Log file
Error file
Output file
Database file
Results file:
structural or
coupled
File Name
File Format
Jobname.LOG ASCII
Jobname.ERR ASCII
Jobname.OUT ASCII
Jobname.DB Binary
Jobname.RST
Binary
Communicating Via GUI
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The easiest way to communicate with the ANSYS
program is by using the ANSYS menu system,
called the Graphical User Interface (GUI).
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The GUI consists of windows, menus, dialog boxes, and
other components that allow you to enter input data and
execute ANSYS functions simply by picking buttons with
a mouse or typing in responses to prompts.
Communicating Via Commands
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Commands are the instructions that direct the ANSYS
program. ANSYS has more than 1200 commands, each
designed for a specific function. Most commands are
associated with specific (one or more) processors, and work
only with that processor or those processors.
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The ANSYS Commands Reference describes all ANSYS commands
in detail, and also tells you whether each command has an equivalent
GUI path. (A few commands do not.)
ANSYS Procedures
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A typical ANSYS analysis has three distinct steps:
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Build the model.
Apply loads and obtain the solution.
Review the results.
Build the Model
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Building a finite element model requires more of an
ANSYS user's time than any other part of the
analysis. First (optionally), you specify a jobname
and analysis title. Then, you use the PREP7
preprocessor to define the element types, element
real constants, material properties, and the model
geometry.
Defining the Jobname
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The jobname is a name that identifies an ANSYS
job. When you define a jobname for an analysis, the
jobname becomes the first part of the name of all
files the analysis creates. (The extension or suffix
for these files' names is a file identifier such as .db)
Command:
/FILNAME
GUI:
Utility Menu>File>Change Jobname
Defining the Title
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The /TITLE command defines a title for the
analysis. ANSYS includes the title on all graphics
displays and on the solution output.
Command: /TITLE
Utility Menu>File>Change Title
Defining Units
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The ANSYS program does not assume a system of units for
your analysis. You can use any system of units (except in
magnetic field analyses) so long as you make sure that you
use that system for all the data you enter. (Units must be
consistent for all input data.)
Using the /UNITS command, you can set a marker in the
ANSYS database indicating the system of units that you are
using. This command does not convert data from one system
of units to another; it simply serves as a record for
subsequent reviews of the analysis.
Defining Element Types (I)
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The ANSYS element library contains more than 150
different element types. Each element type has a unique
number and a prefix that identifies the element
category:BEAM4, PLANE77, SOLID96, etc.
The element type determines, among other things:
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The degree-of-freedom set (which in turn implies the disciplinestructural, thermal, magnetic, electric, quadrilateral, brick, etc.)
Whether the element lies in two-dimensional or three-dimensional
space.
The degree of the interpolation function within each element
Defining Element Types (II)
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BEAM4, for example, has six structural degrees of freedom (UX,
UY, UZ, ROTX, ROTY, ROTZ), is a line element, and can be
modeled in 3-D space. PLANE77 has a thermal degree of
freedom (TEMP), is an eight-node quadrilateral element, and can
be modeled only in 2-D space.
Many element types have additional options (e.g. integration
method), known as KEYOPTs, and are referred to as KEYOPT(1),
KEYOPT(2)…
Command: ET
GUI:Main Menu >Preprocessor >Element Type >Add/Edit/Delete
Defining Element Types (III)
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You define the element type by name and give the element a
type reference number. For example, the commands shown
below define two element types, BEAM4 and SHELL63,
and assign them type reference numbers 1 and 2 respectively.
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ET,1,BEAM4
ET,2,SHELL63
While defining the actual elements, you point to the
appropriate type reference number using the TYPE
command (Main Menu> Preprocessor> Create> Elements>
Elem Attributes).
Defining Element Real Constants
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Element real constants are properties that depend on the
element type, such as cross-sectional properties of a beam
element. For example, real constants for BEAM3, the 2-D
beam element, are area (AREA), and moment of inertia
(IZZ), height (HEIGHT). Not all element types require real
constants, and different elements of the same type may have
different real constant values.
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Cmd: R
GUI: Main Menu >Preprocessor >Real Constants
As with element types, each set of real constants has a
reference number. While defining the elements, you point to
the appropriate real constant reference number using the
REAL command.
Defining Material Properties
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Most element types require material properties. Depending
on the application, material properties may be:
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Linear or nonlinear
Isotropic, orthotropic, or anisotropic
Constant temperature or temperature-dependent.
As with element types and real constants, each set of
material properties has a material reference number. Within
one analysis, you may have multiple material property sets
(to correspond with multiple materials used in the model).
ANSYS identifies each set with a unique reference number.
Linear Material Properties
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Linear material properties can be constant or temperature-dependent, and
isotropic or orthotropic. To define constant material properties (either
isotropic or orthotropic), use one of the following:
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Command(s): MP
GUI: Main Menu>Preprocessor>Material Props>Material Models
the appropriate property label; for example EX, EY, EZ for Young's
modulus, KXX, KYY, KZZ for thermal conductivity.
For isotropic material define only the X-direction property; the other
directions default to the X-direction value. Other material property defaults
are built-in to reduce the amount of input. For example, Poisson's ratio
(NUXY) defaults to 0.3 (?), shear modulus (GXY) defaults to
EX/2(1+NUXY)), and emissivity (EMIS) defaults to 1.0. See the ANSYS
Elements Reference for details.
Creating the Model Geometry
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Once you have defined material properties, the
next step in an analysis is generating a finite
element model, nodes and elements, that
adequately describes the model geometry.
Creating the Model Geometry
There are two methods to create the finite element model: solid
modeling and direct generation.
With solid modeling, you describe the geometric shape of your model,
then instruct the ANSYS program to automatically mesh the
geometry with nodes and elements. You can control the size and
shape of the elements that the program creates.
With direct generation, you "manually" define the location of each node
and the connectivity of each element. Several convenience
operations, such as copying patterns of existing nodes and elements,
symmetry reflection, etc. are available.
Details of the two methods are described in the ANSYS Modeling and
Meshing Guide.
Apply Loads and Obtain the Solution
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In this step, you use the SOLUTION processor to
define the analysis type and analysis options, apply
loads, specify load step options, and initiate the
finite element solution.
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You also can apply loads using the PREP7
preprocessor.
Defining the Analysis Type
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choose the analysis type based on the loading conditions
and the response you wish to calculate. For example, if
natural frequencies and mode shapes are to be calculated,
you would choose a modal analysis.
analysis types in the ANSYS program: static (or steadystate), transient, harmonic, modal, spectrum, buckling, and
substructuring.
not all analysis types are valid for all disciplines. Modal
analysis, for example, is not valid for a thermal model.
Defining the Analysis Options
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Analysis options allow you to customize the analysis type.
Typical analysis options are the method of solution, stress
stiffening on or off, and Newton-Raphson options.
If you are performing a static or full transient analysis, you
can take advantage of the Solution Controls dialog box to
define many options for the analysis.
You can specify either a new analysis or a restart, but a
new analysis is the choice in most cases. See Restarting an
Analysis for complete information on performing restarts.
You cannot change the analysis type and analysis options
after the first solution.
Applying Loads
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The word loads as used in this manual includes boundary
conditions (constraints, supports, or boundary field
specifications) as well as other externally and internally
applied loads. Loads in the ANSYS program are divided into
six categories:
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DOF Constraints
Forces
Surface Loads
Body Loads
Inertia Loads
Coupled-field Loads
Applying Loads
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You can apply most of these loads either on the
solid model (keypoints, lines, and areas) or the
finite element model (nodes and elements).
However, the loads assigned on the solid model will
be transferred to the mesh model before solution.
Load Step & Substep
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Two important load-related terms you need to know are load
step and substep. A load step is simply a configuration of
loads for which you obtain a solution. In a structural analysis,
for example, you may apply wind loads in one load step and
gravity in a second load step. Load steps are also useful in
dividing a transient load history curve into several segments.
Substeps are incremental steps taken within a load step. You
use them mainly for accuracy and convergence purposes in
transient and nonlinear analyses. Substeps are also known as
time steps-steps taken over a period of time. (The word
“time” in here is not always the same as the “time” in
physical.)
Initiating the Solution
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To initiate solution calculations, use either of the following:
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Command(s): SOLVE
GUI:
Main Menu>Solution>Current LS
Results are written to the results file (Jobname.RST,
Jobname.RTH, Jobname.RMG, or Jobname.RFL) and also to
the database.
The only difference is that only one set of results can reside
in the database at one time, while you can write all sets of
results (for all substeps) to the results file.
Review the Results
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Once the solution has been calculated, you can use the
ANSYS postprocessors to review the results. Two
postprocessors are available: POST1 and POST26.
You use POST1, the general postprocessor, to review results
at one substep (time step) over the entire model or selected
portion of the model.
You use POST26, the time history postprocessor, to review
results at specific points in the model over all time steps.
Other Manuals
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Modeling and Meshing Guide
Structural Analysis Guide
Advanced Analysis Techniques Guide
APDL Programmer's Guide
ANSYS Verification Manual
ANSYS Commands Reference
ANSYS Element Reference
DEMO
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ANSYS 2D bracket tutor
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ANSYS Structure Analysis Guide
Structural Static Analysis