FEA Input Files

FEA simulations are run using NairnFEA. Some of the key features of this code engine (with links) are listed here. The documentation on this page explains how to run FEA calculations by describing all the options of input command files.

Input Command File Overview

When using NairnFEAMPM or NairnFEAMPMViz to set up calculations, the input command files are usually script files written using the custom scripting language. When you interpret these scripts, the applications will create an XML input command file that is needed to drive the code engines. Click this link to see a sample scripted FEA input command file for analysis of an end-loaded cantilever beam.

Alternatively, you can directly create XML input command files. Because this approach cannot use the language control options of a scripting language, it is much less powerful then scripting methods and much harder to customize for a range of simulations. If you do use the manual XML method, the files must conform to the structure defined here. Click this link to see a sample XML FEA input command file for analysis of an end-loaded cantilever beam.

Header

This section defines the type of analysis to run, provides an optional description of the calculations, and sets some output options. When using a scripting language, the header is created automatically and its content is controlled by commands defined below. When manually writing XML files, the header, which is required, is defined in a single header block:

<Header>
  (commands)
</Header>

The possible header commands are

• Analysis Command - pick the type of FEA analysis.
• ConsistentUnits Command - pick input and output units for the calculations
• Annotation Commands - these commands let you annotate the calculations.
• Controlling FEA Output
• Processors Command - pick number of processors for parallel code.
• Setting Developer Flags

Defining the FEA Mesh

FEA calculations are done within a mesh of elements. There are two ways to specify the mesh. The most common is to generate the mesh by defining keypoints and paths and then using them to mesh areas. The second mesh method is to explicitly define the mesh by defining every node and element. This uncommon method can only be used in XML input files. The generate and explicit methods cannot be mixed.

An advanced mesh generation option is to create the mesh with image and shape commands. When using this procedure, you first generate or explicitly define a mesh and then follow that with image and shape commands to define material types, define material angles, and create the final mesh geometry.

Element Types

Calculations can use various types of elements and can mix compatible element types.

Explicit Cracks

Defining explicit cracks in a finite element mesh is simply a matter of having two separate paths joined at a crack tip. When FEA calculations include cracks in the proper form, NairnFEAMPM can do crack closure calculations to find energy release once the calculation are done (crack closure calculations are not yet available in NairnFEAMPMViz).

Imperfect Interfaces

NairnFEA supports modeling of imperfect interfaces between phases in composite materials by including imperfect interface elements at phase boundaries.

Defining Materials

Each element in the mesh is assigned a material type. NairnFEA support numerous material models, but currently all are linear elastic. The information on these materials is found in are in FEA Material Models.

Boundary Conditions

All FEA calculations require boundary conditions to define the problem. This conditions specify nodal displacements, nodal loads, or stresses on element faces. In NairnFEA, additional boundary conditions can set up periodic problems.

Temperature

FEA calculations can do thermal calculations involving thermal stresses.

XML Insertion

When writing scripted input files, it is sometimes useful to insert raw XML commands into the automatically-format XML input file. The two commands that help in this task are:

• Entity Command - to define entities that will appear in the automatically-format XML input file.
• XMLData Command - to insert raw XML data into various sections of the automatically-format XML input file.