FINITE ELEMENT MODEL VALIDITY CHECKS
Mathematical Model Validity Checks1
After a finite element model is created and before results are used from that model, Code 542 performs several standard validity checks on the model. This document will describe these standard validity checks.
There are four mathematical validity checks. They are:
- Unit Enforced Displacement and Rotation
- Free-Free Dynamics with a Stiffness Equilibrium Check
- Unit Gravity Loading
- Unit Temperature Increase
The first three checks should be performed on all models used for either statics or dynamics analyses. For models developed for thermal distortion analyses, the Unit Temperature Increase check should also be performed.
For each check, the following information will be provided:
- PURPOSE of the check
- OUTPUT RESULTS
- INPUT REQUIRED in EXECUTIVE DECK, CASE CONTROL and BULK DATA
Computer-Aided Accuracy Checks2
In addition to the mathematical validity checks given above, there are many other model checks that can and should be performed. The checks listed below require a pre/post processor, such as FEMAP, Pro/Engineer, or Patran. Furthermore, you should be aware of any warnings or potential errors your solver gives you, such as grid point singularities. This is not an exhaustive list, just some suggestions.
- Free Edge - Use a free-edge check to make sure elements are connected properly, particularly where 1D, 2D, and 3D elements are connected to zero-length elements (springs).
- Coincident Nodes - Check for erroneous coincident nodes; but be careful not to merge intentional coincident nodes.
- Coincident Elements - Check for coincident elements; this can happen if you are not careful when creating multiple meshes.
- Bar/Beam Orientation - Use orientation visualization to determine if bar and beam elements are oriented properly.
- Plate Element Distortion - Check elements for problems like aspect ratio, internal angles, warping.
- Plate Element Normals - Check that plate normals are in the same direction where necessary.
- Mass - Check to be certain the model's mass is reasonable and accurate.
Common Sense Accuracy Checks2
The following checks should speak for themselves. They are just simple, common sense checks we should make on every model. If you've been bitten by a common sense check that's not on this list, e-mail it to us and we'll consider putting it on the list.
- Material Properties - Make sure the property values are correct, match the units you're using elsewhere, and include all the information you need. For example, thermal expansion coefficient, reference temperature, density, etc. And know if you're using mass density or weight density. It could cause problems when retrieving forces.
- Element Properties - Verify beam cross sections and corresponding area moment of inertias, plate thicknesses and element offsets. Verify mass and inertia properties for concentrated mass elements and stiffnesses for elastic elements.
- Units - Are your units consistent? NASTRAN doesn't care what units you use and won't give you any warnings. Some pre-processors will also let you use inconsistent units. Take particular care with strange, mixed units (e.g. - mm and Kg). Are you using mass or weight units? You may need a PARAM,WTMASS card. For inch/pound models, the WTMASS value is 0.00259.
- Dimensions - Check to make sure that your overall geometry is correct. Check your FEM vs. drawings or CAD. This is especially important if you've transformed units.
- Grounding Conditions - Make sure your constraints are in the correct locations and DOFs. Check NASTRAN SPC forces to make sure only your mount points are listed.
- Proper Element Use - Are you using the right element for the job? Are you using it correctly? For example, RBE3's aren't really "rigid" and shouldn't be used as structural elements. They are primarily for calculating average motion or distributing loads. CELAS elements should be between coincident grids or may generate artificial forces during rigid body rotation.
- MPC's and RBE's - Are your MPC's formulated correctly? Are you using the appropriate RBE? For example, is that component you're modeling with that RBE2 really stiffer than the structure you're attaching it to? Will you artificially lock up your structure with a big RBE? Avoid over-constraining your structure with RBE's.
- Coordinate Systems - If you're using multiple input and output coordinate systems, make sure that you know which ones your elements and constraints are referencing. For example, RBE elements will use the grid output coordinate system when assigning DOFs.
- Understand singularities - If you have singularity errors, are they because of rotational degrees of freedom from plates and solids? Are those removed by AUTOSPC? If you are unsure, then you should try to remove the singularities and re-run the model. If some still exist, they may indicate grounding problems.
1 Validity checks, i.e., validation of the finite element model, are defined as checks that ensure the model is mathematically accurate. Validity checks do not ensure the accuracy of the model in representing a physical system, just that the model will give mathematically correct results.
2 Accuracy checks help the modeler to determine and ensure that the model represents the physical system he or she is modeling.
- Contrast these checks with the verification of a model, which is generally done by correlating a model with actual mechanical test results.
Sandra Irish, Ryan Simmons
July 2001, Updated November 2006