Analyze This!1 Jan, 2003 By: Greg Jankowski
When designing a component, an engineer makes decisions concerning the type of material, wall thickness, related part-geometry, and how the assembly components will interact. These factors all have a far-reaching effect on the function, cost, and quality of a design. The role of an engineering analyst has typically been to perform FEA (finite element analysis) tests on these components to ensure they will function within the intended parameters. If there is a problem with the design, it typically will go back to the engineer or designer for modifications. After this step, the component can then be physically tested to ensure the engineering analyst's assumptions are correct.
Included with SolidWorks, COSMOSXpress can be used to perform engineering analysis and do a number of quick "what-if" scenarios to fine-tune the design. Using COSMOSXpress during the initial design process can lead to better, more informed decisions by the design team, which can ultimately reduce the number of iterations involving FEA and physical testing. However, COSMOSXpress does not eliminate the need to do more extensive FEA or physical testing.
A typical design environment will include the following steps:
- Designing the part
- Repeating FEA (not always done)
- Modifying the design based on the analysis (not always done)
- Building prototypes
- Testing prototypes
- Modifying the design based on the results
- Repeating until the desired results are obtained
To get good results in an engineering analysis or FEA test, one needs to make good assumptions and understand the limitations of an analysis. This allows the part to be optimized based on its purpose or function. The following items need to be understood and defined for the engineering analysis:
Material. The material properties must be defined. These properties can be modified based on the results of the analysis. Based on the results, an engineer may be able to use a different material (for instance, cheaper, lighter, or stronger material). A standard set of materials is included with COSMOSXpress. Users can also define a material if they know its elastic modulus, Poisson's ratio, yield strength, and mass density.
Restraint. Restraints define the way in which a component is constrained within the analysis. There can be more than one restraint on a component based on the design. The mounting pins on the clip shown in Figure 1A were defined as restraints. These restraints show up with green arrows pointing out normal to the face.
Load. The loads can be applied to multiple faces and given force values. The clip shown in Figure 1B has the load applied to the face shown with pink arrows pointing in the direction of the applied force.
COSMOSXpress can only be used to perform linear, static stress analysis on a part design. If the analysis does not fit within these assumptions, the user will need to use a full-blown FEA package.
The assumptions made by a linear static stress analysis package are as follows:
Linearity. The induced response is directly proportional to the applied loads. For example, if the user doubles the magnitude of loads, the model's response (displacements, strains, and stresses) will double.
Elasticity. No permanent deformation occurs when the loads are removed.
Static. Loads are applied gradually. Loads applied suddenly will cause additional displacements, stresses, and strains.
The COSMOSXpress wizard, as shown in Figure 2, is used to guide users through the information required produce an engineering analysis. After the unit of measure, material, restraint(s), and load(s) are defined, you can run the analysis and view the results.
Figure 2. The wizard in COSMOSXpress will guide a user through the steps of an FEA session.
When reviewing the results, the areas that are below the lowest factor of safety (FOS) can be highlighted. When the results page is displayed, it will determine the lowest FOS found within the design. Users can optionally ask to display an area below a specified FOS. The FOS allows users to build in an extra level of safety into the design. Typically, the FOS used is determined by the designer and FEA analyst and is based on the product usage and level of liability.
For example, if you design the component with a safety factor of 3, you would want to display all areas that were below this FOS. You may also want to see the areas with the lowest FOS. So you could display these results a couple of times, using different levels of FOS. Then you can determine how to change the design (in relation to material type, wall thickness, loads, constraints, and so on) to meet the requirements.
This analysis may point out areas where material type, material cost, weight, and safety factors could be adjusted.
At this point, you can go back and change the criteria or modify the design. Then the analysis can be rerun to see how the results have changed. You can go through several iterations of designs, material selection, loads, and restraint definition before arriving at the desired results. This is all done quickly in an early phase of the design process prior to intensive FEA and physical testing.
In the example shown in Figure 3, the stress concentrations around the front posts were above the desired values. The material was changed and the post geometry was increased to help reduce the stresses in that area. After these changes were made, the analysis was re-run and the results were within the requirements.
Figure 3. The results of the FEA session can be displayed as a stress-distribution model.
The final results can be created in a number of different forms. The results can be displayed as stress distribution on a model, as shown in Figure 3, a deformed shape plot, an HTML report, or an eDrawing. The first two options also create an animation of the stress distribution or deformed shape. The animation provides a visual representation of how the part will deform. The animation can be saved as an AVI video file for later playback or analysis documentation.
COSMOSXpress enables the engineer or designer to quickly review an analysis based on the part design. These results can be used to drive changes in geometry, material selection, or design. These quick analyses can be used to help reduce the number of iterations required to produce and test a design.
By driving the basic engineering analysis early in the process, the engineer or designer can determine what material, load, and restraints are required for the design. Changes are easier to implement earlier rather than later, when there are many interdependencies, physical prototypes, and other considerations.
The challenge for the designer is to set up the requirements properly and to ensure that the results fit within the assumptions made by linear, static stress analysis.
About the Author: Greg Jankowski
Autodesk Technical Evangelist Lynn Allen guides you through a different AutoCAD feature in every edition of her popular "Circles and Lines" tutorial series. For even more AutoCAD how-to, check out Lynn's quick tips in the Cadalyst Video Gallery. Subscribe to Cadalyst's Tips & Tricks Tuesdays free e-newsletter and we'll notify you every time a new video tip is available. All exclusively from Cadalyst!
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