Tools for interpreting FEA results

Oct. 7, 2004
There is more to a solution than just the high-stress locations.

Bob Williams
Product Manager
Algor Inc.
Pittsburgh, Pa.

Tools for evaluating and displaying results, such as those in Algor's Fempro interface, help engineers quickly interpret FEA output and assess designs. The underwater instrumentation housing, modeled by DeepSoft Inc., Columbia, Md., shows several different ways to review results.

Precision-contour displays (left) let engineers with Kerotest Manufacturing Corp. judge how well the mesh complied with the assumptions of FEA theory. Based on the display, the mesh could be revised by adding refinement points as needed (right).

Engineers with Unverferth Manufacturing Co. Inc. specified allowable stress values in Algor FEA. The color contours show the factors of safety to highlight where stresses in the model are below and above those allowable.

The pressure vessel is modeled with four elements across its thickness. Algor's stresslinearization utility lets users place a Stress Classification Line through the thickness of a model and then dynamically view the area using clipping planes and zooming. The graph tracks stress along the line.

A Report Wizard includes options for generating organized, professional reports.

One of the most important steps in the FEA process begins after the solution completes: interpreting results. Determining what the contours and graphs are saying tells whether or not models need refinement or designs need modification. But interpreting FEA results is more than highlighting a high-stress value. It's when users apply their knowledge and experience to make sense of the numerical and graphical output.

FEA vendors have made it fast and easy to build and analyze models. To further increase the user's productivity, FEA software should also have powerful, flexible tools for examining results and assessing designs quickly, thoroughly, and accurately.

For example, the single-user interface in our software includes a wide range of evaluation and presentation options. These let users easily examine analysis outputs as graphical displays of color-coded contours that make it easy to find detailed numerical data at locations of interest. FEA software should also create graphs using built-in, virtual instrumentation, such as our Monitor utility, along with animations and graphics for use with other presentation features.

One important interpretation tool is the display of precision contours. It shows changes in stress from one element to the next. The software checks the model for compliance with the assumptions of FEA theory before generating the contours. For example, high-stress areas might come from a modeling error, which would show up as low-accuracy figures. When this happens, the mesh in the questionable high-stress area could be refined and the analysis rerun. High stress with good precision can be taken as useful information.

Another result-interpretation tool — factor of safety contours — takes userspecified allowable stress values and presents results as either above or below the allowable. Factor-of-safety contours help users balance material and cost-reduction issues on one hand while ensuring a safe product on the other.

It's also good to know the distribution of stress through the thickness of a thinwalled part. This is particularly useful when relating 3D solid FEA models of thin-walled pressure vessels to the relevant ASME codes. A stress-linearization utility, such as the one built into Algor, helps evaluate a design's compliance with industry standards, such as the ASME Boiler and Pressure Vessel Code. After the user defines a Stress Classification Line, the utility calculates the linearized stress distribution along the line, and formats results for easy comparison with code requirements.

Result evaluation and interpretation tools, such as precision contours, factor-of-safety contours, and stress linearization, help automate frequently encountered FEA and engineering processes so engineers can focus on determining what the results say about the design. A number of visualization features support these tools. For example:

Annotations that pinpoint the minimum and maximum results make it easy to locate areas of interest. Zooming in on these areas and toggling between result and precision contours helps determine if the mesh is adequate in those areas.

A slider bar controls the display of elements based on a user-specified limit. The slider can also set threshold values, letting the software display only results within a specified range. This lets engineers quickly see areas that need closer examination and where changes may be needed.

A transparent display lets users see results below a surface, over, or inside complex assemblies while keeping areas of interest in proper context.

Knowledge and experience must also be used to interpret FEA results and decide whether design changes are needed. Doing so often leads to an iterative process of redesigning, retesting, and reevaluating.

Once satisfied with the results and the design, it's important to report the information to others. Reporting tools, such as our Report Wizard, collect information related to an analysis and generate an organized, professional report supported by a wide range of image and animation options along with report customization tools. These reports can then be shared with managers, colleagues, and clients by publishing them to any Internet or Intranet site. Alternatively, the generated report can be attached as an appendix within any design report.

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