Accurately Report the Factor of Safety in Autodesk Simulation Multiphysics4 Aug, 2012 By: Radu Stancescu
IMAGINiT Tricks Tutorial: Adjust your reports to display the simulation results accurately.
Editor's Note: This tutorial courtesy of IMAGINiT Technologies.
When we work on an FEA (finite element analysis) project, we must be mindful that the results are displayed in a proper range, or — as you will see below — it is be possible to display stress results that may not tell the entire story accurately. For example, let's consider an analysis run to evaluate the safety factor of a model. The "factor of safety" is used to provide a design margin over the theoretical design capacity to allow for uncertainty in the design process. (Factor of safety is recommended by the conditions over which the designer has no control, to account for variables such as properties of material, machining processes, effect of time on strength, nature and type of load applied, assumptions and approximations made in the nature of surface conditions of the machine element, etc.) Factor of safety guidelines include the following:
- A factor of safety less than 1.0 at a location indicates that the material at that location has failed.
- A factor of safety larger than 1.0 at a location indicates that the material at that location is safe.
- The material at a location will start to fail if you apply new loads equal to the current loads multiplied by the resulting factor of safety.
- A factor of safety for cables in elevators (when people are involved) could be 10 or 11; in the aircraft industry it could be as high as 15.
- For many applications, a safety factor of 4 is a common goal, especially if product durability is an issue.
Generally we would look for a blue color to indicate the factor of safety has met or exceeded our target, but the result shown below is almost completely red! Even though the information is accurate, and the design criteria meet our goals, this picture could lead to a misleading conclusion.
We actually have two ways to display the FEA results: a default and a customized report, which can be saved as HTML, PDF, Word, or RTF. The user can decide which images are displayed and how, and what information will be contained in the report.
Let's talk about a couple of tips to help make these reports display the simulation results accurately. First, to change the default legend for the Factor of Safety display, go to: Results Contour tab > Settings panel > Legend Properties command > Setup option > Range Settings tab, and uncheck Automatically Calculate Value Range under the Current Range. After adjusting the values, it could look like this:
The results display more accurately with a "blue / safe" picture in the report. Another option that helps the report more precisely portray the analysis is to display the results based on different Allowable Stress Values.
We can also display the Ultimate Tensile Stress (the maximum stress that a material can withstand while being stretched or pulled) rather than the Yield Stress. You can use any value you wish in the Allowable Stress field.
These little tricks will allow you to display more accurate results in your FEA reports and make sure that decision makers get the information needed to convey your design characteristics and intent.