Should you be concerned about fatigue? Not the kind of fatigue that affects Air Traffic Controllers, but the kind that causes a product to fail after repeated use. If you have the luxury of designing products that are only used once, you can stop reading now! For the rest of us, we need to be aware of the expected service life of our designs. If you knew your product would last “forever”, would you run around the office high-fiving your co-workers? In Simulation Professional and Simulation Premium, we have a simple tool to quickly evaluate if your product can have an ‘infinite’ life. The tool is the ‘Fatigue Check Plot’.
Fatigue is the localized structural damage that occurs due to cyclic loading conditions. Fatigue also has cumulative effect on a structure – once damaged, always damaged. If the loads applied to the structure are high enough, microscopic cracks will appear on the surface of the part, eventually leading to a failure. Knowing the loading conditions (and, thus, the stresses that occur in the structure) and the number of expected cycles the product will see during its lifetime allows us to determine if our product is safe for the expected life of the product. If the stresses are high, the number of loading cycles the product can withstand are reduced. If the stresses are low enough, the product will have ‘infinite’ life. This stress level is usually referred to as the fatigue limit or endurance limit – a stress level that can act on the material without causing failure due to cyclic loading.
After conducting a static analysis study on the design, right-click on the Results Folder and select “Define Fatigue Check Plot”. The Fatigue Check Plot will be available if the static analysis used solid elements, shell elements or a mixed mesh with solid and shell elements. The calculations for a Fatigue Check Plot are based on an infinite number of constant amplitude cycles (loading events) acting on the product. Let’s take a look at the typical Fatigue Check Plot setup.
When creating this plot, there are several options. Under ‘Modifying factors’ the first is the loading type. You specify ‘ON/OFF’ loading, where the loads are applied and completely removed or ‘Fully reversed’ loading, where the full load is applied in nominal and reversed polarities. The second option is the Surface Finish Factor – surface finished can positively affect fatigue life (shot peening) or negatively affect fatigue life (electroplating). The third option is Loading Factor where you are specifying the loading type the material is experiencing; axial, bending, torsion, etc. The ‘Material’ section of the Fatigue Check Plot property manager allows for additional control of the results. You can enter values from 1 to 100 for ‘Scale this value’ and values from 1 to 10 for ‘Minimum safety factor’. ‘Scale this value’ multiplies the fatigue strength of the material by the scaling factor entered. ‘Minimum safety factor’ divides the fatigue strength of the material by the factor entered. Finally, what’s really nice is the preview of the results during the setup – for the (finished) plot above, we see the green check mark for the selected options and the caveat that you probably do not need to be concerned about fatigue in this design.
In this second plot, I have modified the loading conditions from ‘ON/OFF’ to ‘Fully reversed’ and obtain a warning that the design may possibly fail due to fatigue. I also receive the suggestion to run a complete Fatigue Analysis study on the design. Areas of concern on the part are shown in red on the finished plot.
So the next time you are wondering if your product will last forever, create a Fatigue Check Plot as a first step in analyzing the fatigue life of your design. If your initial results for a Fully Reversed, As Forged, Torsional Loading with a Minimum Safety Factor of 5 is in the green, take that victory lap and high-five your co-workers! Now go make your products better with SOLIDWORKS Simulation!