Designers can also alleviate stress concentrations by removing material near notches, creating an additional groove called a relief notch. Even though relief notches add a bit of additional geometry to the design, designers can use relief notches to control the lines of stress in a part. These features can be tested and perfected through successive FAE analyses. In contrast to the previous technique, where a single groove is used to correct any irregularities, this approach involves surrounding each intentional notch with several smaller notches to smooth out potential stress concentrations.
Engineers can remove additional material to create miniature notches while maintaining the original notch. Sharp corners should be avoided on general principle, especially when CNC machining internal part geometries, but designers should also avoid them if they are worried about creating opportunities for stress concentrations.
If the pattern allows for it, designers should always use a fillet radius at sharp corners. This design factor ensures that the cross-section area decreases gradually instead of suddenly, and distributes stress throughout the part more evenly.
Luckily, product teams can prevent stress concentrations by incorporating the aforementioned design tips and workarounds into their part designs. A global manufacturing partner like Fast Radius can help product teams optimize their designs to reduce stress concentration and take their parts to the next level of functionality and aesthetics. Our team of seasoned designers, engineers, and machinists have access to the latest design technologies available and comes to the table with decades of experience in helping teams perfect their designs for optimal performance.
For more design lessons and key considerations for building clean, functional parts , visit the Fast Radius learning center.
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The stress concentration factor is a multiplier that greatly increases stress. It is recommended that an inside radius be a minimum of one times the thickness. In addition to reducing stresses, the fillet radius provides a streamlined flow path for the molten plastic, resulting in an easier fill of the mold.
At corners, the suggested inside radius is 0. A bigger radius should be used if part design allows. It is from this plot that the general rule for fillet size is obtained : i. As can be seen in the plot, very little further reduction in stress concentration is ois obtained using a larger radius. From a molding standpoint, smooth radii, rather than sharp corners, provide streamlined mold flow paths and result in easier ejection of parts.
The radii also give added life to the mold by reducing cavitation in the metal. The minimum recommended radius for corners is 0. You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account.
Notify me of new comments via email. Notify me of new posts via email. Next, we look at the ratio of the radius to the smaller diameter. Now that we understand the basics, we can step into some examples of correcting stress concentrations. If you recall, we had a very small 0. The first image is the original stresses, and the second is the reduced stresses with the larger radius. We can see that the stresses have gone from 14, psi all the way down to 3, psi.
While this differential is pretty extreme because of the extremely small original radius, it drives the point home of just how much a stress concentration can influence the stresses in the part. The first part is a support bracket that holds a brass pin.
The brass pin typically has an upward load applied, and the base is bolted to a fixed plate, as shown in the setup image below. As you probably expected, we ran the FEA again to get a baseline stress value before making any changes. The results of this study are shown below. The stresses at the base with the small radii 0. By now, we can guess that a larger radius should help lower the stresses here, even with a much different load case and geometry. In the next simulation, I have increased the radii from 0.
If I wanted to get even lower stresses, I could do so by thickening the flange to which this assembly is attached. You can see the light blue coloring that gives an indication of how this part is deflecting under a load.
In our next example, we have a flat plate with a diamond shaped hole cut out of the center. There is a load applied on one end, while the other end is fixed, as shown in the setup below. If that is the case, you were absolutely correct. The next image shows the results of this simulation.
We can see the stress concentration is exactly where we expected. We know we can make the radii bigger, but what if we wanted the option of drilling that hole out, instead of requiring a mill or punch? Can we use a larger diameter circle, effectively reducing the total material in the part, and still lower the stresses?
As the image below details, the same simulation was run with a round hole that is greater in diameter than the distance from the top to the bottom of the diamond-shaped hole above.
In a similar approach but different application, it is common for repair centers to drill a hole at the end of a crack to relieve the high stress concentration associated with the very small radius at the tip of a crack. I hope by now the idea of locating and reducing stress concentrations is clear, and you are well on your way to improving a design. While I used an FEA program here to determine the magnitude of stresses, there are some general guidelines that can be used to improve a design.
When it comes to common methods of reducing stresses, the following list includes some simple items to get you started quickly:. Add stress-reducing holes at the end of slits, sharp angles, or cracks to relieve high stress concentrations. Refer to stress concentration charts to understand when you are in a region of diminishing returns with respect to radius size.
Do not make a large size transition between loaded features. The stiffness mismatch will drive the stress concentration much higher. Remember that the stress concentration is based on a ratio, not a magnitude. This list is not fully comprehensive, but it should cover the basic concepts that every designer should know in order to improve their design skills.
Through the examples and analysis above, it should be clear exactly why we need to be concerned with stress concentrations. By incorporating these concepts through your design, you should be able to achieve higher load ratings, reliability, and fatigue life. I encourage design teams to talk through product requirements and design choices to ensure the proper blend of aesthetics and function. There will always be tradeoffs, but proper analysis can help achieve an optimized solution. Introducing Fictiv Enterprise: A new partnership solution for the speed you want and the security and quality you need.
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