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SPRING 2019

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Distributor's Link Magazine Spring 2019 / Vol 42 No2

170 THE DISTRIBUTOR’S

170 THE DISTRIBUTOR’S LINK LAURENCE CLAUS FATIGUE IN FASTENERS from page 158 If threads are rolled and subsequently heat treated, the advantageous compressive stress is lost. Therefore, for cold rolling threads to have a positive benefit for reducing fatigue risk, the process must be accomplished after heat treatment. If the parts are relatively high strength, this is very difficult on the roll dies used to create the threads and can diminish tool life by many times, making this an effective but sometimes costly solution. ¤ Add a radiused root thread (MJ or UNJ) - In the inch thread series the standard “UN” thread has minimal relief in the root (meaning it does not become a sharp v-notch). Within the series, however, one can choose a UNR or a UNJ thread. Figure 3 illustrates how both of these are versions of a more highly radiused root, with the UNJ being the most. The advantage this brings is that it relieves the root to be less V-notch like and, thus, not only strengthens but reduces the stress concentration. Metric threads act similarly with an MJ variation providing a more highly radiused root than the standard Metric thread. FIGURE 3 ¤ Increase the fillet radius- for that matter increasing any transition radius on the part is helpful. The fillet is often the site of highest stress concentration in a bolted joint. Therefore, the less sharp this juncture is the less the localized stress concentration. Additionally, increased radii increase part strength. ¤ Roll the underhead fillet radius- the theory behind rolling the underhead fillet radius is identical to that of rolling threads. However, in the same way as rolling the threads, the only way for this to be advantageous is to do so after heat treatment. Just like rolling the threads, employing a fillet rolling process places a positive compressive stress in this area. (It is key to remember that once in service, likely the most highly stressed area of the fastener is the head to shank junction.) ¤ Control shot peening- this process is perhaps the most effective of reducing fatigue risk in this group of strategies. It is important to understand, however, that this is a very specialized process and not just simply blasting a part with some shot. The key word in this process is “controlled”. Unlike normal shot peening whose purpose is usually to blast or clean off a dirty or scaly surface, controlled shot peening is conducted in a highly controlled fashion resulting in a uniform and consistent compressive stress layer over the entire part. Once again, cracks initiate in tension, so that any areas or surfaces that are in compression must first be overcome before they can flip into tension and initiate a crack. Controlled shot peening utilizes controls where the size and geometry of the shot, intensity of the impinging shot, and the length of time the process is conducted are closely defined and precisely controlled. The combination of these controls results in an effective compressive stress layer over the entire area which this process is applied to. ¤ Eliminate surface finishes that are in tensionsome surface finishes such as Electroless Nickel, by nature, are in tension. These should be avoided for fatigue applications when and where possible. CONTINUED ON PAGE 171

THE DISTRIBUTOR’S LINK 171 LAURENCE CLAUS FATIGUE IN FASTENERS from page 170 Special Material and Process Steps ¤ Use fine grain alloy steels- these materials do better in fatigue than coarse grain, plain carbon steels ¤ Utilize NDT methods- NDT is an acronym for Nondestructive Testing and refers to Magnetic Particle testing for ferrous materials and Dye Penetrant testing for non-ferrous materials. These processes are very effective in identifying surface flaws and defects such as cracks, laps, folds, etc… Imperfections that are identified during these tests are referred to as “indications”. If a part has apparent indications, it triggers a closer look at the part to see if these are potential and critical flaws. ¤ Use and meet Surface Discontinuity standards- Fastener Surface Discontinuity standards such as ASTM F788 for externally threaded fasteners and F812 for internally threaded fasteners help bring attention to common manufacturing flaws. Being aware of and following these documents can help produce a better quality part. Returning to the story that leads this article off. The root cause for the failure to pass the required durability test was an under designed bracket. Addressing the root cause, although surely the best solution was, in this case, not practical due to the costs and timing. Therefore, our challenge became how we could sufficiently fortify the Caliper Pin to increase its fatigue resistance and pass the test. To this end, we employed as many of the above strategies as we could, improving the material, increasing the radii on the part, rolling the threads after heat treatment, converting to an MJ thread form, and adding a controlled shot peen. I’m not quite sure if one of those actions carried the day, but combined they did the trick. Once these strategies were employed the parts well surpassed the requirements of the test. In the same way, designers can and are regularly employing these same strategies to improve the fatigue life of their critical fastener components. LAURENCE CLAUS

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