In fastened joints subjected to external loading, the design and selection of fasteners become very important. The bolt or bolts joining the pieces together should create more clamp load than the external forces can generate. If the clamp load is correct, the bolts will never feel the outside forces. But, if the clamp load is too low, fatigue failure may result, especially in cyclical loading.
If the external loads on the joint exceed the clamp load, joint decompression, or joint separation will occur. The repeated joint separation will likely lead to exceeding the fatigue endurance limit of the thread, which is much lower than the tensile strength of the bolt.
Avoid Loaded Joint Fatigue Failure
How can we avoid fatigue failure in high cyclical loaded joints? Performing a joint study to ensure proper clamp load is key, as well as knowing your external loads and selecting the proper grade/property class of fasteners. Often fatigue failure tempts engineers to increase the strength of the bolt. If tightened, this may help to create more clamp load, but the fatigue endurance limit of higher strength bolts is very like lower strength versions.
Consider other safeguards such as special root radius threads, or rolling threads after heat treat, to increase the fatigue endurance limit.
For more information on keeping your joints together, contact us at ProvenProductivity@bossard.com.How To Avoid Fatigue Due To Low Clamp Load by Bossard