As stated in our newsletter we will be covering different aspects, factors, and results of fatigue over the next several weeks.
Fatigue is a very common failure mode for metal. Although there are plenty of other modes for metal failure, fatigue can be commonly found in items that are used on a regular basis. For example, springs, gear shifters, door handles, light poles, airplane wings, locomotive parts, wires and cables, hinges, anything that is subject to repeatedly applied (cyclical or alternating) loads over time are perfect vessels for fatigue. The fatigue failure usually occurs in the area that the material is the weakest, and/or the applied stress is the highest or most frequent.
When examining fatigue failures, marks are commonly found on the outer perimeter of the fracture surface of the failed material; these marks can often look like small steps on the periphery of the failure surface and usually vanish moving towards the interior. These are called Ratchet Marks. Usually these are observed on a microscopic level but sometimes they can be seen with the naked eye depending on the material and size of the failure fracture face. Ratchet marks originate when multiple cracks, commenced at different points, join together, creating this common step like appearance on the fracture surface. The number of ratchet marks found can be closely related to the number of crack origins and can often play a key role in helping identify the failure mode, type, origin location and root cause.
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