10 Aug 2021 Fatigue
Since the beginning of mechanical engineering, static (https://www.fea-solutions.co.uk/static-vs-dynamic-analysis/) tests have been used to determine the strength of designs. This was suitable for structures such as houses, towers and bridges, subjected to static loads due to self weight (https://www.fea-solutions.co.uk/mass-and-weight/). With the Industrial Revolution, machines were introduced which were undergoing many thousands of load cycles. It was soon noticed that many of them were failing after some time of operation although they were tested to be strong enough to withstand the maximum static loads, they were designed for. This failure phenomenon was called ‘fatigue’, a term coined in 1854.
Fatigue describes the weakening of a material undergoing repeated loading and unloading, so-called cyclic loading. This results in increasing localised structural damage and eventually the initiation of a crack. This crack will propagate through the material due to the cyclic loading, which will in time cause the material to fail.
Often, fatigue will cause a component to fail even when the stresses in the material are well below its yield strength. Therefore, looking at the static stress of the component will not highlight that it might fail after a certain number of load cycles. To realistically predict this failure mode, a fatigue analysis (https://www.fea-solutions.co.uk/fatigue-analysis/) is required.
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