Fatigue has been around since we started making things. While we have always understood that every material will have a breaking point when it is overloaded, but engineers in mid-1800s became aware of the failure and coined it fatigue. Jean-Victor Poncelot was the first person to observe and coin the name “fatigue”, he was a designer of cast iron axles for mill wheels. Later in his book on mechanics, he stated that the axles became ‘tired’ or ‘fatigued’ after being in use for a long time and then began to crack. Since then, fatigue has been studied by many and a lot of research helped to understand the nature and types of fatigue. However, it is still one of hardest to predict in terms of failure due to fatigue.

Mechanisms of Fatigue:

Essentially, fatigue failure can be broken down into three phases – initiation, propagation and ultimate failure.

Initiation:

Initiation is the initial step towards a components finite life. There are many variables which can affect fatigue such as mean stress, material microstructure, blow holes in weld, surface roughness of finished part. Fatigue related failure can occur more often when the material or part undergoes cyclic stress, which causes localized hardening of material. Once material is hardened, the amplitude of stress in that region is increased and cause it to break down. Once the amplitude of stress level rises, they form intrusions in the structure. Once the intrusions are large enough, they can be classified as micro cracks. Fatigue failure is initiated.

Crack Propagation:

Propagation of crack can be further broken down into two categories – when the material is in cyclic plastic zone and is smaller than the characteristic grain size, then the crack may be considered to be in Stage – 1 of its Propagation phase. The path of the crack is highly dominated by the dimensions and the orientation of the grain structure. Once the plastic zone outgrows the grain size, the crack begins to grow more perpendicular to the high stress region.

Solving Fatigue in FEA

Solving Fatigue in FEA solver essentially can be approached in these methods –

1) Run FEA simulations on the model to predict stresses and strains for cyclic loads.  

2) Collect all relevant output data from each node into the fatigue solver, and

3) Now use the fatigue solver to calculate life of the component based on the FEA results from above iterations and some additional inputs load scaling, surface finish, endurance limit etc).

Ultimately these FEA fatigue simulations can help to predict until initiation of cracks, but fracture mechanics and propagation of cracks are too complex and requires writing codes.

Conclusion :

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