Fatigue defined as the progressive and localized structural damage that occurs when a material is subjected to cyclic loading.
Now if the loads are beyond a certain limit for that material, microscopic cracks will begin to form at the stress concentrators which eventually would reach a critical size, and the material will suddenly fracture.
Fatigue is affected by the shape of the material, presence of any square holes which might act as stress risers. Presence of sharp corners would also lead to increase in local stress.
In contrast, round holes and smooth transitions are important to increase the fatigue strength of the structure.
Failure is the end result of a process involving the initiation and growth of a crack.
Fatigue strength is determined by applying different levels of cyclic stress to individual test specimens and measuring the number of cycles to failure.
Fatigue as a phenomenon is important in orthopedic implant material and designing the shape of the implant.
After an implant is put, it has to bear the cyclical loading till the bone unites and is strong enough to take over. If the bone fails to unite and implant is continued to bear weight, it would at some point of time undergo fatigue and eventually fail.
Our bones bear so much load but does not fatigue normally. Why?
It is because bone is a living tissue and its structural integrity is continually maintained by remodeling.
A bone is completely remodeled in approximately 3 years. Thus every time bone undergoes microstructural damage, it repairs itself and the damage does not accumulate.
However, if the rate of repair is not equivalent to the quantum of the damage, it might lead to microstructural damage and accumulation of the damage over the time.
This may occur with age or pathology of bone.
Microstructural damage due to repeated loads below the bone’s strength occurs when muscles become fatigued and less able to counter-act loads during continuous strenuous physical activity
It is affected by the magnitude of the load, number of cycles, and frequency of loading
The bone undergoes relatively rapid loss of stiffness, bears less load and cracks develop rapidly.
Sometimes, the bone may stabilize the crack quickly without much propagation
After a crack forms interlamellar tensile and shear stresses are generated at its tip tend to separate and shear lamellae at the fiber-matrix interface.
Cracks tend to grow parallel to the load and may lead to stress fractures.
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