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What is Fatigue Phenomenon in Materials?
Fatigue is the progressive, localized, permanent structural change that occurs in materials subjected to fluctuating stresses and strains that may result in cracks or fractures after a sufficient number of fluctuations.
Fatigue it’s a mechanical phenomenon of progressive degradation of material o components, subjected to different loads in a time, that may result in the material or component fracture. During this fatigue, test is possible to notice a plastic deformation on the material. In its smaller form, this deformation can be called microplastic deformation. this damage can be enlarged with a continuous solicitation and eventually caused the fracture of the material or component
Why is Fatigue Testing important? From the automotive till the dental industry
From the chassis of Formula 1 cars, to dental prostheses, the phenomenon of fatigue and related breakages is a key issue in the design of any component subjected to cyclic loads. Consequently, it is essential for designers to know precisely the fatigue performance of the materials used. To this end, materials and finished products are increasingly characterized with fatigue tests in laboratories, before being placed on the market.
The fatigue test is executed to obtain an estimate of the durability of the component or material. Then, at the end of the tests, the designer can evaluate the duration of the components in function of the applied sollecitation.
What is the origin of Fatigue Testing?
The scientific analysis of fatigue fail has its origin in 1842 with the investigation after the damage reported in the Versaille accident in Meudon, France. On the occasion, the axis on the locomotive broke, setting fire and destroying the other carriages leading to the death of 55 people. William John Macquorn Rankine, a mechanical engineer, decided to study the event to understand what was the cause of the brake. His investigation highlighted the importance of stress concentration factors in components and the impact of repeated cyclic stress on these areas that lead to the component’s failure. But his theory of the damage was caused by the progressive formation of cracks due to repeated cycling of the load was ignored by other engineers. The fatigue continued to remain a serious and misunderstood phenomenon for a while.
John Braithwaite was the one defining “fatigue ” as we know it thanks to his experts between 1850 till the beginning of 1900.
Starting from 1910, the origin of fatigue failure of microscopic cracks was identified and the relationship between cyclic stresses and the number of cycles to failure was demonstrated.Olin Hanson Basquin proposed a log-log relationship of stress (S) and cycles to failure (N) based upon the test data generated by August Wöhler. Wöhler concluded that the cyclic stress range was more important than peak stress and introduced the concept of an “endurance limit” of a material.
The real foundation for modern fatigue testing was the 1960s when P.C. Paris proposed a method for predicting the rate of growth of individual fatigue cracks. He demonstrated the rate of change in the crack length versus the number of cycles. His work is popularly referred to as da/dN curves. Still, M.A. Miner’s popular phenomenological approach from the mid-1940s overshadowed Paris’.
What do you obtain from Fatigue Testing?
-S-N curve of Wöhler
Most commune types of Fatigue Testing?
-LCF High cycle fatigue test: Normal fatigue test at frequencies up to approximately 1-5000 Hz.
-HCF ( din 50100) High cycle fatigue test: Normal fatigue test at frequencies up to approximately 1-5000 Hz.
-Bending fatigue tests on rotating bars, DIN 50113
-Tension-compression fatigue tests
-Evaluation of endurance (fatigue strength) against cyclic stress by using a combination of tension, unloading and compression.
-Gigacycle fatigue test : Fatigue test in which approximately 1 × 109 cycles or more are applied in a short time by ultrasonic vibration, etc
-Fatigue crack propagation testing machine
-Evaluates the propagation rate of fracture crack under cyclic stress.