Fatigue Test :

Failure of a metal can take place at much lower stress as compared to tensile strength when it is subjected to repetitive or fluctuating load. Such failure is referred to as fatigue failure, which always result in brittle fracture.

● It takes place instantaneously without any warning. Many components such as crank shafts, gears, connecting rods, springs and blades of power driven machines, which are subjected to cyclic loading are prone to failure by fatigue.

● Such failure has been observed to be promoted mainly by application of sufficiently high tensile stresses, large fluctuations or variation in the applied stress or large number of cycles of applied stress.

Fatigue Testing Machine :

Fatigue testing machine
Block dia of fatigue testing machine
And stress development in specimen

● The principle of operation of a fatigue testing machine is very simple. The test specimen is mounted on the machine and subjected to rotation. Due to rotation, the upper surface of the test specimen is subjected to tension, where as the lower surface experience compression.

The rotation of specimen continue till failure occurs. The test is performed for given metal at varing cyclic stresses. Thus a graph is obtained between cyclic stress and number of cycles. This grape is known as a S-N curve.
S-N curve
S-N curve for ferrous and non ferrous metal

● Fatigue failure is generally starts at the surface. Hence in addition of stress, surface finish should be given due to attention for components subjected to cycling loading.

Remedies of Fatigue Failure :

It has been experimentally observed that resistance to fatigue failure improved significantly by improving the surface finish, removal of decarburized layer, hardening of surface and electro polishing. The compressive residual stresses in the surface considerably enhances resistance towards failure of fatigue. Shot peening and surface rolling are two commercial method available for the introduction of the compressive residual stresses in the metal surface.

Factor Affecting Fatigue Strength :

● Here certain metallurgical factors such as heat treatment, grain size, presence of alloying element and non-metallic inclusions also affect fatigue strength. The presence of equilibrium products in steel greatly reduces the fatigue strength.
● In general a hardened and tempered steel has optimum fatigue properties. Further improvement in fatigue properties can be achieved by bainitic structure.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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