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Fatigue failure

Fatigue is an interesting phenomenon. Fatigue is a condition in which the material cracks and fails due to cyclic loading under the ultimate strength of the material. In fatigue, the materials are not loaded to their limit. However, the materials fail, and the reason behind failure is the repeated cycles of loads. Fatigue often occurs suddenly, and it can result in catastrophic consequences. 

Figure 1

When a material is loaded, a crack will be initiated or nucleated in the materials. The continuous cyclic loading of the material will result in growing the crack size. Finally, when the crack is large enough, the material will fail. The phenomenon of fatigue can't be studied from the engineering side only. We should also study the properties of the material thoroughly. 

The figure below showing the phases of fatigue failure. At the beginning stage, the crack start as a slip band in material grains. The slip band will result in forming slip steps; in the presence of oxygen, the freshly exposed surface of the material in slip steps gets oxidized, which prevents slip reversal. The slip reversal, in this case, occurs in some adjacent slip planes, thereby leading to the formation of extrusions and intrusions on the surface of the material. The cyclic slip will develop to crack nucleation. The growth of these cracks on materials will result in material failure.  

Figure 2

The endurance or fatigue limit for a material can be found by testing. The endurance limit is the stress value that will not result in fatigue failure regardless of the cycle number. The figure below showing the plot of stress and cycle numbers. This diagram is also called the S-N diagram. We can notice that the aluminum alloy has no clear endurance limit. While the other material has a clear endurance limit at almost 550 Mpa.
Figure 3

Surface defects such as scratches, notches can affect the resistance of a material to fatigue failure. These kinds of defects can result in reducing the material resistance to fatigue failure. However, resistance can be different for different materials. Also, corrosion can affect the material's fatigue resistance severely. Corrosion may result in reducing the fatigue resistance of a material by 90%—table no:1 showing the endurance limits for different materials.

Table 1


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