Fatigue

In most service conditions the parts of structures and elements of machines are subjected to loads that change with respect to time. The aerodynamic loads on aircrafts, the earth resistance to earth moving machines, the wind load on buildings and direct load on rail road rolling stock etc. all change with time and so do the stresses, caused by these loads. The important point to be noted is that material behaves differently under these variable loads than under static load. The behaviour of material under variable loads is referred to as “fatigue”. In recent past several failures of structures and machinery have been attributed to fatigue and this recognition of fatigue problem has given a great impetus to researches in the area because of huge loss of life and capital resulting from such failures. However, the present day understanding of fatigue phenomenon has reached a stage where exploitation of material without endangering human life and compromising economical gains has become already possible.

The main reason for fatigue behaviour to have assumed such significance is the weakness of material under variable load. A stress level at which failure under static condition will not take place is not safe if the stress repeats with respect to time. Theoretically there is no stress level for most materials which will be indefinitely safe. However, practically some safe stress level can be defined and determined for some specified number of cycles. Yet another characteristics of fatigue which makes it an important design consideration, is that fatigue takes place in brittle manner. So there is not plastic deformation giving a warning for impending failure.