SIDEBAR: Fatigue Properties
The mechanical properties of steel when a load is repeatedly applied is known as its fatigue strength.
Fatigue testing determines the ability of a material to withstand repeated applications of a load.
The load by itself may be too small to produce a failure.
There are three factors involved when considering fatigue strength:
1) The number of cycles at which a crack initiates;
2) The number of cycles at which the crack starts to grow;
3) The number of cycles at which the fitting fails.
One accepted method of fatigue rating fittings is to test them to one and a half (1-1/2 times the working load limit for 20,000 cycles, without failure.
This standard test is accepted as indicating indefinite life when used within the working load limit under normal circumstances.