Consider two identical discs suspended by two wires of same material as shown in the fig. Now set one of the discs (say A) into torsional vibrations daily, keeping the disc B undisturbed. After few days, now set both the discs into torsional vibrations. It will be seen that the vibrations of disc A will die out soon as compared to the vibrations of disc B.
Lord Kelvin suggested that the wire with which disc A is attached gets tired due to repeated strains. Such wire is said to have tired or fatigued due to repeated strains.
Thus, the loss of strength of the material due to repeated strains on the material is called elastic fatigue. That is why bridges are declared unsafe after a long period of their use.
