• Damping can give information on the orgin of defects such as the formation of quenching cracks.• When a solid specimen vibrates, its free oscillations decay when isolated from their environment. some of the energy is always convert into heat.
• The various mechanisms by which this transfer of energy occurs are collectively termed as internal friction.• For EXAMPLE: An increase in damping was found in a steel specimen which had been quenched . with cylindrical pieces it is possible to determine the position of a crack in the piece .
• The method of determining the damping factor consists of finding the time for amplitude to decrease by a specific fraction, the ratio between the amplitudes of successive free oscillations is constant .• The natural log of this ratio delta, called the logarithmic decrement is taken as a measure of the internal friction.
The figure shows the damping factor varies in the case of a sound specimen and for a cracked specimen.
• Using torsional vibrations, sensitivity with which flaws can be detected is least, when they are situated in the centre of the bar.• High strain damping is sensitive to the presence of cracks and this fact has been applied successfully in the routine detention of season cracks in brass catridge cases.