The resonant frequency and Damping Analyser (RFDA) is a non-destructive testing device based on the impulse excitation.
A vibration is induced by a small mechanical impulse. The energy is dissipated by the material into a vibration. This vibration is expressed as a damped sine.

In practice the vibration of any product will consist of a sum of several resonant frequencies. This vibration has a frequency spectrum according to its resonant frequencies which are depending on :

• the elastic properties of the material
• the geometry
• the density
• the micro- and macroscopic structure of the material

Each frequency will damp according to the energy absorption of the material. The exact micro structural origin of damping varies from one class of materials to the other. The mechanism of damping or internal friction can be understood as the rearrangement of defects in the crystal structure of the material. The capacity of damping k is dependent of the dimensions of the specimen. The specific damping or loss factor on the other hand is a material property defined as:

This mechanical loss is reflected in the measured vibration of the specimen and can be seen as the relative decrease of the signal:

where: x1/x2 = ratio of the amplitude of the signal in one period T

In this way the loss factor, an important property in characterizing the material is easily calculated from the measured damping and frequency.

For simple shapes such as rectangular bars, cylindrical rods and discs there are well defined vibration modes. The supports are placed in the nodes of the test specimen so there is minimal influence on the vibration by the supports. The nodes are the locations where the vibration has zero displacement.