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The IET is based on the analysis of the vibration of a test sample or component after it was 'impulse excited' (= gently tapped). The resonance frequencies are characteristic for the test object, as they are related to its stiffness, mass and geometry. The relation is accurately known for isotropic objects of simple shape such as bars or discs, thus making IET a standard method to determine elastic moduli of isotropic materials (ref. ASTM E 1876, ENV 843-2).
Also, small geometrical distortions, density fluctuations, cracks etc. influence the value of the resonance frequencies. Therefore, IET is also used for quality control in materials processing.
Several instruments are available for the non destructive measuring of resonance frequency. The RFDA gives extra added value by determining the damping or internal friction of the material. These accurate measurements have not only a scientific value, but also a substantial industrial relevance in material testing and quality control.
