The impulse excitation technique (IET) is a non-destructive material characterization technique to determine the elastic properties and internal friction of a material of interest. It measures the resonant frequencies in order to calculate the Young’s modulus, shear modulus, Poisson’s ratio and internal friction of predefined shapes like rectangular bars, cylindrical rods and disc shaped samples.

The measurement principle is based on tapping the sample with a small projectile and recording the induced vibration signal with a microphone or laser vibrometer. Afterwards, the acquired vibration signal in the time domain is converted to the frequency domain by a fast Fourier transformation. Dedicated software will determine the resonant frequency with high accuracy to calculate the elastic properties based on the classical beam theory.

• Non-destructive measurement of elastic and damping properties
• Large temperature range: -50 °C – 1700 °C
• Reliable, fast and easy accessible measurement technique
• Limited restrictions on sample geometry and dimensions
• Applicable to porous and brittle materials due to small strains
• Information about internal structure, global behavior, damage,…

Different resonant frequencies can be excited dependent on the position of the support wires, the mechanical impulse and the microphone. The two most important resonant frequencies are the flexural which is controlled by the Young’s modulus of the sample and the torsional which is controlled by the shear modulus for isotropic materials. For predefined shapes like rectangular bars, discs, rods and grinding wheels, the dedicated software calculates the sample's elastic properties using the sample dimensions, weight and resonant frequency (ASTM E1876-15).

To determine the Young's modulus, the equipment measures the flexural vibration frequency (see picture) and calculates the Young's modulus using the mass and dimensions of the sample according to the different standards (ASTM E1876-15, ISO 12680-1, EN 843-2).

To determine the shear modulus, the equipment measures the torsional vibration frequency (see picture) and calculates the shear modulus using the mass and dimensions of the sample according to the different standards (ASTM E1876-15, ISO 12680-1, EN 843-2).

The Poisson’s ratio is a measure in which a material tends to expand in directions perpendicular to the direction of compression. After measuring the Young's modulus and the shear modulus, the dedicated software determines the Poisson’s ratio using Hooke’s law which can only be applied to isotropic materials according to the different standards (ASTM E1876-15, ISO 12680-1, EN 843-2).

Material damping or internal friction is characterized by the decay of the vibration amplitude of the sample in free vibration as the logarithmic decrement. The damping behaviour originates from anelastical processes occurring in a strained solid i.e. thermoelastic damping, magnetic damping, viscous damping, defect damping,… For example, different materials defects (dislocations, vacancies,..) can contribute to an increase in the internal friction between the vibrating defects and the neighboring regions.