Impulse excitation technique
The impulse excitation technique (IET) is a non-destructive material characterization method to determine the elastic properties and internal friction of a material of interest. Impulse excitation equipment measures the resonant frequencies and measurements 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 non-destructive impulse excitation technique is based on tapping the sample with a small projectile and recording the induced vibration signal with equipment such as 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.
Advantages of impulse excitation technique
- Non-destructive impulse excitation measurements 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,…
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Different resonant frequencies can be excited dependent on the position of the support wires, the mechanical impulse excitation method 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 impulse excitation 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 measurements and standards (ASTM E1876-15, ISO 12680-1, EN 843-2).
To determine the shear modulus, the impulse excitation equipment measures the torsional vibration frequency (see picture) and calculates the shear modulus function 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 impulse excitation technique 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.
Relevant IET standards
- ASTM standards
- ASTM E1876-15: Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio by Impulse Excitation of Vibration
- ASTM C1259-15: Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio for Advanced Ceramics by Impulse Excitation of Vibration
- ASTM C1548-02(2012): Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio of Refractory Materials by Impulse Excitation of Vibration
- ISO standards
- ISO 12680-1:2005: Methods of test for refractory products – Part 1: Determination of dynamic Young’s modulus (MOE) by impulse excitation of vibration
- ISO 20343:2017: Fine ceramics (advanced ceramics, advanced technical ceramics) – Test method for determining elastic modulus of thick ceramic coatings at elevated temperature
- EN standards
- EN 843-2:2006: Advanced technical ceramics – Mechanical properties of monolithic ceramics at room temperature – Part 2: Determination of Young’s modulus, shear modulus and Poisson’s ratio
- EN 820-5:2009: Advanced technical ceramics – Thermomechanical properties of monolithic ceramics – Part 5: Determination of elastic moduli at elevated temperatures