Impulse excitation technique
[2] The measurement principle is based on tapping the sample with a small projectile and recording the induced vibration signal with a piezoelectric sensor, microphone, laser vibrometer or accelerometer.
Dedicated software will determine the resonant frequency with high accuracy to calculate the elastic properties based on the classical beam theory.
[3] Different resonant frequencies can be excited dependent on the position of the support wires, the mechanical impulse and the microphone.
For predefined shapes like rectangular bars, discs, rods and grinding wheels, dedicated software calculates the sample's elastic properties using the sample dimensions, weight and resonant frequency (ASTM E1876-15).The first figure gives an example of a test-piece vibrating in the flexure mode.
Considering the importance of elastic properties for design and engineering applications, a number of experimental techniques are developed and these can be classified into 2 groups; static and dynamic methods.
IET is mostly used in research and as quality control tool to study the transitions as function of time and temperature.
A material is called orthotropic when the elastic properties are symmetric with respect to a rectangular Cartesian system of axes.
In case of a two dimensional state of stress, like in thin sheets, the stress-strain relations for orthotropic material become: E1 and E2 are the Young's moduli in the 1- and 2-direction and G12 is the in-plane shear modulus.
The figure above shows some examples of common orthotropic materials: layered uni-directionally reinforced composites with fiber directions parallel to the plate edges, layered bi-directionally reinforced composites, short fiber reinforced composites with preference directions (like wooden particle boards), plastics with preference orientation, rolled metal sheets, and much more... Standard methods for the identification of the two Young's moduli E1 and E2 require two tensile, bending of IET tests, one on a beam cut along the 1-direction and one on a beam cut along the 2-direction.
Major and minor Poisson's ratios can be identified if also the transverse strains are measured during the tensile tests.
The non destructive Resonalyser procedure allows a fast and accurate simultaneous identification of the 4 Engineering constants E1, E2, G12 and v12 for orthotropic materials.
The question is now how to extract the orthotropic Engineering constants from the frequencies measured with IET on the beams and Poisson plate.
Problems with inverse methods are: · The need of good starting values for the material properties · Are the parameters converging to the correct physical solution?
The requirements to obtain good results are: In the case the Young's moduli (obtained by IET) are fixed (as non variable parameters) in the inverse method procedure and if only the Poisson's ratio v12 and the in-plane shear modulus G12 are taken as variable parameters in the FE-model, the Resonalyser procedure satisfies all above requirements.
