## 2 Experimental Methods  
### The Resonalyser Procedure  
The Resonalyser procedure is a multi-sample IET that extracts the resonance frequencies and damping ratios from the IRF measured on a thin rectangular test plate and two test beams. The orthotropic engineering constants are parameters in numerical models. The parameters are iteratively updated till the numerical models become digital twins of the test samples. The test beams are cut along two in-plane orthotropic directions (Figure 1). The length/width aspect ratio of the actual test plate _L_/W is adjusted according to the following formula (3):  
\[\frac{L}{W}=\sqrt[]{\frac{f_{1}L_{2}}{f_{2}L_{1}}} \tag{3}\]  
The frequencies f\({}_{1}\) and f\({}_{2}\) are associated with the fundamental bending vibration modes of the two test beams. L\({}_{1}\) and L\({}_{2}\) are the lengths of the beams. The aspect ratio _L_/W creates a so-called \"Poisson\" plate. The first three resonance frequencies of the Poisson plate are always associated with torsion, saddle, and breathing vibration modes (see Figure 3). If the length-to-thickness ratio of the plate is higher than 50, the vibration modes are nearly not influenced by transverse shear deformations. Since the vibration modes of the first three resonance frequencies are known, no OMA or EMA to identify the type of vibration mode sequence is necessary. An interesting property of a Poisson plate is that the saddle and breathing vibration modes are very sensitive to variations in Poisson's ratio, as shown by [PERSON] [39]. The knowledge of the type of vibration mode, together with the measured resonance frequencies, allows generating good starting values for the engineering constants G\({}_{12}\) and v\({}_{12}\) with the virtual field method (see, e.g., [PERSON] [40]). Detailed mathematical derivation can be found in [PERSON] et al. [41]. Good starting values for E\({}_{1}\) and E\({}_{2}\) are obtained by applying IET on the two test beams. After obtaining good starting values, further convergence to the final values can be performed using a sensitivity-based gradient method, as shown by [PERSON] [15]. The same three test specimens (two beams and one Poisson plate) used for the identification of the elastic part of the engineering constants are used for identification of the imaginary part. The IET provides the modal damping ratios associated with the fundamental bending vibration modes of the test beams and the modal damping ratios of the first three vibration modes of the Poisson plate. The decaying signal after impact (see Figure 2) is curve-fitted in the time domain with the formula:  
\[x(t)=Xsin(\omega t-\varphi)\cdot e^{-\xi\omega t} \tag{4}\]  
with vibration amplitude \(X\), circular frequency \(\omega\), phase \(\varphi\), and the modal damping ratio \(\xi\). The tangents \(\delta\) of the [PERSON]'s moduli \(E_{1}\) and \(E_{2}\) are found from the measured damping ratios of the freely suspended beam bending modes. \[ \begin{cases}2\xi_{Beam1}\\ 2\xi_{Beam2}^{2}\end{cases} =\begin{bmatrix}1&0\\ 0&1\end{cases} \begin{cases}tan\delta(E_{1})\\ tan\delta(E_{2})\end{cases}  \tag{5}\]  
Figure 3: Torsion (**a**), saddle (**b**), and breathing (**c**) vibration modes of a Poisson plate. The frequencies associated with these modes are mainly sensitive to the indicated engineering constants below the figures. A second set of Equation (6) yields the tangents \(\delta\) of the orthotropic stiffness matrix \(C_{ij}^{*}\) using the weighting coefficients \(G\) for the three first-plate vibration modes. \[\left\{ \begin{array}{l}2\xi_{Torsion}\\ 2\xi_{Saddle}\\ 2\xi_{Breathing}\end{array} \right\}=\left[ \begin{matrix}G_{11}&G_{12}&G_{13}&G_{ 14}\\ G_{21}&G_{22}&G_{23}&G_{24}\\ G_{31}&G_{32}&G_{33}&G_{34}\end{matrix} \right]\left\{ \begin{matrix}tan\delta (C_{11})\\ tan\delta(C_{22})\\ tan\delta(C_{66})\end{matrix} \right\} \tag{6}\]  
The detailed mathematical derivation of components G can be found in [PERSON] et al. [41].