## 2 Experimental Methods
### The Resonalyser Procedure
The knowledge of \(tan\delta(E_{1})\) and \(tan\delta(E_{2})\) and relationship (1) allows solving for \(tan\delta(v_{12})\) and \(tan\delta(G_{12})\). For room temperature measurements, the test plates and beams and beams are freely suspended on a frame. A small accelerometer fixed on the test samples with bee wax is connected to a signal conditioner box and a data acquisition card through a USB connection with a PC (see Figure 4). The Resonalyser software guides the measurement of the IRF and the creation of the digital twins (see Figure 5). The numerical models of the beams and plate are based on the finite element method. These models must replicate the geometry and mass of the physical specimens. The sole unknown parameters within the models are the engineering constants, which are iteratively refined until the computed resonance frequencies, derived from solving an eigenvalue problem, match with the experimentally measured values. The models that possess the  
Figure 4: Resonalyser room temperature setup: portable flight case with an adjustable aluminum suspension frame and a PC with automated measuring and identification software (USB connection with signal conditioning box). Figure 5: The created digital twin can reproduce the IRF. accurate engineering constants at the conclusion of the iterative process are referred to as digital twins. Due to the low excitation level of the IET, the obtained values for the engineering constants of the Resonalyser are situated near the origin of the stress-strain curve (Figure 6). The global uncertainty of the results for a mixed numerical experimental method (MNET) receives contributions from the measurement errors and inaccuracies of the numerical model (digital twin). Even with simple affordable equipment, the measured frequencies with the IET are typically accurate within 0.1%. The accuracy of a finite element model (FEM) of a thin beam or a thin plate can also be kept easily lower than 0.1%. The real source of uncertainties of an MNET for composite materials is the sample heterogeneities (thickness, material stiffness, density, and local anisotropies). The Resonalyser is a multi-sample method and therefore can estimate the uncertainty caused by heterogeneities in the different samples. The digital twins use the same engineering constants for the computation of the resonance frequencies of the beams and the plate. Perfect sample homogeneity would lead to perfect agreement between experimental and computed resonance frequencies (within the limit of the 0.1% experiment and 0.1% model uncertainty). The software gives high confidence to the plate measurements and less to the beam measurements, resulting in nearly perfect matches for the plate and--in case of heterogeneities--lesser-quality matches for the beams. The deviations for the beams in the Resonalyser software are used to estimate the uncertainties of the identified engineering constants using the sensitivity matrix method [16]. All isotropic and orthotropic materials can be tested. The precision of the test outcomes is critically dependent on the quality of the test specimens, which must exhibit flat, homogeneous material properties and maintain a consistent thickness. The plates and beams are modeled as thin samples, necessitating a length-to-thickness ratio preferably exceeding 50. Identification with the Resonalyser procedure can be carried out with three plate frequencies (torsion, saddle, and breathing modal shapes) or with only two (torsion and breathing). An example of an output screen of the Resonalyser software is given below (Figure 7). More examples and illustrations can be found on the Resonalyser website [42]. Processing three frequencies is recommended in case the heterogeneity of the plate is high. Two frequencies are sufficient for good-quality plates (see Figure 7). Figure 7 shows the identification results of a PA6/Organo glass composite at 20 \({}^{\circ}\)C using two plate frequencies and two beam frequencies.