## 4 Discussion  
Measuring damping is more challenging compared to resonance frequencies. Damping is associated with energy dissipation during a vibration cycle, whereas frequency is related to potential energy. Since the amount of dissipative energy per cycle is significantly lower than the potential energy, the measurement uncertainty for damping is higher, as illustrated in graphs (a-d) in Figure 14. The noise at each temperature step is also influenced by variations in the temperature settings of the climate chamber. At each temperature step, the climate chamber must rapidly achieve and uniformly maintain the new temperature within the cavity. The graphs indicate that damping is more sensitive to temperature variations than resonance frequencies. Damping is mainly related to the matrix, while stiffness is more dominated by the fibers. Temperature influences the matrix more than the fibers. An extensive review on damping in composites was presented by [PERSON] et al. [47]  
The resonance frequencies of the torsional, breathing, and bending modal shapes of the two test beams peaked at \(-20\)\({}^{\circ}\)C and decreased continuously to a minimum at \(60\)\({}^{\circ}\)C, which is typical for composites with a thermoplastic matrix. The damping ratios of these modal shapes exhibited maximum values between \(5\)\({}^{\circ}\)C and \(20\)\({}^{\circ}\)C, related to physical phenomena in polymers such as the glass transition temperature.  
After smoothing the damping ratio curves, the orthotropic engineering constants were identified using the Resonayser procedure. The results are presented in the graphs of Figure 15. As the measured resonance frequencies decreased continuously with increasing temperature and since modulus values E\({}_{1}\), E\({}_{2}\), and G\({}_{12}\) are directly related to the frequencies, the modulus values also decreased continuously with increasing temperature. [PERSON]'s modulus E\({}_{1}\) evolved from \(20.5\) GPa at \(-20\)\({}^{\circ}\)C to \(16.5\) GPa at \(60\)\({}^{\circ}\)C (\(20\)% reduction). [PERSON]'s modulus E\({}_{2}\) showed comparable values typical for bidirectionally reinforced composites and evolved from \(19\) GPa to \(14\) GPa (\(26\)% reduction). The in-plane shear modulus in bidirectionally reinforced composites G\({}_{12}\) is primarily dependent on the matrix, showing a dramatic reduction from \(5\) GPa at \(-20\)\({}^{\circ}\)C to \(1\) GPa at \(60\)\({}^{\circ}\)C (divided by a factor five!). [PERSON]'s ratio remained low and nearly constant across all observed temperatures. The tangent deltas of the modulus values were mainly determined by the measured damping ratios of the test samples, with maximum values observed between \(5\)\({}^{\circ}\)C and \(15\)\({}^{\circ}\)C. Tangent delta measures the phase delay between the sinusoidally varying stresses and strains at each point of the sample. This value was averaged out over the area of the samples, similarly to the modulus values obtained by the Resonayser procedure. The tangent delta of Poisson's ratio represents the phase angle between the strains in the two orthotropic directions for a sinusoidal vibration at the associated frequency. The developed measurement instrument will enable us to study material behavior in future research on different composite material types. The primary benefit of the proposed extension of IET over conventional IET is its capability to concurrently identify all orthotropic engineering constants.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Resonalyser Results at 20 \({}^{\circ}\)C** & **Results of Automated** \\  & **(see Figure 7)** & **Measurement at 20 \({}^{\circ}\)C** \\ \hline Young’s modulus E1 & 17.4 GPa & 17.4 GPa \\ \hline Young’s modulus E2 & 15.5 GPa & 15.5 GPa \\ \hline Poisson’s ratio & 0.097 & 0.10 \\ \hline Shear modulus G12 & 2.13 GPa & 2.10 GPa \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of results at 20 \({}^{\circ}\)C.