## 4 Conclusions
The Rayleigh-Ritz formulation is firstly validated with experimental data taken from the literature and then, for each retained material, it is used to calculate the frequencies for the different material orientations and mass positions, which constitute the reference values. Then, the elastic properties are handled as the design variables of the optimization process, which minimizes the discrepancy between the resonant frequency calculated at each iteration and the corresponding reference value. The results show that according to the material orientation, the mass differently affects the first resonant frequency in two locations. When dealing with anisotropic materials, the isolines of the modal shape are not parallel to the clamped boundaries, as in the case of an orthotropic material whose axes of orthotropy are not parallel to the boundaries. Particularly, in the case of the unidirectional laminate, the modal shape follows the rotation of the material with respect to the clamped boundaries. Hence, the discrepancy between the reductions of the first resonant frequency for the two mass spots is a measure of the material anisotropy. Thus, information related to the modal shape can be obtained only through a concentrated mass, without the need for expensive equipment. Furthermore, results show that by increasing the number of information, particularly by retaining the modal shape, the accuracy in the assessment of the four elastic properties is increased. In particular, it is shown that even in the numerical analysis here pursued with the further information of the modal shape, the shear modulus and the Poisson's ratio can still present very limited discrepancies with respect to the nominal values. In particular, these are 0.3% for the shear modulus and 3% for the Poisson's ratio. Instead, when the information related to the modal shape is not accounted, the discrepancies reach the values of 12% for the shear modulus and 76% for the Poisson's ratio. Indeed, the sensitivity of the first resonant frequency on these parameters is low but not null. Therefore, by increasing the information retained within the optimization problem, the allowable solutions of the inverse problem accordingly decrease, thus allowing also estimating the two low-influencing parameterswith very limited discrepancy. Thus, the optimization is able to catch all the four elastic constants for all the considered materials without the need of limiting the domain of the elastic properties. The promising results of this study can increase the robustness of the previously described experimental methodology for the nondestructive, quantitative, and local assessment of damage level, i.e., of residual elastic properties, in composite laminates. The local characterization of the health state of composite components allows evaluating the quality of a manufacturing process or damages due to in-service loads with consistently enhanced confidence. This research received no external funding. This work has been supported by the Research Project AMICO (code ARS01_00758) funded by the Italian Ministry of Education, University and Research. The author declares no conflict of interest.