Variational mode decomposition framework for modal shape visualization of honeycomb sandwich structures for full-field vibration measurements in non-uniform temperature fields

Thermal effects in hypersonic vehicles, such as severe aerodynamic heating, can greatly influence their thermodynamic behavior. These effects can cause uneven temperature distributions and drastic vibrations, which in turn impact the vibration characteristics of the vehicle structure. To address this issue, a comprehensive approach was developed to study the behavior of honeycomb sandwich structures in high-temperature environments. Firstly, a full-field thermal vibration testing system was established, which allowed for free-boundary multi-temperature region heating at 900 degrees C while applying unconnected excitation. Through this setup, we were able to obtain modal frequencies and modal shapes of the honeycomb sandwich structures under hightemperature conditions. To accurately simulate the heat transfer distribution of the honeycomb sandwich structure, the Q-learning algorithm was employed, which proved effective in achieving accurate and detailed predictions of the full-field heat transfer distribution. Furthermore, for the purpose of further enhancing the accuracy of the finite element model, a multi-state non-uniform temperature field model updating method was utilized. This updated model accounted for the influence of the multi-temperature region heating, resulting in a more reliable representation of the structure's behavior. Additionally, a variational mode decomposition (VMD) framework was applied to visualize and analyze the modal shapes of the honeycomb sandwich structures. By decomposing the time-domain signals with VMD, it is possible to efficiently obtain the modal shapes of the output-only response signals. Experimental results demonstrated that the identification method used in this study produced modal frequencies and modal shapes that closely matched those obtained using the PolyMAX method at room temperature. Therefore, the proposed approach offers an effective means of identifying the modal characteristics of structures subjected to non-uniform temperature fields, particularly in output-only scenarios.