The Role of Three-Directional Functionally Graded Materials in Controlling Supersonic Flutter in NEMS-Enhanced Structural Components of Tunnel Systems

For the first time, this study explores the role of viscoelastic three-directional piezoelectric functionally graded materials (3D-PFGMs) in suppressing supersonic flutter in nanoelectromechanical systems (NEMS)-enhanced structural components of advanced tunnel systems. By incorporating piezoelectric materials with mechanical properties that vary spatially in three orthogonal directions, the adaptive structural behavior of tunnel systems under extreme aerodynamic environments can be significantly optimized. The analysis leverages the nonlocal strain gradient theory (NSGT) to capture size-dependent effects, particularly relevant at the nanoscale, and employs Hamilton’s principle to derive the governing nonlinear equations of motion. A harmonic balance solution procedure is utilized to solve the resulting equations and to characterize nonlinear flutter boundaries of tunnel systems. Additionally, this study emphasizes the importance of nonlinear group and phase velocity analysis in understanding wave propagation phenomena under supersonic airflow conditions. The integration of 3D-FGMs with piezoelectric functionality enhances the tunability of the system, offering improved control over vibrational instabilities. Results reveal that material gradation along three directions provides a superior capability for modulating stiffness and mass distribution, thereby extending the stability region and delaying the onset of flutter. These findings contribute valuable insights into the development of smart, resilient infrastructure systems where NEMS-integrated, directionally graded materials can ensure both structural integrity and aerodynamic performance in high-speed environments such as tunnel systems exposed to supersonic flow.