Green composite materials reinforced with natural fibers offer environmental benefits, such as reduced energy consumption and carbon emissions. However, their limited mechanical strength and poor aging resistance restrict their practical use. This study investigates sandwich structures composed of flax and cotton fiber reinforced polylactic acid (PLA) (FFRP and CoFRP) facesheets and balsa wood cores, in comparison with glass fiber reinforced PLA (GFRP) facesheets. Tensile and flexural tests were conducted before and after accelerated aging. The results reveal pronounced aging sensitivity in natural fiber reinforced facesheets, with tensile strength reductions of up to 26.3% in FFRP, primarily attributed to interfacial degradation and fiber deterioration. In contrast, GFRP exhibited minimal aging effects, showing a slight increase in tensile strength. Aging also weakened balsa wood cores, leading to reductions in tensile and flexural properties. For the sandwich structures, aging reduced flexural stiffness, while the flexural modulus remained relatively stable. Increased flexural strain capacity contributed to enhanced flexural strength, indicating a transition toward more ductile behavior in aged sandwich structures. This study offers a mechanics-based foundation for optimizing natural fiber composites for structural applications by addressing aging effects.
Abstract Overhead transmission lines are critical components of the power grid. Long-span conductors are particularly susceptible to wind-induced vibrations in service, which can lead to fatigue damage and strand breakage. A coupled conductor-damper model is developed using the transfer matrix method, and the resonance frequencies and energy-dissipation characteristics of the system are investigated. A full-scale laboratory vibration test setup is built and excitation experiments are carried out to measure the resonance frequencies, input powers, and vibration amplitudes of the conductor-damper system. The theoretical model reproduces the main trends observed in the experiments, including the two coupled resonance frequencies of the damped system and the variation of the wind input power and vibration amplitude with frequency. The results confirm the effectiveness and reliability of the proposed model for describing the dynamic characteristics of conductor-damper system.
Abstract Thermal energy management is fundamental to human life and a wide range of engineering systems, including electronic cooling, thermal protection, energy conversion, sensing, and aerospace technologies. Thermal metamaterials provide a powerful platform for manipulating heat flow beyond the capabilities of conventional homogeneous materials, while thermal invisibility cloaks offer the additional ability to protect a prescribed region without significantly disturbing the surrounding temperature and heat-flux fields. Their practical realization, however, is complicated by transient thermal effects and imperfect bonding between constituent materials. This study develops an analytical and numerical framework for a transient spherical thermal cloak with low-conducting (LC) and high-conducting (HC) imperfect interfaces. In the quasi-static limit, the coated object becomes thermally undetectable to leading order when two universal relations, referred to as the weak-invisibility conditions, are satisfied through cancellation of the monopole and dipole scattering modes. The monopole condition establishes the effective heat-capacity balance required to suppress transient thermal-storage mismatch, whereas the dipole condition ensures long-term conductive neutrality. The formulation accounts for temperature jumps across LC interfaces, and normal heat-flux jumps associated with surface conduction and interfacial thermal storage across HC interfaces. Transient finite-element simulations validate the analytical invisibility conditions and demonstrate substantial recovery of the undisturbed exterior temperature and heat-flux fields while retaining controllable thermal behavior within the cloaked region. The proposed framework advances the design of robust three-dimensional thermal metamaterials for applications requiring heat-flow regulation, thermal shielding, and reduced thermal detectability.
Abstract This study presents a theoretical, numerical, and experimental investigation into the vibration characteristics of rectangular elastic plates under simply supported boundary conditions. A three-dimensional state-space formulation is used to obtain the vibration characteristics of the simply supported plate. The calculated natural frequencies are compared with finite element simulations and experimental measurements, and the theoretical mode shapes agree well with the finite element results. In the experiment, a separable V-groove support system combined with elastic adhesive materials is proposed to emulate simply supported boundary conditions. This method provides vertical support to the plate edges while allowing rotational freedom and maintaining operational flexibility and good repeatability. The effects of different adhesive media are systematically examined. The results indicate that neutral silicone performs the best. For the thin plate, the discrepancies in natural frequencies between the experimental measurements and theoretical analysis are within 1.6%. For the thick plate, the proposed method accurately captures the first two natural frequencies, with errors <2.6%. The main contribution is a nondestructive experimental method for realizing simply supported boundaries, validated by analytical and numerical results.
Abstract Nonlinear dynamic instabilities in blunt-body atmospheric entry vehicles can lead to self-sustained oscillatory motion during transonic and low-supersonic flight, driven by unsteady shock–wake interactions. This study presents an integrated analytical and computational framework for assessing limit cycle oscillations in atmospheric entry capsules equipped with retropropulsive opposing jets. A reduced-order nonlinear pitch model is developed using the multiple time scales method to characterize the onset and relative severity of oscillatory behavior as a function of aerodynamic damping and nonlinear stiffness. These stability-governing parameters are obtained from steady and time-accurate computational fluid dynamics (CFD) simulations using a forced oscillation methodology. To ensure predictive accuracy, the multiple time scales framework is quantitatively validated against direct numerical integration of the governing nonlinear differential equation, demonstrating exact agreement in transient amplitude growth, phase characteristics, and steady-state saturation limits. The computational analysis examines the influence of jet pressure ratio and nozzle placement on static drag characteristics, unsteady aerodynamic response, and shock–wake topology for an Orion-class entry capsule. Results show that jet configurations and varying pressure ratios producing large upstream shock displacement can achieve a drag reduction of upto 50% but may increase flow compliance and promote oscillatory growth. In contrast, distributed jet configurations located near the capsule shoulder stabilize the bow shock, reduce wake unsteadiness, and introduce effective aerodynamic damping that reduces drag overshoots by 14%. Furthermore, the analytical time-history reconstruction is validated directly against the high-fidelity CFD dataset, successfully capturing the complex waveform distortions and aerodynamic phase lags generated by the jet plumes. The combined analytical-computational approach provides a computationally efficient tool for identifying stable design regimes and understanding the mechanisms governing nonlinear dynamic stability during atmospheric entry.
