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The main scope of this work is to study the effect of embedding a periodic pattern inside a porous material, in order to passively improving its acoustic performance in terms of sound transmission loss. A contemplated application is the improvement of classical aeronautical soundproofing packages. In order to reach this goal, numerical models of an acoustic package including periodic patterns are implemented using the finite element method and the Transfer Matrix Method. Firstly, some of the proposed configurations are experimentally tested, providing a comparison and validation of the obtained numerical results. Afterwards, several configurations of inclusions are numerically studied, and incorporate hollow cylindrical inclusions, half-cut hollow cylindrical inclusions and cylindrical Helmholtz resonators. The improvements in terms of transmission loss, essentially brought by a periodicity peak, are evaluated under plane wave excitation with various incidence angles. The main novelties of the present work are represented by an experimental validation of the proposed acoustic meta-materials that were only numerically studied in previous works. The effect of the inclusion of a periodic pattern of Helmholtz resonators inside the foam core is also considered. The presented numerical results are also evaluated for different incidence angles of an exciting acoustic plane wave. [ABSTRACT FROM AUTHOR]

Subcritical damage in sandwich panels refers to damage that occurs in the skins or core of the panel due to loading below the ultimate strength of the materials, and it can reduce the strength and durability of sandwich panels, leading to premature and unpredicted failure. In this work, a computational tool was developed for designing sandwich structures and optimizing their subcritical impact response. The model was validated experimentally on sandwich panels comprising an aluminium honeycomb core with carbon fiber reinforced epoxy skins. The study investigates subcritical damage in aluminium honeycomb and CFRP sandwich panels through mechanical experiments and finite element simulations. A novel computational tool was developed to optimize the impact response of these structures. Experimental validation was performed using standardized tests, and numerical simulations were conducted using Abaqus. Key findings indicate that the developed tool accurately predicts damage and stiffness reduction, offering significant potential for optimizing sandwich panel designs. [ABSTRACT FROM AUTHOR]

The aim of this work is to provide in-depth examination of metal-composite hybrid gears that successfully merge significant weight reduction with improved or maintained Noise, Vibration, and Harshness (NVH) properties. A nonlinear finite element method was employed for designing hybrid gears, optimizing mass reduction and Static Transmission Error (STE) as main source of excitation. Experimental methods, including static and dynamic transmission errors, run-ups, and transmitted vibrations were analyzed to compare hybrid gear pairs with their steel counterpart. Results demonstrated significant discrepancies in vibration characteristics between standard and hybrid gear pair configurations. Notably, the hybrid configurations, particularly the innovative solution incorporating a viscoelastic layer, showed superior performance in vibration dampening despite being 50% lighter than its steel counterpart. The research underscores the inherent differences between hybrid and standard gears, relating the observed behaviors to static and dynamic excitations and their impact on the energy distribution between meshing harmonics. Findings suggest that hybrid gear pairs offer a potential pathway for improving noise and vibration comfort in mechanical systems without compromising on weight reduction. [ABSTRACT FROM AUTHOR]

The present research activity aims to bridge the gap between gear geometry tolerances, eventual test rig uncertainties, and the resulting gear excitations represented by the static transmission error (STE) as the main source of whining noise. This article offers a detailed experimental investigation addressing the impact of manufacturing errors in the form of lead slope and profile slope errors. One of the major contributions of this article is related to the number of carried measurements, including six spur gear pair configurations. While most literature focuses on gear defects related to wear, cracks, or pitting, this study emphasizes the need to consider gear micro geometry tolerances to minimize vibroacoustic response at the source. The experiments under static conditions are conducted for various operating conditions and compared with numerical computational schemes. Latin Hypercube sampling is selected to compute the peak-to-peak static transmission error bounds. Results yield reliable prediction of the gear STE response even in the presence of deviations. The manufacturing errors cause different trends of STE response, with up to 8 μm difference at low torque. The robustness of the predictive numerical tool is validated, which enables further confident parametric studies. Finally, the dynamic behavior is investigated. Important differences are detected for gears having the same macro geometry and different manufacturing errors. The meshing excitations resulting from gear uncertainties lead to notable fluctuations in the dynamic responses. [ABSTRACT FROM AUTHOR]

This study proposes a novel semi-active vibration control technique for mechanical transmission systems. The backbone of current research technology is the adaptive stiffness and damping properties of metal rubber (MR) which plays the role of tuning to the vibrational system eigenmodes. The adaptive properties of MR material were proved by a previous study, but the present research investigates the effect of this feature on the structural response and designs an MR-based semi-active vibration controller for industrial application. For this purpose, the semi-active control strategy is created based on an optimal tuning using experimental harmonic test results. The control strategy is applied in practice using a linear actuator controlled by a double pole double throw (DPDT) relay, and National Instruments hardware and software for reading data and writing the control task. The isolator attachment position is defined through the analysis of eigenmode shapes that are predetermined by numerical simulation. A maximum isolation rate of 55.17% is measured near the vibration isolator position. It is observed that the isolation position has a prominent effect on structure vibration behavior where the elastic wave has been reduced in amplitude by passing the isolator position. The effectiveness of the MR-based semi-active control approach is eminent and its application for vibration isolation of commercial mechanical transmission systems could be studied in the future. [ABSTRACT FROM AUTHOR]

In transport engineering applications, flow-induced vibrations is an interesting topic to address since it may negatively affect the operation and the response of the system. Wind tunnel facilities are mandatory to test the structure design efficiency or to analyse new material performances under aerodynamic load. However, these experimental tests can be expensive and take a long time to set up and operate; hence, alternative methods for the reproduction of the structural response to a turbulent boundary layer excitation are required to accelerate and improve the experimental setups and provide more data for uncertainty analysis. In this paper, an alternative approach, the eXperimental Pseudo-Equivalent Deterministic Excitation method (X-PEDEM), is here extended for applications in the low frequency domain. An investigation about the applicability of the method in the low frequency domain is conducted, together with an analysis of its main properties. The reliability of the method is then tested numerically by considering different conditions: two different panels, two different boundary conditions, and different asymptotic flow velocities are considered. [ABSTRACT FROM AUTHOR]

The need to reduce pollutant emissions leads the engineers to design new aeronautic combustors characterized by lean burn at relatively low temperatures. This requirement can easily cause flame instability phenomena and consequent pressure pulsations which may seriously damage combustor’s structure and/or compromise its fatigue life.