Vibro-acoustic analysis of additively manufactured acoustic metamaterials

In the last decade it emerged the interest in new types of acoustic insulating materials, called acoustic metamaterials. These materials are composed by a host and inclusions and are arranged periodically or non-periodically in sub-wavelength elements called meta-atoms. Their inclusions and internal geometries can be manipulated to tailor the acoustic properties, reducing weight, and increasing at the same time their efficiency. Thanks to the high absorbing characteristics that they can achieve, their usage is of particularly interest as material of the core in sandwich panels of aerospace structures to reduce vibrations and noise inside passengers aircraft’s cabin. In addition, since the low frequency signals are difficult to be damped with conventional materials, their usage can guarantee a high transmission loss at low frequencies, obtaining a positive benefit on passengers’ comfort. The performances and efficiency of these materials are enhanced thanks to the new additive manufacturing techniques opposed to the conventional ones uncapable to pro- duce such complex internal geometries. The aim of this work is to study, produce and redesign micro-perforated sandwich panels of a literature case study to achieve high performances in the low frequency range, e.g., below 2000 Hz. Some geometrical parameters, such as perforation ratio and diameter of holes, were considered to realize different models and see the differences in the sound transmission loss. The models were produced by means of Fused Deposition Modelling using an Acrylonitrile Butadiene Styrene (ABS Plus p430) material on a commercial additive manufacturing system. Finally, the frequency response analysis was carried out with Mul2 software, based on the Carrera’s Unified Formulation (CUF) to understand the acoustic and structural properties of the material employed, analyzing the plates’ displacements and the TL results.

Mitigation of Train-Induced waves by resonant filled-trench (RFT)

The increase of railways near the urban areas is a significant cause of discomfort for inhabitants due to train-induced vibration and noise. Vibration characteristics can vary widely according to the train type: for high-speed trains, if train speed becomes comparable to the ground wave speed, the vibration level becomes significant; for freight trains, due to their heavier weight and lower speed, the vibration amplitudes are greater and propagate at a more considerable distance from the track; for urban tramways, although the vibration amplitude is relatively low, they can have a negative structural effect on the closest buildings [51]. Therefore, to dampen the vibration level, it is possible to carry out some interventions both on the track and the transmission path. This thesis aims to propose and numerically investigate a novel method to dampen the train-induced vibrations along the transmission path. The method is called "resonant filled-trench (RFT)" and consists of a combination of expanded polystyrene (EPS) geofoam to stabilize the trench wall against the collapse and drowned cylindrical embedded inclusions inside the geofoam, which act as a resonator, reflector, and attenuator. By means of finite element simulations, we show that up to 50% higher attenuation than the open trench is achievable after overcoming the resonance frequency of the inclusion, i.e., 35Hz, which covers the frequency contents of the train-induced vibration. Moreover, depending on the filling material used for the inclusions, trench depth can be reduced up to 17% compared to the open trench showing the same screening performance as the open trench. Also, an RFT with DS inclusion installed in dense sand soil shows a high hindrance performance (i.e., IL≥6dB) when the trench depth is larger than 0.5λ_R while it is 0.6λ_R for the open trench.

Mineral paragenesis and fluid inclusion constraints on the Torre di Rio skarn, Island of Elba

The historical iron ore deposits of eastern Elba held great importance for the region and were its primary source of iron. The Torre di Rio skarn, despite its easily accessible outcrop and vicinity to the larger Rio Marina deposit, was never properly characterized. The results of petrographic and microthermometric study presented in this work provide new constraints on the Torre di Rio skarn. Mineral assemblage of ilvaite, calcite, quartz, iron oxides and sulphides combined with textural evidence indicate that Torre di Rio skarn does not fit into classical skarn model. The complex paragenetic sequence and overlapping of skarn and ore mineralogy is result of fast formation at relatively low temperatures evidenced by the silicon enrichment and pervasive nature of limonite alteration. Hematite-magnetite textural relationship points to boundary conditions of the ore fluid in terms of oxygen fugacity. Eutectic temperatures range from -16 to -33 °C indicating complex fluids. Calculated salinities range from 1.4 to 17.4 wt% NaCleq suggesting multiple fluids of different compositions. Total homogenization temperatures vary from 330 °C to 150 °C with both homogeneously and heterogeneously trapped FIAs. Ore deposition is concentrated where skarn formation was controlled primarily by phase separation during boiling. Calculated fluid pressure at boiling suggest shallow formation depth of a few hundred meters and constrains maximum temperature of ore deposition to c. 260 °C. This work suggest that relatively low salinities of fluid inclusions could indicate dominant marine origin of the hydrothermal fluids that were activated by the Porto Azzurro pluton emplacement and that scavenged Fe from sedimentary host rocks. During boiling at shallow depths and decreasing iron solubility, these fluids started precipitating Fe-minerals at Torre di Rio mineralization. Mixing with batches of more saline fluids at around 236 °C increased salinity abruptly and marked the end of ore deposition.