Mechanical metamaterials are architected structures with unique functionalities, such as negative Poisson's ratio and negative stiffness, which are widely employed for absorbing energy of quasi-static and impact loads, giving improved mechanical response. Acoustic/elastic metamaterials, their dynamic counterparts, rely on frequency-dependent properties of their microstructure elements, including mass density and bulk modulus, to control the propagation of waves. Although such metamaterials introduced significant contribution for solving independently static and dynamic problems, they were facing certain resistance to their use in real-world engineering problems, mainly because of a lack of integrated systems possessing both mechanical and vibration attenuation performance. Advances in manufacturing processes and material and computational science now enable the creation of hybrid mechanical metamaterials, offering multifunctionality in terms of simultaneous static and dynamic properties, giving them the ability of controlling waves while withstanding the applied loading conditions. Exploring towards this direction, this review paper introduces the hybrid mechanical metamaterials in terms of their design process and multifunctional properties. We emphasize the still remaining challenges and how they can be potentially implemented as engineering solutions.

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The global energy sector is increasingly shifting towards sustainable solutions, driving the expansion of offshore wind farm installation to meet rising energy demands. The upcoming wind farms will be constituted of larger wind turbines, requiring larger monopile foundations. The monopiles are typically installed using impact hammers that blow several strikes until the monopile reaches a specific penetration depth. The installation process generates high underwater noise levels, posing severe risks to marine life, including hearing loss, behavioral changes, and even death.

Existing noise mitigation systems struggle to attenuate low-frequency noise below1000 Hz due to the mass law limitations of natural materials. This thesis proposes a design methodology for a novel metamaterial-based noise mitigation system–the meta-cushion–designed to brake this limitation. Placed between the monopile and the hammer, the meta-cushion filters out mechanical waves generated by the hammer’s blow that cause high underwater noise levels. The design methodology ensures adaptability to various monopile installation specifications, facilitating the implementation of the meta-cushion in real world pile driving cases.

The thesis begins with an analysis of underwater noise during pile driving and the limitations of existing mitigation solutions, emphasizing the need for novel approaches to reduce low-frequency noise. The analysis shows a strong relation between high underwater noise levels and the low-frequency vibrations of the monopile caused by the hammer’s blow. Then, the concept of metamaterials is introduced, highlighting how their unique properties contribute to reducing low-frequency vibrations. Based on the understanding of the underwater noise-vibration relationship, an initial meta-cushion design exhibiting such filtering features is defined.

The numerical modeling of the meta-cushion is then detailed, with finite element analyses being conducted to investigate the behavior of the meta-cushion when interacting with mechanical propagating waves. The attenuation capabilities are evaluated via analysis of dispersion-curve and transmission loss diagrams. To ensure both mechanical integrity and noise reduction under impact loads, a design optimization strategy is developed, from which genetic algorithm is used to investigate the trade-off behavior between mechanical and attenuation performances. Once verified, the meta-cushion’s functionality is experimentally validated through modal impact analysis and small-scale pile hammering tests, demonstrating effective noise reduction at frequencies below 1000 Hz as also indicated by the numerical results.

Finally, this thesis presents a systematic design methodology–encompassing meta-cushion design selection based on pile specifications, numerical verification, and experimental validation–that can be adapted to full-scale monopile installations. By integrating the meta-cushion into offshore pile driving operations, this work contributes to mitigating underwater noise and its environmental impact.@en

The use of optimization procedures for designing acoustic/elastic metamaterials (A/E MMs) has gained significant interest since they enable the efficient attainment of unique functionalities often contradicting. When it comes to vibration attenuation caused by mechanical stress waves, such as impact loads, the dynamic properties of A/E MMs are optimized so that their wave-control ability is maximized. However, the mechanical performance of A/E MMs during the propagation of such waves is normally not evaluated into the design optimization stages. This may compromise not only the load-bearing capacity of MMs, but also their ability in attenuating vibrations. To prevent such effects, we propose a design strategy that incorporates the stress analysis in the early design phase of A/E MMs subjected to an impact load. The effective mass density approach is applied, from which the vibration attenuation is identified at frequency ranges where the resonator moves out-of-phase in relation to the applied excitation. Regarding to the A/E MM mechanical behavior, maximum von Mises stress is calculated through the transient analysis of a unit cell array subjected to a dynamic load. A Pareto front shows a trade-off behavior between the A/E MM functionalities. With that, we emphasize the importance of incorporating the mechanical performance into the design stage of A/E MMs for vibration attenuation of structures undergoing high impact loads, such as installation of foundations by impact hammering. This brings A/E MMs closer to real applications involving energy filtering at specific frequencies from transient loads, designed in an optimized and efficient way.

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Underwater noise resulting from the monopile driving process can cause severe damage to marine wildlife, such as hearing injury, behavioral disturbance, or even death. Although current noise-attenuation techniques used in this process have shown a significant noise reduction at high frequency ranges, mitigating low-frequency noise is still extremely challenging. To address the problem, here we propose an elastic metamaterial-based structure composed of single-phase resonant structures. The proposed structure, which we call a meta-interface, is introduced between the monopile and the hammer and is used to remove energy from the input signal associated with high noise levels. To that end, we first identify the frequency ranges associated with high sound pressure levels, which were shown to be related to the monopile's eigenmodes. Then we design the meta-interface's periodic unit cells so that the elastic/acoustic waves at identified frequency ranges are attenuated. A meta-interface is then realized by replicating the unit cell along the monopile wall (matching the thickness) to form a ring-shaped layer, and then by stacking up these concentric layers. A frequency analysis of the pile driving system with the meta-interface shows that the new noise levels attain a significant attenuation in frequency ranges lower than 1000Hz. This demonstrates a novel solution for the low-frequency underwater noise issue during the hammering of offshore monopiles.

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The performance improvement of renewable energy sources has become nowadays a crucial topic to boost the energy transition. With respect to offshore wind farms, larger wind turbines are currently being designed to increase their power capacity. To support such turbines into the seabed, monopile foundations are mainly used, due to their low-cost manufacturing and transportation. Because of the increased wind turbines’ dimensions, larger monopiles are needed, which have shown to shift high sound waves to lower-frequency ranges during their installation. A precise prediction of the high noise level is necessary to properly design noise-cancelling devices, which prevent negative impacts on marine life. Because the evaluation of sound levels at a full-scale monopile installation is time-consuming and expensive, small-scale tests are being explored. Here we propose a small-scale test for investigating the high noise levels source. To that end, proper scaling laws and instrumentation have been adopted to assure the similarity between the small- and full-scale cases. The experimental results were also compared to a simplified numerical model. The findings of this work showed that investigating a priori the monopile’s features provides an underwater noise prediction, which can be used to design the upcoming noise-cancelling devices.@en

Elastic metamaterials have been mostly designed to control structural vibration for a specific frequency range. However, the stresses developed due to the wave propagation are not commonly consider in their design phase. This work proposes a multi-objective design optimization of a metamaterial-based interface to tune its dynamic characteristics, while keeping stresses below the allowed value.@en

Elastic metamaterials – man-made resonant structures exhibiting unusual functionalities – have shown promising results for controlling structural vibration, specially at a low frequency regime. Such functionalities rely on the presence of resonant bandgaps, which consists of a frequency band where waves cannot propagate in response to the out-of-phase motion of the local resonators. Usually, the contrast between the properties of different material phases in such resonators results on the resonant effect, however, the manufacturing of such multi-phase structures is challenging and can be a high-cost process. This work proposes a parametric investigation of an elastic metamaterial constituted by single-phase local resonators. The bandgap formation in such structure depends on the geometrical properties of the resonators, instead of the material parameters. This analysis allows us to understand which geometrical features are sensitive to the position of the resonant bandgaps and its width. Designing such single-phase resonators provides an alternative to manufacture low-cost structures for engineering application.@en

This paper discusses an elastic metamaterial for filtering energy from certain frequencies of an incoming wave. The unit cell, which is composed by a single material, is built to obtain a local resonance band gap. Since Bloch-Floquet's periodic condition is enforced to the unit cell, the dynamic characteristics of the metamaterial is obtained by only evaluating such structure. The attenuation mechanism is guaranteed by visualizing bandwidths in which the wave propagation is prohibited. Such bandwidths, denoted as band gaps, are observed in the band structure diagram of the unit cell and in the transmissibility loss over the metamaterial. The assemble of unit cells with different internal geometries results in a filtering mechanism. The proposed single-phase metamaterial could be applied in structures requiring vibration control for selective frequencies.

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