Resonant metamaterials for the control of Rayleigh waves

Since their emergence, locally resonant metamaterials have found several applications for the control of surface waves, from micrometer-sized electronic devices to meter-sized seismic barriers. The interaction between Rayleigh-type surface waves and resonant metamaterials has been investigated through the realization of locally resonant metasurfaces, thin elastic interfaces constituted by a cluster of resonant inclusions or oscillators embedded near the surface of an elastic waveguide. When such resonant metasurfaces are embedded in an elastic homogeneous half-space, they can filter out the propagation of Rayleigh waves, creating low-frequency bandgaps at selected frequencies. In the civil engineering context, heavy resonating masses are needed to extend the bandgap frequency width of locally resonant devices, a requirement that limits their practical implementations. In this dissertation, the wave attenuation capabilities of locally resonant metasurfaces have been enriched by proposing (i) tunable metasurfaces to open large frequency bandgaps with small effective inertia, and by developing (ii) an analytical framework aimed at studying the propagation of Rayleigh waves propagation in deep resonant waveguides. In more detail, inertial amplified resonators are exploited to design advanced metasurfaces with a prescribed static and a tunable dynamic response. The modular design of the tunable metasurfaces allows to shift and enlarge low-frequency spectral bandgaps without modifying the total inertia of the metasurface. Besides, an original dispersion law is derived to study the dispersive properties of Rayleigh waves propagating in thick resonant layers made of sub-wavelength resonators. Accordingly, a deep resonant wave barrier of mechanical resonators embedded inside the soil is designed to impede the propagation of seismic surface waves. Numerical models are developed to confirm the analytical dispersion predictions of the tunable metasurface and resonant layer. Finally, a medium-size scale resonant wave barrier is designed according to the soil stratigraphy of a real geophysical scenario to attenuate ground-borne vibration.

Advanced electrospun nanofibrous mats for hindering delamination, improving damping and for sensing of composite laminates

Carbon Fiber Reinforced Polymers (CFRPs) display high specific mechanical properties, allowing the creation of lightweight components and products by metals replacement. To reach outstanding mechanical performances, the use of stiff thermoset matrices, like epoxy, is preferred. Laminated composites are commonly used for their ease of manipulation during object manufacturing. However, the natural anisotropic structure of laminates makes them vulnerable toward delamination. Moreover, epoxy-based CFRPs are very stiff materials, thus showing low damping capacity, which results in unwanted vibrations and structure-borne noise that may contribute to delamination triggering. Hence, searching for systems able to limit these drawbacks is of primary importance for safety reasons, as well as for economic ones. In this experimental thesis, the production and integration of innovative rubbery nanofibrous mats into CFRP laminates are presented. A smart approach, based on single-needle electrospinning of rubber-containing blends, is proposed for producing dimensionally stable rubbery nanofibers without the need for rubber crosslinking. Nano-modified laminates aim at obtaining structural composites with improved delamination resistance and enhanced damping capacity, without significantly lowering other relevant mechanical properties. The possibility of producing nanofibers nano-reinforced with graphene to be applied for reinforcing composite laminates is also investigated. Moreover, the use of piezoelectric nanofibrous mats in hybrid composite laminates for achieving self-sensing capability is presented too as a different approach to prevent the catastrophic consequences of possible structural laminate failure. Finally, an accurate, systematic, and critical study concerning tensile testing of nonwovens, using electrospun Nylon 66 random nanofibrous mats as a case study, is proposed. Nanofibers diameter and specimen geometry were investigated to thoroughly describe the nanomat tensile behaviour, also considering the polymer thermal properties, and the number of nanofibers crossings as a function of the nanofibers diameter. Stress-strain data were also analysed using a phenomenological data fitting model to interpret the tensile behaviour better.

Innovative electrospun nanofibers for improving delamination resistance and damping of CFRP laminates

Carbon Fiber Reinforced Polymers (CFRPs) are well renowned for their excellent mechanical properties, superior strength-to-weight characteristics, low thermal expansion coefficient, and fatigue resistance over any conventional polymer or metal. Due to the high stiffness of carbon fibers and thermosetting matrix, CFRP laminates may display some drawbacks, limiting their use in specific applications. Indeed, the overall laminate stiffness may lead to structural problems arising from their laminar structure, which makes them susceptible to structural failure by delamination. Moreover, such stiffness given by the constituents makes them poor at damping vibration, making the component more sensitive to noise and leading, at times, to delamination triggering. Nanofibrous mat interleaving is a smart way to increase the interlaminar fracture toughness: the use of thermoplastic polymers, such as poly(ε- caprolactone) (PCL) and polyamides (Nylons), as nonwovens are common and well established. Here, in this PhD thesis, a new method for the production of rubber-rich nanofibrous mats is presented. The use of rubbery nanofibers blended with PCL, widely reported in the literature, was used as matrix tougheners, processing DCB test results by evaluating Acoustic Emissions (AE). Moreover, water-soluble electrospun polyethylene oxide (PEO) nanofibers were proposed as an innovative method for reinforcing layers and hindering delamination in epoxy-based CFRP laminates. A nano-modified CFRP was then aged in water for 1 month and its delamination behaviour compared with the ones of the commercial laminate. A comprehensive study on the use of nanofibers with high rubber content, blended with a crystalline counterpart, as enhancers of the interlaminar properties were then investigated. Finally, PEO, PCL, and Nylon 66 nanofibers, plain or reinforced with Graphene (G), were integrated into epoxy-matrix CFRP to evaluate the effect of polymers and polymers + G on the laminate mechanical properties.