The objective of this project is to theoretically and experimentally advance the field of vibration mitigation employing novel engineered devices and materials to achieve seismic and shock protection in buildings and sound-proof barriers for railways and flutter control in bridges.
To this end, hysteretic tuned mass dampers (TMD) made of wire ropes of different materials (steel, shape memory material, carbon nanotube nanocomposite) and shock absorbers will be employed using a unique combination of analytical, numerical and experimental tools coupled with genetic-type optimization algorithms to drive the search of the optimal TMD design parameters while resorting to nonlinear mechanical models subject to various excitation scenarios. A new TMD architecture for bridges will be developed proposing a metamaterial deck concept which integrates multiple arrays of TMDs. Contrary to conventional architectures making use of a few TMDs, the metamaterial bridge deck concept aims to strategically distribute a large number of TMDs away from the torsional center in order to exert point-wise distributed forces and moments counteracting the negative damping effects of aerodynamic lift forces and moments which cause flutter or other self-excited oscillations and instabilities. To this end, a new concept of nanocomposite TMD is proposed leveraging on the exploitation of the hysteretic properties of carbon nanotube (CNT) nanocomposite materials investigated by this group in previous projects. The softening stick-slip interfacial dissipation that takes place at the interfaces between CNTs and polymer chains combined with geometric nonlinearities allow tuning of the TMD nonlinear frequency to the frequency of the bridge/structure as well as tuning of the device damping to optimally control the TMD phase relative to the bridge oscillations. The constitutive parameters of the nanostructured materials will be identified via a new technique based on nonlinear guided wave propagation.
This project will lead to a theoretical and technological advancement in the field of vibration mitigation by developing, investigating and testing enhanced hysteretic TMDs and shock absorbers for a large variety of structural engineering applications (seismic control, sound-proof barriers, bridge flutter control).
Classical TMDs suffer from the limitations of constituting a weight penalty for the structure to be controlled, as well as of being operationally effective only in narrow frequency excitation bandwidths. The hysteretic TMDs here proposed are conceived with a novel semi-active design which exploits strategically nonlinearities to overcome the above limitations.
The first proposed semi-active TMD made of steel/NiTiNol wire ropes as rheological components (the SMA tension is changed by letting current flow in the SMA wires) can allow the high stiffness and damping tunability to be largely leveraged for enhanced frequency/phase tuning with both linear and nonlinear dynamic responses. This is expected to make the TMD suitable for vibration mitigation in multi-story buildings subject to seismic excitations. While the use of linear TMDs is not considered effective for seismic control, preliminary works of this team showed that hysteretic TMDs can be optimized also for applications in the field of earthquake engineering as well as for sound-proof barriers employed in high-speed railways. The increased running speeds of trains imply higher pressure distributions on the barriers. Until the last decade, only a few countries focused on this aspect and evaluations were made in absence of codes or specific requirements when considering nonlinear dynamic effects induced by the train passage. In addition, the proposed TMD architecture will improve the device compactness and save weight and stroke making the application of such hysteretic devices amenable in the field of bridge flutter control. The new compact anti-flutter TMD can be introduced as a repeated, periodic element in the metamaterial deck structure of the bridge. The exploitation of multiple TMDs is a further novelty with respect to current use of one or a few TMDs to avoid or delay the onset of dynamic instability phenomena (flutter, self-excited oscillations, etc.) in suspension bridge structures. The distribution of the TMDs and their parameters optimization will be carried out adopting the most recent analytical/computational strategies and genetic-type optimization algorithms taking into account the nonlinearities of both the TMDs and the bridge. To address the issue of introducing several distributed oscillating heavy masses along the deck span, the rheological components of the proposed anti-flutter TMD, besides compactness, is made of a new lightweight material. Multifunctional, highly-damped and flexible CNT nanocomposites will be employed for the first time as wire ropes and flexures replacing the most common polymeric/elastomeric materials in TMDs. The benefit of exploiting CNT nanocomposites stems from their capability to provide tunable energy dissipation in a large frequency/strain amplitudes bandwidth, thanks to nano-frictional mechanisms occurring at the nanofiller/polymer interfaces. Novel interface engineering techniques, developed for the first time by the PI and co-workers, will be exploited to tailor the material damping capacity of the nanocomposite components of the hysteretic devices. Moreover, the resulting nonlinear constitutive response of the nanocomposites will be experimentally investigated and identified via guided waves propagation, a technique well consolidated for linearly elastic media but only recently applied also to nanostructured hysteretic media.
As a result, the nanocomposite anti-flutter TMD combines geometric nonlinearities - due to its operating principles - with lightweight nonlinear materials.
A step forward will be also made in the field of vibro-impact protection for isolated structures undergoing large oscillations and accelerations when subject to seismic excitations. In this framework, different configurations of double unilateral constraint/bumpers will be experimentally investigated to seek optimal designs.
All targets set in this project aim to provide a substantial and useful progress in the mentioned engineering technical fields of interest supported by the use of analytical models which handle well the nonlinearities of the investigated structural systems. The nonlinear devices design will be optimized through genetic algorithms and the effective TMDs and shock absorbers functionalities will be experimentally tested in the context of in-scale structures or in-situ.