Passive mechanical damping through bioinspiration and hierarchical structuring
Embargoed until 2022-12-29
- Doctoral Thesis
Mechanical vibration is widespread in modern society and manufacturing technologies. Vibrations typically occur in systems with dynamically moving parts. The development of materials capable of damping mechanical vibrations is crucial to prevent structural damage and to enable a reliable and seamless functioning of such systems. At the same time, materials need to be stiff, strong and light to also save resources and meet environmental demands. However, stiffness and damping are often antagonistic properties. Amongst the vast selection of construction materials, composites provide attractive potential solutions to this challenge because of their excellent property-to-weight ratio, large design freedom and in general good ability to suppress unwanted mechanical vibrations. While synthetic composites with high damping performance have been developed, current solutions are still limited by the trade-offs between stiffness and the energy dissipation mechanisms required for damping. Recent modelling studies have shown that biological composites known to combine stiffness and damping, such as nacre and bone, might provide powerful design principles for the manufacturing of synthetic materials with enhanced damping performance. Inspired by the evolved design of biological composites, the goal of this thesis is to study the damping behaviour of nacre-like staggered composites and to enhance the dissipation properties of natural fibre composites. To the best of our knowledge, the damping properties of bio-inspired composites featuring reinforcing elements on a similar length scale as observed in biological systems, is studied for the first time. Furthermore, the microstructure of highly anisotropic natural fibre composites is systematically altered to provide materials that offer a wider transverse design space without reducing the inherently high damping properties in the direction of the fibre. Polymers reinforced with a nacre-like staggered arrangement of stiff inorganic platelets were first prepared using a previously reported magnetic alignment technique. These bio-inspired composites contained up to 30 vol% of aligned platelets distributed in a polymer matrix consisting of either epoxy or poly(methyl methacrylate). Mechanical characterisation of the nacre-inspired composites showed that the loss modulus, which is defined as the damping figure of merit, can be systematically increased by a factor of 5 upon the addition of platelets to the polymer matrix. Interestingly and counter-intuitively, the composite’s loss factor remained nearly unaffected. The rise of the damping performance with increasing platelet volume fraction is explained on the basis of micromechanical models developed for compliant materials reinforced with discontinuous stiff elements. Such models can be used to describe the effect of two different structural parameters, namely the platelet volume fraction and the aspect ratio, on the damping behaviour of the bio-inspired composite. This analysis shows the importance of replicating the design principles rather than copying per se the microstructure of the biological material. To leverage the high damping characteristics of flax fibres, natural composites comprising flax-based laminates reinforced by chopped carbon fibres were also developed and investigated. To achieve enhanced mechanical properties and damping behaviour, ultra-high modulus carbon fibres were introduced into the flax fibre laminate as discontinuous fillers blended into the matrix and aligned during composite processing. Using matrix suspensions with up to 15 vol% of carbon fibres, a local fibre fraction of 48 vol% can be achieved within the flax fibre laminate. The alignment of such high volume fraction of carbon fibres orthogonally with respect to the principal direction of the flax fibres leads to a composite flexural stiffness that is 1.46-fold higher compared to the pristine coupons, without compromising the longitudinal performance. Furthermore, the damping properties are significantly enhanced, which is manifested by an increase of the loss modulus of the composite up to 2.6-times relative to the laminate without carbon fibres. Such carbon fibres are also introduced into printable inks in order to deposit three-dimensional reinforcing ribs onto pre-fabricated flax fibre composites. Two-layer ribs with a carbon fibre volume fraction of 10 vol% were found to increase the bending stiffness of the composite by 60% and 600% for co-aligned and orthogonal ribs, respectively. Finally, the role of microstructural features on the viscoelastic response of nacre and nacre-like composites was investigated by creating brick-and-mortar structures with tuneable density of mineral bridges and nanoasperities. By keeping the platelet volume fraction of our nacre-like composite constant, we show that the damping performance is enhanced by increasing the fraction of such microstructural features. Samples exhibiting the highest fractions of mineral bridges and nanoasperities display 150% higher loss modulus and 31% higher storage modulus compared to composites displaying a low fraction of these reinforcing elements. This indicates the importance of nanoscale structural features in controlling the stiffness and the energy dissipating behaviour of nacre-inspired composites. In summary, the research presented in this thesis provides useful guidelines for the design and fabrication of composite materials with ultra-high damping performance. Composites that exploit the inherent hierarchical structure of natural fibres or replicate design principles of nacre and bone can reach damping response that significantly exceed the properties of biological and state-of-the-art materials. The implementation of these design strategies in future composites should enable the fabrication of passive damping elements for a broad range of structural applications. Show more
External linksSearch print copy at ETH Library
ContributorsExaminer: Studart, André R.
Examiner: Ermanni, Paolo
Examiner: Bismarck, Alexander
Examiner: Masania, Kunal
Organisational unit03831 - Studart, André R. / Studart, André R.
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