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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.

Hybrid preforms are used in thermoplastic composite manufacturing processes to reduce the potentially long consolidation times caused by the high viscosities of thermoplastic melts. Darcy's law for fluid flow through a porous medium indicates that the negative effects of high viscosities on impregnation time can be offset by reducing the maximum distances the thermoplastic melts must flow for complete consolidation. In thermoplastic composite preforms, the total impregnation length is reduced by increasing the degree of mingling between reinforcement and matrix. Currently, mingling in such preforms is found on the level of the laminate down to the level of the yarn, but may occur on any of the hierarchical tiers found in fibre-reinforced composite materials. The level and quality of mingling in existing arrangements - such as organosheets, commingled yarns, powder-impregnated yarns and fibre impregnated thermoplastics (FITs), co-woven yarns or stacked laminates - greatly influence the flow lengths, cycle times, achievable part complexity, raw material costs, and suitable manufacturing routes. Given the limited selection of commercially available preform architectures, manufacturers must choose between the low cycle times of organosheets and the better drapeability of unconsolidated hybrids, e.g. commingled yarns, in thermoforming. The development of a material architecture which combines the fast processing of fully impregnated products with the flexibility of unconsolidated preforms would render thermoplastic composites significantly more attractive to high volume production markets, e.g. automotive parts. This thesis proposes hybrid bicomponent fibres - which consist of continuous reinforcement fibres individually sheathed in a thermoplastic polymer - as a new class of preform materials for thermoplastic composites. By reducing the scale of mingling between the reinforcement and matrix materials to the level of the fibre, a full wet-out of the fibres is ensured while the unconsolidated nature of the material allows the fibres to shift and deform with respect to each other to ensure drapeability even at room temperature. It is hypothesized that preforms made from hybrid bicomponent fibres can be stamp formed with cycle times similar to those of pre-consolidated blanks. Furthermore, it is expected that the void content of laminates stamp formed from hybrid bicomponent fibre preforms is greatly influenced by sintering mechanisms and the removal and/or collapse of air pockets. The presented research aims to answer these hypotheses by developing suitable methods to manufacture such hybrid bicomponent fibres and by processing them into consolidated laminates. The basic idea of hybrid bicomponent fibres is motivated and introduced in further detail in part I. Part II moves on to discuss materials and their corresponding processing methods for their suitability in realizing bicomponent fibres. A fibre forming approach based on glass-melt spinning combined with an in-line coating process is chosen. Multiple versions of the latter are investigated empirically, namely dip-coating of newly spun glass fibres in either a polymer solution or a sparsely nanofilled polymer melt, as well as the so-called kiss-roll coating method. It is found that drawing glass monofilaments of finite length over a rotating roll which is partially immersed in a dilute polymer solution yields a coating method which can endure high fibre velocities while ensuring the deposition of a sufficiently thick thermoplastic sheath for down-stream conversion into a structural grade composite laminate. The validity of this strategy is proven in the realization of a pilot plant which employs solution kiss-roll coating in-line with melt-spinning of a glass monofilament for the continuous fabrication of bicomponent fibres. Unidirectional layups of specimens of aluminium borosilicate glass fibres clad in polycarbonate produced with the pilot plant were characterized for their consolidation behaviour, the results of which are reported in part III. Supported by theoretical treatments on issues related to void collapse and autohesion, a parameter study on rapid stamp forming of these preforms was performed and complemented with stamp forming trials processing cross-ply layups of different thicknesses. All experiments yielded excellent laminate qualities with void contents

ISBN:978-3-907234-12-9

The use of composite materials in structural applications is limited by the high costs associated with the production of the components. The research of cost-effective solutions leads to the exploration of innovative manufacturing approaches. One of those is the combined prepreg / liquid composite molding (LCM) process. This manufacturing technique consists in the combination of prepreg materials with dry fabrics, subsequently impregnated by the injection of a liquid resin. The main ad-vantage of the combined process is the possibility to exploit the characteristics of the two manufacturing approaches in different regions of the same component. This shall enable the manufacturing of structures with a high level of integration and increase the effectiveness of the production. New challenges arise when combining different manufacturing techniques. A characteristic phenomenon of the combined prepreg/LCM process is the bleeding of the prepreg resin into the dry fabric. Such consolidation problem has a relevant influence on the properties of the cured laminate and needs to be understood to guarantee a good reliability of the process. The main objective of this thesis is the description and study of the mechanisms driving the consolidation of a stack of reinforcement plies. The first part of the work aims at establishing the manufacturing procedures for the combined out-of-autoclave (OOA) prepreg / vacuum assisted resin transfer molding (VARTM) process. Following an experimental approach, the main characteristics of the process are described and the influence of the process parameters on the composite quality is assessed. The derived procedures are then validated with the production of a representative aerospace demonstrator component. In the second part of the thesis, the material properties sup-porting the modeling of the consolidation process, i.e. fiber bed through-thickness compaction and permeability, are investigated. A new approach for the characterization of the quasi-static and dynamic (time-dependent) compaction response of fiber beds is proposed and validated for different conditions. Furthermore, an experimental setup and procedure for the derivation of the “true” fiber bed through-thickness saturated permeability is established. The characterization method is validated exploiting a numerical model to assess the deformation the fiber bed sample during the experiment. Finally, the derived material properties are implemented in coupled finite element models describing the consolidation of reinforcement plies and the resin flow in composite manufacturing processes. The simulation of a saturated consolidation is validated against experimental data measured in a dedicated setup. The observations made in this thesis contribute to a better understanding of consolidation phenomena in composite manufacturing processes. The insight shall support the definition of manufacturing and design guidelines, aiming at increasing the reliability and robustness of the production of composite components.

ISBN:978-3-906916-50-7

Various methods for controlling wave propagation, and their ensuing vibrations have been proposed. This thesis will focus on one such method, whereby periodic piezoelectric inductancecapacitance (LC) shunts will be interconnected following different schemes. The electrical components will then be tuned to tailor the wave propagation properties of one (1D) and two (2D) dimensional materials. The periodicity of the mechanical and electrical domains will be exploited to design these electromechanical materials based on the characteristics of their bandstructure, namely by identifying weak eigenvalue coupling phenomena where the exchange of energy, mechanical to electrical, leads to wave attenuation. Moreover, the bulk of the work presented in this thesis will aim at attenuating transverse mechanical waves, however, another part of the research will delve into utilizing both local and interconnected shunts for controlling longitudinal waves with the purpose of modifying the depth and/or width of Bragg-scattering band-gaps, thus promoting wave propagation. By interconnecting the piezoelectric LC shunts, the design space is vastly expanded, as the latter are no longer individual elements, but rather components of a secondary discrete domain along which energy can propagate parallel to the mechanical medium. Albeit, a myriad of electrical interconnection schemes of increased elegance, and complexity could be envisioned, this work will focus on the interconnection of unit-cells comprised of low-pass, band-pass, high-pass filters, tailored to achieve specific effects on the mechanical domain of 1D and 2D media. Furthermore, this thesis will have a strong focus on the design, fabrication, and testing of improved electrical components to achieve wave propagation control over low frequency ranges, that could otherwise not be attained using passive components. Particular attention will be devoted to the development of programmable floating virtual inductances that satisfy the geometric form factor required to achieve full, and seemless integration of the electrical, and mechanical domains of the unit-cell. As a result of the integration and digitalization of the electrical components, it is the goal of this thesis to engender truly "smart" materials with the potential of bridging the gap between theory and practical applications. Analytical models based on the transfer matrix method will be derived to the extent possible, and validated with numerical results obtained using the multi-physics FEM software. Analytical, and numerical work will be validated using experimental test setups.

This thesis presents and explores a set of novel approaches for the development of hybrid, integrated, lightweight, structures by combining additive manufacturing (AM) with fiber-reinforced polymers (FRP) in layup processes. The presented methods combine the strengths of both technologies: While AM takes a forming, structural and functional role, FRPs primarily contribute with their outstanding mechanical properties at low weight to the performance of the structure. From these characteristics three major design concepts are derived, including AM tooling for composite fabrication, AM structural sandwich cores with additional functionalities and AM load introduction elements. The understanding of materials and manufacturing approaches is a precondition for the embodiment of the design concepts. Therefore, various manufacturing routes are investigated including AM technologies such as fused deposition modeling, binder jetting and selective laser sintering. The thermo-mechanical stability of polymeric AM parts is crucial for autoclave curing of hybrid high-performance structures. To successfully design complex AM elements for the curing process of FRP, the thermo-mechanical creep properties of polymeric AM materials are characterized in three-point bending creep and tensile tests. Alternative concepts include sand and salt structures to produce sacrificial tooling. The embodiment of the three design concepts comprises two directions: designing for the performance of the part during service operation and designing to support the manufacturing of the structure. In this thesis both directions are addressed: Design for Performance: The specific mechanical performance of AM-CFRP structures is assessed in comparison to state-of-the Art structures by comparing the weight, the first failure load, the breaking load and the bending stiffness. Hat-stiffeners with structural cores made by AM were over-laminated with CFRP prepregs, cured in an autoclave and tested in three-point bending. AM core designs include honeycombs, trusses oriented along principal stresses, hollow structures filled with salt and a machined PMI foam core with a local load introduction element made by AM. AM-CFRP hat-stiffeners exhibit an increase in the specific fracture load ranging from 54% to 107% compared to the reference. Weights vary from an increase by up to 55% to a reduction of 5%, and the specific bending stiffness is increased by up to 41%. Results thereby confirm the mechanical competitiveness of AM-CFRP structures. Design for Processing: Additive manufacturing can support the production of composite parts along the process chain ranging from tooling, layup, handling, curing and post-processing. Four major design principles are presented and classified into integrated positioning and fixation elements, layup and handling aids, structural curing aids and post processing aids. Case studies show that the consideration of the processing during the design phase can reduce the deformation of the part during curing, the number of parts and the number of work steps and even eliminate drilling operations for the post-processing of inserts. The AM-CFRP approach is validated by incorporating the material data, the design principles and concepts into three components on system level. First, a novel aircraft instrument panel consisting of a multi-functional sandwich core shows that AM-CFRP can reduce the structural weight by 40%, the part count by 50% and the assembly steps by 50%. The second case study validates the mechanical performance of the AM-CFRP approach on component level by assessing the ultimate and the fatigue strength of a novel AM-CFRP prosthetic knee. The third case study consists of a robot leg structure and yields in weight reductions by 54% compared to state-of-the art references. The combination of AM with CFRP thus is a suitable approach for the manufacturing of individualized, lightweight, and geometrically-complex structures with integrated functionalities for low-volume applications, e.g. in the field of aerospace, flying vehicles, robotics and biomedical structures.

ISBN:978-3-909386-46-8