MULTISCALE DESIGN AND MANUFACTURING OF ADVANCED COMPOSITES INTEGRATING DAMPING FEATURES.
[Tesi di dottorato]
Multi-functionality is a current issue in materials design, in particular, the fast growing application of advanced composites in commercial aeronautic is raising the need to design primary structures with composite material that perform multiple functions: i.e. able to fulfil not only mechanical allowable but also functional requirement such as vibroacoustic and fire reaction. The development of multifunctional design tools integrating structural and damping features enables a next step toward the exploitation of the composite materials benefits. It is worth noting that the structural damping in the case of a composite fuselage is a multiscale problem. The fuselage vibroacoustic requirement is determined by the behaviour of stringers reinforced skin, that is determined by the panel damping behaviour which owns its damping features to its laminate architecture and constituents materials. The requirement chain for a composite structure is formulated by a top-down approach determined by the behaviour of sub-structures which compose the final structural component. Aim of this work is to individuate and implement a design procedure able to describe a composite structure starting from its constituents, moreover for each dimensional scales the behaviour have to be modelled. The through dimensional scales model proposed for describing composite materials use the formulation of constitutive equation for describe the material behaviour at each sub-component. From the homogenization of fibres and hosting matrix it is possible to formulate a micro-scale constitutive matrix describing mechanical and dissipative lamina behaviour, with analogous approach the laminate behaviour is described by the homogenization of the constituents layers. The potential of describe mechanical and dissipative feature for a laminate starting from its elementary constituents gives the chance of imagine hybrid architecture able to improve a desired feature. Keeping in mind the passive damping feature, three possible hybrid architecture have been proposed for suit the requirement of increment material performance, moreover the composite have to maintain its mechanical properties above a defined level to preserve structural safety. The insertion of a viscoelastic layer within the laminate has been individuated as promising architecture for increase damping performance although this configuration is susceptible to interlaminar stress and prone to de-bonding. From theoretically study on the energy allocation within the laminate is formulated the novel idea of an hybrid laminate where the viscoelastic material is embedded as long fiber in the reinforcement preform, this architecture contribute to increment the damping properties withstand the mechanical properties but enhancement level is less than an interleaved containing the same volume of added material. Rather than modifying the fiber arrangement the lamina passive damping could be increased by means of introducing high damping nano-fillers within the hosting matrix. For the prediction of the overall laminate properties an hierarchical procedure has proposed accounting the hybridization at each laminate level. Considering elementary structures, such as a beam, subjected to boundary condition which induce that energy is allocated in only one component, the damping predicted is the overall damping capacity for the considered energy component. A valuable technology for manufacturing composite materials have to be flexible in changing constituents properties as well as the insertion of a softer material as lamina or the use of hybrid layer stacking the fibres or the use of a pre-hybridised hosting matrix. Process technologies allowing the listed item are based on the liquid moulding, in particular the VARTM process is selected as this process could be easily extended on large scale fabrication. Unidirectional composites of the proposed lamina architecture were manufactured and tested. In each case a valuable increment in passive damping were measured. Both the interleaved layer and the hybrid preform lead to a loss in mechanical performances, whilst the hybrid laminates manufactured by the nanofilled hosting matrix kept the its mechanical features leading to an enhancement of loss factor until 40% at temperatures suitable for aeronautical applications. The most promising architecture selected from experimental study was the multiscale laminate, as they are reinforced by microscale long fibres and nanoscale nanotubes. As proof of the industrial feasibility of this solution a simple typical aeronautical component has manufactured. A stiffened composite plate is designed and manufactured for further acoustical testing. In addition the angle ply laminate has fabricated and mechanical tested.
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