Effects of design parameters on damping of composite materials for aeronautical applications

• Autor(a) principal: Lopes, João Pedro dos Santos
• Data de Publicação: 2013
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.6/2016
• topic: Indústria aeronáutica; Materiais viscoelásticos; Materiais compositos; Cortiça

Many engineering projects have shown a great concern with the dynamic response of generalized mechanical systems where a set of rigorous demands are placed on the design of structures in sectors such as the aerospace or the automobile industries. Aerospace materials, for example, in most cases have been developed for a specific purpose, like some particular metal alloys and composites. It is their operation requirements that influence the optimization of certain intrinsic mechanical or physical properties. The dynamic behaviour of a structure can be refined by anticipating any performance-related problem during the design process, where vibration and others parameters can be measured and optimized for the desired applications. It is often desirable to be able to predict accurately the dynamic response of a structure under certain excitation conditions. Given that, it is necessary to understand how its mass, stiffness or damping properties could be modified to obtain a desired response and vibration control, taking into account structural margin of safety and life-time limits under service. Thus, the damping of such structures, which is associated to the energy dissipation capacity, is a key aspect regarding the fatigue endurance and noise/vibration control as it controls the amplitude of resonant vibration response. Envisaging to create a cheaper, direct and maintenance-free alternative to active damping systems, passive methods are a straightforward solution for certain industrial demands. In fact, active damping systems typically imply more structure weight, considerable energy consumption, reliability issues and limited strain/force response, which are undesired features in general technological applications, especially in the aerospace sector. To achieve high damping properties discarding the use of an active system, the use of some isolation techniques, the inclusion of high damping materials or even the need for physical structural modifications are often necessary in the standpoint of a new component’s development for passive control applications. As an example, in most recent investigations, co-curing/embedded viscoelastic damping constituents in composites has been a successful way to increase the damping capacity. In the context of the present work, a passive damping treatment method based on cork utilization as viscoelastic material has been used to improve the damping properties of fiber-reinforced composites. A numerical and experimental study was made to predict and understand the benefits of such method and characterize any inherent effects on modal loss factor and respective structural natural frequencies regarding the use of cork. The excellent energy absorption properties of cork under static and dynamic loading conditions, its lightness, near-impermeability and lower thermal conductivity, are the base of a recent and crescent interest in aeronautical, railroad and automobile applications for cork based materials. These intrinsic characteristics are also the main reasons for considering it as viscoelastic layer applicable in passive damping treatments with a great potential for vibration control in future aerospace applications. As far as the numerical study concerns, a finite element model (FEM) was developed to analyze the main dynamic properties of the composite structure samples, for example, the modal frequencies and respective loss factors, and compare it with the experimental results, allowing to assess the accuracy of numerical data. Distinct design variables were considered to determine their influence in the loss factor variation, namely: damping layer thickness and its relative position within the laminate, number of viscoelastic layers and effect of different layup stacking sequences. Results are encouraging about the possible use of cork based composites as a viable passive solution to improve the damping properties of high performance composites, giving rise to an increase of the loss factor as well as a change of the natural frequencies of the structure according to the design requirements for particular applications.