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- Thermal recycling of glass fibre composites: A Circular Economy ApproachPublication . Iglésias, Maria; Santos, Paulo Sérgio Pina dos; Lima, Tânia M.; Leite, LauraComposite materials are used in a wide range of applications, but due to their inherent nature of heterogeneity, particularly for thermoset-based polymer composites, their recycling is a problem, and their life cycle management remains one, too. This study applies a circular economy approach to the problem of excess waste of glass fibre (GF) composites and seeks a solution by testing a methodology for thermal recycling of GF composites by combining different times and temperatures. Through the by-hand lay-up process, diverse laminates were manufactured with recycled GF, and the mechanical results were compared with those of the control laminate; in this way, we sought to reinsert recycled fibres into a new life cycle, closing the loop of the material. The static properties, tensile tests, and three-point bending (3PB) tests were studied as well as the viscoelastic behaviour of the recycled fibres and respective laminates. For woven fibres, we highlight the recycling process at 600 °C for 15 min, which revealed a loss of only 15.3% of the tensile strength. The laminates with fibres recycled at 400 °C for 180 min presented a reduction of 52.14% and 33.98% for tensile and flexural strength, respectively, representing the best solution. For all laminates, the bending stress, stiffness, and strain are sensitive to the strain rate, and the tendency observed for these properties can be supported by linear models. Subsequently, for the best results, the stress–relaxation and creep behaviour were analysed, and it is possible to conclude that temperature and time of fibre recycling influence the viscoelastic response of laminates.
- Dielectric barrier discharge plasma actuators based on thermosetting composites for flow control in wind turbines bladesPublication . Nunes-Pereira, João; Rodrigues, Frederico; Abdollahzadehsangroudi, Mohammadmahdi; Santos, Paulo Sérgio Pina dos; Silva, Marco; Pereira Silva, A; Pascoa, Jose; Lanceros-Mendez, SenentxuThe massive use of fibre reinforced plastics (FRP) in several industrial sectors such as aeronautics, aerospace, or wind turbines, raises serious environmental concerns with respect to the end-of-life of these structures, based on their poor recyclability. The low density of FRP gives it outstanding specific properties, such as strength and stiffness, when compared to materials with high recyclability rates, such as metals1. FRP find several applications in structures in which the aerodynamic performance is of extreme importance to guarantee a proper operation. In this sense, to endow these structures with the ability to control or modulate the flow around their surface could be an extraordinary feature to improve the performance of the structure, thus saving considerable amounts of energy and increasing its useful life cycle, contributing in this sense also to reduce the environmental impact of the materials. This work is focused on the development of dielectric barrier discharge (DBD) plasma actuators supported on fibre reinforced thermosetting composites, which can be used in manufacturing of wind turbine blades: epoxy resin and glass, aramid (Fig. 1) and natural flax fibres. DBD plasma actuators are electrohydrodynamic devices capable of generating induced flows of a few m/s through nonthermal plasma, that can be successfully leveraged for flow control applications2, 3. The characterization of the system was carried out in terms of mechanical (flexural strength, strain and stiffness), electrical (power consumption, capacitance, charge-discharge cycles) and electromechanical (induced flow velocity, electromechanical power and efficiency) properties. The results showed the multifunctionality of the composites, demonstrating their suitability for the application, in particular, the epoxy/glass composite with a bending stress of ≈600 MPa, which obtained an induced flow velocity of ≈2.1 m/s with a power consumption of ≈15.1 W, when powered by an AC signal of 11 kVpp and 24 kHz.
- Resistência ao impacto de compósitos híbridosPublication . Santos, Paulo Sérgio Pina dos; Reis, Paulo Nobre Balbis dosActualmente vem-se a assistir a um aumento significativo do uso de materiais compósitos reforçados com fibras em aplicações de engenharia e existe uma forte indicação de que este fenómeno tende a continuar. Contudo, a resistência e rigidez destes materiais é fortemente afectada pelos defeitos que decorrem durante o processo de fabrico e/ou de cargas de impacto que surgem ao longo da sua vida activa. Neste contexto ocorrem enormes decréscimos da resistência residual destes materiais. O presente trabalho pretende assim contribuir para um maior conhecimento e melhoria da resistência ao impacto dos materiais compósitos. Para tal foi utilizada uma resina epóxi nano-transformada tendo-se concluido que a adição de nanopartículas promove melhorias significativas nas performances ao impacto.
- The Effect of Carbon Nanofibers on the Mechanical Performance of Composite LaminatesPublication . Santos, Paulo Sérgio Pina dos; Reis, Paulo Nobre Balbis dos; Silva, Abílio Manuel Pereira daCarbon fibre reinforced polymer composites are now widely used in various industries/sectors of activity, replacing traditional materials due to their ease of processing, excellent specific resistance, excellent fatigue behaviour, durability and low specific weight. However, the development and optimisation of these materials is faced with new challenges every day, either by optimising their properties or by changing the paradigm in terms of energy consumption. The incorporation of nanofillers significantly improves the mechanical response by reinforcing the polymer matrices and the fibre/matrix interface. This improvement is due to several synergistic mechanisms operating at the nanoscale. In addition, the dispersion of these nanoscale reinforcements within the matrix can inhibit crack propagation and increase the resistance of the material to deformation and fracture. At the fibre/matrix interface, nanofillers can bridge gaps and promote stronger adhesion, resulting in more effective load transfer between the reinforcing fibres and the surrounding polymer. The use of carbon nanofibres (CNFs) offers advantages over other nanofillers such as graphene and carbon nanotubes (CNTs), primarily due to their comparatively lower specific surface area, which facilitates the implementation of accessible and cost-effective manufacturing processes. The lower specific surface area of CNFs, in contrast to the high surface areas of graphene and CNTs, results in reduced interparticle interactions and a lower tendency to agglomerate. In this work, the manufacturing process and the fraction of CNFs that maximises mechanical response, viscoelastic behaviour and impact and fracture resistance were optimised for two commercial epoxy matrices. To assess the improvements, an extensive static mechanical characterisation was carried out, studied the viscoelastic behaviour in bending, interlaminar fracture, low-velocity impact, multiimpact and residual strength and viscoelastic behaviour after impact. The results obtained show that even with small amounts by weight of CNFs (0.5 and 0.75) added to the epoxy resins, significant benefits were obtained, for example: improvements of more than 10% in bending stress and bending stiffness of the matrices and laminates. As a result, the resistance under different strain rates and the interlaminar shear strength were higher in the additive matrices and laminates. In terms of multiple impacts and for an impact energy of 3 J, approximately 66 - 89 impacts are required to achieve full perforation for the nano-enhanced laminate with CNFs, whereas only 17 - 20 impacts are required for the control laminates.