La Defensa Doctoral tendrá lugar el 20 de noviembre a las 12:00 hr. La asistencia será a modo remoto.

La disertación se titula «High Strain Rate Mechanical Behavior of Advanced High Strength Steels»

Abstract:

While FRP composites are extensively used in structural components due to their remarkable mechanical properties, a major weakness of these materials is their low through-the-thickness properties which permits the easy formation of impact-induced delamination, limiting their more widespread applications in structural engineering. CNTs carry the promise of enhancing this poor out-of-plane performance, although their integration has been challenging. One of the main objective of this thesis is the development of strategies to integrate CNTs into structural laminate composites for interlaminar toughening. Two different routes, vacuum assisted resin transfer moulding and prepreg consolidation by compression moulding, are explored to interleave different CNT fiber veils into FRP laminates in a facile and scalable way. The interlaminar fracture toughness under Mode I and Mode II loading cases are investigated and discussed comparatively, followed by a systematic analysis of failure and toughening mechanisms. Results showed that CNT veil is a promising candidate for interleaving application. The toughening effects of CNT veils depend highly on their thickness, degree of compaction, host fabric architectures as well as loading conditions etc. Interlaminar crossing plays a dominant role amongst toughening mechanisms. The crack front propagates alternatingly between interfaces of the laminate, triggering multi-level fiber bridging and significantly improving the fracture toughness of the laminate.

In addition, CNT fiber veils, with a porosity close to that of an activated carbon and tensile properties in the high-performance range as well as the remarkable electrical conductivity, could be ideal electrodes and current collectors for structural power application. In this thesis, CNT veils as essential part of supercapacitor were integrated into FRP composites to achieve simultaneously both load-carrying and electrochemical energy storage capabilities. A simple fabrication method was demonstrated through embedding CNT veils-based EDLC interleaves between carbon/glass fabrics, followed by resin infusion via vacuum bagging process. Coupled mechanical and electrochemical tests were carried out to realize the efficacy of the resultant structural supercapacitors under the application of mechanical forces. Possible degradation mechanisms in the samples were analysed through scanning electron microscopy and 3D X-ray computed tomography.
It was found that a high coulombic efficiency and stability of EDLCs can be maintained during resin infusion. However, further studies on coupled mechanical and electrochemical properties were hampered by the large presence of defects in the structure, most notably those arising from the presence of thick metallic current collectors, which essentially act as severe delaminations. By using CNT veils not only as active material but also as current collectors, a true multifunctional composite material was produced, which can sustain mechanical deformations without degradation of electrochemical properties. Furthermore, grid-shaped EDLC interleaves were found to be effective to improve interlaminar properties of the composite and offer a wide range of design parameters to obtain desired composite performance. A finite element model approach was employed to explore the effect of rivet structures on the lap shear behaviour of laminated materials containing EDLC devices. In parallel, experimental tests were performed to validate the proposed simulation model. The resulting model is particularly useful to design structural power composites with tailored balance between the energy/power densities and structural properties, thus opening the multifunctional spectrum.