Cancelled event - IMX Seminar Series - High-performance discontinuous composites: material and structural design

High-performance discontinuous composites (also known as tow-based discontinuous composites (TBDCs), advanced-SMCs, or randomly-oriented strands) are composed by chopped carbon-fibre tows randomly oriented and distributed in a polymeric matrix. This discontinuous and random microstructure allows components with complex 3D shapes to be moulded using fully automated processes, with processing times down to a few minutes. In addition, the tow-based microstructure allows these materials to achieve a high content of carbon fibres (up to 60% in volume) and, consequently, to achieve good mechanical properties. Due to this combination of manufacturability and high-performance, TBDCs are now being used to manufacture lightweight (semi-)structural components in the aeronautics, automotive and sports industries.
While the multi-scale nature of the microstructure of TBDCs (reinforced at both the tow- and fibre-levels) is key for their unique combination of manufacturability and performance, it also creates two challenges for the effective use of these materials. Firstly, it widens the design space of the material, since its mechanical properties are dictated not only by the fibre and matrix types, but also by the dimensions of the tows. Secondly, the large dimensions of the tows (up to 50 mm long and 20 mm wide) lead to significant variability of local mechanical properties (e.g. stiffness and strength) from one point of a component to another; this makes TBDCs extremely damage tolerant, but it also complicates structural design.
This talk will address these two challenges through a combination of experiments and modelling. Regarding the first challenge, we have experimentally characterised the mechanical response of TBDCs with a range of material microstructures, to assess the effect of tow geometry and preferential orientation on the properties of the composite; we show that the thickness (or filament count) of the tows has a very significant impact on performance, with thicker tows leading to a knock-down on both strength and stiffness. We also propose computationally-efficient models which can be used to perform virtual experiments, identify optimal material microstructures, and support the development of improved materials.
Regarding the second challenge, we have characterised the damage tolerance of TBDCs using a combination of unnotched and notched specimens; we show that the fracture toughness of TBDCs can be higher than that of continuous-fibre composites. Moreover, notched specimens present no reduction in load-bearing capacity (compared to unnotched specimens), and often fail away from the notch; this makes TBDCs the ultimate “damage tolerant” material, but makes it difficult to predict the behaviour of structures with complex geometries. We overcome this challenge by proposing a stochastic framework based on Finite Element (FE) Monte-Carlo simulations, which accounts for the spatial variability of local mechanical properties of TBDCs, and uses non-local homogenisation criteria to predict failure in a mesh-independent way.
Bio: Dr Soraia Pimenta obtained her PhD from Imperial College London in 2013, and she is now a Senior Lecturer at the Department of Mechanical Engineering. Soraia’s research interests include developing accurate and efficient models for the mechanical response of composites, and promoting a new generation of easy-to-manufacture, damage tolerant and sustainable materials. Soraia won the SAMPE Schliekelmann Award in 2009, the International Committee for Composite Materials Tsai Award in 2011, the Japan Society for Composite Materials Hayashi Memorial International Award in 2015, and the Imperial College President’s Medal for Outstanding Early Career Researcher in 2015. She has also been a Research Fellow of the Royal Academy of Engineering since 2015.