Natural fibre (NF) reinforced composites offer high specific mechanical properties and are an ecological alternative to synthetic fibre-reinforced composites. While having great potential, their use today is limited to non-structural applications, mostly with epoxy or polypropylene matrices. This work studies suitable highperformance thermoplastic matrices and characterises their bulk properties, fibre-wetting and composite mechanical behaviour. Thermoplastic polymers such as poly-L-lactide (PLLA) and polyoxymethylene (coPOM) are matrices with bulk properties similar to epoxy. The results show that PLLA matrix NF-composites have a longitudinal modulus and strength of 27 GPa and 308 MPa. The tougher coPOM matrix NF-composites show both high transverse stiffness and strength of 2.6 GPa and 41.5 MPa and show that even the drawback of creep can be overcome by the use of hierarchically structured coPOM. The developed NF-composites demonstrate inplane properties comparable to those with epoxy matrices and can outperform them by up to 26% in the transverse direction.
a b s t r a c tThe formation of resin fillet between honeycomb core cell walls and skin in light sandwich structures was studied to gain a better understanding of the bonding process. A method was developed for tailoring the amount of adhesive between 8 and 80 g/m 2 . The size of the adhesive menisci and the contact angles between the adhesive and the skin and the core materials were measured. A model was developed to predict the size of the menisci. Their shape was driven by the surface energy of skin and honeycomb materials. When adhesive films were used for bonding, up to 50% of the adhesive did not form the menisci whereas 100% did when the newly developed adhesive deposition method on honeycomb was used, which allowed better bonding with lower weight.
a b s t r a c tThis work evaluated the possibility of using silicon solar cells as load-carrying elements in composite sandwich structures. Such an ultra-light multifunctional structure is a new concept enabling weight, and thus energy, to be saved in high-tech applications such as solar cars, solar planes or satellites. Composite sandwich structures with a weight of $800 g/m 2 were developed, based on one 140 lm thick skin made of 0/90°carbon fiber-reinforced plastic (CFRP), one skin made of 130 lm thick mono-crystalline silicon solar cells, thin stress transfer ribbons between the cells, and a 29 kg/m 3 honeycomb core. Particular attention was paid to investigating the strength of the solar cells under bending and tensile loads, and studying the influence of sandwich processing on their failure statistics. Two prototype multi-cell modules were produced to validate the concept. The asymmetric sandwich structure showed balanced mechanical strength; i.e. the solar cells, reinforcing ribbons, and 0/90°CFRP skin were each of comparable strength, thus confirming the potential of this concept for producing stiff and ultra-lightweight solar panels.
Skin wrinkling phenomenon is investigated in the case of ultra-light sandwich structures with a honeycomb core manufactured by one-shot vacuum bag processing. The interplay between process pressure and compressive strength of the skin is established. It is observed that the size of the adhesive menisci between honeycomb cell walls and skin, and the waviness of the skin increases with process pressure. As these two effects exert opposing influences on the compressive strength of the skin, an optimal process pressure equal to 0.7 bar is identified experimentally and confirmed by an analytical model.
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