The mode-I interlaminar fracture toughness of flax, glass and hybrid flax-glass fibre woven composites was studied by using a DCB test. The acoustic emission signals recorded during the tests and scanning electron microscope images were used to analyse the damage mechanism of each composite. The crack initiation for the flax-fibre laminate needs the highest energy (1079 versus 945 for hybrid flax-glass fibre and 923 J/m² for glass-fibre laminates). The morphology of the flax fibres, short and bonded together in bundles to manufacture the twill fabric, allows the creation of a larger amount of fibre bridging as the origin of this highest energy. Furthermore, hybridisation of glass fibres with flax fibres in an appropriate combination offers an interesting solution when the toughness of glass fibre composites needs to be increased. More interesting is the considerable advantage of the composite structure weight reduction due to the low flax fibre density.
Fused Filament Fabrication is a very common additive manufacturing technology and several manufacturers have developed commercial 3D-printers that enable the use of fibre-reinforced filaments in order to improve the mechanical properties of the printed parts. The obtained material is a composite that exhibits complex mechanical properties. The aim of this study is to characterize the mechanical behaviour of 3D-printed continuous glass fibre-reinforced polyamide composites. In a first step, we focus on the reinforced filament: the heterogeneity of its microstructure is evidenced as well as its brittle elastic tensile behaviour. In a second step, parts of different fibre orientations and fibre volume contents are manufactured using a Mark Two 3D-printer (MarkForged®), their microstructure is analysed and tensile, flexural and short beam bending tests are performed. As expected, the results show a significant influence of fibre volume content and fibre orientation. Standard homogenization methods are used to compare the predicted mechanical behaviour to the experimental results. Regarding the elastic stiffness, a good correlation is observed when the material is loaded in the direction of the fibres. Regarding the tensile strength, the results show that no benefit is obtained above a fibre volume content of about 11%. These results highlight the importance of choosing an optimised stacking sequence prior to the printing process, in order to obtain composites with the desired mechanical properties. The mechanical results are analysed in the light of Scanning Electron Microscopy observations of specimen cross-sections before and after testing.
Adding self-healing properties to coatings is a promising way to increase their lifetime. Despite an increasing popularity, lots of self-healing polymers are not suited for commercial coatings because they exhibit poor mechanical properties or require expensive products or high healing temperature (> 100 C). One way to obtain self-healing abilities with good mechanical properties is by using dynamic H-bonds as they also limit the healing temperature. To do so, urea groups are used since they are well-known for their bonding capacity and can be readily synthesized. In this study, several methacrylate monomers containing urea groups in their side-chain were synthesized from easily accessible amines and isocyanates in a one-step, high-yield synthesis. They were afterwards used in a copolymer containing methyl methacrylate and butyl acrylate monomers. The self-healing properties of the resulting coatings were evaluated with gloss recovery and optical microscopy and the presence of H-bonds in the most promising polymer was investigated with FTIR. The mechanical properties of the coatings as a function of time were checked by nanoindentation creep test. We were able to obtain an affordable, easily prepared polymer that suits the requirements for protective coating applications and shows a complete healing after heating at 75 C for 1 h.
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