Recently, due to environmental and sustainability issues, the scientific community has been attracted to renewable natural resources, even for the development of materials intended for structural applications. This work deals with the pre-treatment of a flax fiber fabric by exploring the effects of the exposure time to nitrogen plasma on the ultimate performance of polypropylene matrix composite laminates potentially usable for the realization of internal parts in the naval sector. Flax fabrics were treated with three different exposure times (5, 10, and 15 min) with the aim to improve the adhesion between the hydrophilic fibers and the hydrophobic matrix. At first, transverse and longitudinal wicking tests were carried out on the treated fabrics in order to evaluate the improvement of the absorption constants after the formation of reactive groups on their surface. The results collected so far in terms of fabric water uptake, tensile, and flexural properties as well as morphological aspects (SEM analysis) and damage evolution (ESPI technique), have indicated that the optimal pre-treatment time of the reinforcing fabrics is equal to 15 min. Compared to the laminate with untreated fabrics, the specimen with flax fibers treated for 15 min displays higher tensile properties with an increase of 6.8% and 22.31% in ultimate strength and modulus, respectively. More in general, the achievements of this research might be useful to extend the current range of applications of flax fibers even in industrial fields still dominated by conventional fibers.

Nowadays, due to the competitive economic scenario, industries ever more need to focus on manufacturing speed, increasing efficiency and steady quality. More industrial sectors see additive manufacturing (AM) as a possible way to enhance their processes and increase production efficiency. Thanks to its versatility the Fused Deposition Modelling (FDM), also known as Fused Filament Fabrication (FFF) technique, is one the most attractive processes in Industry 4.0 paradigm. This technique, thanks to its low-cost, is spreading widely in industrial sectors, from biomedical to aerospace to cite some. In this frame a valid solution is to use composite materials Among many, particular attention is paid to thermoplastic systems based on polyether-ether-ketone (PEEK) reinforced with short carbon fibre (CF). The PEEK is a high-performance semi-crystalline thermoplastic polymer that belongs to the polyaryl-ether-ketone family (PAEK). 3D printing, being a novel technology, it must be validated by understanding the behaviours of components and structures. At this purpose, we used different optical techniques for analysing advanced short fibre composites realized by 3D printing. Different CFR-PEEK samples with short carbon fibre at 10% by weight were realized by the FFF technique and characterized in terms of failure mode and mechanical behaviour. Optical tools have been used to retrieve full-field data and expand information about the mechanical behaviour of the investigated material, i.e. 2D Digital Image Correlation (2D-DIC) and Electronic Speckle Pattern Interferometry (ESPI), Optical Microscopy (OM) and Scanning Electron Microscopy (SEM).

In the industrial field, it is of great interest to identify fast and low-cost manufacturing procedures to guarantee reliable and functional products. In this frame, 3D printing is a fast and low-cost innovative technique and its capability to produce fibre-reinforced polymer matrix composites widely opens the application possibilities. 3D printing technology can form also composite materials made by short fibres that can be exploited for manufacturing parts and structures in high-tech sectors such as aerospace, automotive or naval, to cite some. Here we show how a multimodal approach can furnish a whole characterisation of reinforced composites produced with a 3D printing technology during and after tensile and flexural tests. In the multimodal strategy, we employed a couple of full-field and non-contact optical techniques, i.e. 2D-Digital-Image-Correlation (2D-DIC) and Electronic Speckle Pattern Interferometry (ESPI) in combination with SEM and optical microscope for analysing the sample at the micro and macroscale. The result of the work allows us to achieve a complete characterisation of the components realized by 3D printing.

In the last decades, fibre-reinforced composites have attracted outstanding interest especially for weigh-sensitive applications as well as for their specific mechanical properties as strength and stiffness. However, with the awareness that, like most materials, fibre-reinforced composites also exhibit the so-called strength versus toughness dilemma and, over the years, many different strategies have been proposed to improve their damage resistance. One of the most accounted strategies consider the use of at least two different reinforcing fibers distributed in the same matrix with typical configurations best known as interlayer, intralayer or intrayarn, depending on whether the fibres of different nature are arranged on as many laminate of the composite, in the same lamina or side-by-side in the strand making up the reinforcing phase. In this frame, the research was focused on polypropylene-based composite laminates manufactured by film-stacking and hot-pressing steps and reinforced by a commercial hybrid fabric obtained by weaving flax and basalt fibres. Specimens, consisting of 6 plies and 3.0 mm laminate thickness, were cut from the prepared plates along the direction of both flax and basalt fibres and subjected to Quasi Static Indentation (QSI) tests. The tested specimens were investigated by combining Optical microscopy (OP) and Electronic Speckle Pattern Interferometry (ESPI) to analyze the complex damage which can be generated as a result of stress on this kind of fibre-reinforced composites.

Composites represent the evolution of the material science and technologies. They are obtained by combining two or more materials of different nature with the aim of exploiting any synergies between the characteristic performances of the raw materials. Their properties, in fact, are influenced by those of the starting components but also by the quality of the interface generated between the combined phases as well as by their mutual distribution. The interphase, even if of minimal extension with respect to the main phases constituting the composite, plays a significant role in the control of the damage mechanisms, determines the breaking strength and the stress / deformation behavior of composite materials. In this work we study the effect of low pressure plasma treatment on intrinsically hydrophilic flax fiber fabrics to improve their adhesion to a hydrophobic polypropylene matrix. The fibers are treated using nitrogen (N2) plasma with four different exposure times. The interfacial adhesion actually achieved was indirectly quantified by interlaminar shear strength measurements. After this, the damaged areas were measured with non-destructive techniques, i.e. Electron Speckle Pattern Interferometry and Lock-in thermography.

Drop-weight experiments studied the damages due to repeated low-velocity impacts of hybrid composite laminates. The laminate made by carbon woven fabric and glass woven fabric impregnated by vinyl ester resin was subjected at single impact, at 5 repeated impacts and 10 repeated impacts, for an energetic level of U=20J. The final damage after the single and repeated impact events was analysed by no-destructive methods, Pulse Thermography and Holographic Interferometry to evaluate the influence of the multi-hit events on the damaged area's evolution.

In aerospace, it is of great interest to identify safe and low-cost manufacturing procedures to guarantee reliable and functional products. In this perspective, 3D printing is a fast and low-cost innovative technique to produce fiber-reinforced polymer matrix composites. This new technology, among other, forms short fiber composite that can be used as a shim material to fill voids left by manufacturing defects. The purpose of this work is to characterize reinforced composites developed for these applications and obtained with the innovative 3D printing technique.