Dottorato in INGEGNERIA ELETTRICA, DEI MATERIALI E DELLE
NANOTECNOLOGIE

Titolo della tesi: Experimental and modeling analysis of the dynamic response of bio-based sandwich structure

Mankind took advantage of synthetic materials for decades neglecting the huge consequences that this attitude could have had on the planet in terms of a massive contamination of air, water and soil because of the considerable emissions of pollutants and chemicals in the production process and the difficulties connected with the disposal of these non-biodegradable materials. The use of materials from renewable resources is an appropriate way to face these problems and to meet the more restrictive regulations promulgated in many countries. In light of this, one of the aims of the work is to propose a bio-based sandwich structure with an agglomerated cork core and with PP skins reinforced with a flax/basalt hybrid textile. Agglomerated cork is a good alternative to traditional synthetic foams, being from renewable resources, biodegradable and produced taking advantage of the wastes of wine stoppers production. Three agglomerated corks with different densities were considered in order to investigate the influence of this parameter not only on the physical, thermal and mechanical properties of the sole core but also on the overall sandwich structures to optimize their properties obtaining the most performing structure with the lowest possible weight. Well-established PVC foams, with the same cork densities, were subjected to the same experimental campaigns to act as benchmark and legitimate cork feasibility as core material. To increase sandwich structure eco-friendliness, a flax/basalt hybrid textile was selected as skin reinforcement to obtain a composite with satisfying mechanical properties, thanks to the high-tech basalt fibres, together with a partial biodegradability and a reduction of laminate weight, thanks to the presence of flax. The application of basalt fibres rather than glass ones in the hybrid textile is a strong contribution to the achievement of a greener structure. Sandwich composites are heavily susceptible to impact events that can compromise their structural integrity and stability thus reducing significantly their residual mechanical properties. In light of this, the second aim of the work is to provide an extensive characterization of the impact behaviour of the novel sandwich structures in order to be aware of the numerous failure mechanisms that can intervene and to grant their feasibility in all those industrial fields where impact resistance needs to be included in the design criteria. To reach this goal different impact conditions were considered. Puncture and CAI low velocity impact tests allows to evaluate the influence of different boundary conditions on composites behaviour in fact CAI constraints are less restrictive and further mechanisms of deformation and damage intervene to face the impact. Ballistic impact tests permit to assess the effect of impact mass and velocity moving from low velocity impacts performed with a heavy mass to more localized high velocity impacts with a lighter mass. The sole core and skins were also subjected to the same type of testing to split up the impact behaviour of the single components and to evaluate at a later time how it evolves as a consequence of composite ensemble. Temperature is another parameter that can totally modify structure reaction to impact determining an unpredictable structural failure and that is why CAI impacts were performed at room temperature but also at low and high temperatures, i.e. -40 °C and 60 °C. The resulting damage scenario and the morphology of the overall structures and of the single components were investigated through the help of non-destructive techniques such as profilometry and scanning electron microscopy. Since agglomerated cork main feature is the peerless dimensional recovery further impact investigations were carried out on the sole cores to examine more in depth their compressive behaviour. Drop weight tower dynamic compression tests and split Hopkinson pressure bar tests allowed to study the effect of different strain rates, to investigate the anisotropy induced by the production process and the effect of temperature. Moreover, multiple-impact tests permitted to evaluate the effect of an accumulation of damage over time induced by minor impact events rather than the detrimental effect caused by a single impact. The use of PP skins is fundamental because a thermoplastic polymer can be easily reprocessed and recycled at the end of its life cycle thus increasing the eco-compatibility of the composite. Moreover, it is useful to face sandwich structures predisposition to impact damage being more ductile than the brittle thermosetting matrices and with huge plastic deformations providing additional energy dissipation mechanisms. The poor fibre-matrix interfacial adhesion in natural fibre laminates can be enhanced through the addition of a coupling agent that acts as a bridge between the hydrophilic fibre and the hydrophobic matrix. For this reason, the skins in this work were produced with and without a maleic anhydride grafted PP as coupling agent to evaluate its effect on both quasi-static and dynamic properties. If the enhancement of fibre-matrix interface plays a positive role on the quasi-static performances of the composite, it causes embrittlement when the laminate is subjected to impact determining a decrease of its perforation threshold. To legitimate the extensive work carried out with the experimental campaign a finite element modelling analysis was carried out on agglomerated cork and PVC foam in order to standardize the modelling of a relatively new material as agglomerated cork, encouraging its spread at an industrial level where the use of well-established materials is normally preferred to reduce the possibility of setbacks. The results show a good matching between the experimental curves and the numerical data obtained.