Compositi a base di lino: ViscoElastoPlastic Caratterizzazione sperimentale e modellazione

In recent times, the need in the world for more eco-friendly materials is ever increasing. Therefore, scientists and researchers focus on developing new materials that would improve the environmental quality of products. This demand for greener materials has led to the usage of composites realized by natural fibers (NFs) and polymer matrices; nowadays this topic represents one of the most largely investigated research aim The use of natural fibres as reinforcement in composites has gained attention thanks to an increasing requirement for developing sustainable materials. NFs help control pollution issues and represent a viable alternative to conventional synthetic fibers that are harmful to the environment. Moreover, NFs have attractive features such as good mechanical properties, low production energy consumption, and low cost. Among others, flax fibers (FFs) are cost-effective and offer specific mechanical properties comparable to those of conventional glass fibres (GFs). Recently, composites based on FFs and bio resins also show encouraging mechanical properties. Keeping in view all the benefits of natural fiber reinforced polymer composites, the present work aims to mechanical characterization of composite material based on flax fiber reinforcements. The first section presents the results of an extensive experimental campaign on unidirectional flax fiber-based composites (FFRCs) produced through Light and Vacuum Resin Transfer Moulding techniques (L- and VRTM). Conventional composites such as glass and carbon fiber reinforced polymers (G - and CFRPs) are approximated to linear-elastic materials with brittle failure in most cases. Contrarily, on the other side, natural fiber-reinforced composites (NFRCs) are characterized by non-linear elasticity, viscous effects, and plastic strains far before failure. Then, in the second section of this work, the nonlinear phenomena characterizing the stiffness evolution and its relations with damage propagation and accumulated inelastic strain are investigated and analyzed through pure monotonic loading tension, cyclic tension, and repeated progressive loading tests. In fact, the analysis of the experimental results focuses mainly on the assessment of the stiffness evolution as the inelastic strain is accumulated in the sample. Results highlighted the need for a complex material model in order to predict the mechanical behavior of flax-based composites. The growing demand for use of bio-based composites in different engineering and structural applications requires proper test methods and models for predicting their long-term behavior. Creep resistance in natural fiber-reinforced polymer (NFRP) laminates is a very important characteristic to assess and is still not well understood: poor information is available about the creep deformation of the composite material based on natural fibers or biodegradable matrices obtained from renewable sources. Hence, in 5 the third section, the long-term performance of flax fiber-based composites (FFRC) will be presented. The mechanical properties and the creep behavior of unidirectional FFRC under tensile stress at ambient temperature were investigated. Again, the related results highlight the role of viscoelastic, viscoplastic, and pseudo-plastic components in the full mechanical response. Then, based on the complete experimental data campaign, rheological predictive models that are candidates to fit the measured stress-strain-time behavior of the flax-based laminates were analyzed. In the fourth section, the implementation of a genetic algorithm-based method to identify the models’ parameters able to predict successfully the behavior of FFRCs loaded at different strain rates and for different load histories will be discussed