• Composite materials
• Hybrid composites
• Mechanical properties
• Micromechanics
• Non-linear Finite Element Method
• Statistical analyses

Composite materials are a consolidated reality in the worldwide technological scenario. Their specific stiffness and strength, merged to high flexibility in manufacturing, render composites ideal materials for aerospace structural applications.
While ensuring many advantages, only a little part of their true potential can be exploited since some of their degradation and failure mechanisms have not been completely understood yet. One of the more concerning aspect is the intrinsic brittleness of traditional aerospace graded composites that causes undesirably abrupt structural failures. A possible way to mitigate such behaviour is to develop composites with hybrid fiber systems. This approach can be effectively pursued once failure mechanisms at the constituents scale (microscale) have been clearly understood and tools developed to analyze composite at such scale.
The aim of this thesis is twofold: firstly to develop high-fidelity micro-mechanical models capable to give an insight of the damage onset at the constituents level and, secondly, to move first steps into the field of composite hybridization as a toughening mechanism.
The proposed micro-mechanical modeling approach ensures an accurate reproduction of unidirectional composites together with a detailed characterization of their constituents: fibers, matrix and inter-phase. In order to quantify the fibers dispersion, an analysis tool for unidirectional composite micrographs has been developed and endowed with an algorithm for statistical spatial analyses.
Moreover, information on the fibers pattern have been collected to feed a code that automatically generate of representative volume elements with statistically equivalent fibers distributions.
Once the material geometries have been validated, the other crucial point in the definition of high-fidelity models is the characterization of the constituents mechanical behaviour. The matrix is modeled as an isotropic elasto-palstic damageable solid. A pressure dependent yield surface has been implemented and hardening functions calibrated with experimental data reported in recent literature.
The fibers have been characterized as brittle damageable transversely isotropic solids. Crack band theory has been implemented for both fiber and matrix damage modes in order to avoid mesh sensitivity issues. A bilinear traction separation cohesive zone model approach has been introduced for the simulation of the interphase. All these constitutive behaviours have been defined into the commercial software Abaqus by means of a user-defined material subroutine (UMAT).
Linear elastic simulations have been performed to check the modeling procedure and results have been compared to experimental values of material moduli.
Non-linear simulations have been performed in order to investigate the effect of the fiber-matrix interphase and the composites failure modes due to microdebonding and to the curing process. Micro-debonding between the matrix and the fibers has been introduced via a state variable initialization procedure through a dedicated Fortran subroutine (SDVINI). A numerical reproduction of single fiber fragmentation test has been performed and fed with experimental data.
The effects on material stiffness and damage onset and propagation are investigated under different loading conditions. By tailoring the matrix constitutive behaviour with a cure kinetic model, the effect of resin stiffening and shrinkage during the curing has been introduced and results have been produced for the transverse tensile loading condition.
Nowadays the possibility of manufacturing very thin plies allows the use of hybridization techniques as a toughening strategy. In the second part of the thesis the effect of the hybridization is investigated for carbon composites of aeronautical interest built by using micro-ply CFRP prepregs. An experimental campaign has been carried out to investigate the mechanical behaviour of different fiber systems both in thin-ply hybrid and non-hybrid configuration. An analytical tool for the prediction of hybrid composite behaviour has been developed and used to asses the pseudo ductility of a number of hybrid solutions. Specimens have been manufactured for the testing of the selected fiber systems: T800 and HR40.
Single-fiber tests have been performed on fibers and data analyzed via a statistical approach. Material configurations were compared in terms of longitudinal tensile response and fiber fracture toughness through the double edge notched test. Transverse tensile tests and in-plane shear tests have been carried out for a full definition of the reference hybrid system as well as for the thin-ply composite used in the hybridization process.