For many decades, fiber reinforced polymers (FRPs) have been extensively utilized in load-bearing structures. Their formability and superior in-plane mechanical properties have made them a viable replacement for conventional structural materials.  A major drawback to FRPs is their weak interlaminar properties (e.g., interlaminar fracture toughness). The need for lightweight multifunctional structures has become vital for many applications and hence alleviating the out-of-plane mechanical (i.e., quasi-static, vibration, and impact) and electrical properties of FRPs while retaining minimal weight is the subject of many ongoing studies. The primary objective of this dissertation is to investigate the fundamental processes for developing hybrid, multifunctional composites based on surface grown carbon nanotubes (CNTs) on carbon fibers' yarns. This study embraces the development of a novel low temperature synthesis technique to grow CNTs on virtually any substrate. The developed method graphitic structures by design (GSD) offers the opportunity to place CNTs in advantageous areas of the composite (e.g., at the ply interface) where conventional fiber architectures are inadequate. The relatively low temperature of the GSD (i.e. 550 C) suppresses the undesired damage to the substrate fibers. GSD carries the advantage of growing uniform and almost aligned CNTs at pre-designated locations and thus eliminates the agglomeration and dispersion problems associated with incorporating CNTs in polymeric composites. The temperature regime utilized in GSD is less than those utilized by other synthesis techniques such as catalytic chemical vapor deposition (CCVD) where growing CNTs requires temperature not less than 700 °C. It is of great importance to comprehend the reasons for and against using the methods involving mixing of the CNTs directly with the polymer matrix, to either fabricate nanocomposites or three-phase FRPs. Hence, chapter 2 is devoted to the characterization of CNTs-epoxy nanocomposites at different thermo-mechanical environments via the nanoindentation technique. Improvements in hardness and stiffness of the CNTs-reinforced epoxy are reported. Long duration (45 mins) nanocreep tests were conducted to study the viscoelastic behavior of the CNT-nanocomposites. Finally, the energy absorption of these nanocomposites is measured via novel nanoimpact testing module. Chapter 3 elucidates a study on the fabrication and characterization of a three phase CNT-epoxy system reinforced with woven carbon fibers. Tensile test, high velocity impact (~100 ms⁻¹), and dynamic mechanical analysis (DMA) were employed to examine the response of the hybrid composite and compare it with the reference CFRP with no CNTs. Quasi-static shear punch tests (QSSPTs) were also performed to determine the toughening and damage mechanisms of both the CNTs-modified and the reference CFRP composites during transverse impact loading. The synthesis of CNTs at 550 C via GSD is the focus of chapter 4. The GSD technique was adjusted to grow Palladium-catalyzed carbon filaments over carbon fibers. However, these filaments were revealed to be amorphous (turbostratic) carbon.  Plasma sputtering was utilized to sputter nickel nano-films on the surface of the substrate carbon fibers. These films were later fragmented into nano-sized nickel islands from which CNTs were grown utilizing the GSD technique.  The structure and morphology of the CNTs are evaluated and compared to CNTs grown via catalytic chemical vapor deposition (CCVD) over the same carbon fibers. Chapter 5 embodies the mechanical characterization of composites based on carbon fibers with various surface treatments including, but not limited to, surface grown CNTs. Fibers with and without sizing were subjected to different treatments such as  heat treatment similar to those encountered during the GSD process, growing CNTs on fabrics via GSD and CCVD techniques, sputtering of the fibers with a thin thermal shield film of SiO₂ prior to CNT growth, selective growth of CNTs following checkerboard patterns, etc. The effects of the various surface treatments (at the ply interfaces) on the on-axis and off-axis tensile properties of the corresponding composites are discussed in this chapter. In addition, the DMA and impact resistance of the hybrid CNT-CFRP composites are measured and compared to the values obtained for the reference CFRP samples. While the GSD grown CNTs accounted for only 0.05 wt% of the composites, the results of this chapter contrasts the advantages of the GSD technique over other methods that incorporate CNTs into a CFRP (i.e. direct growth via CCVD and mixing of CNTs with the matrix). Understanding the behavior of the thin CFRPs under impact loadings and the ability to model their response under ballistic impact is essential for designing CFRP structures.  A precise simulation of impact phenomenon should account for progressive damage and strain rate dependent behavior of the CFRPs. In chapter 6, a novel procedure to calibrate the state-of-the-art MAT162 material model of the LS-DYNA finite element simulation package is proposed. Quasi-static tensile, compression, through thickness tension, and in-plane Isopescu shear tests along with quasi-static shear punch tests (QSSPTs) employing flat cylindrical and spherical punches were performed on the composite samples to find 28 input parameters of MAT162. Finally, the capability of this material model to simulate a transverse ballistic impact of a spherical impactor with the thin 5-layers CFRP is demonstrated. It is hypothesized that the high electrical conductivities of CNTs will span the multifunctionality of the hybrid composites by facilitating electromagnetic interference (EMI) shielding. Chapter 6 is devoted to characterizing the electrical properties of hybrid CNT-fiberglass FRPs modified via GSD method. Using a slightly modified version of the GSD, denser and longer CNTs were grown on fiberglass fabrics.  The EMI shielding performance of the composites based on these fabrics was shown to be superior to that for reference composites based on fiberglass and epoxy. To better apprehend the effect of the surface grown CNTs on the electrical properties of the resulting composites, the electrical resistivities of the hybrid and the reference composites were measured along different directions and some interesting results are highlighted herein. The work outlined in this dissertation will enable significant advancement in protection methods against different hazards including impact, vibrations and EMI events. / Ph. D.

Nature develops several materials with remarkable functional properties composed of comparatively simple base substances. Biological materials are often composites, which optime the conformation to their function. On the other hand, synthetic materials are designed a priori, structuring them according to the performance to be achieved. 3D printing manufacturing is the most direct method for specific component production and earmarks the sample with material and geometry designed ad-hoc for a defined purpose, starting from a biomimetic approach to functional structures. The technique has the advantage of being quick, accurate, and with a limited waste of materials. The sample printing occurs through the deposition of material layer by layer. Furthermore, the material is often a composite, which matches the characteristics of components with different geometry and properties, achieving better mechanical and physical performances. This thesis analyses the mechanics of natural and custom-made composites: the spider body and the manufacturing of metallic and non-metallic graphene composites. The spider body is investigated in different sections of the exoskeleton and specifically the fangs. The study involves the mechanical characterization of the single components by the nanoindentation technique, with a special focus on the hardness and Young's modulus. The experimental results were mapped, purposing to present an accurate comparison of the mechanical properties of the spider body. The different stiffness of components is due to the tuning of the same basic material (the cuticle, i.e. mainly composed of chitin) for achieving different mechanical functions, which have improved the animal adaptation to specific evolutive requirements. The synthetic composites, suitable for 3D printing fabrication, are metallic and non-metallic matrices combined with carbon-based fillers. Non-metallic graphene composites are multiscale compounds. Specifically, the material is a blend of acrylonitrile-butadiene-styrene (ABS) matrix and different percentages of micro-carbon fibers (MCF). In the second step, nanoscale filler of carbon nanotubes (CNT) or graphene nanoplatelets (GNP) are added to the base mixture. The production process of composite materials followed a specific protocol for the optimal procedure and the machine parameters, as also foreseen in the literature. This method allowed the control over the percentages of the different materials to be adopted and ensured a homogeneous distribution of fillers in the plastic matrix. Multiscale compounds provide the basic materials for the extrusion of fused filaments, suitable for 3D printing of the samples. The composites were tested in the configuration of compression moulded sheets, as reference tests, and also in the corresponding 3D printed specimens. The addition of the micro-filler inside the ABS matrix caused a notable increment in stiffness and a slight increase in strength, with a significant reduction in deformation at the break. Concurrently, the addition of nanofillers was very effective in improving electrical conductivity compared to pure ABS and micro-composites, even at the lowest filler content. Composites with GNP as a nano-filler had a good impact on the stiffness of the materials, while the electrical conductivity of the composites is favoured by the presence of CNTs. Moreover, the extrusion of the filament and the print of fused filament fabrication led to the creation of voids within the structure, causing a significant loss of mechanical properties and a slight improvement in the electrical conductivity of the multiscale moulded composites. The final aim of this work is the identification of 3D-printed multiscale composites capable of the best matching of mechanical and electrical properties among the different compounds proposed. Since structures with metallic matrix and high mechanical performances are suitable for aerospace and automotive industry applications, metallic graphene composites are studied in the additive manufacturing sector. A comprehensive study of the mechanical and electrical properties of an innovative copper-graphene oxide composite (Cu-GO) was developed in collaboration with Fondazione E. Amaldi, in Rome. An extensive survey campaign on the working conditions was developed, leading to the definition of an optimal protocol of printing parameters for obtaining the samples with the highest density. The composite powders were prepared following two different routes to disperse the nanofiller into Cu matrix and, afterward, were processed by selective laser melting (SLM) technique. Analyses of the morphology, macroscopic and microscopic structure, and degree of oxidation of the printed samples were performed. Samples prepared followed the mechanical mixing procedure showed a better response to the 3D printing process in all tests. The mechanical characterization has instead provided a clear increase in the resistance of the material prepared with the ultrasonicated bath method, despite the greater porosity of specimens. The interesting comparison obtained between samples from different routes highlights the influence of powder preparation and working conditions on the printing results. We hope that the research could be useful to investigate in detail the potential applications suitable for composites in different technological fields and stimulate further comparative analysis.