The contribution of nanostructures to the structural adhesives fracture toughness

This work of thesis investigates the effects of nanostructures on the fracture toughness of bonded joints and to evaluate their applicability at an industrial level. Fracture toughness was estimated by calculating the critical value of the Mode I strain energy release rate based on Double Cantilever Beam (DCB) test data. Three types of nanostructures were investigated: electrospun nanofibres, multiwalled carbon nanotubes, and graphene nanoplatelets. The effect of the integration of nylon and rubbery nanofibres, developed and produced at the University of Bologna, was studied. These nanofibres were impregnated with a structural epoxy resin to produce prepreg layers. The prepregs obtained were used to manufacture S235 steel joints. The fracture toughness of joints bonded with both nylon and rubbery nanofibers was lower than that of virgin samples. However, rubbery nanofibers exhibited improved performance compared to nylon ones. The SEM analyses of fracture surfaces of nylon nanomodified specimens revealed no presence of microdimples, which are typically observed in virgin samples, and no evidence of fiber bridging or pull-out mechanisms. The rubber modified specimens show no fiber bridging and pull-out phenomena, similar to that seen in nylon specimens. The increased fracture toughness values for the rubber modified specimens can be attributed to the presence of a rubbery phase, which absorbs more energy than nylon nanofibers. The addition of nanofibers to an adhesive matrix does not increase adhesive fracture toughness, but it does provide a preferential pathway for crack propagation that exfoliates the nanomat embedded in the epoxy matrix. Furthermore, adhesives modified with nanofibers lead to more reproducible results, cohesive fracture, and constant fracture toughness values. The effect of commercial XD 10 Polyamide nanofibers (XantuLayr) within composite joints bonded with the epoxy film was also studied. Materials and bonding techniques commonly employed in the automotive and aerospace sector were used for joint manufacturing. The nanomat was used in two different ways. The first consists of applying the nanofibres to the adhesive/adherends interfaces, and the second consists of interleaving the nanofibres between two layers of adhesive. The application of XanturLayr nanofibers at the adhesive/adherend interface of composite joints significantly improves their fracture toughness, as evidenced by SEM images showing nanofibers stretched and pulled out from the matrix. However, when the same nanomat is applied at the center of the adhesive layer, it does not contribute to the fracture toughness of the joints. Finally, the toughening effects of multiwalled carbon nanotubes (MWCNT) and graphene nanoplates (GNP) integrated with different concentrations were investigated. Carbon-based nanofiller were integrated in an epoxy film used to bond composite substrate. At low MWCNT concentrations the crack propagates into the adherend, indicating that the surface preparation is adequate and that the use of solvent did not generate a weak layer at the interface. Increasing the fraction of MWCNTs promotes crack propagation in the adhesive, but the it is less prone to plastic deformation mechanisms. Further addition of MWCNTs promotes crack deflection mechanisms, mainly in the vicinity of CNT-rich zones near the interface. The addition of GNP shows similar behavior: at low concentrations GNP caused a sharp decrease in fracture toughness, likely due to the reduced ability of the adhesive to plastically deform and absorb energy. However, at higher concentrations crack deviation and bridging mechanisms may positively contribute to the fracture toughness of the joints, resulting in fracture toughness values similar to those of virgin joints. The use of carbon-based nanofillers improves the electrical conductivity of composite joints. The electrical resistance between two points through the thickness of the joints was measured during mechanical testing to investigate the potential relationship between resistance variation and crack length. The initial electrical resistance of the joints was measured and was found to be affected by a variety of factors, such as composite resistance and the quality and stability of the electrodes. Further optimization of the adhesive layer and of the test set up may allow for precise control over the electrical resistance of the joints.