Issue 51

C. Bellini et alii, Frattura ed Integrità Strutturale, 51 (2020) 442-448; DOI: 10.3221/IGF-ESIS.51.32 443 corrosion [4]. Indeed, the composite material nature strongly determines the structural characteristics of the whole hybrid laminate; in fact, a higher impact toughness is given by the aramid fibres, while carbon fibres are not suitable for this purpose; furthermore, the latter is more appropriate for high-cycle fatigue applications, while the former for low-cycle fatigue ones [5]. In the present work, the influence of both the metal/composite bonding and the layer thickness on the ILSS value of different hybrid laminates was investigated. This kind of mechanical test was chosen because it highlights the characteristics of the interface between the constituents of the FML. In a composite material, the ILSS is a property that depends on the matrix, since it relies on the adhesion between matrix and fibres. In an FML, the adhesion between metal sheets and composite material layer holds the same role; in fact, the rupture generally happens at the interface between these materials. The surface preparation is a factor that must be evaluated when designing FMLs and composites in general for critical applications [6]. The ILSS of a laminate is an important parameter since the separation of the layers, that is the delamination, makes the bending stiffness drop and, consequently, the bending deformation grow. It is important to emphasize that the attention was paid not only to the maximum reached loads but also to the whole force-displacement curves, in order to evaluate the behaviour prior to and after the main damage occurrence. Other significant singularities of the present work consist in the reinforcement architecture and the stacking sequence: other researchers considered unidirectional reinforcements and metal sheet as external layers, while this work deals with woven fabric and the external layers are made of composite material. In the literature there are some other works dealing with the ILSS of FMLs, but, to our best knowledge, the analysis of the effect of both the layer thickness and the metal/composite interface on the ILSS and the loading curves has never been investigated. Several works aimed at improving the bonding between metal and composite material. Three different solutions for CARALL were compared by Ning et al.: the enhancement of the composite-metal interface due to the addition of nanoparticles and the chemical etching or mechanical patterning on the surface of the metal sheet. The influence of the surface treatment of the sheet for the production of GLARE was investigated by Mamalis et al. [7] and Park et al. [8]. The advisability of a glass layer at the metal/composite interface of a CARALL was evaluated by Jakubczak et al. [9]. The influence of the fibre treatment on the mechanical characteristics was compared for a composite laminate and a FML by Lawcock et al. [10,11]. The effects of adhesive thickness between metal and composite on the structural characteristics of GLARE was investigated by Li et al. [12]; instead, the influence of loading conditions on the shear strength and the damage mechanism were studied by Liu et al. [13]. The influence of the hygrothermal ageing on the ILSS of CARALL was investigated by Botelho et al. [14] and by Pan et al. [15], while that of testing temperature on the ILSS of GLARE was studied by Hinz et al. [16]. An in-depth study on the flexural behaviour of different types of CARALL, characterized by different layer thickness and composite/metal bonding, was carried out by Bellini et al. [17–20]. M ATERIALS AND METHODS s mentioned in the introduction paragraph, in this work the shear strength of CFRP/Al hybrid laminates was examined. In particular, the aim consisted in assessing the influence of the layer thickness and the interface bonding on the ILSS of this type of material. An experimental plan was conceived, that considered two levels for each explored factor; therefore, a total of three different laminates were produced. As concerns the layer thickness, a type of laminate presented three CFRP layers and two aluminium sheets, while the other was constituted by two aluminium sheets and a CFRP layer. It must be remembered that all the laminates presented the composite material as external layers, while the metal was inside. Moreover, the thickness of the various layers was chosen in order to obtain a constant composite material/metal ratio and a total laminate thickness equal to 5 mm. Consequently, the metal sheet thickness was 0.3 mm or 0.6 mm (for the FML with two sheets or a single sheet, respectively) and the number of prepreg plies was 12, that were grouped in threes or fours to produce both types of FML. As regards the other parameter, that is the bonding solution between aluminium and CFRP, a kind of laminates was produced inserting a layer of structural adhesive, the AF 162 2k, at the interface, while in the other one any adhesive was absent, hence the adhesion relied on the prepreg resin only. All the FML analysed in this work were manufactured considering the prepreg hand layup process, also known as vacuum bag process, a manufacturing technique common in the aeronautical industry. Several steps were necessary to make the laminates. The process started with the mould preparation: a release agent was spread on the surface of a steel plate, that had a thickness of 10 mm, in order to allow part removal at the end of the curing process. After, the prepreg plies and the metal sheets were cut in the suitable dimensions and stacked on the mould, paying attention to respect the established stacking sequence. Then, the vacuum bag was prepared using all the necessary ancillary materials, as the release film and the breather fabric, and the laminates were sealed under the vacuum bag. Once the sealing operation was concluded, the mould A