PSI - Issue 26

Costanzo Bellini et al. / Procedia Structural Integrity 26 (2020) 120–128 Bellini et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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adopted to produce several parts in the aeronautical and automotive industries. Moreover, FMLs are characterized by other properties, as the high damage tolerance, the high fatigue strength and the high impact strength, that are conferred by the combination of the constituting materials, as stated by Vermeeren (2003). In fact, an FML consists in a stack of metal sheets alternating to composite material layers: in such a manner, the best characteristics of both constituents are combined together and a new material with outstanding characteristics is produced, as specified by Bellini et al. (2019a). As known, in the last years composite materials are used for the construction of modern aircraft structures, such as the vertical and horizontal stabilizers, flaps, ailerons, rudders and elevators, but today the FMLs are considered too for aeronautical applications, in fact the fuselage panels of the A380, the biggest passenger plane in the world, are made of GLARE (Glass Laminate Aluminium Reinforced Epoxy), as indicated by Li et al. (2015). Today, there are different types of FML, that can be classified based on the reinforcement fibre types of composite materials. For example, ARALL (Aramid Fibre Reinforced Aluminium Laminate) and CARALL (Carbon Fibre Reinforced Aluminium Laminate) are made of aluminium sheets and composite material presenting aramid and carbon fibre, respectively. Moreover, not only the aluminium is adopted to manufacture fibre metal laminates, but also other metals with a low specific weight can be found, as the titanium that is used in the TIGR (Titanium Graphite). One of the most frequent failure modes is connected to flexural stress, since the frame parts are subjected to bending loads, as stated by Bellini et al. (2019b), and the flexural behaviour of several types of FML has been studied in the past by several authors. For example, the enhancement at both room and high temperatures of the CARALL flexural behaviour through polyimide and titanium sheets was proposed by Hu et al. (2015), while the effect of the fibre pre-treatment on the residual strength was studied by Lawcock et al. (1998). Instead, the structural characteristics of FML were improved through the physical and chemical treatments of the metal sheets by Mamalis et al. (2019). The surface preparation is an important step that must be taken into consideration when considering bonding, and this is valid not only for the FML but also for composites in general, as attested by Sorrentino et al. (2018). The influence of the layer thickness on the flexural behaviour of the FML was investigated by Wu et al. (2017), while that one of the adhesive thickness was explored by Li et al. (2016). The effect of the position of the metal sheets along the thickness of some FMLs was investigated by Dhaliwal and Newaz (2016). In particular, they studied two kinds of laminate: one with aluminium sheet as external layer and the other one with carbon laminate in that position, and they found that the latter one was stiffer, even if the failure strength of both FMLs was similar. A comparison of the corrosion resistance between carbon-based FML with aluminium sheet and bulk metallic glass was carried out by Hamill et al. (2018), that found a higher strength in the latter ones. As stated by Sen et al. (2015), the composite plies orientation, together with the number of layers and their thickness, is able to control the mechanical characteristics of an FML. In fact, an increase of the bending strength with the longitudinal fibres volumetric fraction was found by Xu et al. (2017), that carried out some experimental tests on laminates presenting plies with different reinforcement orientation. The bending stiffness and the bending deformation depend on the delamination between layers, that is damage consisting in the separation of the layers, that is evaluated through the ILSS (Interlaminar shear strength) test and it makes the bending stiffness decrease, as it appears, as indicated by Bellini et al. (2019c). In the literature, there are different works concerning the ILSS. For example, Ning. et al. (2015) studied three different toughening methods for the ILSS improvement: acid etching, mechanical patterning and vapour grown of carbon fibre at the metal/composite interface, and they tested these methods both individually and in combination. They found that the best solution was represented by the combination of acid etching and vapour grown of carbon fibre. Park et al. (2010) evaluated the effect of the surface morphology, varying the surface texture and roughness through different treatments, like etching, sanding and nylon-pad abrasion. They found that the surface modification improved the interface between aluminium and composite material, and the highest ILSS were found for the sandpaper abrasion with a grit size of #100 grit. Jakubczak et al. (2018) added a glass fibre interlayer at the interface between carbon laminates and aluminium and they found that this modification, typically used to avoid galvanic effect between carbon and aluminium, did not affect the ILSS of the CARALLs. The aim of the present work is the analysis of the CARALL flexural behaviour. In particular, the attention was focused not only on the flexural strength but also on the ILSS. In fact, both long beam and short beam specimens were produced and tested: the former was suitable to determine the flexural strength, the latter the ILSS, since the length to-thickness ratio influences the stress type arising in the material and, consequently, the failure mode. Moreover, the