Issue 49

C. Bellini et alii, Frattura ed Integrità Strutturale, 49 (2019) 739-747; DOI: 10.3221/IGF-ESIS.49.66 740 properties to the FMLs, such as high strength, high damage tolerance and low density [1]. These extraordinary structural properties are due to the particular characteristics of the materials the FMLs are made of; in fact, the FMLs made of aluminium and glass fibre composite, that are widespread in the aeronautical field, are less robust than those based on CFRP (carbon fibre reinforced polymer) [2]; in fact, CARALL (carbon fibre-reinforced aluminium laminates) are about 10% tougher than GLARE (glass fibre reinforced aluminium laminates) [3]. In general, carbon-based FMLs present superior characteristics, as regards the ability to withstand crashes, energy absorption capacity, tensile modulus, yield strength and fatigue strength, in comparison with glass fibre or aramid-based FMLs [4]. The FMLs can be considered for ballistic applications too, together with aramid fibre composites [5]. Usually, structural parts are subjected to flexural loads, that represents the most diffused, and consequently most studied failure mode. The elements an FML is made of have dissimilar properties, causing a complex damage behaviour; in fact, the composite layers are brittle, while metal layers are ductile. The most diffused failure mechanisms of FLMs are matrix cracking, metal layers plastic deformation, fibres fracture, composite layers delamination and debonding between metal and composite [6]. Hu et al. [7] analysed the flexural properties of FMLs made of carbon fibres reinforced PMR polyimide and titanium, finding a good structural strength at both room and high temperature. They also found that the micro roughness of the titanium surface layer improved the adhesion between composite material and titanium itself. Lawcock et al. [8] found that the bending characteristics of CARALL depend on the adhesion between the composite laminate and the aluminium layers, while the tensile properties are not affected; in fact, a weak bonding can give rise to a decrement of about 10% for the interlaminar shear strength. They determined that the bonding strength did not influence the residual strength of notched specimen, even if a small decrease for a specimen with a stronger bonding was found. A similar result was stated by Botelho et al. [9] that treated the aluminium surface with two different processes: sulphuric chromic acid etching and chromic acid anodization. They found that the latter method brought to better surface wetting, but the interlaminar shear strength of both GLARE and CARALL was not influenced. The effect of the metal sheet location along the laminate thickness was studied by Dhaliwal and Newaz [10], that produced and tested some CARALL specimens presenting carbon fibre laminate as outside layers. They compared the flexural properties of those laminates with that of common CARALL, that had aluminium layers outside, determining a superior strength for their material. They also made a comparison between CARALL and bulk aluminium specimens, determined a higher bending strength of the former ones. The effect of the aluminium sheet strength and the fibres directions on the in-plane flexural behaviour of CARALLs was analysed by Xu et al. [2]. They discovered an increment of the flexural strength as both the amount of the longitudinal fibres and the metal strength were increased. As regards the progressive failure process, at first the aluminium sheets yield appeared, together with tension damage of fibre and resin in the section bottom and resin compression crushing in the section top; after, the delamination started in the laminate mid-span, caused by the unstable deformation. A work has been carried out aiming at analysing the behaviour of laminates with the same composite/metal volume fraction ratio and different layers thicknesses by Wu et al. [11]. These authors studied the flexural behaviour of carbon fibre/magnesium FMLs, discovering that the flexural modulus linearly decreased with the layer thicknesses, while no differences were observed for the flexural strength. However, the layer thickness decrease made the failure area shift from compression to tension region. Moreover, the bonding behaviour is fundamental for composites and FMLs [12,13]. The aim of the present work regards the analysis of the flexural behaviour of CARALL specimens, studying the influence of both layer thickness and the adhesion solution between CFRP laminate and aluminium sheet. In particular, the attention was paid not only to the maximum flexural strength reached by the specimens, but also to the stress-strain response after the first elastic phase and the first stress peak, that represents the maximum stress. In this manner, it is possible to determine also which parameter combination presents the highest toughness and the highest safety after the first peak stress. Moreover, unidirectional reinforcements are usually employed in the design of FMLs, while in this study a carbon fabric was considered as composite material reinforcement. The effect of the stacking characteristic on the laminate mechanical behaviour was studied by several researchers, even if they keep the composite laminate thickness [14,15] or metal sheet thickness [16,17] constant and focused the attention on the dynamic characterization. The layer thickness is a significant factor concerning the structural behaviour of FMLs that should be taken into consideration for the product design, even if the study of this topic has been infrequently explored [11]. M ATERIALS AND M ETHODS n this work, the effects of layer thickness and of the layer adhesion were investigated both separately and combined. As reported in Tab. 1 , four different specimen types composed the full factorial plan of the experimental activity, that I