PSI - Issue 33

Costanzo Bellini et al. / Procedia Structural Integrity 33 (2021) 824–831 Author name / Structural Integrity Procedia 00 (2019) 000–000

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indicated by Vermeeren (2003) and Bellini et al. (2020). In fact, FMLs offer very interesting mechanical properties, as high impact strength, high fatigue resistance, and high damage tolerance together with high stiffness and low weight, which are characteristics typical of composite material, as stated by Bellini et al. (2019a and 2019b). As known, in the last years, composite materials have been more and more adopted for the production of modern aircraft structures, such as vertical and horizontal stabilizers, ailerons, flaps, elevators and rudders. Today, also the fibre metal laminates are considered for aeronautical applications: for example, some fuselage panels of the biggest passenger plane in the world, the A380, are made of GLARE (Glass Laminate Aluminium Reinforced Epoxy), that is an FML based on glass fibre reinforced polymer, as said by Li et al. (2015). There are several kinds of FMLs, whose categorization can be carried out according to the type of reinforcement fibre of the composite materials. For instance, CARALL (Carbon Fibre Reinforced Aluminium Laminate) and ARALL (Aramid Fibre Reinforced Aluminium Laminate) are both constituted by aluminium sheets, but the former has carbon fibre composite material layers and the latter aramid fibre one (Bellini et al. (2019c and 2019d)). Moreover, not only aluminium sheets can be used for the realization of FMLs, but also other low specific weight metals can be taken into consideration: for instance, the TiGr (Titanium Graphite) is made of carbon fibre and titanium (Li et al. (2017)). In this work, attention has been paid to the CARALL, which is a hybrid material composed of aluminium sheets alternated to carbon fibre composite material. Being used in several applications, some studies have been carried out proposing numerical models for the simulation of the mechanical behaviour of FMLs. Bikakis and Savaidis (2016) implemented a non-linear model to calculate the static loading and unloading response of simply supported FML plates, considering a lateral indentation through a circular hemispherical indentator, and they found relations among the indentator radius, the maximum load and the permanent dent depth. Zolkiewski (2013) found that the incongruities between numerical and experimental results are given by the incorrectness of material properties values, but the lamination sequence too can affect the accuracy of the numerical results. In another work, the same author asserted that mesh characteristics influenced the calculation, as well as the accordance between physical and numerical boundary conditions (Zolkiewski (2015)). Hundley et al. (2011) proposed a three-dimensional numerical model for the simulation of the bolt bearing strength in TiGr, adding a progressive failure constitutive model for the composite material layers to enhance the prediction accuracy. Dhaliwal and Newaz (2016) used a numerical model, based on the explicit time integration scheme, to simulate the statical behaviour of an FML, considering different lamination sequences too. The same authors implemented a numerical model also to evaluate the delamination induced by compression after impact (Dhaliwal and Newaz (2017)). The in-plane flexural behaviour of an FML was investigated both numerically and experimentally by Xu et al. (2017), that considered a cohesive zone model to calculate the inter-laminar failure mode characteristics of the composite material. Wittenberg et al. (2001) proposed finite element analysis to substitute experimental tests in designing the fuselage shear panels made of FML. Nam et al. (2001) combined genetic algorithms and finite element simulation to determine the best ply orientation angles of an FML, finding that FML is better than composite material in several loading conditions. Iaccarino et al. (2007) modified the classical lamination theory in order to take into account the inelastic behaviour and the anisotropy of the aluminium in the FML, obtaining a simplified model suitable to simulate both the tensile and shear stress-strain behaviour. Vasumathi and Murali (2013) substituted carbon fibres with natural ones in FML based on aluminium and magnesium, and the introduced numerical model was able to simulate the tensile, flexural and impact behaviour of the new material. Sadighi et al. (2012) used a numerical model to simulate the low-velocity impact behaviour of glass fibre FML, concluding that the proper choice of the mesh element was more important than the failure criterion to determine acceptable results. Lopes et al. (2008) simulated the double cantilever beam, the end notched flexure, and the mixed mode beam tests implementing a finite element model and considering the cohesive zone modelling to simulate the bond between two layers. The present work deals with the prediction of the flexural behaviour of FMLs by using a numerical model, that was implemented in FEM software. In particular, the attention was focused not only on the flexural strength, but also on the interlaminar shear strength, so the model took into account several aspects of the phenomena happening in the loaded specimen, such as the material failure due to the normal stress, the progressive damaging, and the delamination due to the shear stress, in order to be able to simulate different load conditions. This work is organized in several steps: first of all, there is the definition of mechanical tests to be considered for the simulation, and for the experimental activity too, that was carried out to validate the model. The dimensions of the specimens, the materials