PSI - Issue 18

Costanzo Bellini et al. / Procedia Structural Integrity 18 (2019) 373–378 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Aluminium Reinforced Epoxy), even if it possessed a lower stiffness value compared to monolithic aluminium; therefore, other combinations of composite and metal materials have been presented in the last years, as asserted by Cortés and Cantwell (2005), Lee et al. (2014) and Li et al (2015). FMLs can be employed for ballistic applications too, even if composite laminates based on aramid fibre are also applicable (Sorrentino et al. (2014)). Among these hybrid materials, one of the most widespread is the CARALL (Carbon Fibre-Reinforced Aluminium Laminates), since its stiffness is higher than that of GLARE; on the other hand, this material suffers from galvanic corrosion, as observed by Hamill et al. (2018) in their researches. The mechanical properties of the FML depend on the reinforcing fibre type. In fact, composite laminates based on aramid fibre present high impact tolerance, instead that one based on carbon fibre and glass fibre possess lower impact toughness; moreover, the carbon fibre ones better support the high-cycle fatigue, instead aramid fibre ones the low-cycle one, as stated by Botelho et al. (2004). In general, for a material, one of the most critical structural property to be kept in view is the ILSS (InterLaminar Shear Strength), as affirmed by Hundely et al. (2011). In the years, several methods have been implemented to enhance the interlaminar strength of FMLs. For example, Ning et al. (2015) evaluated different methods: the metal surface pre treatment by acid etching or mechanical patterning and the improvement of the metal-composite interface by nanoparticles. Jakubczak et al. (2018) studied the effect of additional glass interlayer on the ILLS of CARALL subjected to thermal cycles, while Park et al. (2010) and Mamalis et al. (2019) analysed the influence of surface pre treatment on the structural properties of GLARE. Lawcock et al. (1998a and 1998b) investigated the effect of fibre treatment on the static and dynamic mechanical characteristics of both bulk composite and FML. Li et al. (2016) carried out studies on how the thickness of the adhesive layer affected the mechanical behaviour of an FLM, while Liu et al. (2016) analysed the effect of loading conditions on the ILSS of such laminates, examining the failure mechanism by SEM investigation. The effects of the test temperature on the ILSS of GLARE were investigated by Hinz et al. (2005), while the hygrothermal ageing of CARALL was analysed by Botelho et al. (2007). In some recent works, Bellini et al. (2019a and 2019b) studied the flexural behaviour of CARALL, carrying out three-point bending tests on hybrid laminates with different metal/composite interfaces and different number of metal sheet. The surface preparation and the in-service bonding behaviour are two questions that must be taken into account when adopting FMLs and composites in general for critical applications, as asserted by Bellini et al. (2018) and Sorrentino et al. (2018). This article deals with a survey of the ILSS behaviour of CFRP/aluminium laminates samples, exploring the influence of the interface between composite and metal and the number of sheet metal on this mechanical parameter. In fact, the aim of this research activity was the evaluation of the interface strength, so ILSS was chosen as reference tool since this kind of structural parameter is dependent on the adhesion between the sheet metal and the composite material. In the past, this theme has been almost not explored: Li et al. (2016) carried out some works on the impact of the adhesive thickness, while Pan et al. (2017) studied the effects of hygrothermal ageing. Moreover, it is worth to highlight that in this work the fibrous reinforcement of the composite material is in the form of a fabric; on the contrary, the other researchers usually chose unidirectional fibres as reinforcement type. 2. Materials and methods The current research activity investigates the ILSS behaviour of different types of FMLs based on CFRP laminate and aluminium sheet; for this aim, some hybrid laminates were produced with structural characteristic in compliance with the full factorial plan showed in table 1. As can be seen, the experimental tests plan consisted of two factors: one was the adhesive type and the other one was the number of metal sheets. As regards the former factor, in a kind of laminates a structural adhesive, the AF 163 2k, was used to realize the adhesion between the aluminium and the composite layer, while in the other type the resin of the prepreg material was used to guarantee the interface. As concerns the latter factor, one laminate had one aluminium sheet, instead the other one had two metal sheets, with the composite material layers on the exterior surfaces in both cases. Moreover, the produced laminates had a constant composite material/metal ratio and a thickness of 5 mm. Four specimens were extracted from each laminate and tested. The traditional vacuum bag process, a manufacturing process usually adopted for composite laminates fabrication (Sorrentino et al. 2009), was employed for manufacturing the hybrid laminates studied in this work. First of all, the mould, that consisted in a 10 mm thick steel plate, was coated with a release agent, then all the materials constituting the hybrid laminates were prepared and stacked according to the desired stratification sequences. Afterwards, the