Issue 47

F. Cucinotta et alii, Frattura ed Integrità Strutturale, 47 (2019) 367-382; DOI: 10.3221/IGF-ESIS.47.27 368 stiffness-to-weight ratio. These parameters are highly influenced by the type of fibres used to produce the composite. A general overview of these parameters in function of the mass density is shown by Kelly [3]. In many fields of engineering there is the need to reduce the weight in order to reduce the energy consumption, which impacts in the air pollution. For this reason, the mechanical properties of composites are of great interest in many engineering fields. Aeronautical engineering proved to be the pioneer in the use of composite materials. In this field the use of composites started many years ago, and the principal applications are shown by Soutis [4]. With the increase of the requirements in terms of weight and pollution performances, also automotive and naval engineers started to use these materials in different ways. An example is in the production of automotive components: Ding et al. [5] showed that with the use of the composite materials in a rear bumper beam it is possible to achieve a reduction of the weight of about 40% keeping the same performance requirements. In the naval field, the use of composite material is isolated to the yacht design. Kimpara showed the developments and the increase of use of these materials in this field in the last years [6]. The reduction of environmental impact in the usage phase of the product represents another important aspect directly linked to the reduction of weight. Timmis et al. [7] proposed an analysis aimed at assessing the reduction of environmental impact thanks to the use of composite materials in the aviation field. The study has been conducted for a Boeing 787 and successively expanded to the global feet. The use of composite materials was proven to reduce the CO x and NO X emissions of about 20% [7]. Also, in the automotive field the stringent rules for pollution pushed the engineers to use composite materials to reduce environmental impact. Witik et al. [8] proposed an environmental and costs comparison between carbon fibres, magnesium and steel materials for a representative component of a car, i.e. the bulkhead between the passengers compartment and luggage space. The final results showed that the weight reduction sometimes does not improve the environmental performance because of the disposal process of synthetic fibres. For this reason, many researchers pushed the studies on the use of natural fibres in automotive components. A review of all green useful fibres for automotive applications is reported by Koronis et al. [9]. The use of this type of fibre allows to reduce the environmental impact in production phase and successively in usage phase, as shown by Boland et al. [10]. The environmental benefits produced by the application of composites is taken into account also in mechanical industry. Barone et al. [11] shown the different impacts between utility poles manufactured in steel and composite, highlighting that the weight reduction achieved through glass fibres led to an important reduction of the environmental impact. However, there are also critical aspects in the use of composite materials. The manufacturing process of a product with composite material is complex, requiring highly skilled workers. The mechanical properties of each product are highly influenced by the manufacturing process. Kim et al. [12] shown the difference of the mechanical properties between the same product manufactured with two different process technologies (hand lay-up and vacuum infusion), highlighting that the samples manufactured with vacuum infusion have larger ultimate strength and modulus in both tension and compression tests than the hand lay-up ones. Composite materials have different failure modes and so it is very complex to predict the type of failure and the evolution of damage during operations [13]. In order to give general laws for the prediction of the possible failure modes many researches focused on experimental tests. A characterization of a honeycomb sandwich with aramid fibres and aluminium core is proposed by Belouettar et al. [14]. The study concerned the evaluation of mechanical properties of the sandwich during four-points bending tests in static and dynamic conditions. Other tests has been conducted by Manalo et al. [15]: the composite sandwich beams under study were subject to four-points static bending test to determine their strength and failure mechanisms in the flatwise and edgewise positions. Another crucial aspect is represented by the response of the composite material to dynamic loading. Many researchers developed new indexes in order to quantify the capacity of the sandwich composite to absorb energy during the low velocity impacts [16]. Also the influence of the thickness of the laminates has been evaluated by Belingardi et al. [17]. In the last years, many researchers, in parallel to the experimental tests, proposed also numerical simulations. A validation of this approach is very important for structures in composite materials in order to have a useful tool in preliminary design. Russo et al. [18] proposed a numerical investigation using non-linear simulations and accurate failure criteria with an experimental validation. Also Manalo et al. [15] used the experimental tests for the validation of a numerical approach developed with a commercial software. Hassan et al. [19] investigated the potentiality in the use of numerical simulations for the analysis of impact tests on composite material. In the aforementioned works, the main reason of the discrepancies between experimental and numerical simulations is determined by the insufficient information on actual mechanical properties of the materials. The main goal of this work is to develop a Finite Element (FE) model for the simulation of a particular family of composite materials, widely used in naval field and, in particular, in the High Speed Craft and motorboats field. The model validation was achieved through the comparison with experimental four-points bending tests and impact drop tests at different energy levels. Two different sandwich specimens were considered, having a different lay-up sequence. The first validation concerns the four-points bending test in terms of load-displacement curves. The second validation concerns the impact drop test