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the passive safety of aircraft passengers is such an important aspect in the design of aircraft structures as strength and fatigue concerns. The development of methods and devices to prevent passenger injuries is the subject of continuous efforts. The mission is to minimize stresses and accelerations on passengers during a crash. Over the years, studies on crash phenomena have been focused on experimental tests, using full-scale structures and Anthropomorphic Test Devices (ATDs) to assess the consequences of impact phenomena on the human body. However, due to the high costs of experimental campaigns and the difficulty of controlling all relevant parameters, the need of efficient numerical models capable of validating experimental data has increased. This is specifically relevant for tests on ATDs. In the frame of this work, the side-impact of an aircraft passenger have been numerically investigated positioned on a window-side seat of an aluminium commercial aircraft fuselage a World SID-based dummy. An attempt to increase the aircraft crashworthiness was made placing in correspondence with the head and the shoulders of the dummy hybrid sandwich shock absorbers. In order to validate the considered dummy model, a lateral impact against a flat barrier has been carried out. The obtained numerical results have been cross-compared with literature experimental data. Then, the side-impact behaviour of the dummy within a fuselage section has been investigated, with the aim to verify the absorption capability of the shock absorbers and to quantify their effect on the safety of the dummy. The employment of the shock absorbers allowed to reduce the acceleration peaks experienced by the dummy's head up to 50%. [ABSTRACT FROM AUTHOR]

One of the ongoing challenges in aeronautics is the development of increasingly lighter structures. This has become a crucial goal because structural lightening means improving aircraft performance in terms of speed, manoeuvrability, fuel efficiency, and at the same time reducing manufacturing and service costs. The aerospace industry continues to focus more and more on this goal and every improvement in technology and materials can bring significant advantages in performance, efficiency, and safety of aircraft. The present work is part of this framework, as it analyses the feasibility of achieving very high weight reduction in sandwich panels using a new manufacturing approach. It introduces an experimental limited sensitivity analysis on the mechanical responses of composite sandwich structures for shock-absorption applications lightened by appropriately setting a specific additive manufacturing process parameter: the infill value. The investigated sandwich structures are characterised by a core Designed for Additive Manufacturing (DfAM) in order to maximise their performance in terms of energy-to-weight ratio and damping of impact loads. The material chosen for the inner part of the sandwich structure (core and internal face sheets) is polypropylene (PP) while the external face sheets are in Carbon Fibre Reinforced Polymers (CFRP). By comparing the post-impact responses at 20 J impact of three sandwich configurations with the same external shape but different material layer densities related to different setting of the infill parameter in the frame of the printing process, the work proves that this approach leads to lightening of this specific sandwich structures by up to 28% and at the same time improves their structural effectiveness in terms of energy absorption characteristics. The comparison was made by relating specific absorption indices, force-time and force-displacement graphs and CT scans. [ABSTRACT FROM AUTHOR]

• The combination of innovations in manufacturing and materials makes the development of compact and high-performance shock absorbers possible. • The manuscript analyses an engineering product starting from the design up to the manufacturing and to the testing phases. • The manuscript analyses an engineering product manufactured by machines which are the state of the art of additive manufacturing techniques. • Mechanical responses of different sandwich shock absorbers was compared and a minimum-mass configuration that maximises energy absorption was identified. • A correlation between numerical and experimental data was performed. The capability to guarantee passenger safety is a core feature of any transportation system. For this reason, a considerable effort is being committed, by researchers, to the study of innovative shock-absorbing devices able to increase the safety performance. According to this topic of great interest, this paper presents a numerical/experimental study on a new effective shock absorber concept achievable by means of the Additive Manufacturing technology. Indeed, additive technologies exhibit some fundamental advantages, such as the possibility to produce complex microstructures, with superior impact energy absorption capabilities, which cannot be made with standard manufacturing processes. Hence, this manufacturing technique could be preferred for the development of high-efficiency shock absorbers cores. In the present work, to achieve shock absorbers high mechanical efficiency while limiting mass and volume, an innovative sandwich shock absorber concept is introduced, which uses additive manufactured solutions for the core by combining the advantages offered by thermoplastics (polypropylene), such as their ability to absorb energy through plasticisation and their recyclability, to those offered by fibre-reinforced thermoset composites (Carbon Fibre Reinforced Polymers), i.e. high stiffness/mass and strength/mass factors. First, numerical low-velocity impact analyses have been carried out to compare the mechanical response of several shock absorber configurations, Designed for Additive Manufacturing (DfAM), characterised by a polypropylene (PP) honeycomb core and CFRP composite external skins. These PP-CFRP sandwich configurations have been compared to full polypropylene configurations (with polypropylene skins and core PP-PP). Comparisons have shown that the PP-CFRP configurations are characterised by better overall crashworthiness performances (energy absorption and peak-force smoothing). Finally, an experimental activity, including ASTM D7136 based impact tests, have been carried out on the best performing investigated PP-CFRP configuration, to preliminary validate the numerical results. [ABSTRACT FROM AUTHOR]

The increasing use of composite materials in the aviation industry for the manufacture of higher performance components required the introduction of efficient optimisation methods for the design of composite laminates. Metal-replacement with composites provided the first step in the design of higher performance aviation structures. Nowadays, the additional step is to optimise these structures to make them even more performing and lighter. Mass reduction drives the optimization with number and orientation of plies as main parameters. In this perspective, the Double-Double (DD) laminates approach provided an effective method to optimise composite structures for weight reduction and strength purposes, without restrictions on symmetry and orientation angles. Especially for optimized components, crashworthiness remains a key aspect in evaluating the performance of composite aircraft structures, such as for fuselages. In this work, the crashworthiness features and the dynamic response of a regional fuselage barrel section in a vertical drop test has been numerically investigated performing a comparison between a configuration with conventional composite skin and a configuration with optimized and lightened skin through the Double-Double (DD) approach. The comparison proved that the configuration with the optimised skin has been characterised by adequate performance in terms of crashworthiness and passive safety for passengers, when compared to the conventional configuration. The effectiveness of Double-Double method in designing crashworthy composite components has been demonstrated, while simultaneously reducing the structure's total mass and enabling compliance with restrictive fuel consumption and emission regulations. The employment of optimized skin resulted in a total mass reduction for the whole structure up to 34%, due to mass reduction of the skin of 69.8% with respect to conventional configuration. [ABSTRACT FROM AUTHOR]