Analysis and optimization of thermoplastic fuselage struts under crashworthiness requirements

Abstract

The safety of a commercial aircraft is a factor to consider from the early stages of its design to guarantee that this one's structure can protect the occupants inside in a low-speed crash. In an aircraft's primary structure such as a fuselage, where the occupants and cargo should be protected, the design regulations imposed by the EASA have been applied for decades to conventional metallic designs. These designs had aluminum as the main constituent for being a lightweight material. Nevertheless, this project improves a component's design from a fully thermoplastic fuselage. While two commercial aircraft (A350, and B787) have already implemented some composite parts in their design to aim for sustainable and lightweight components, thermosets were the composites used in primary structures mainly. Composite materials do not present the plasticity metals do, which means that the regulations from EASA can no longer apply in the A350 and B787 hybrid designs. With the introduction of composite materials in these aircraft, organizations like the FAA and EASA had to write some new guidelines for these aircraft structures. These enable the protection of the passengers and cargo from the loads exerted on the aircraft during a survivable crash. Therefore, the new regulations are the ones taken into account for this thesis project, since these can be applied to similar composite structural designs, such as the fully thermoplastic fuselage section, whose numerical model is inherited from the Clean Sky STUNNING project.

What is wanted for the fuselage section's crashworthiness is for its structure to absorb as much energy as possible from the crash. All while minimizing the peaks of force that this one experiences. That way, the loads that arrive at the cabin of the aircraft are reduced and the structure becomes safer for a low-speed crash of under 30 ft/s (about 10 m/s), meaning that the passengers inside have a higher chance of survival and it is less probable they suffer from severe injuries. That is the aim of this project, focusing only on the structure's behavior caused by the changes in the fuselage struts.

The numerical analyses performed in LS-DYNA increase single components’ structural complexity until optimizing the layup of a tube to improve its crashworthiness behavior. It is after this optimization that the component is introduced as a strut in the STUNNING fuselage section. To further improve its global crushing in low-speed crushing conditions, the struts (or energy absorbers) keep the square geometry cross-section from the tube exercise, as well as the optimized layup. However, these are changed in size to improve the fuselage’s crushing behavior with little to no increase in mass. This study considers a fuselage section that neglects the passengers’ mass, their cargo, and the upper part of the fuselage airframe in the section's model. Considering only the cargo mass for six passengers on the model causes a change in the fuselage’s final design crashworthiness, which proves that a more representative crushing conditions setup should be considered in future studies to better predict the structure’s crushing behavior numerically.