(A) study on the design of energy absorbing structures for aircraft crashworthiness

Abstract

Crashworthiness refers to the ability to safely protect occupants and cargo in the event of an aircraft accident. Energy absorbing structures are primarily employed for ensuring crashworthiness, serving as passive safety systems that convert the kinetic energy generated during a crash into other forms of energy, thereby minimizing the energy transferred to occupants and cargo. In the case of aircraft, energy absorbing structures are utilized in landing gear, subfloor fuselage structures, seats, and other components. In the event of an aircraft crash, the energy absorbing structures typically experience a combined shear-compression loading, necessitating a design that considers both shear and axial loads. Failure to account for shear loads can significantly compromise the energy absorption performance due to shear-induced damage, impacting passenger survival rates. In this dissertation, we conducted a study on the design of energy absorbing structures that effectively utilize the energy absorption principles of the material under combined shear-compression loading. We applied concave and convex designs to energy absorbing structures made of isotropic and anisotropic materials, studying the energy absorption characteristics accordingly. Metals and braided composites represent typical isotropic and anisotropic materials, respectively, commonly used in energy absorbing structures. The energy absorption principles vary depending on the characteristics of each material. For metal energy absorbing structures, a design maximizing energy absorption through plastic deformation, utilizing the material's high ductility, is necessary. Concave and convex designs induce initial damage due to stress concentration when subjected to loads, reducing the maximum load. Subsequently, the load is distributed throughout the entire structure, improving load-bearing performance and enhancing energy absorption performance through plastic deformation. This is effective in improving energy absorption performance for combined shear-compression loads regardless of the load angle. Braided composite materials exhibit superior characteristics in terms of specific stiffness, strength, and energy absorption performance compared to metals, with brittle failure being the predominant mode due to their lower ductility than metals. In the case of braided composite materials, they absorb energy through damage accumulation in the laminate. Therefore, to maximize the performance of the energy absorbing structure, it is necessary to induce a progressive failure mode. Applying a straight design results in shear failure at the ends, followed by a progressive crushing mode induced by stress concentration. Therefore, in the case of braided composite materials, effective energy absorption performance can be achieved without altering the geometric shape, unlike metals. Through finite element analysis, structural characteristics were analyzed based on geometry and material. Energy absorbing structures were fabricated based on the results of parametric studies, and quasi-static compression tests and drop impact tests were conducted. In conclusion, it was observed that the geometric shapes effective in enhancing crashworthiness vary according to the energy absorption principles of each material. The metallic energy absorbing structure requires the design of geometric shapes to maximize plastic deformation, while for composite materials, a straight design is sufficient to induce a progressive failure mode that maximizes damage accumulation in the laminate. The energy absorption per unit volume was found to be superior for metal energy absorbing structures, whereas the energy absorption per unit mass was better for composite energy absorbing structures. Therefore, depending on the application, both metals and composites can be effectively utilized as materials for energy absorbing structures. Additionally, experimental verification confirmed that the energy absorption principles of braided composite materials are equally applicable to combined shear-compression loading, an area less explored in existing research.