Optimal design of composite fuselage frames for progressive failure and energy absorption by genetic algorithms

Abstract

This work presents a method for preliminary design of laminated composite fuselage frames with improved crashworthiness. The proposed method uses a progressive failure analysis and genetic algorithms to obtain frame designs optimized for maximum energy absorption. To demonstrate the method, a representative composite fuselage frame is designed for failure at two different load levels. The analytical results indicate that significant increases in crashworthiness can be achieved with this frame design methodology. * National Kesearch Council Associate. Correspondence author. ** Associate Professor. member AIAA and ASME *** Professor: Associate fellow AIAA This paper is declmed a work of the 1 J.S. Government and is not subject to copyright protection in the lJnited Statcs. Vehicle structural designs are considered crashworthy if occupants are protected from exposure to high accelerations and other crash hazards('). A key to achieving such protection is controlled absorption of crash impact energy through progressive failure of the structure. The structural designer attempts to control the failure process so as to tailor the magnitude and duration of the crash acceleration pulse for increased survivability. One of the failure mechanisms commonly used for crash energy management of an aircraft structure is fuselage crushing below the main passenger deck'. This failure mode is governed primarily by fuselage frame design, Ideally a frames load response will have an elastic-plastic character. We seek a controlled failure load followel by a sustained crushing force as a function of displacement at the point of contact. The objective is to maximize the energy absorbed, without peak occupant accelerations exceeding the limits of human survivability. To achieve this, fuselage frames must be designed for an optimal crushing response. Frame design requirements established by the normal flight and service loads, are constrained by the additional design requirements of a potentially survivable crash. Fuselage structures constructed from composite materials require a special attention to crashworthiness4. Conventional aluminum skin-stringer designs will absorb kinetic crash energy through a process that involves ductile crushing and yielding of the fuselage structure. This type of crushing behavior is attributed to the formation of plastic hinges in the frames at the locations where local bending moments are high. For a laminated composite skin-stringer fuselage design, the thin walled frames are predisposed to fail by brittle fracture at these same locations. Since less material is involved in the failure process, this brittle failure mode can result in lower energy absorption. As a consequence, designing composite fuselage structures to provide sufficient levels of crash energy absorption requires special attention. Maintaining or improving upon the existing levels of crashworthiness exhibited by conventional aluminum fuselage structures is an important design criterion for composite fuselage structures In the present work, the design of hselage frames for improved crashworthiness is approached as an optimization problem We seek to maximize the ener$y absorbed subject to a constraint on maximum failure load. Energy absorbed is equal to the area under the load response curve, and passenger accelerations are directly related to failure loads The optimum structural response for crashworthiness is a controlled failure load followed by a sustained crushing force. (See Figure 1 ).