The objective of this research effort has been to gain a better understanding of energy absorption behavior in composite laminates. Crush tests of nine flat plate specimens with three different lay-ups of T650-35/F584 graphite/epoxy fabric were performed to evaluate the mechanisms that contribute to the energy absorption process. A nonlinear finite element approach was used to model the sustained crushing of the flat plate, and a progressive failure model was implemented as part of the finite element analysis to enable modeling of the fundamental mechanisms involved in the crushing behavior. The progressive failure model was based on linear elastic fracture mechanics for prediction of crack growth and a set of failure criteria for predicting fiber/matrix failures that occurred as a result of large deformations. Friction between the specimen and the crushing surface was included in the model. Predicted values of sustained crushing stress agreed with experimental results for the flat plate specimens within nine percent.
The vertical component of impact forces that is often present in aircraft crashes makes an energy absorption capability essential for seats in small aircraft and helicopters. The US Army has required dynamic testing of seats for its aircraft since 1971, and the energy-absorbing seats that have been designed to meet its requirements have proven successful. Amendment of the Federal Aviation Regulations (FAR) in 1988 and 1989 to require dynamic testing of civil aircraft seats has generated increased interest among airframe and seat manufacturers in development of new energy-absorbing seat concepts. New analytical techniques are required to model these new seat concepts efficiently. The research effort described in this paper has involved a comparison of the performance of various energy-absorbing seat concepts with respect to the FAR-specified dynamic test criteria for general aviation and for rotorcraft. Three different energy-absorbing configurations were modelled using the SOMLA program, which has been developed for aircraft seat analysis. For a type of seat with guided energy-absorbing stroke, energy absorber parameters were determined that would limit the maximum lumbar force in the 50th-percentile dummy to 1500 lb (6.7 kN), as required by the FAR. Seats with energy absorption systems designed for the 50th-percentile occupant were found to limit lumbar force to tolerable levels for other sizes of occupant as well, based on assumptions of simple scaling rules. The seat models and results obtained with the models are described.
The genetic algorirhm (GA) is a search algorirhm that mimics the pamrns of natural selection and reproduction displayed by biologi'cal populations. The GA's operation as a non-calculus-based search and optimization method makes it a good candidare for use in conceptual design. This paper presents a discussion of the use of a G A as an automated approach to conceptual design of helicopters. To accomplish this task, a G A war combined with an industry-standard helicopter sizing code. This resulting genetic algorithm-based design code mas then used to generare conceptual designs for the medium 113 replacement helicopter and a hypothetical attack helicopter. Results of these design effom are discussed, providing insight into the use of a G A to automate the conceptual design of helicopters.
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