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Abstract
This thesis presents an investigation into the progressive crushing of composite materials with the aim of improving energy absorption for crashworthy rotorcraft sub-floor structures. The thesis formed part of the research program of the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) contributing to a project that designed and tested a demonstrator helicopter frame and sub-floor energy absorbing structure. The aim of the thesis was to validate the approach taken to the design process and undertake research to provide supporting data that was not available in the research literature.
Advanced composite materials are becoming increasingly used in all aeronautical sectors, and the crash performance of these materials is an area where designers are in need of more data. Whilst numerical methods are advancing, experimental testing remains the only conclusive way to assess new materials and design concepts. Coupon level crush tests were performed using specimens known as “DLR crush segments” to characterise the specific carbon-fibre/epoxy materials used and assess a variety of parameters including manufacturing methods, lay-up, loading rate and novel methods for improving energy absorption. Two types of more complex test elements were then used to investigate the effect of joints on the crushing process, the additive nature of energy absorbing sections and the off-axis performance of structural elements.
Overall this body of work validates a building block design approach of composite structures for crashworthiness. The issues addressed in the testing program included the investigation of the effect that manufacturing tooling, varying lay-up and interleaving of laminates using thin thermoplastic materials have on crushing performance. Relatively simple joined elements are then tested to assess the performance of different assembly techniques, and recommendations are made as to the applicability of different joint types for crashworthy design. These tests included the first reported crush testing of thermoplastic welded composite joints using the CRC-ACS patented Thermoset Composite Welding process. This thesis also designed an alternative novel energy absorbing element for improved off-axis performance and tested the element at a variety of inclinations to validate the design. The building block design approach was demonstrated through the design and testing of this element and also the CRC-ACS demonstrator crashworthy helicopter frame.