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Abstract
This thesis presents an investigation of triggering mechanisms designed to ensure energy absorbing composite structures exhibit a progressive crushing mode of failure. The crushing of composite structures provides significant dissipation of energy in a crash if a brittle buckling mode is prevented from occurring. Triggers are needed to ensure that the failure initiation involves splitting and folding of the laminate and that the failure develops into a stable crushing mode.
The research reported in this thesis focuses on practical triggers that can be incorporated into the subfloor structure of helicopter airframes. The work was part of a collaborative research project including the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) and the German Aerospace Center (DLR). A small-scale specimen, namely the DLR crush element, was tested with a variety of triggers. Finite Element models were developed to predict the failure of these structures using the explicit solver PAM-CRASH. The test program included quasi-static testing carried at UNSW and at the Defence Science and Technology Organisation in Australia, and dynamic testing at the DLR laboratories in Stuttgart. Finally a large scale test article was manufactured by the CRC-ACS and the thesis details finite element prediction of the quasi-static and dynamic tests in Stuttgart and comparison against experimental test data.
This research included the application of a stacked-shell finite element modelling methodology for predicting the initial triggering and progressive crushing of laminates. This analysis methodology was validated against experimental test data. The study included chamfer, steeple and ply-drop triggers and makes recommendations as to the most appropriate form of trigger for inclusion in structures that are expected to carry static loads but trigger when dynamically loaded. A further significant contribution is the inclusion of crush tests and the design of ply-drop triggers for sandwich panels. These panels are widely used in helicopter airframe structures and contribute to the energy absorbed in a crash. To the authors knowledge this is the first work on ply-drop triggering of sandwich panels and design of a constrained pi-joint to ensure progressive crushing of the core material and surface plies continues as the panel fails.