Energy absorption of composite structures under axial loading : design, manufacturing and analysis
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Date
2023
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Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023.
Abstract
Geometry has a significant influence on Specific Energy Absorption (SEA) of composite structures. However, the variation of crush induced failure mechanisms with the change in geometry and the way they affect energy absorption capability has not yet been fully clarified. Moving from simple to complex structures, this dissertation aims to conduct a systematic study investigating the influence of geometry on SEA and to design a novel crash box geometry which has a better performance than those currently in use. For that purpose, an experimental investigation is conducted first for the crushing process of the flat, semi-circle, and sinusoidal structures with different dimensions made of both unidirectional and woven fabric prepregs. The results are compared, and variation of the observed failure mechanisms are discussed. The sinusoidal geometry having the highest SEA is identified to utilize in the design of a novel crash box geometry. Design process based on trial and error through manufacturing and testing, on the other hand, is time consuming and expensive so that researchers are currently interested in Finite Element (FE) Model predictions. However, due to the complexity of the process, FE models applied to estimate the SEA of composites require high computational time. This dissertation proposes a novel numerical approach applied for the crushing process of composite structures in which computational cost is decreased by reducing the number of interfaces between plies and modifying the relevant properties. The novel approach is first applied to composite flat plates and later validated by modelling sinusoidal geometries. The same approach, which can estimate the SEA of the structures in more than 50% less time than conventional methods, is then utilized in the design process of a crash box geometry. As a result of various modelling attempts, a novel geometry, which is called "Sun-like Structure" is manufactured and tested. Results show that the SEA of the final geometry is 2.3 times and 16% higher than the flat plates and the single sinusoidal structures, respectively. In the final part of the thesis work, tapering is applied to edges of the crash box structure so that the initial peak load is decreased to prevent an excessive deceleration causing a fatal damage to the passengers during a crush event.