Analysis And Design Improvement Of Thin-walled Structures For Energy Absorption Under Axial Crush

Energy absorption of structures is essential for security service of structures if extreme load conditions such as explosion and impact occure. Thin-walled structures are used widely as kinetic energy absorbers as they are high energy absorption and weight efficient. Numerious investigations are carried out to explore energy absorption of thin-walled structures to meet the strong demand of the automotive industry. Howerer, the energy absorption behavior for most type of thin-walled structures under impact load is still not clear, due to the complexity of this problem. The energy absorption of thin-walled structures is closely relate to factors such as structural configuration, material properties and load conditions. It is still a major goal for scientists and engineers to understand the energy absorption mechnism and deformation mode of thin-walled structures and guide their industrial applications. In this thesis, the following aspects to the energy absorption behavior of thin-walled structures under low velocity impact are investigated. A strategy named non-convexity of cross-section is proposed and two types of thin-walled structures named as non-convex multi-corner columns (abbr. NCMC) and non-convex multi-cell columns (abbr. NCMCELL) are developed. A type of bio-inspired bulkhead reinforced non-convex multi-corner columns (abbr. BI-NCMC) and A type of cylindrical multi-cell columns (abbr. CMC)are proposed. The main contents and conclusions of this dissertation are as follows:(1) Cross-section non-convexity and energy absorption of non-convex multi-corner thin-walled columns subjected to axial crushing. A strategy to improve energy absorption efficiency of thin-walled columns by introducing extra non-convex corners on the cross section is proposed. Several profiles of NCMC obtained through this strategy are presented and their energy absorption capacities under axial crush are investigated analytically and numerically. Explicit formulations for predicting the mean crushing force of NCMC are derived based on the theory of Super Folding Element (abbr.SFE) method, and the predicting results of these formulations have good agreement with the numerical simulation performed by explicit non-linear finite element method. The comparisons of the NCMC and square column show that the former behaves better energy absorption performance.(2) Comparative study of energy absorption performance of NCMC and multi-cell square columns (abbr. SMC)subject to axial crushing. Energy absorption of NCMC subject to axial crush is investigated, and their performance is compared with square and SMC. First, the three type of columns with the same wall thickness is investigated using explicit non-linear finite element software ANSYS/LS-DYNA. And then, investigation of their energy absorption performance under three different requirements is carried out, which are the same amount of material usage, the same energy absorption and the same maximum peak crushing force. The results show, NCMC are superior to square column as energy absorber no matter in specific energy absorption or crush force efficiency. NCMC performance better than SMC except for crush force efficiency in some cases.(3) Axial crushing of non-convex multi-cell columns. A strategy to improve energy absorption efficiency of thin-walled columns is proposed, which is by introducing extra non-convex corners as well as introducing internal webs in the cross section. Both the two ways increases the material distribution proportion near severe deformation corners, which can improve energy absorption efficiency of structure. Several NCMCELL obtained through this strategy are presented and their energy absorption capacities under axial crush are investigated analytically and numerically. Explicit formulations for predicting the mean crushing force of NCMCELL are derived based on the theory of Super Folding Element method. Numerical simulation performed by explicit non-linear finite element method shows that the NCMCELL subjected to axial crash deforms progressively and absorbs a large amount of energy. The comparisons of the NCMCELL with square, NCMC and SMC show that the NCMCELL behaves better in energy absorption.(4) Energy absorption of boi-inspired bulkhead reinforced non-convex multi-corner columns. A type of BI-NCMC is proposed to improve energy absorption performance. First, the non-compact expansion-contraction deformation mode of NCMC subjected to axial crush is discussed. And then, transverse bulkheads are introduced to restrain the non-compact expansion-contract deformation mode, which is inspired by the phenomenon of bamboo node and nodal diaphragm enhanced the transverse strength of bamboo. Energy absorption of this type of BI-NCMC is investigated numerically. Progressive deformation mode is achieved and this structure shows high energy absorption performance. The role of the transverse bulkheads is to change the deformation mode from expansion-contract mode to progressive mode, while the energy absorption of the bulkheads is little. Finally, a parametric analysis of the column is carried out, and it is found that the column with highest energy absorption efficiency is the one with least transverse bulkheads and still maintain progressive deformation mode.(5) Energy absorption of cylindrical multi-cell columns. The structural effectiveness of circular tube and square tube is compared and the forner peforms better than the latter. A modified solidity ratio based on the cross-section area coefficient is difined to ensure the geometric equivalent structure with the same value of solidity ratio. A type of CMC is proposed to improve energy absorption performance, which is inspired by the high energy absorption performance of circular tube. Numerical examples illustrate that CMC is more efficient than square column and SMC in energy absorption. In addition, a parametric study is conducted to investigate the influence of geometrical parameters on crashworthiness. And it is found that the wall thickness, the number of cells alone the radial and circumferential directions have a distinct effect on the energy absorption.This research is supported by National Natural Science Foundation of China (No.90816025), National Basic Research Program (973Program) of China (No.2011CB610304) and National Science and Technology Major Project of the Ministry of Science and Technology of China (No.2009ZX04014-034).The financial supports are greatly acknowledged.