Dynamic Failure Of TypeⅡ Structures Under Impact

Research on the dynamic failure and energy absorption of energy absorbing structures is one of the most important subjects in impact engineering.After the summary of the present research status and some key issues of energy absorbing structures, theory algorithm and Finite Element Method (FEM) are investigated to find the mechanical behavior and dynamic failure modes of the type II structure and a series of type II cells under impact, some valuable results are presented in this thesis:1 Firstly, the impact behavior of Type II structure (A pair of pre-bent plates with initial imperfection) is obtained, and it is compared with that of Type I structure (A compressed circular ring). The results from theoretical analysis and FEM show that, under the constant impact energy, the case with smaller impact mass (i.e. higher impact velocity) will present higher impact force and smaller residual deformation. That is to say, the Type II structure is inertia-sensitive, and which couldn't be found with Type I structure. We also found that, with smaller impact mass, more plastic energy was dissipated in axial compression, and less plastic energy was dissipated in plastic hinges when they were rotated, hence the residual axial compression displacement would be smaller.2 Secondly, FEM is introduced to see the dynamic failure process of a series of Type II cells. With the constant impact energy, four failure modes were observed with different impact velocity: forward progressive failure mode, backward progressive failure mode, bi-directional failure mode and random failure mode etc. Meanwhile, the same failure modes are found with the constant impact velocity. The finding of these fruitful failure modes is valuable to impact engineering.3 Since the FEM for impact problem is time consumption, a simple model is introduced in this thesis to substitute the above series model of Type II cell. Based on the pulse response of a same cell, the equivalent stiffness can be obtained, and the equivalent mass can be obtained through the hypothesis of the same kinetic energy. Therefore, a simple mass-spring system with nonlinear stiffness can be replaced to reduce the dimension of the model. The simulation results show that such model with small impact velocity is feasible. All the results in this thesis are valuable to the further research on cellular structures.