Study On Key Techniques Of NPR Structure And Its Application In Vehicle Body Design

The Negative Poisson’s ratio(NPR) materials could be widely used in automotive engineering as having excellent mechanical properties, good energy absorption characteristics and light structure, they could be used to enhance the structure properties and reduce the weight and so on. In recent years, lots of scholars carried out their research on the NPR materials and a lot of achievements are made, however there are still many problems of the NPR materials need to be solved and most of the research focuses on the manufacture and application of the materials. The research for the NPR material is is still in its infancy. Systematic studies of the NPR materials are carried out in the work based on the relationship between the micro structural parameters and macroscopic mechanical properties, research methods on the properties, design optimization for the microstructure and application of NPR materials in the automotive industry etc. The following researches are carried out in this work.1. Research on the mechanical properties and energy absorption characteristic of the NPR materials. The constitutive equations between micro structure parameter and effective elastic properties are established to provide the theoretical basis for studying the relationship between the two. The method of effective performance is applied to evaluate the effect of micro structure on the macro performance. The research on the static mechanical properties, inner surface compression characteristics and energy absorption characteristic of the NPR material are also conducted in this work, proving the research basis for design optimization and engineering applications.2. Research on the simulation method of NPR material. To make the simulation of the NPR materials more accurate and efficient, a new Timoshenko beam model large deformation is proposed in this work and then applied to simulate the material. The FEM model based on the traditional beam model, tetrahedral elements and hexahedral element are the main methods for simulating the NPR materials, and there are problems of poor accuracy or lower efficiency. The absolute nodal coordinate formulation and the geometrically-exact beam formulation are combined effectively in the new Timoshenko beam model and make it suitable for simulating the large deformation of the complex systems. The numerical problems which are easily aroused are good solved by applying the strategy of chosen appropriate shape functions and the strategy of approach similar. By applying the new Timoshenko beam model, traditional beam model, tetrahedral elements and hexahedral elements, the model of the two-dimensional cellular unit, two-dimensional multi-cells and three-dimensional multi-cells for the NPR material are established respectively. The analysis results show that the accuracy and efficiency of the simulation model based on the new Timoshenko beam model could be well enhanced. Quasi-static compression tests are also conducted to verify the accuracy of the models.3. Research on the design optimization for the micro structure of the NPR material. Based on the parametric model, approximate model and genetic algorithms, a new design optimization method for micro structure of the NPR material is proposed in this work. As having the excellent mechanical properties, the NPR materials could be used widely in vehicle manufacture and the performance is affected by the micro structures. There are few effective optimization methods for NPR materials since the honeycomb characteristic. The parametric model of the micro structures are first established in the new optimization method, optimal Latin squares design method and response surface model method are combined to establish the response surface model for the structure, genetic algorithms are then applied to optimized the geometry parameters, thickness and length of the micro structures for improving the mechanical properties. Different single objective optimizations and multi-objective optimization are conducted separately in this work and the optimal topology, shape and thickness of the micro structures are achieved successfully.4. Research on the application of NPR material in vehicle parts. A new NPR crash box is proposed in this work by combining the traditional crash box and NPR inner core. Crash box has great impact on the performance of the vehicle crashworthiness. To reduce the injuries to passengers and damages to other parts in the vehicle collision, the crash box should absorb as much energy as possible through steady and controlled deformations. The traditional design method is to optimize the structure of the crash box and the optimization potential is limited, while the traditional crash box and NPR inner core are combined in this work to improve the energy absorption capability. To verify the validity of the new crash box, the performance of the traditional crash box, aluminum foam filled crash box and NPR crash box are studied at the conditions of given initial speed, constant collision speed and vehicle frontal crash respectively. The absorbing energy, impact force, energy absorption efficiency, crushing displacement and component mass are mainly compared and analyzed. The influences of collision speed, materials and parameters of micro structures on the performance are also detailed studied in this work. The multi-objectives optimization of the crash box is also conducted successfully in this work. These provide the theoretical basis for design and applications of NPR crash box.