Reasearch On The Responses And Failure Of The Complex Structure With Bolted Connection Subjected To Impact Loading

Method of meticulous FEM modeling for threaded bolt is studied in the dissertation, and the numerical method for impact fracture of complex structure, according to the impact response and impact fracture of the complex structure with bolted connections, such as weapon warhead, spacecraft etc. Then, the impact fracture of one typical complex structure with bolted connections is researched by the numerical methods presented in the paper, and the calculated results consistents with the experiment nicely. The main contents in this paper are as follows.Firstly, the numernical modeling method for threaded bolt is studied. A method of meticulous FEM modeling for threaded bolt named as two guide line method, which consider the helical model of the thread, is presented based on the basic modeling technique of the pre-processing software patran. Comparing to the existing methods, it has many advantages such as needing not dig a ostiole factitiously in the screw or without complex mathematics modeling etc. It is simple to implement, and the FEM meshes gained by it are all hexahedron element without malformation. The convergence of the thread contact is solved by the balanced penalty function method with the exponential pressure-overclosure relationships. Pretension load is defined as a couple on the nut directly, since the helical model of the thread is obtained. Compared to the traditional pretension modeling method, it is not only simpler but also more reasonable. A method used to solve the static and dynamic coupling problem is presented, since the structure is under the static pretension load and the impact load synchronously.Based on the meticulous FEM modeling method, the numerical rearch of the static response and impact response of the bolted connection structure are implemented. The static response of the structure under various pre-tension loads is analyzed primarily. The distribution of stress in the thread, the stress in the thread bar and the maximum equivalent stress are gained. The stress in the thead bar is identical to the experimental results perfectly, and the maximal relative error is less than five percent. Then the impact response of the structure hit by one steel ball is cacultated, and the strain history of the thread bar is identical to the experiment results perfectly too, and the maximal relative error of the pulse width or amplitude of the strain wave were less than twenty percent.Secondly, the numerical method to simulate the impact fracture of structure is researched base on XFEM-based cohesive model in the paper. On this basis, the impact fracture process of three point specimen under three load cases with bullet impact velocities of 12.2m/s,15.1m/s and 26.3m/s is studied respectively by the method, and the failure pattern obtained by numerical calculation consists with the experimental results. The simulation results show that the opening mode I is the major fracture mode of the specimen, and the crack propagates primarily along the initial crack direction. The crack makes a turn at the place where is 3 mm to 4 mm from the impacted part of the specimen, and the fixed fracture mode is dominating in this domain, which fits the experimental results in the paper. Then the static initiation of crack and fracture process of a typical structure with bolted connection is simulated by the XFEM method used in the paper, and fracture mode consisting with the experimental results is obtained.Finally, referring to one typical complex structure with bolted connections, its impact response and fracture process is researched by the numerical methods in this paper. The impact response characteristic is gained by the numerical method, and the fracture mode consistents with the experimental results perfectly. It shows that the numerical methods pretended in the paper are reasonable. On this basis, the factors that influence the fracture characteristic of the structure are discussed further,and the results show that the major factors are angle of roll, degumming of foam material with PBX degumming explosive, variety of the pre-tension load etc.The research results of the paper show that the numercal simulation of the impact response of the complex structure with bolted connections can be achieved with reasonable results, using the meticulous FEM modeling method for threaded bolt and the numerical method for impact fracture based on XFEM synchronously.