Modeling And Identification Of Nonlinearities In Structural Connections

Bolted joints, which are widely used in assembled structures, have a significant effect on the flexibility and stiffness of structures. The structures connected by bolted joints behave nonlinearly because their dynamical behaviors vary with external loading. Depending on the design requirements, sometimes a linear model, without taking the nonlinearities of bolted joints into consideration, may be justified. However, determining the physics of bolted joints is of vital importance for precision instruments like satellites because neglecting nonlinearities introduced by bolted joints can result in major difference between theoretical analysis and experimental results. Therefore, it is critical to accurately model the nonlinearities in such cases.This thesis aims at studying structures with nonlinearities introduced by bolted joints, especially with nonlinear boundary conditions. The main work and results include:1. The describing function is employed to quasi-linearize the nonlinear model of structure. Nonlinear index is introduced to detect, localize and determine the type of the nonlinearities in structures. Meanwhile, an iterative method based on describing function is exploited to predict the response of structures under different excitation levels.2. For most assembled structures, it is hard or even impossible to directly measure the boundary responses. To overcome these difficulties, a new method combining describing function and model updating is presented.3. All the methods are demonstrated by numerical case studies. Firstly, the location and type of nonlinearity are determined by using nonlinear index. Secondly, the nonlinear parameters are identified by curve fitting method and the response of structures under different excitation levels are calculated by iterative method. In addition, the parameters of nonlinear boundary conditions are identified based on the assumption that the boundary response cannot be measured.4. Experiments are conducted to investigate a honeycomb board with nonlinear boundary conditions. The nonlinear type and parameters of the board are determined. Finally, a restoring force model is established to represent the nonlinear boundary condition of the board.