Researches On The Numerical Simulation And Fatigue Life Prediction Methods Of Response Characteristics Of Thin-walled Structures Via FEM

Thin-walled structures are widely used in aeronautics, watercrafts, automobiles, vessels and so on because of its light weight and good load-carring property. But the theoretical stress and strain results of these structures can be hardly derived due to the influence of geometrical shape, boundary conditions and so on. Hence, the basic theory and numcerical simulation methods of these structures have been the research focus for decades. In the dissertation, the following topics are studied based on the problems existing in numerical simulation, stability analysis and fatigue life prediction:1. Research on the finite element modeling method and error correction method of the response characteristics of thin-walled structuresThe modeling method and discrete characteristics of curved thin-walled structures with chamfers, bosses, holes and so on are studied systematically based on its governing equations and the discrete principles of finite element method (FEM) at first. Then, the error mechanism of discrete preocess and the correction method of the calculated stress results are studied by theoretical deduction and a display method of the corrected stress results is proposed. Finally, a probe inside the turboprop is taken as an example to analyze the variation of buckling load and structure geometric features thoroughly, which lays a foundation for structure stability analysis and optimization.2. Research on the finite element method for analyzing thin-walled structures under foundation vibrationThe dissertation proposes a new method named static inertia method to calculate the structural response under foundation vibration with FEM by theoretical deduction. The original problem is transformed into an ordinary vibration problem with fixed base by applying the equivalent nodal loads from the new method and the response results can be calculated accordingly. The new method overcomes the restrictions of existing methods and provides a new thought to handle the problems under flush and anharmonic vibration from the base.3. Research on the fatigue life prediction of thin-walled structures and display method of fatigue livesBased on the cumulative fative damage theory, the dissertation estabilishes a procedure for fatigue life prediction of thin-walled structures under quasi-static and dynamic loads as well as a display method of fatigue lives with FEM package. Moreover, A program is developed accordingly and a probe is taken as an example to verify the procedure and the display method. The results indicate that the procedure above is effective and the display method is feasible.4.Research on the fatigue crack growth (FCG) model and prediction method of thin-walled structuresThe dissertation presents a new model named integrative influence factor model (IIF) to account for the intergrative influence of specimen thickness, cyclic stress ratio, material property and so forth on FCG rates by analyzing the characteristics of existing models and experimental results. Furthermore, predictions of the crack opening ratioγand effective stress intensity factor range ratio U by Newman, Huang and Codrington models are used to validate the IIF model, which indicates that the IIF model predicts the results more accurately and describes the influence of Poisson's ratio, specimen thickness, R ratio and so on explicitly. In addition, a general model of fatigue crack growth in ductile alloys under variable amplitude loading is developed based on the IIF model and Passivation-Lancet theory. Several sets of test data are used to validate the general model and the predictions are in good agreement with the test data.5.Research on the numerical simulation method of FCG processIn the dissertation, a new procedure of numerical simulation of FCG process under constant and variable amplitude loading is established via extended finite element method (XFEM), level set method and the general model in part 4. In addition, a matrix update algorithm based on LDU decomposition is introduced. After that, the validity of the procedure and the matrix updata algorithm are verified with several sets of test data from academic publications, which shows great consistency.6. Research on the redevelopment of ANSYS package for numerical simulation and fatigue life prediction of thin-walled structuresThe dissertation proposes a method for secondary development of ANSYS package based on the conclusions in part 1~5 and a special software for numerical simulation and fatigue life prediction of probes is developed accordingly. The originalities of this dissertation are summarized as follows:1. Estabilishment of the static inertia method for simulation of the continua responses under foundation vibrationThis method overcomes the restrictions of existing models and can be extended to the problems under single- and multi-points anharmonic vibration from the base, flush, transient loading and so on.2. Development of the IIF model to account for the influence of Poisson's ratio, specimen thickness, R ratio and so forth on FCG rates The IIF model describes the influece of material property, specimen dimensions, cyclic stress ratio and so forth onγ, U and FCG rates explicitly. Furthermore, the fatigue property of real structures and specimens can be estimated by substituting the geometric parameter, experimental conditions and material parameter into IIF model. It is very helpful for fatigue tests, specimen selection and real application.3. Estabilishment of the general model for FCG rates under constant and variable amplitude loadingThe model describes the retardation and acceleration effect of single overloading, single underloading, sequential loading and spetrum loading effectively and quantitatively, owns explicit physical meaning and is suitable for modeling the FCG process of structures with various ductile materials under different loading conditions.4. Development of a numerical simulation procedure of FCG process with XFEM and a new matrix update algorithm for equilibrium equationThe procedure inherits the advantages of level set method and XFEM, and describes the FCG process of real structures effectively. In addition, the whole continua domain is divided into two parts by the level set functions in the procedure, only the nodal data in updating region needs to be calculated and integrated, which improves the computational efficiency significantly.