| Fatigue is a very complicated problem for the structure and machinery. And fatigue researches contain a quantity of empirical laws and contradictory views. There are many influencing factors on fatigue strength and fatigue life. At present, most influencing factors can't quantitatively describe fatigue phenomenon with physical models or mathematical models. And there are big differences between test conditions and practical applications. Although a lot of theory results and test data have been obtained, accurate prediction and analysis for the fatigue strength and fatigue life can't realized better. One hand, fatigue destroy can be prevented efficiently with fatigue research results and taking related measures in actual engineering application. On the other hand, though the same principles guide the same applications, fatigue destroy accidents still can't be prevented. All facts show that fatigue problem still can't be solved completely on theories and practice. With the more requirements of rapid development diversity in society, fatigue more and more has been an important problem on academic circles and industry circles in home and abroad.The dissertation mainly studies the fatigue problem of the engineering machinery members. The engineering machinery is one of important tools in the human society application. With the development of modern science technology, more and more high demands are put forward for the engineering machinery. Not only high bearing capacity is demanded in order to decrease the material use, but also long fatigue life is requested for structure. The researches of machinery strength valuation, design and fatigue life estimation are paid more attention and developed. When the researches are done with the existing theories, the shape characteristics and case environments of machinery members become important influencing factors. Besides, calculation steps and use methods are also very different.Fatigue life researches were developed based on the destroy of actual machinery member in the dissertation. On the basis of fatigue crack propagation theory, macroscopic mechanism and new improved formula of fatigue crack propagation was proposed. Simultaneously, the important physical parameters of the formula were studied due to the actual application. Also, the FEM simulation of machinery member was done according to the working process of machinery. And the stress load spectrums were drawn in combination with the mechanical analysis of all working cases. All these results could provide theory basis for the later fatigue analysis. Moreover, the normalized fatigue analysis steps were presented. The popular theory formula and the improved formula were adopted to analyzed the fatigue life of actual machinery member, respectively. And the results were compared. At last, with the neural network intelligent technology, the optimization analysis of fatigue life was carried out because of many complex influencing factors in fatigue researches. In summary, the research results of this dissertation are as followed.Firstly, many existing theories were summarized in the dissertation, such as Fatigue theory, Fracture Mechanics theory, Damage Mechanics theory, fatigue crack propagation theory results, models and criterion. The related important factors of influencing crack propagation were found out by means of analysis of all factors, which provide preparation for the following research.The dissertation mainly focus on the researches of mechanism and formula for the fatigue crack propagation. Though the interior stress is far less than the yield strength or fatigue limit, the structure member bearing dynamic load still may yield or destroy. The existing theories don't explain this problem with reasonable methods. So, the macroscopic mechanism of fatigue crack propagation was proposed in the dissertation. Inertial effect of dynamic load was added to the member. That is, though stress magnitude acting on machinery member is far less than the yield strength of material, the actual stress magnitude may exceed the bearing capacity of member and induce further crack propagation.At present, there are two most wide used fatigue crack propagation criterions. They are crack propagation threshold and material fracture toughness. For the threshold, the criterion is that the crack will grow if the driving force is more than the threshold. And for the fracture toughness, the crack will grow steady if the driving force is less than the fracture toughness. Else, the member will destroy. On the basis of these two criterions, the improved fatigue crack propagation formula was presented. Physical meaning of the improved formula is simple and it is easy to be applied. The improved completed formula was induced by two crack propagation curve and it is suitable for the whole crack propagation process. Furthermore, the crack propagation curve contains all important factors affecting crack propagation.The two important parameters, crack propagation threshold and fracture toughness, in the improved formula, were also studied. For the crack propagation threshold, according to the stress distribution, plastic zone size around crack tip and fatigue limited curve of crack propagation, the new formula was put forward with routine test parameters of material capacity and loading parameters, such as, yield strength, Poisson ratio, elastic modulus and stress ratio. For the fracture toughness of material, based on the simplified curve, the formula is described with material capacity parameters and geometry size. The two formula were checked by test data, respectively. Allowable error is in the permission scope of the project.In order to check the effect of the improved formula, Fatigue life researches were developed based on the destroy of actual machinery member in the dissertation. Based on the analysis of bearing load case of the actual members in engineering machinery, the dissertation makes use of the FEM numerical simulation and obtains the stress-strain distribution of the members and builds the stress distribution load spectrum that is needed in the analysis of practical members fatigue life. Fatigue life analysis is only a sort of strength analysis, it must follow clear steps in practical application. This dissertation offers such a method.For the study object in this dissertation, firstly, it should justify whether it belongs to fatigue problem, if so, fatigue life calculation method is in need. Then, fatigue strength correction is needed. It shows that the members in this dissertation suffer the fatigue accumulative damage. Thus, linear accumulative damage theory and nonlinear damage theory are used to calculate life separately. Then, making use of Finite Element method, fatigue life was also obtained. Comparing the three calculation results, it can be found that the results satisfy the requirement of the engineering design without the fatigue damage within the scope of the bearing load in project. But practical members broke up within a short use time. For this problem, using the reverse analysis and supposing the size of initial crack and applying the crack estimation formula that is most acceptable and the improved formula in the dissertation, the life was estimated for the crack member. In compared with actual cases, the results of the improved formula are more suitable for the crack propagation process.In the dissertation, the fatigue life was also be predicted with neural network intelligent technology. By this technology, the importance of each influencing factor on crack propagation was ranked. It shows that the initial angle, initial crack size and stress ratio are all the most important influencing factors. But that of the plate thickness is secondary. Moreover, the initial angle is more important than the initial crack size, which is very different from the past researches. It also shows that the importance of crack threshold is far more than that of fracture toughness, which isn't mentioned in the previous researches. All researches in the dissertation will be tested by more tests and data in future. And all works also present new aims of the fatigue researches. |
PDF Full Text Request