The Numerical Simulation And Analysis Of Stress And Strain Field At Crack Tip

Welded structures are widely used in many large engineering applications including the shipbuilding、pressure vessel and pipeline、offshore drilling platform. However, these components are often fabricated by welding, which inevitably introduces undesirable residual stresses. When welded components with defects work in special environment, such as high temperature、high pressure、fluctuating loads and the in-service performances, they will be affected by both external loads and residual stresses. The micro-defects can increase the risk of fatigue crack initiation and propagation, leaving welded structures prone to catastrophic failure. For one thing, the supporting capacity and corrosion resistance of welded components will be seriously weakened by crack. For another, the stress concentration generated during the operation will be the sources of various kinds of fracture, result in low stress damages, and even disastrous failure. Therefore, study the crack problem has significant theoretical significance and engineering application value.Studying the crack matter, especially the stress intensity factor KI、J integral and the crack tip opening displacement in stress and strain field at crack tip, not only has important indication for researching the crack propagation at crack tip, but also has significance for preventing and controlling fracture behaviors. Currently, experiment、theoretical analysis and numerical simulation are generally used to study these parameters. However, experiment is restricted due to the limitations of equipments and expensive costs. For theoretical analysis, a large amount of assumptions should be made to ensure the accuracy of the analysis. Furthermore, it is difficult to solve the problem with completed material properties and boundary conditions. Therefore, due to the excellent ability to solve the problems of various materials, structural types and loads, numerical simulation turn out to be an important tool for people to analyze fracture behaviors. Therefore, the finite element method and the extended finite element method were used to investigate the matter of quasi-static static crack and dynamic crack in this paper, respectively. The results indicated that:As for quasi-static static crack, the distributions of crack tip stress and strain field、KI and J integral were obtained by loading the FQ at the condition of linear-elastic and elastic-plastic, respectively. Results showed that they firstly increased and then tended to be stable and finally decreased at the direction of specimen thickness. Furthermore, they increased along with the crack length increased. Besides, the critical stress intensity factor KQ and the critical J integral were also gained, respectively. It was found that the3D results were more accurate than2D and the fracture model could be used to analyze KQ and J integral. Investigating the quasi-static static crack problem, especially the critical-stable state at crack tip, not only verified the accuracy of the fracture model, but also acquired the distributions of stress and strain field at crack tip、KI and J integral. Therefore, it was a foundation for study the quasi-static dynamic crack in the following chapter.As for quasi-static dynamic crack, firstly, the displacement increased along with the load increased before the critical load. Then the crack tip was in the state of elastic propagation. Secondly, the crack tip was in the critical-stable state when load reach the critical value. Then the crack tip become inactivated and grew to be the plastically deforming area. When load exceeded the critical value, the fast crack propagation happened and the specimen fractured. Here, the displacement under the maximum load was regarded as the crack tip opening displacement δs. In addition, δs increased after the critical load increased along with the specimen thickness increased.Furthermore, as for the influence of the residual stress field on the crack tip stress and strain field, it was found that, to a different extent, KI、J integral and the crack tip opening displacement increased by the residual stress field. And the crack growth will be accelerated in the residual stress field.There are five chapters in the thesis:In the first chapter, a summary of the relative researches on the fracture problem of quasi-static crack was introduced, including background and significance, the researches state at home and abroad, and the chief contents and methods.In the second chapter, the principles of fracture mechanics, the finite element method and the extended finite element method were reviewed.In the third chapter, taking the compact tension specimen as an example, the2D and3D numerical simulation methods of quasi-static static crack based on ABAQUS were studied, and the calculated results were also analyzed.In the forth chapter, taking the compact tension specimen for instance, the3D numerical simulation methods of quasi-static dynamic crack based on ABAQUS were investigated, and the computed results were also investigated.In the fifth chapter, the stress and strain field at crack tip in the residual stress field and the influence of crack length and specimen thickness on KI、J and the crack tip opening displacement were investigated.