| Dynamic fracture behaviors of shipbuilding steel 907A and its melded joint have been studied in this thesis. A method used for measuring the dynamic fracture toughness with a Hopkinson pressure bar system is presented. By using this method, the dynamic fracture toughness of 907A is obtained under stresswave loading (K1d = 106"7 MPay m /s) and is compared with the results of waveimpact. The influence of temperature and loading rate and inertia force on dynamic fracture behaviors are discussed. The microstructure evolution of 907A under high-velocity load has been studied systematically by optical metallography scanning electron micrography(SEM), transition electron micrography(TEM) etc.The results show that the influence of the inertia force on the dynamic stress intensity factor response curve is less, but more on load-time curve. With the temperature decreasing or the loading rate increasing, the fracture model will change in the 907A steel from ductility to brittleness, from elastoplastic plane-stress stable to linear elastic plane-strain stable, from micro-voids intergranular fracture to crystalline fracture. The dynamic fracture toughness of weld metal is less than that of base metal.Dynamic fracture toughness of 907A steel increases with the increase of the temper temperature. The result of the sample tested by aging embritle shows that 907A steel doesn't exist the trend to temper embritlement.Fracture characteristic Analysis Map has been established. With this map, the quantitative analysis for the material choice in marine design and building, evaluation of structure integrity and reliability, and the failure prediction and prevention will become possible in practice of engineering. |
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