Study Of Ⅰ+Ⅱ Mixed Mode Crack In16MnDR Pressure Vessels

Fracture is one of the main failure forms of engineering materials, which is mainly caused by the crack initiation and propagation under the external force. There are three basic crack modes, namely opening mode^sliding mode and tearing mode. But most cracks in engineering are mixed mode which is combined by the three basic modes. Therefore, many studies have been carried out to investigate mixed mode crack growth behaviors. In this thesis, Ⅰ+Ⅱ mixed mode fatigue crack propagation in low alloy steel made from16MnDR was experimentally investigated. Numerical simulations were performed for the growth path of Ⅰ+Ⅱ mixed mode crack in a pressure vessel.Firstly, experimental specimen with a slant crack in the center was designed, and fatigue experiments for Ⅰ+Ⅱ mixed mode crack propagation were carried out under uniaxial fatigue loading. Crack growth rate was measured and crack propagation path was observed with stress on the crack mode transition. For quantitative analysis, finite element calculation with the software ANSYS was performed to compute the stress intensity factor ahead of the crack tip and simulate the crack growth path. Results show that whatever the orientation of the initial crack is, the newly formed crack grows in a direction to be perpendicular to the applied load, making crack mode transformation from Ⅰ+Ⅰ mixed mode to Mode I.Secondly, metallurgical and fractography analyses of the crack growth path were analyzed. It is found that the crack growth path is transgranular without clear bifurcations although there is some damage around the crack tip. Striations and dimples are found on the fatigue fracture surface and the final fracture surface, respectively, showing that the Ⅰ+Ⅱ mixed mode crack changes to Mode I when propagation and the material is quite ductile in nature.Finally, based on the experimental results about crack growth criterion and crack growth rate, fatigue crack growth path was simulated under biaxial stress loadings, which actually is the case of mixed mode fatigue crack propagation in a pressure vessel. Structural parameters affecting crack growth rate and path were investigated and the crack tip was positioned as a function of the cycles, which is helpful to take measures to prevent crack propagation.