Study Of Ⅰ+Ⅱ Mixed Mode Fatigue Crack Propagation In An Austenite Stainless Steel06CR19NI10

Fatigue fracture is one of the main failure forms of pressure vessels. A lot of efforts have been spent on the study of the fatigue fracture of pressure vessels. Cracks can be divided into three basic modes according to the different loadings based on fracture mechanics, namely opening mode, sliding mode and tearing mode. In engineering, most cracks are mixed mode combined by the three basic modes. In this thesis, Ⅰ+Ⅱ mixed mode fatigue crack propagation in austenitic stainless steel06Crl9Nil0which is usually used to construct pressure vessels in China is tested and analyzed.Firstly, fatigue experiments were carried out on the specimens with inclined notch-cracks and crack growth path and rate were investigated. Results show that regardless of the magnitude of the inclined angles of the notch-cracks, the new formed cracks at the notch-crack tips grow in a direction to be perpendicular to the applied load. Calculation of the stress intensity factors along the crack growth path show that compared with the mode Ⅰ stress intensity factor range, mode Ⅱ stress intensity factor range is very small. All these indicate that the crack propagates with the mode transferred from Ⅰ+Ⅱ mixed mode to mode Ⅰ. The relationships between the crack growth rate and mode I stress intensity factor range conform to the Paris-Erdogan equation.Secondly, the strain-strengthening effect on Ⅰ+Ⅱ mixed mode fatigue pre-crack propagation was studied. To do that, a3.5percent tensile strain was gradually applied first on the specimens with angled through-thickness cracks. Then fatigue experiments were carried out and the results were compared with the specimens without strain-strengthening. It turns out that the strain-strengthening has no effect on the new crack growth path. However, the new crack initiation is more difficult and the crack growth rate is slower at first but faster at last compared to those without strain-strengthening. Finite element calculation indicates that the reason for different crack growth rates is owing to the residual stress produced during strain-strengthening process.Finally, experimental study was carried out for the Ⅰ+Ⅱ mixed mode fatigue crack transformation propagation with consideration of the material orientation. Results show that after initiation from the notch-crack tips, the new formed crack propagates in Mode Ⅰ. The material orientation has no effect on the crack growth path and a very small effect on the crack growth rate. In addition, effects of the material orientation on the tensile property and impact toughness of the material were also tested and metallurgical analysis was performed to explain the obtained results.