Effects Of Residual Stress On The Failure Mode Of Pressure Vessels

Leak-before-break (LBB) acts as an rule in the structure design and safety evaluation of pressure vessels. LBB can be indicated as such a kind of fracture behavior: the surface cracks propagate through the wall of the structures, and the contained liquid starts leaking out before the collapse of entire structures. Thus we can check the defects of structure in order to avoid disastrous accidents due to the rapid collapse of whole structure timely. Amounts of simulations for the processes of the surface crack propagations should be conducted to estimate whether the structure satisfies LBB requirement. The prediction of geometric features of surface crack propagation under quasi-static loading is recognized as one of the attractive topics in LBB assessment. The stress intensity factor (SIF) is employed as the basic parameter to analyze the crack propagation in the field of engineering. Moreover, some researches on surface crack propagation under the exterior load have been presented recently. However, the case only under the exterior load is not sufficient to satisfy LBB requirement under the exterior load only. The influence of residual stress induced in the manufacture process of press vessels should be taken into account.In this study, with the use of commercial FEM code ABAQUS, the quasi-state crack propagation of surface crack occuring in a pressure vessel model with a 3-D surface crack is simulated under the given residual welding stresses. A new method is proposed to predict the geometric features of surface crack propagation by introducing the concept of scale factor. The conclusions obtained are listed as follows:1) The residual stresses restrain the crack propagation in the direction of wall thickness. The effect of residual stresses makes the surface cracks become slender to induce the bursting fracture consequently. Therefore the influence of residual stresses must be taken into account in the LBB rule;2) The geometric features of surface crack is not be optional. The correct one is that the points along the crack front tip are in the same propagation states.3) The ratio of SIFs near the surface and those in the direction of thickness varies with the ratio of the major axis and the minor axis of the elliptical crack (b/a). The actual geometry of surface crack depend on the appropriate choice of ratio.The simulation method for the crack propagation proposed in this study can be applied in the case of applying of both exterior load and residual stresses. After the distributions of SIFs are obtained, the geometric features can be determined according to the scale factors. Based on the above results, we can estimate whether the structure satisfies LBB requirement.