Creep crack growth analysis of elliptic surface cracks in pressure vessels

摘要

It is common practice in nonlinear fracture mechanics based methodologies to model an elliptic axial surface crack located at the internal or external surface of a cylindrical pressure vessel as an infinitely long axial crack. This simplification considerably reduces the amount of computation needed for evaluating high temperature fracture parameters such as C and C_t and for determining crack growth life. However, the inaccuracies introduced by this simplification on the estimated component life have never been systematically assessed in detail except stating that the assumption leads to a conservative estimate of life. In this study, an analysis method is developed that does not require such an assumption. Using this newly developed method, a procedure for assessing creep crack growth life was demonstrated for an elliptic axial surface crack located on the internal surface of a pressurized vessel at high temperature. Equations are proposed for estimating G_t at both the deepest point along the crack front and also at the crack tip on the surface. Using these equations, the aspect ratios of the surface crack were updated for every time step of crack growth calculations and were reflected in determining G_t value of the next step. Changes of crack depth, crack size and G_t values at the deepest point and surface point of the crack tip are estimated and used to predict the evolution of crack shape during creep crack growth.