Stress Intensity Factor Equations for Internal Semi-Elliptical Cracks in Pressurized Cylinders

In order to calculate the fatigue life of cylinders subjected to internal pressure using the fracture mechanics approach, the stress intensity factors (SIFs) for internal semi-elliptical cracks are needed. Nowadays, the most accurate procedure for fatigue life calculation consists in starting from a postulated internal semi-elliptical crack and updating the flaw aspect ratio during the crack propagation. In this sense, assuming a semi-elliptical shape during crack propagation, SIFs both at the deepest crack point and at the surface point must be calculated in order to update the crack aspect ratio through its fatigue propagation. This continuous crack shape updating cannot be done using the conventional tabulated solutions, as those provided in the main design codes and SIF handbooks. This paper presents a number of closed-form equations, which very accurately fit the tabulated results in ASME Boiler and Pressure Vessel Code, Section VIII, Division 3, to calculate the SIF for internal semi-elliptical cracks in cylinders subjected to internal pressure. These equations can be used to avoid the use of the tabulated solutions, and, as a consequence, an automatic numerical integration of the propagation law can be done.