Natural Crack Growth of Nozzle Corner Crack Under Cyclic Pressure and Thermal Transients Using XFEM

Abstract The nozzle corner region, particularly the inside blend radius, is subjected to significant stresses due to internal pressure and thermal transients. In some cases, a postulated or detected flaw at this location must be evaluated for life assessment or fitness-for-service purposes. To assess the remaining life, crack growth calculations are typically performed using stress intensity factor (K values), assuming a semi-circular or semi-elliptical crack shape. These K calculations are generally limited to the deepest and surface points of the crack. However, due to the complex geometry of nozzle corner cracks, there is a need to compute K along the entire crack front. The extended finite element method (XFEM) in Abaqus offers a solution, allowing for K to be calculated along the entire crack front without the need to explicitly model the crack-tip. This paper aims to evaluate the fatigue crack growth (FCG) behavior of a nozzle corner crack under cyclic pressure and thermal transients using automated XFEM Python scripts. Comprehensive verification and validation of the XFEM results were performed by analyzing stress distributions, K solutions along the entire crack front, and FCG comparison against experimental results. In addition, the FCG results obtained through automated XFEM were compared against various analytical solutions, including the existing nozzle corner crack solution based on the universal weight function (UWF) method and the ASME Section XI flat plate solution. Based on the natural FCG behavior observed in the representative nozzle geometries, a method is proposed to predict the nozzle corner crack growth using the flat plate solution.