NOZ-FLAW: A Finite Element Program for Direct Evaluation of Stress Intensity Factors for Pressure Vessel Nozzle-Corner Flaws

摘要

This report describes a linear elastic finite element computer program (NOZ-FLAW) which is used for the direct evaluation of mixed-mode stress intensity factors along user-defined crack flaws at pressure-vessel nozzle corners. Special three-dimensional crack front elements are used to model the immediate vicinity of the flaw. These crack front elements have the proper square root and inverse square root variations for displacements and stresses, respectively. Regular isoparametric elements are used away from the flaw front. Inter-element displacement compatibility between singular and regular elements is satisified by assuming an independent boundary displacement field and using a Lagrange multiplier technique to enforce the compatibility constraint. The stress intensity factors at various points on the crack front are solved for directly along with the unknown nodal displacements. The program provides for automatic generation of a finite element model incorporating either a mathematical or user-defined crack flaw. Generation and analysis of the model are performed with program input consisting of 8 to 12 cards. Applications of the program to several crack flaws in an intermediate test vessel are described.