Stress distribution, as obtained by finite element method (FEM), and corresponding fracture mechanics parameters, have been used for the integrity assessment of a pressure vessel. Once the stress distribution is known, the most critical area can be analysed with assumed crack to estimate its critical length according to fracture mechanics parameters and basic laws. Toward this end, the Failure Assessment Diagram (FAD) is used to calculate the crack length corresponding to the limit curve. As one can see, coordinates in FAD are the ratio of net- and critical stress (x axis, S-r = S-n/S-c), and ratio of stress intensity factor and its critical value (y axis, K-r = K-i/K-Ic). Once net stress and the stress intensity factor are known and their critical values, coordinates of a point corresponding to any crack length can be calculated and positioned in the FAD
