Evaluating the Static Cracking Resistance of Thin-Walled Pressure Vessels

The failure criteria and limit states for thin-walled pressure vessels are analyzed taking into account the influence of plastic deformations. The introduction describes the main problems in operation of thin-walled vessels working under internal excess pressure associated with technological defectiveness and reduction of the residual life. Typical technological and operational defects in welded joints of vessels and statistical data on their number and types are presented. The share of welding defects is 62% of the total number of defects, and the share of remaining types of defects is significantly less. Histograms of the dimensions of welding defects are drawn, and the distribution laws are determined: the length of undercuts is described by a lognormal distribution law, whereas the depth of undercuts is described by a normal distribution law. Further, the limiting states and fracture criteria of vessels with defects and cracks under the conditions of elastoplastic deformation of material are indicated. The advantages of using the generalized equations of the form of J-curves are shown for calculation of the cracking resistance. A formula is given for calculating the J-curves that associate the dimensionless J-integral with the dimensionless load. The stress-strain state of a thin-walled vessel with a surface half-elliptical crack and an internal elliptical crack is analyzed in a volumetric setting. The peculiarities of the stress and strain fields in the local region of the crack zone under elastoplastic deformation are investigated. For the vessel model with a surface half-elliptical crack and an internal elliptical crack under elastoplastic deformation, the energy criterion of fracture mechanics, the J-integral, is calculated, and the results of calculations are presented. The results are presented in the form of graphs of the dimensionless J-integral versus the geometrical dimensions of the vessel and crack. The equations of J-curves are obtained and the limit load for thin-walled vessels is determined which depends on geometrical dimensions, loading parameters, strength properties of the material, and characteristics of cracking resistance and deformation. From the J-curves and the deformation curve, a formula is derived for determining the dependence of the limit load on the crack size, loading parameters, and characteristics of the material. Using this formula, the dependences of the limiting pressure of the vessel under elastoplastic deformations on the ratio of crack length a to vessel wall thickness S (a/S) is plotted for surface and internal cracks with different ratios R/S (R is the shell radius) and J_c which allow estimating the limiting pressure levels of the safe operation of thin-walled vessels.