EVALUATION OF THE STATIC CRACKING RESISTANCE OF THIN-WALLED PRESSURE VESSELS

Abstract

The analysis of failure criteria and limit states for thin-wall pressure vessels is carried out, taking into account the influence of plastic deformations. The introduction describes the main problems of operation of thin-walled vessels operating 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, their grouping and statistical processing are carried out. The share of welding defects was 62 % of the total number of defects, the remaining types of defects are much smaller. The histograms of the dimensions of welding defects were constructed and the distribution laws were determined: the length of the cuts was described by the lognormal distribution law, the depth by the normal distribution law. Further, the limiting states and criteria for the destruction of vessels in the presence of defects and cracks in the conditions of elastoplastic deformation of the material are indicated. The advantages of using generalized equations of the form «J-curves» when calculating for fracture toughness are shown. A formula is given for the calculation of «J-curves» connecting a dimensionless J-integral with a dimensionless load. An analysis is made of the stress-strain state of a thin-walled vessel with a external half-elliptical and internal elliptical cracks, in a volumetric setting. The peculiarities of the stress and strain fields in the local region of the crack zone under elastoplastic deformation were investigated. The accountings are performed and the results of evaluation of the fracture mechanics energy criterion — J-integral for vessel model with the external half-elliptical and internal elliptical cracks in elastic-plastic deformation — are presented. The results are presented as graphs of the dimensionless J-integral dependency on the geometrical sizes of the vessel and crack. The equations of «J-curve» were obtained and the ultimate load for thin-wall pressure vessels were determined. That ultimate load depends on the geometrical dimensions, loading parameters, structural behaviors of material, the characteristics of crack strength and deformation. On the «J-curves» and the deformation curve, a formula is obtained for determining the dependence of the ultimate load on the crack size, loading parameters and material characteristics. Using this formula, we plotted the dependences of the vessel’s limiting pressure for elastoplastic deformations on the ratio a/S for surface and internal cracks for different ratios R/S and Jc, which allow estimating the levels of limiting pressure for the safe operation of thin-walled vessels.