PSI - Issue 33

D.D. Okulova et al. / Procedia Structural Integrity 33 (2021) 1055–1064 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Keywords: Pitting corrosion; Stainless steel; Random distribution; Strength; Surface defects; Finite element analysis; FEM; Spherical shell; Stress state; Pressure vessels

1. Introduction Pressure vessels are widely used in various industrial processes, for storage and transportation of compressed gases, liquids and other substances (Li et. al. (2017), Dehrouyeh-Semnani et al. (2019), Dastjerdi et al. (2020)). For these purposes, a sphere is the ideal shape to hold internal pressure (Nilsen, 2011). Spherical vessels have the smallest surface area per unit volume. Accordingly, the temperature conditions of the environment have the least effect on the substance within the sphere. Various loads, severe operating and transportation conditions, aggressive environmental factors can damage the surface of the vessel. Since a weighted part of pressure vessels are manufactured with metal materials, corrosion processes are a common cause of the damage (Obeyesekere, 2017). One of the most common and dangerous types of corrosion is pitting corrosion. This is a localised corrosive attack with relatively small area compared to the surface of a corroded object (ASM International Corrosion, 2001). This type of localised corrosion leads to minimal volume loss in the structure but can force catastrophic failure (Zhao et al., 2018). Gradually accumulating pitting defects result in stress concentration and can initiate premature plastic behaviour (Wang et al. 2019). To assess and ensure long-term reliability and strength of the structure, it is necessary to analyse stresses concentrated in the vicinity of defects and accurately predict the behaviour of a structure under corrosion conditions. Many authors investigated the effect of multiple surface defects on strength and buckling behaviour of structures experimentally and numerically, that significantly enhanced the qualitative understanding of the corrosion processes. Most of the works concerning structures with multiple pitting defects focus on metal pipes and plates. For example, Zhao et al. (2018) investigated the influence of pit shape, specification, corrosion thickness, diameter and thickness of randomly corroded thin-walled circular tube on its bending capacity. Wang et al. (2019) experimentally and numerically studied the structural performance of steel tubular members subject to pitting corrosion under compression. The paper showed the impact of the distribution pattern of pitting corrosion and volume loss of the material on a strength decrease. The paper of Sun et al. (2021) investigated the effect of interaction between corrosion defects circumferentially aligned on a pipeline under axial tensile stresses. It was found that an increasing spacing between corrosion defects leads to reduction of stress concentration and the plasticity near defects. When the spacing exceeds a certain value, the mutual interaction between the defects is negligible. The effect of random pitting corrosion on the collapse pressure of pipe was investigated by Wang et al. (2018). Khedmati et al. (2014) developed an analytical method for derivation of the average stress–average strain relationship for imperfect steel plates with random corrosion defects on both sides with taking into account both geometric and material nonlinearities. The ultimate strength of the plate was predicted using an empirical formulation. Zhang et al. (2016) obtained an estimate based on the corroded volume loss for ultimate strength of hull plate with pitting corrosion damage under combined loading. The effects of some corrosion characteristics on the ultimate strength with respect to the corroded volume loss were also investigated. A parametric study on effects of pitting corrosion on the ultimate strength of stiffened plates subjected to uniaxial compression was carried out by Feng et al. (2020). It was found that the ultimate strength significantly decreases with an increase in the degree of pit corrosion intensity on the plate surface and strongly depends on the distribution of corrosion defects. The authors also stated that the ultimate strength reduction of stiffened panels with pitting corrosion is associated with the loss of volume due to corrosion. There are also a number of papers where spherical structural elements are considered. For example, Zhao et al. (2019) studied the influence of randomly located corrosion defects on the loading capacity of welded hollow spherical joints. Authors pointed out that corrosion can reduce the effective thickness and, therefore, considerably decrease the ultimate loading capacity. Stress concentration in the vicinity of a growing corrosion pit on the outer surface of a hollow sphere was studied by Sedova et al. (2014). A spherical shell with multiple surface defects was considered by Okulova et al. (2019). It was shown that an increase in the number of defects and in the area of surface damage causes the maximum normal stress to grow. In the paper of Sedova et al. (2021) the stress state of a pressurised sphere with hemispherical notches on its outer surface was investigated; uniform and random location of defects along the equator of the sphere was considered in the framework of the linear theory of elasticity. Closed-form solutions for the lifetime