PSI - Issue 30

Nikolay I. Golikov et al. / Procedia Structural Integrity 30 (2020) 28–32 Nikolay I. Golikov / Structural Integrity Procedia 00 (2020) 000–000

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from the lower end. The areas of their propagation deep into the walls are observed on these cracks. On the outer side of the pipe opposite the section, an accumulation of small holes was found (Fig. 2, b). The RWS of a longitudinal factory weld of a pipe from the same batch as those with cracks was investigated. Figure 3 demonstrates the graphs of RWS distribution on the inner and outer sides of the pipe. The stress was examined beginning from the weld metal (point 0 on the graph) to 50 mm in the transverse direction from the weld. As Figure 3 shows, the level of circular and axial residual stresses is low on the outer side of the pipe and ranges from -100 to +100 MPa. On the inner side, high tensile RWS, which reaches 370 MPa in the axial direction and 280 MPa in the circular, was detected near the weld metal. According to the study results, there is a considerably high level of tensile RWS near the weld metal on the inner side of the pipes. It is consistent with the views presented in the reference book edited by Nikolaev (1979) that the longitudinal weld causes bending of the long shells due to restraint as a result of shrinkage. Owing to bending, tensile RWS develops on the inner side of the pipe.

Fig. 3. Distribution of RWS of the longitudinal seam (0 – weld):1 (  ) and 2 ( ▲ ) are the circular and axial stresses in the inner near-surface layers; 3 (  ) and 4 (  ) are the circular and axial stresses of the outer near-surface layers

The cracks on the inner side of the longitudinal weld and their small development on the outer side of the pipes indicate the influence of the RWS field on their propagation. The branching of the crack in the pipe No. 2 (Fig. 2, b) depends not only on the rate of its development but also on the level of accumulated strain energy of the structure, as noted by Ivanov et al. (2010). It is known that tensile RWSs are carriers of elastic strain energy. The energy of residual stress enhances the dynamics of the fracture process and accelerates the crack development. The formation of cracks in the longitudinal welds of the gas pipeline is caused by disbanding due to poor-quality welding. During operation, the linear part of the gas pipeline experiences cyclic loads associated with seasonal changes in gas pressure, temperature fluctuations, heaving, and subsidence of soils. Herewith, the residual stress is algebraically summed with the stress caused by the external load, as noted by Vishnyakov and Piskarev (1989). Consequently, tensile RWS has a significant effect on crack growth. 4. Conclusion It has been revealed that wall cracks formed in the examined longitudinal welds of the inner side of the pipe with no visible cracks on the outer side. It has been established that tensile residual stress is formed on the inner surface of the pipe at a level close to the yield strength of the base metal. Moreover, the level of residual stress is close to zero values on the outer surface of the pipe near the weld. The distribution of residual stress of the longitudinal welded joint explains the crack formation on the inner side of the pipe wall. Tensile residual stress contributes to the development of cracks in the welded joints of the linear part of the main gas pipeline. The conducted studies confirm