The impact of residual technological stresses on the opening and stability of through cracks in pipeline elements

Abstract

The impact of residual stresses in a DN850 pipe (steel 10GN2MFA) with austenitic cladding, welding stresses in the mounting annular seam of the pipeline, and residual stresses arising in a curvilinear branch DN350 (steel 08Kh18N10T) during manufacture by plastic deformation on the opening and stability of through cracks is considered. Calculations of residual stresses are performed using the finite element method (FEM). It is shown that residual stresses cause a change in the size and shape of the outflow channel, the coolant flow rate, and the value of the J-integral at the crack tip. In case of short cracks and relatively low operating stresses, the crack edges can close on the inside of the pipe wall due to the action of residual stresses thus leading to a decrease or cessation of the leak. A reversed effect of residual stresses on extended cracks is observed at rather high operating stresses: change in the shape of the outflow channel (an increase in the opening of the crack edges on the outer surface of the pipe) leads to a decrease in the friction of the coolant flow against the crack edges and, hence, to an increase in the leak volume. The results of testing full-scale models of elements of a straight section of the pipeline with a welded seam and a curvilinear branch DN350 with artificially created defects by internal pressure and bending moment are presented. It is shown that local through cracks develop from initial defects, which remain stable at maximum design loads (normal operating conditions plus maximum design earthquake) which matches the calculation results and meets the requirements of the applicability of the concept of «leak before break».