Critical State of a Thin-Walled Cylindrical Shell Containing an Interlayer Fabricated from a Material of Lesser Strength

Abstract

The stress state and strength of thin-walled cylindrical shells containing straight and spiral interlayers of a material of lesser strength are investigated. Dependences of the critical internal pressure in thin-walled shells on the slope of the layer of lesser strength, the coefficient of biaxiality of the loading, the relative thickness of the layer of lesser strength, and the coefficient of mechanical nonuniformity are obtained. A comparative theoretical analysis of the strength of seamless, straight-seam, and spiral-seam largediameter pipes containing interlayers of lesser strength in the seams is carried out. Welded joints of pipes, including thermally strengthened pipes, may contain a sector of lesser strength in the nearseam zone, such as a soft interlayer made of a metal with yield strength and ultimate strength lower than that of the base metal. The relative position of soft interlayers in shell and sheet-metal structures, pressure vessels, and spiral-seam pipes of main and field pipelines influences the strength of the entire structure. The efficiency of welded joints with soft sloping interlayers depends on the mechanical nonuniformity of the joints and the geometric parameters of the interlayer and its orientation relative to the acting force. The action of an axial load caused by temperature fluctuations and the pipe laying conditions, which lead to biaxial loading of the wall of the pipe, may exert a substantial influence on the magnitude of the critical pressure. In order to assure that welded joints possess a high load-carrying capacity, parameters that satisfy the basic requirements of welding technology and assure the maximal effect of local and structural strengthening must be selected. In certain cases, full strength of a welded joint may be achieved. The influence of biaxial loading of the wall of a pipe on the efficiency of a joint with an interlayer of lesser strength situated oblique to the main stress axes has not been studied to a sufficient degree. This influence on the static strength of spiral weld seams of large-diameter pipes that are part of main pipelines subjected to the action of internal pressure and an axial load was studied in [1‐9]. Certain important features of the behavior of the material of the layer at a critical moment of loading were not taken into account in the mathematical models of the stress state of a layer of lesser strength of a weld joint that were employed in these studies. This could have led to the overstatement of the theoretical estimates of the magnitude of the local strengthening and (in certain cases) the critical load. In the present study, the theoretical approaches of the previous studies [1‐9] are developed and extended on the basis of results found by the present authors [10‐14]. The objective of the study is to develop a mathematical model of the stressstrain state of a layer of lesser strength of a thin-walled cylindrical shell at a critical loading moment; approximate analytic expressions and computer programs (for calculating the critical pressure and other characteristics) necessary for carrying out a comparative analysis of the strength of longitudinal, spiral, and annular “soft” layers of such shells and for estimating the load-carrying capacity of straight-seam and spiral-seam pipes are obtained on the basis of the latter results.