Cyclic Mean Stress Variation Effect on Pipe Ratcheting.

The deformation of a straight pipeline segment under stepwise cyclic loading by internal pressure is investigated. The tests were carried out on a pneumohydraulic bench, allowing one to determine the strength and durability characteristics of full-scale pressure vessels and pipeline elements under conditions close to operational ones. The pipe element was loaded with stress control. The asymmetric loading regime was a sequence of three types of cycles at a constant stress amplitude with a step change in the mean stress after stabilizing the strain rate. The load level corresponded to the lowcycle fatigue region with distinctive elastoplastic strain of the material. The experiment aimed to obtain diagrams of ratcheting (cyclic creep) of the pipe under the action of a biaxial proportional load from internal pressure (pulsating cycle) depending on the change in the mean stress of the cycle. It is shown that for a pipe element, similar to laboratory specimens, an increase in the load level (compared to that applied at the previous stage) enhances the effect of ratcheting in the circular direction. In contrast, a return to the initial lower level eliminates this effect. Unlike laboratory specimens, the accumulation of ratcheting strains in the pipe element during the first 20 cycles is uneven. A steady-state rate of strain accumulation is observed after reaching 40 loading cycles. An increase in the accumulated plastic strains of the ratcheting in the circular direction with an increase in loading cycles significantly affects the initial distribution of strains in the axial direction. [ABSTRACT FROM AUTHOR]