Fatigue ratcheting studies on TP304 LN stainless steel straight pipes

Abstract‘Fatigue Ratcheting’ is a phenomenon which leads to reduction in fatigue life of a structural component by loss of ductility due to cycle-by-cycle accumulation of plastic strain. One real-life example of a situation with fatigue ratcheting possibility is piping of power plants subjected to internal pressure (primary load) and cyclic bending (secondary load). Fatigue ratcheting studies were carried out on TP304 LN stainless steel straight pipes of 168 mm outer diameter subjected to steady internal pressure and four point cyclic bending. The length and average thickness of the pipes were 2800 mm and 15 mm respectively. The thickness was reduced to 12 mm in the gauge length portion of 200 mm at the centre of the pipe. Post-yield two element rosette strain gauges were mounted at various locations within the gauge length to measure the longitudinal and circumferential strains. The pipes were filled with water and pressurized; the pressure was maintained at 35 MPa till the first through-thickness crack was observed. Cyclic bending load was applied on the pipes by subjecting them to different levels of load-line displacements. Number of cycles corresponding to through-thickness crack/s and final failure of the component were recorded. During the tests, load, load-line displacement and deflections at three locations were continuously monitored. Local bulging, ovalization and consequent thinning of the pipe cross-section were observed due to fatigue ratcheting. These ratcheting studies on pressurized piping components of power plants give valuable inputs necessary for designing the components and assuring the integrity of pressure boundary under design basis loads such as loads arising during an earthquake event