The experimental aspects of ratcheting include the effect of stress levels, stress rate, temperature, planar anisotropy, previous loading history, and multiaxial loading paths. The current article addresses the recent progresses made on the experimental front on the ratcheting behavior spanning from the specimen level to the component level and its correlation with microstructural evolution. Ratcheting can be defined as the directional progressive accumulation of plastic deformation of a material when it is subjected to a primary load along with a secondary cyclic load. In the past three decades ratcheting fatigue has attracted lots of research interest. 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. Local bulging, ovalization and consequent thinning of the pipe cross-section were observed due to fatigue ratcheting. During the tests, load, load-line displacement and deflections at three locations were continuously monitored. Number of cycles corresponding to through-thickness crack/s and final failure of the component were recorded. Cyclic bending load was applied on the pipes by subjecting them to different levels of load-line displacements. The pipes were filled with water and pressurized the pressure was maintained at 35 MPa till the first through-thickness crack was observed. Post-yield two element rosette strain gauges were mounted at various locations within the gauge length to measure the longitudinal and circumferential strains. The thickness was reduced to 12 mm in the gauge length portion of 200 mm at the centre of the pipe. The length and average thickness of the pipes were 2800 mm and 15 mm respectively. 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. 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’ 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.
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