Fatigue ratcheting studies on pressurised stainless steel straight pipes under cyclic bending

Abstract

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. Ratcheting occurs in a structure subjected to a combination of steady and cyclic loads such that the material response is inelastic. Fatigue ratcheting studies were carried out on five numbers of SA 312 Type 304LN stainless steel straight pipes of 168 mm outer diameter under four point cyclic bending. The nominal thickness of the pipe specimens was 14.5 mm and inner diameter was 139 mm. Out of the five pipe specimens, four specimens were tested under combined internal pressure and cyclic bending and the remaining one specimen was tested under cyclic bending without internal pressure. Cyclic bending load was applied on the pipe specimens by subjecting them to different levels of load-line displacement applied at the centre (axis of the actuator piston). Loadline displacement is the vertical distance by which the actuator piston moves; the same was measured from the in-built LVDT in the actuator. Number of cycles corresponding to through-thickness crack and final failure of the component were recorded. During the tests, load, load-line displacement and deflections (measured at the bottom of the pipe specimen) at four locations were continuously monitored. Local bulging and ovalization of the pipe cross-section and consequent reduction of the pipe wall thickness were observed due to fatigue ratcheting. The pipe specimens subjected to combined internal pressure and applied cyclic displacement failed either by occurrence of through-wall crack accompanied by simultaneous ratchet swelling (ballooning) or occurrence of through-wall crack accompanied by simultaneous ratchet swelling (ballooning) and bursting.