RATCHETING FAILURE OF STRAIGHT PIPES AND ELBOWS WITH INTERNAL PRESSURE SUBJECTED TO CYCLIC LOADING

摘要

'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 can occur in a structure subjected to a combination of steady/sustained and cyclic loads such that the material response is in the inelastic region. Ratcheting studies were carried out on four straight pipes and four elbows, made of TP 304LN grade stainless steel, subjected to steady internal pressure and cyclic bending. The specimens were filled with water and pressurized; the pipe specimens were subjected to 35 MPa internal pressure and in the case of elbows the internal pressure was maintained between 27.6 MPa and 39.2 MPa. Cyclic bending load was applied on the specimens by subjecting them to different levels of load-line displacement. Number of cycles corresponding to occurrence of through-thickness crack was recorded. In the case of straight pipes, the through-thickness crack was observed either in the top or in the bottom location, at the centre of the specimen in both cases. The pipe specimens failed either by occurrence of through-thickness crack accompanied by simultaneous ballooning, or bursting with simultaneous ballooning. The ballooning was found to be varying from 13.4% to 19.0%. The percentage reduction in thickness varied from 8.0% to 16.3%. In the case of elbows, the ballooning was found to be varying from 3.8% to 5.8% and the reduction in thickness was around 12% to 15%. All the elbows failed by occurrence of through-thickness crack accompanied by simultaneous ballooning. Axial crack was observed in the bent portion at crown locations in all the four elbow specimens. The number of cycles to failure in both the pipe and elbow specimens depend on the applied internal pressure and cyclic displacement.