THE APPLICATION OF A KINEMATIC HARDENING MODEL FOR ASSESSING RATCHETTING IN PRESSURIZED PIPES

Concerns within the nuclear power generation industry regarding the po ssibility of incremental collapse or ratchetting incurred in pressure vessels and large pressurized piping runs during seismic disturbances has led to a programme of experimental work to simulate component and material behaviour under such conditions. As part of this programme, t he plastic deformation of thin-walled cylinders has been experimentall y examined for the loading conditions of +/-1% cyclic axial strain wit h hoop stresses of approximately 0, 1/4, 1/2, and 3/4 of the initial u niaxial yield stress. Two materials similar to those used in the pipew ork of pressurized-water reactor nuclear plant in the UK have been tes ted, namely type 304S11 stainless steel and En6 low carbon steel. Unde r the loading conditions, both materials incurred plastic hoop ratchet strains to varying degrees. These ratchet strains were compared with the limiting ratchet strains predicted by the Prager-Ziegler model of kinematic work hardening. It was concluded that this model could not b e satisfactorily used for design purposes as it did not consistently e ither overestimate or underestimate the measured ratchet strains. Furt hermore, the manner in which the model reaches a limit is not observed in the experimental results.