Numerical Studies on Ratcheting Behaviour of Straight Pipes

Abstract

In order to simulate ratcheting behaviour in SA 312 Type 304 LN stainless steel straight pipes of 168 mm nominal outer diameter used in power plant systems, finite element studies were carried out under combined internal pressure and cyclic bending load. The length and average thickness of the pipe was 2700 mm and 14.5 mm, respectively. Reduced thickness in the gauge length portion of 200 mm at the centre of the pipe was 11.5 mm. Finite element (FE) analysis was carried out using ABAQUS software by direct cyclic method. A complete FE model of the specimen was generated using solid modelling with the dimensions equal to the dimensions of the pipe specimen on which ratcheting experiment was conducted earlier. Analysis was carried out for three different cases under different combinations of internal pressure and cyclic displacements. The internal pressure was varied from 25 to 45 MPa. The bending load was applied in the form of cyclic displacement which was varied from ±25 mm to ±45 mm. The elastic–plastic response of the pipe was simulated by using the Chaboche hardening parameters. Non-linear isotropic/kinematic hardening Chaboche model with three back stresses was used to study the effect of biaxial ratcheting. Ratcheting strain values were obtained at different locations in the gauge length portion of the pipes and significant ratcheting strain accumulation was observed in the circumferential and longitudinal direction. Details of the studies carried out and the results of the investigations are discussed in this paper.