Evaluation of several hardening models in the ratcheting behavior of piping branch with Different diameter/thickness ratios

In this study, the strain ratcheting behavior of piping branch under the influence dynamic bending moments are evaluated. The Chaboche nonlinear kinematic hardening model and combined Armstrong-Fredrick model with isotropic rule are used to predict the plastic behavior of the piping branches. The results of FE method by using the hardening models have been compared with the results of the experimental method and Armstrong-Fredrick kinematic hardening results. The constant parameters of the hardening model and stress-strain data have been obtained from several stabilized cycles of specimens that are subjected to simulated seismic bending cycles. Both the FE and experimental results showed that the maximum strain ratcheting occurred on the flanks in the piping branch hoop stress direction just above the junction. The ratcheting strain rate increases with increase of the dynamic moment levels. The FE results show that initial rate of ratcheting is large and then it decreases with the increasing of loading cycles. In BMS1 sample, the FE hoop strain ratcheting data by using chaboche nonlinear kinematic hardening model comparing with the other hardening models to be near that found experimentally values. In BMS2 and BMS3 components, the FE hoop strain ratcheting data by using chaboche nonlinear kinematic hardening model and combined hardening model comparing with the A-F hardening model to be near that found experimentally values. The hoop strain ratcheting rate by Armstrong-Fredrick model gives overestimated values comparing with the experimental data.