PSI - Issue 60

Rosy Sarkar et al. / Procedia Structural Integrity 60 (2024) 75–92 Rosy Sarkar/ StructuralIntegrity Procedia 00 (2019) 000 – 000

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Comparison of the methods in elastic route at higher pressures shows that at high pressure (beyond 25 bars), the EID and OPD predict initiation of ratcheting at the same displacements. The comparison of initiation of ratcheting at 600 ℃ by various routes has been presented in Figure 18. It may be observed that the strain accumulation is governed by both cyclic plasticity and creep at high temperatures. Hence, the initiation of ratcheting at high temperatures is observed at a relatively lower magnitude of displacement and pressure combination.

Figure 18 Initiation of ratcheting at 600 ℃ temperature by various routes It is observed that RCC MR predicts the highest displacements for the initiation of ratcheting, and elasto-perfectly plastic analysis indicates the lowest displacements. RCC MR strain limits are more conservative when compared with the stress limits, and ASME is more conservative than RCC MR. As the EID (RCC MR) had been developed based on the experimental data [Chellapandi and Lakshmana Rao (2022)], [Matheron et al. (2014)], and ASME ratcheting design rules had been established from analytical routes [Chellapandi and Lakshmana Rao (2022)], ASME is more conservative than RCC MR. The elasto-perfectly plastic analysis predicts the least displacements for the initiation of ratcheting at room temperature and high temperature. Hence the elasto-perfectly plastic route seems to be more conservative when compared to the elastic route. In general elasto-perfectly plastic analysis is more realistic and elastic route is more conservative, contrasting the observations. The reason may be attributed to the considering the number of cycles in the elasto-perfectly plastic route, whereas, in the elastic route, the number of cycles has not come into the picture anywhere. However, the conservativism in the prediction needs to be verified with experiments and elasto-plastic analysis which considers the hardening effect. 5. Conclusion Ratcheting studies have been carried out at room temperature and 600 ℃ , for SGDHR pipe bend using finite element analysis. Prediction of initiation of ratcheting with elastic and elasto-perfectly plastic routes have been compared. It is observed that ASME is more conservative than RCC MR code rules, as ASME predicts the lower displacements for the initiation of ratcheting in the elastic route. Elasto-perfectly plastic analysis, which considers the number of cycles, predicts the least displacements for the initiation of ratcheting. Future work is envisaged on experiments and elastoplastic analysis considering the material hardening effect.