PSI - Issue 60

11 9

S.K. Pandey et al. / Procedia Structural Integrity 60 (2024) 3–12 S. K. Pandey/ Structural Integrity Procedia 00 (2023) 000–000

Fig 18. Ramberg-Osgood parameter ‘n’ v.s. temperature (°C)

Fig 19. Ramberg- Osgood parameter ‘α’ v.s. temperature (°C)

n=1E − 05 ∗ T 2 − 0.007 ∗ T+3.5 α = − 9E − 05 ∗ T 2 +0.058 ∗ T+9.7

(5)

(6)

5. Conclusion The load-displacement curves obtained from FE analysis with the unique Ramberg- Osgood parameter (n, α) are closely approximating the experimental load-displacement curve for the notched specimens. The FEA load displacement curve produced by using experimental stress-strain material data of smooth specimen is not approximated well for the notched specimens. For the material damage parameter evaluation, the large plastic strain data, up to the ultimate value is needed. Hence this approach provides the rules to evaluate the precise stress-strain data for large plastic deformation which can be used for the damage mechanics study of the material as well as designing for the high stress concentration zone. The equations for the best fit curve for RO parameters- n and α are closely approximate with RCC-MRx material stress-strain curve for the austenitic stainless steel 316LN and valid for plastic strain beyond 1.5% which generally occurs in high geometrical discontinuity regions like nozzle junction, vessel head, T-junction etc. This best fit equation is not approximate well for plastic strain less than 1.5%. Hence the RCC-MRx equation of stress-strain can be used when plastic strain is less than 1.5%. These equations of RO parameters as derived in this work can be used for finding the stress-strain curve at any temperature from 25°C to 650°C. It was observed that the optimized values of RO parameters provide a good approximation of the stress strain data which is comparable to that provided on the RCC-MRx curve. The following conclusions can be drawn from this work. • The error minimization approach as followed in this work provides a new approach to evaluate the true stress-strain curve of any material as this is represented in terms of Ramberg-Osgood paramaters. The advantage is that any kind of specimen can be used in the experiment, i.e., both smooth and notched specimens and hence, the effect of stress triaxiality in the notched region of the specimen is automatically taken care of in the FE model. • Appropriate scaling parameter must be used for smooth and notched specimens as the energy absorption magnitude is high for smooth specimens due to presence of large elongation before fracture as compared to notched specimens. • Integration of error between laod-displacement curves of FE and experimental data should be carried till experimental value of displacement at fracture of the specimen. • The disadvantage of the approach is that it requires a hybrid approach of experiment and FE analysis and hence, the engineer must be familiar with FE analysis in order to use the procedure.