PSI - Issue 60

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

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4. Comparison of stress-strain curves with data of RCC-MR design code The unique Ramberg- Osgood parameter (n, α) has been compared with RCC -MRx French design code. RCC-MR provides design curve equations for stress-strain relation, at a temperature, valid up to 1.5% of plastic strain. Design curve equations provided in RCC-MR code can’t be valid if plastic strain increases beyond 1.5% which generally occurs in high geometrical discontinuity regions like nozzle junction, vessel head, T-junction etc. Generally, design is carried out with minimum stress-strain curve and life assessment is carried out with an average stress-strain curve. Beyond the 1.5% of plastic strain, the stress-strain curves are provided at a certain interval of temperature in RCC MR. Conservatively, next higher value is considered if design temperature falls in between the temperature interval. Secondly, for thermo-mechanical analysis of structural components, where temperature varies across the thickness of components, temperature dependent material stress-strain curve can be helpful for more accurate analysis in the form of temperature dependent RO parameter (n, α), instead of using the maximum temperature data which is conservative. 4.1. Evaluation of Ramberg-Osgood parameter from RCC-MRx data RO parameters (n, α) have been derived, using best fit curve, for stress -strain data provided in RCC-MR for SS316LN material at the certain interval of temperatures as provided in Fig.17. It is seen that the curve fitting of RCC-MRx stress-strain curve with RO type stress strain curve is performing well away from the first yield point. Near the yield point, the RO type curve is not approximating the RCC-MRx data. The RCC-MRx provides equations for the stress-strain curve up to the plastic strain of 0.015 (1.5%). RCC-MRx equation can be used up to the plastic strain of 1.5%, and beyond that RO based stress-strain data can be used.

Fig 17. RCC-MRx stress-strain curve and Ramberg-Osgood best fit curve [8]

4.2. Comparison of unique Ramberg-Osgood parameters with RCC-MRx data The variation of RO parameters ‘n’ and ‘ α ’ with temperature have been plotted in Fig.18 and Fig.19 respectively for RCC-MRx data and also compared with unique RO parameter derived by the proposed approach at the temperature of 298K and 923K. The unique RO parameters give a good approximation of RCC-MR curve. The best fit curves for RO parameters are also plotted. The equations for Romberg Osgood parameters (n and α) have been proposed with the function of temperature. The found relations for n and α with temperature are provided in Eq. 5 and Eq. 6 respectively. These proposed equations are good approximate the optimized Ramberg-Osgood parameters at the temperatures of 293K (25°C) and 923K (650°C), as well as the RCC-MRx data. These relations can be used for finding the stress-strain curve at any temperature ranging from 25°C to 650°C for the stainless steel grade 316LN.