PSI - Issue 60

K. Mariappan et al. / Procedia Structural Integrity 60 (2024) 444–455 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

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curve, whereas at 823 and 873 K the 0 N f , 0.05 N f and 0.1 N f samples fall in the increasing hardening segment of cyclic stress response curve and only the 0.3 N f and 0.5 N f fall in the saturation stress regime, with the peak stress value observed at 0.3 N f and marginally decreasing peak stress values at 0. 5N f . The flow curves for 316L(N) SS with varying prior fatigue damage fractions presented as double logarithmic plots of true stress versus true plastic strain are shown in Figs. 6a, 6b and 6c for the test temperatures of 300, 823 and 873 K respectively. At all the temperatures, the overall flow curves exhibited increase in the flow stress values with increase in the extent of prior fatigue damage, which gets pronounced at higher temperatures of 823 and 873 K. The difference in flow stress values at 823 and 873 K is observed to be higher in the lower strain ranges, whereas the difference in the flow stress decreases with increasing strains. Also, at 823 and 873 K, the uniform plastic strain,  u , which defines the limit of stable plastic deformation before instability sets-in, decreases with an increase in prior fatigue damage (Fig. 6). The overall upward shift in the flow curves with increasing prior cyclic damage could be attributed to the increase in dislocation density caused by dislocation multiplication that occurs during the prior fatigue cycling [Pham et al (2013)]. At room temperature the true stress-true strain curve for the 0.05 N f shifts upwards substantially, whereas with further increase in the amount of prior fatigue damage the changes are marginal. The difference in flow stress between the 0 N f specimen and the specimen with prior fatigue damage fractions of 0.05 N f , 0.1 N f , 0.3 N f and 0.5 N f at 823 and 873 K compared to that at 300 K, suggests that the effect of prior fatigue damage on tensile σ - ε curves is more pronounced at higher temperatures. The additional hardening may be attributed to the occurrence of dynamic strain aging (DSA) phenomenon in the 316L(N) SS, at the test temperature range of 823-873 K and strain rate of 3×10 -3 s -1 [Valsan et al (1995), Srinivasan et al (1999)]. DSA is caused by the interactions between the moving dislocations and the diffusing solutes and hence are observed at the optimal combination of temperatures and strain rates. The overall upward shift with increasing percentage of prior fatigue damage could be attributed to the hardening due to generation of dislocations and their mutual interactions and an increase in dislocation density incurred during the prior fatigue cyclic loadings [Mughrabi and Christ (1997), Pesicka et al (2003), Srinivasan et al (1997)]. As it can be seen from the true stress-true plastic strain curves at 300 K in Fig. 6a, the true stress values for specimen with varying prior fatigue damages are found to merge with the as received specimens at higher strain level. At 823 and 873 K (Figs. 6b and 6c), as the uniform elongation reduces with increasing prior fatigue damage, the true stress values reach the peak values comparable to that of the as-received specimen at the respective temperatures, however, at strains lower than the uniform elongation achieved by the as received specimen.

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Fig. 7 (a) The typical three stages of work hardening behaviour observed is demonstrated in the 0 N f specimen tested at 300 K. (b) Influence of various degree of prior fatigue damage on  -  plots of 316L(N) SS at 300, 823 and 873 K.