PSI - Issue 60

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

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intercept method was  80 µm. The heat treated blanks were machined as the low cycle fatigue (LCF) specimen with the gauge section of 6 mm dia and 15 mm length. Figure 2 shows the geometry of the specimen used for both LCF and tensile tests. The surface finish of the specimens was maintained better than  0.4 µm. LCF tests were conducted up to failure at 300, 823 and 873 K to identify the cyclic life, N f , at the respective temperatures. Further, few additional tests were interrupted after 5, 10, 30 and 50% of fatigue life and subsequently tensile tests were carried out on the above specimen till failure. These specimens are referred as 0.05 N f , 0.1 N f , 0.3 N f and 0.5 N f , respectively. Also, a tensile test was carried out on the ‘as - received’ sample without prior fatigue damage for comparison and is referred as 0 N f . The LCF tests were conducted employing constant total strain amplitude of ±0.6% at a nominal strain rate of 3×10 -3 s -1 . The tensile tests were also carried out at the same strain rate and temperature. The wave form employed for the experiment is depicted in Fig. 3. During the tensile tests, the actuator displacement in a servo-hydraulic fatigue testing machine was taken as the specimen elongation. The linear elastic portion of load-elongation data was contributed by the specimen, machine frame, and load-train assembly. The combination of this elastic elongation was subtracted from the total elongation for the calculation of plastic strain. Thus, obtained true stress-true plastic strain data was used in the identification of constitutive equations. The instantaneous work hardening rate θ, was calculated as the differential of the true stress-true plastic strain data using the central difference formula.

Fig. 1 Optical micrographs of Type 316L(N) SS in solution annealed conditions.

Fig. 2 Specimen geometry for sequential LCF and tensile tests.