Issue 48

S.C.S.P. Kumar Krovvidi et alii, Frattura ed Integrità Strutturale, 48 (2019) 577-584; DOI: 10.3221/IGF-ESIS.48.56 582 Figure 8 : The fracture appearance of the fatigue tested specimen at ± 0.8 % strain amplitude (a) crack initiation sites and (b) transgranular crack propagation in the material. Figure 9 : Variation of hysteresis loop energy as a function of number of reversals for 316Ti SS. Creep-fatigue interaction test was conducted at strain amplitude of ± 0.4% with hold time of 1 minute in peak tensile strain. The stabilized hysteresis loops for continuous cycling with and without hold time are shown in Fig. 10. The fatigue life of the material decreased in presence of tensile hold compared to the continuous cycling at same strain amplitude (± 0.4%) and temperature (823 K), Tab. 4. The introduction of the hold time at maximum tensile strain allows for the conversion of elastic strain to plastic strain and results in the stress relaxation. The stress relaxation results from the creep deformation in the specimen resulting in creep cavitation on the grain boundaries. The amount of stress relaxation was found to be significantly lower than that of ferritic steels and superalloys, Tab. 4 [11,22]. The stress relaxation is generally found to occur from the creep deformation in the specimen interior that can result in cavities on grain boundaries. These internal grain boundary cavities interact with a propagating fatigue crack to result in an enhanced crack growth rate. The material properties generated in these experiments are useful for design of bellows made of SS316Ti as per RCC-MR design code for SFR applications.