Issue 48

S.C.S.P. Kumar Krovvidi et alii, Frattura ed Integrità Strutturale, 48 (2019) 577-584; DOI: 10.3221/IGF-ESIS.48.56 580 Pure fatigue tests were carried out on SS316Ti specimens at 823 K at different total strain amplitudes. The cyclic stress response of the alloy is shown in Fig. 4. The material showed continuous hardening followed by saturation for a brief time period and the decrease in peak stress due to the formation of macro-cracks. SS 316 and other grades of stainless steels are used in solution annealed condition; an initial hardening is quite expected during fatigue cycling in this material [6,7,14]. The initial hardening in this material generally occurs due to the dislocation generation and their mutual interactions as well as interactions between mobile dislocations and the solute atoms present in the matrix. DSA during cyclic deformation is observed in the form of serrations in the hysteresis loops, Fig. 5. Various other manifestations include pronounced cyclic hardening, increase in stress response with increase in temperature, decrease in plastic strain with increasing temperature or decreasing strain rate (negative strain rate sensitivity) [15]. The influence of DSA on dislocation substructure in austenitic stainless steels has been studied by Rao et al. [16]. At room temperature, well developed dislocation cell structure was observed. However, in the DSA range, the tendency from cell to planar slip bands was observed [16,17]. It is interesting to note that the serrations in the hysteresis loop disappear slowly with cycling, Fig. 5. Similar observation has been reported by Goyal et al. on 316 LN SS at 873 K [18]. Sarkar et al. observed the DSA during LCF of 316 LN SS in the temperature range of 823-923 K [19]. Figure 4 : The cyclic stress response of 316Ti SS at 823 K under total strain controlled cycling. Figure 5 : Occurrence of dynamic strain ageing during low cycle fatigue testing at 823 K and at strain amplitude of ± 0.5%. The influence of total strain amplitude on fatigue life of the material is shown in Fig. 6. The fatigue life decreased with increase in total strain amplitude. The variation of low cycle fatigue life with total, plastic and elastic strain amplitudes has been analyzed on the basis of strain-life relationship, which is defined by the following equation:     2 2 2 f b c t f f f N N E         (1)