PSI - Issue 14

A. Poonguzhali et al. / Procedia Structural Integrity 14 (2019) 705–711 Poonguzhali et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig.3. Variation of open circuit potential with time during corrosion fatigue test at σ mean = 475 MPa

Fig.4. LRS spectra of fatigue tested specimen as a function of CW at σ mean = 375 MPa.

The fracture surface of the fatigue tested specimen were examined under Scanning Electron Microscope (SEM) for as-received material, 5% and 20% CW 316LN SS at σ mean = 375 MPa as seen in fig.5 a through c. It is known that the fatigue life consists of both initiation and propagation of fatigue cracks and fatigue life is governed predominantly by crack initiation. As, cold work introduces higher strains at the surface than at the mid-thickness, resulting in the introduction of residual stresses in the material. Higher crack initiation resistance due to an increase in the yield strength by CW generally increases crack initiation resistance. Hence, the role of microstructural examination is important for assessing the crack initiation and growth mechanism which controls the fatigue life (Qian et al., 1997). From figure 5d, it clear that the crack initiated at the surface of the specimen and finally, switched over to the transgranular mode with striation irrespective of the material condition and environment and faster crack propagation leading to ductile fracture mode known as final stage fracture mode decreases for CW as compared to base metal. The dimpled features in the rapid fracture area during the overload of the specimen shows final rupture zone. Slip as the primary plastic deformation process gets accumulated during cyclic loading resulting in strain localization in the form of slip bands which are the precursors to crack initiation (Poonguzhali et al., 2016). As-received fatigue tested specimen indicated high slip width as seen in the statistical distribution of slip height and width as compared to the other 5% and 20 % CW material in fig.6(a-c) It clearly shows that the coarse slip in base metal, promotes slip irreversibility due to higher slip offset. On the other hand finer slips for 5% and 20 % CW material is an indicative of more slip reversibility; thereby it enhanced the crack initiation resistance.