Issue 48

A. Ghosh et alii, Frattura ed Integrità Strutturale, 48 (2019) 585-598; DOI: 10.3221/IGF-ESIS.48.57 596 As indicated from the fracture surface the material exhibit fatigue crack propagation via appreciable amount of plastic deformation of the matrix and hence fracture analysis can be carried out in the elastic-plastic regime. Accordingly, the numerically calculated crack nucleation time (t n ) and damage evolution (D) following the relation given in [26] and [27], respectively has been plotted in Fig. 12. It shows that as the orientation of the loading axis changes from 0 to 90 degree the Schmid factor for primary slip system i.e, prism slip decreases and with decrease in Schmid factor the fatigue crack nucleation time increases. While the damage evolution rate due to nucleated crack has been found to be comparable in 45 and 90 degree orientation, hence, 90R sample showed higher ratcheting strain to failure compared to 45R sample. Figure 12 : a) Schmid factor of prism slip, crack initiation time and damage accumulation rate plotted against loading direction and b) Damage evolution curve. C ONCLUSIONS he present investigation leads to following conclusions: 1) As the orientation of the loading direction of basal grains change from 1010  in 0 degree to 1120 in 90 degree, deformation mechanism changes from predominant prism + contraction twin activity to extension twin + pyramidal slip activity leading to higher yield strength and tensile strength but poor fatigue life of 90 degree sample compared to 0 degree sample. 2) During cyclic deformation in 0R orientation continuous contraction of yield surface indicates cyclic softening due to high prism slip and contraction twin activity leads to longer fatigue life in 0R orientation. While, in 45R, there is expansion of yield surface due to reduced prism slip activity and due to extensive detwinning of extension twins during compression half cycle but induced cross slip in 90R contributes to higher cumulative strain accumulation in 90R. A CKNOWLEDGEMENTS he author would like to thank Department of Science and Technology, Government of India for funding this work through INSPIRE fellowship of Atasi Ghosh (research grant no. DST/INSPIRE/04/2016/001217). The author would like to thank Dr. Subhasis Sinha, postdoctoral fellow and Prof. Nilesh Gurao, Associate professor at IIT Kanpur for providing their help in manuscript preparation. The author also thanks the imaging facility available in ACMS at IIT Kanpur for carrying out the microstructure characterization successfully. R EFERENCES [1] Kuruvilla, M., Srivatsan, T.S., Petraroli, M., Park, L. (2008). An investigation of microstructure, hardness, tensile behavior of a titanium alloy: role of orientation, Sadhana, 33(3), pp. 235-250. DOI: 10.1007/s12046-008-0017-2. [2] Li, H., Mason, D.E., Yang, Y., Bieler, T.R., Crimp, M.A. and Boehlert, C.J. (2013). Comparison of the deformation behaviour of commercially pure titanium and Ti–5Al–2.5Sn (wt. %) at 296 and 728 K, Phil. Mag. 93 (21), pp. 2875- 2895. DOI: 10.1080/14786435.2013.791752. T T