Issue 48

A. Ghosh et alii, Frattura ed Integrità Strutturale, 48 (2019) 585-598; DOI: 10.3221/IGF-ESIS.48.57 592 C ORRELATION BETWEEN TENSILE AND RATCHETING PROPERTY he strength-ductility property of commercially pure titanium as a function of orientation of the loading direction (0  45  90 degree) during monotonic tensile loading gets manifested in different way during cyclic loading. This is evident from the comparision of different tensile and ratcheting properties shown in Fig. 7. Fig. 7a shows orientation in which strain hardening coefficient is high, ratcheting life is low. Interestingly, ratcheting strain and tensile strain to failure are inversely related, however, the increase in ratcheting strain for 45 degree orientation is much lower compared to 90 degree orientation as indicated by red dotted circle in Fig. 7b. Basically, ratcheting is a phenomenon which occurs during stress controlled cyclic loading when the cross slip activity is promoted by internal stresses generated in the material [22]. The internal stresses arise from dislocation substructures formation and which obviously depends on the slip/twin activity. However, in order to understand the correlation between tensile and ratcheting property, the manifestation of slip/twin system as a function of orientation influencing cyclic deformation in each tension-compression cycle is required and has been predicted using VPSC simulation. Figure 7 : Variation of a) ratcheting life and strain hardening coefficient and b) Ratcheting strain and tensile strain to failure with respect to loading direction. During ratcheting the cumulative accumulation of strain proceeds in three stages- initial stage of slow rate of increase in ratcheting strain, followed by constant rate of increase in ratcheting strain and finally rapid rate of increase in ratcheting strain leading to failure as indicated in Fig. 3d. The yield locus for the 1 st cycle tension-compression-reload tension shown in Fig. 8a depicts contraction of yield surface continuously with each compression and reload-tension half cycle w.r.t the initial tension half cycle in 0R and 90R orientation. On the other hand, in 45R orientation yield surface expands w.r.t the initial tension half cycle during compression half cycle and reload tension half cycle. It is because the average number of active slip system in the tension half cycle is higher in 0R compared to 45R and 90R, while the trend is opposite in compression and reload tension half cycle as indicated from Fig. 8b. The increase in average number of slip system in 0R is mainly due to prism slip while in 45R and 90R, is due to increase in basal slip, pyramidal slip. The pyramidal slip activity is slightly higher in 45R compared to 90R leading to higher yield locus during reload tension of 45R. Another interesting observation is the detwinning of extension twin during compression half cycle as predicted from VPSC simulation shown in Fig. 9. The alternating activity of extension twin and pyramidal slip between tension-compression cycle has also been shown by Elasto-plastic self consistent (EPSC) simulation during strain controlled cyclic deformation of CP-Titanium [17]. The reason behind lower extension twin activity during compression half cycle has been attributed to detwinning of extension twin because of backstress generated during forward loading. Thus, due to higher prism slip activity and suppressed pyramidal slip activity during compression half cycle, 0R exhibits longer life compared to 45R and 90R. However, the predicted slip/twin activity could not explain the reason behind lower ratcheting strain of 45R, which needs analysis of the role of twinning interpreted from EBSD analysis of deformation microstructure. T