PSI - Issue 14

Digendranath Swain et al. / Procedia Structural Integrity 14 (2019) 207–214 Swain et al./ Structural Integrity Procedia 00 (2018) 000–000

211

5

initiation of stable crack growth- point 2 (>1100 MPa). The stable crack growth was under severe plastic deformation at point-3 (>1200 MPa) leading to unstable crack growth and failure at 4 (>1295 MPa). In Fig. 4, at red dashed lines specific trend jumps also were seen. However, since the trend changes are not visible in all the parameters; hence they are not identified for any phenomena inside the specimens. The data obtained from the PT specimens are discussed next.

Fig. 3. Longitudinal strain maps obtained with DIC for (a) SCT specimen-1 at 1300 MPa and (b) SCT specimen-2 at 1295 MPa. Similarly, the longitudinal displacement field for the specimens are shown in (c) and (d) for specimen-1 and 2, respectively.

The strain field obtained with DIC for PT specimen-1 is shown in Fig. 5a, where certain rectangular areas are marked as R0, R1, R2, R3, and R4. The strains extracted from these areas are used for plotting the stress-strain curves for the PT specimen-1 in Fig. 6a. The photograph of PT specimen-1 after failure (Fig. 5b) and the strain field (Fig. 5a) show clear necking before failure. Similarly, the strain fields and failure of PT specimen-2 are shown in Figs. 5c and 5d, respectively. In Fig. 5c regions namely R0, R1, R2 and R3, and specific points P1 and P2 are shown, where strains are extracted and reported in Fig. 6b. The PT specimen-2 does not show any necking (Fig. 5d) and failed through cracking inside the gauge length. The crack was visible in the DIC strain field before its failure (i.e. at ~1400 MPa <1459 MPa). AE data was correlated to the DIC strains. The stress-strain data obtained at various points on both the PT specimens are shown in Figs. 6a and 6b. It can be seen that the overall strains in PT specimen 1 is one order larger than specimen-2 (Table 4), even if the failure loads are almost similar. It implies that the material has the ability to undergo large plastic deformation. However, in presence of defects, the elongation decreases drastically. This is a major disadvantage for the designers. Figure 6c correlates the AE and DIC data for PT specimen-1, where it can be seen that few AE signals were seen prior to yielding (point-1 in Figs. 6a and 6c). Subsequently, the material remained silent until failure (point-2 in Figs. 6a and 6c). However, in specimen-2, AE signals were observed just after the proportional limit of the material (point-3 in Figs. 6b and 6d). Subsequently, a drastic growth in AE signals was seen (point-4 in Fig. 6d). In the mean-time, at around 1400 MPa, a crack was