PSI - Issue 64

Mao Ye et al. / Procedia Structural Integrity 64 (2024) 1824–1831 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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ε yy

0.06 0.07 0.05 0.08 0.04 0.03 0.02 0.01

Transformation bands

Virtual strain gauges

0

At prestrainlevel ε pre

Photo of specimen

At residual strain ε res

Fig. 4. Typical longitudinal strain contour maps of NiTiNb-SMA plate during prestraining (specimen No.2, both scaled from 0 to 0.08).

During unloading, the stress-strain curve is nonlinear, suggesting the presence of a martensite to austenite transformation. After returning to the zero-stress state, the residual strain comprises a portion of phase transformation, which could be recovered, and a portion of permanent plastic strain, which is irrecoverable. Through DIC, strain contour maps during the prestraining process were obtained. Two typical frames (of specimen No.2) were demonstrated in Fig. 4: the left frame captured the moment when the specimen was loaded to the target prestrain level ε pre , while the right frame shows the specimen ’ s residual strain at the moment of reaching a zero-stress state. Virtual strain gauges were defined to measure the average strain of the gauge length of 50 mm, as well as that of some local zones. The average strain measured by the ceramic extensometer and virtual extensometer closely aligned, proving the accuracy of both measurements. At prestrain level, two bands in red were formed at the top and bottom of the gauge length, which was also easily visible in the specimen’s photo due to its metallic gloss. Other researches on NiTi-SMAs have reported a similar phenomenon (Tan et al. (2004) and Zhang et al. (2010)), calling it the transformation band (also known as the Lüders like band). The prestrain level of this specimen was 4%. The results of the virtual strain gauges set in local zones indicated that the maximum strain was only about 1.2% (smaller than the prestrain level) beyond the bands, while the maximum strain was nearly 7% (larger than the prestrain level) within the bands. These values basically echo with the start and end of the stress plateau (as mentioned earlier). A possible explanation could be that, during tensile loading, a localization of strain could happen due to the stress induced martensitic (SIM) phase transformation, which was accumulated preferentially in some regions to form the bands. Although both contours were produced at the same scale, the strain contour patterns of ε pre and ε res seemed very much alike, suggesting that the unloading process had little influence on the localization of strain. Through a prestrain level of 4%, effective residual strain was accumulated in the NiTiNb-SMA plate specimen, to varying degrees within or beyond the transformation bands. The recovery stress of the activation and re-activation tests are summarized in Table 2 . It’s worth mentioning that results of recovery stress were calculated according to the specimens’ dimensions measured before activation, instead of using the nominal cross-sectional size. This approach ensures accuracy in the calculation of recovery stress. Comparing specimens No. 9, 10, and 11, which were tensioned to a prestrain level of 10% and heated to an activation temperature of 180 ℃ , it could be speculated that heating rate and initial preload had limited influence on the stress recovery behavior of NiTiNb-SMA plates. As for specimens No. 5, 6, and 7, the same prestraining of 8% but activated at different temperatures, their stress temperature curves were plotted in Fig. 5(a). During heating, stress rose below 180 ℃ but slightly declined at 240 ℃ 3.2. Stress recovery behavior (test set-up No.2)