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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Fig. 1. Geometry and dimensions of the dog-bone shaped NiTiNb-SMA specimens (unit in mm).

2.2. Test set-up of tensile failure test and prestraining To obtain the stress-strain behavior and basic mechanical properties of this material, tensile failure tests were conducted. Besides, with the same test set-up, a total of 18 specimens were prestrained (listed in Table 2), as a separate preparation step for the following activation tests (described in section 2.3). Fig. 2(a) shows the test set-up No.1 for tensile failure tests and prestraining. Loading and unloading of specimens were performed by an MTS servo-hydraulic uniaxial testing machine, with a load capacity of 250 kN. A 3D-DIC (three-dimensional digital image correlation) system was adopted to obtain the full-field strain contour of the specimen. DIC is a non-contact measurement technique that utilizes digital images of specimens, whose surfaces are typically prepared with random DIC speckles to enhance the contrast for image analysis. Through comparing and processing images of different deformation, relative displacement and strain fields could be derived with algorithms. In addition, an extensometer (with a gauge length of 50 mm) was mounted to the parallel section of specimens, to control the testing speed at the desired strain rate. Specimens were clamped to the testing machine with 2400 psi, leaving a clamping distance of about 40 mm on both ends. The tests were conducted with a conservative displacement rate of 0.864 mm/min, which is equivalent to the strain rate of 0.02%/second, till specimen failure. As-received specimens were tensioned at the same loading speed to target prestrain level and unloaded, transferring them to prestrained specimens. Prestrain levels ranging 4%~16% were investigated in the current study, as Suhail et al. (2020) and Yang et al. (2023) reported considerable resulting recovery stress in NiTiNb-SMA wires.

Upper clamp

Extension rod

Light

Thermal resistance sensors

NiTiNb-SMA specimen

Upper clamp

NiTiNb-SMA specimen

Extensometer

Imaging system

Lower clamp

Furnace

Extensometer

DIC speckles

Lower clamp

Extension rod

(a)

(b)

Fig. 2. (a) Test set-up No.1 (for tensile and prestraining); (b) Test set-up No.2 (for activation and re-activation).

2.3. Test set-up of activation and re-activation Besides prestrain level, another primary factor to the stress recovery behavior could be the activation temperature. For the application of bonded prestressed strengthening, research (Li et al. (2023)) of SikaPower 1277 adhesive proved that a temperature higher than 180 ℃ could introduce heat damage to the adhesive bond, resulting in a reduction of