PSI - Issue 66

A.R. Pelton et al. / Procedia Structural Integrity 66 (2024) 265–281 Pelton/ Structural Integrity Procedia 00 (2025) 000–000

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Fig. 9: Microscale measurements of major principal strain fields ε 1 ( x , y ) from SEM-DIC are shown at the peak stress intensity Kmax = 8 MPa √ m during a single fatigue cycle. Smaller regions of high major principal strain ( ε 1 ≈ 4 %) are observed around the crack tip for the 10, 18, and 1500 nm grain size samples (a,b,e) than in the 42 and 80 nm samples (c,d), and the 1500 nm sample (e) was observed to exhibit the most microcracking. (f) diagrams with details of strain directions as well as measurement of ∆ v . After (LePage, Ahadi et al. 2018) A more recent investigation (LePage, Shaw et al. 2021) studied the effects of crystallographic texture on the fatigue performance of superelastic Nitinol sheet in order to assess the evolution of plastic deformation and transformation stress (functional fatigue) and the resistance to crack growth (structural fatigue). X-ray diffraction crystallographic investigations indicated a strong <110> texture in the rolling direction with {111} in the normal direction. Fatigue experiments on compact tension specimens showed a 25% lower fatigue threshold and twice the fatigue crack growth rate when loaded in the rolling direction (RD) than in the transverse direction (TD). These macroscopic findings correlated with the microscopic observations of crack-tip displacements, where the crack tip opened at a lower stress intensity (K ≈ 0.9 MPa √ m) when loaded in the rolling direction than in the transverse direction ( ≈ 1.5 MPa √ m). Figure 10 shows schematics of the orientations of the C(T) samples with respect to the rolling direction as well as the SEM-DIC images at K/K max = 0.7 and 1.0 with K max = 6 MPa √ m.

Fig. 10: (left) Schematic drawings of the three directions of C(T) samples with respect to the rolling direction. SEM-DIC strain fields ( ε YY , perpendicular to the crack direction) for RD, 45, and TD samples at two stress intensities K/Kmax = 0.7 and 1.0, with (K max = 6 MPa √ m). The shape of the high-strain lobes was similar in all three samples, but strain magnitudes were about 1.25 x times greater for the RD and 45 than for TD. After (LePage, Shaw et al. 2021)