PSI - Issue 66

10

Pelton/ Structural Integrity Procedia 00 (2025) 000–000

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

274

Fig. 8: (a) Tensile stress-strain curves that compare the mechanical properties of rolled Nitinol sheet after various treatments. The corresponding grain sizes were measured with transmission electron microscopy, after (Ahadi and Sun 2013). The crack growth kinetics and pathways were initially measured on C(T) specimens with optical digital image correlation (DIC); the results from the as-rolled 10nm grain size sample are shown in (b) whereas the results from the annealed 1500nm grain size sample are shown in (c) as functions of cycle count. Note that although the force was applied for mode I crack opening, the crack in (b) grew in mixed mode I+II during both high and low ∆ K cracking (shown here in the low ∆ K stage). For the largest grain size Nitinol sheet, there is a crack bifurcation observed after about 163,000 cycles (blue circle); with subsequent cycles, the original crack stopped growing and the secondary crack continued to grow. After (LePage, Ahadi et al. 2018). Exciting results from novel SEM-DIC analyses from these samples reveals additional details about the crack paths and associated proximal strains at higher special resolutions for this grain size study (LePage, Ahadi et al. 2018). After performing the macroscopic, high ∆ K fatigue crack growth measurements with optical DIC, the samples were prepared for DIC in the SEM. These samples were subjected to a single high ∆ K (K max = 8.0 MPa √ m) load-unload cycle during which crack opening displacements were measured at the microscale by SEM-DIC. As shown in Figure 9, with increasing GS from 10 nm to 80 nm, the strain fields exhibited a progressive broadening of process zones (high strain regions) around the crack tip. However, this trend was reversed in the 1500 nm sample that had a small process zone size, between the GS 10 and 18 nm cases. The GS 42 and 80 nm samples exhibited large regions of high strain ( ε 1 ≈ 4 %) roughly ≥ 10µm around the crack tip, while the GS 10, 18, and 1500 nm samples had relatively small regions of high strain, < 5 µm around the crack tip. The authors concluded that fracture toughness at crack initiation does not seem to correlate directly to fatigue behavior (crack growth and crack opening response) in nanocyrstalline Nitinol. Close examination of Figure 9 shows that the cracks in the GS 10 and 1500 nm samples consisted of multiple microcracks, whereas the GS 18 and 42 nm samples showed occasional slight kinks along their length with no microcracks. The SEM-DIC study also showed non-monotonic GS dependence in the relative crack displacement profiles ∆ v ( x ) and the stress intensity responses with respect to relative crack displacements (LePage, Ahadi et al. 2018). The 1500 nm sample consistently exhibited the lowest ∆ v followed by the GS 10 and 18 nm samples that had the next lowest ∆ v . The GS 42 and 80 nm samples had the greatest and nearly coincident ∆ v profiles. The sample with the 1500 nm grain size was observed to exhibit the slowest crack growth rates and greatest threshold stress intensity at the macroscale. At the microscale, this sample exhibited the minimum crack opening displacements, greatest crack opening stress intensity level, and roughest fracture surfaces at the microscale. The sample with the GS 80 nm exhibited the fastest crack growth rates and largest crack opening displacements.