PSI - Issue 66

9

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

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observed as a function of orientation and texture under uniform tensile deformation (Barney, Xu et al. 2011). Furthermore, the images shown in Figure 7(b) demonstrate the effects of stress associated with the growing crack in that there was a non-uniform distribution of strain and phase transformation due to these crystallographic effects.

Fig. 7: (a) Strain maps from an ~1 mm x 1 mm box around the crack tip for the first load/unload fatigue cycle. The top row shows the resultant strains (  yy ,  xx ,  xy ) and regions of transformed martensite surrounding a crack loaded from 0 to 7.5 MPa √ m, with the black regions that represent the crack and martensite. The second row of contour maps shows a dramatic increase in strains and volume fraction of martensite after the crack was further loaded to a stress intensity of 15 MPa √ m. Finally, the sample was reverse loaded back to 7.5 MPa √ m, where it is apparent that both the residual tensile strains parallel to the crack face (  yy ) and the martensite volume fraction are much larger than during the first forward cycle of loading. (b) The images on the right illustrate the effects of grain orientation on the martensite phase transformation as a function of number of cycles. Grains with <100> orientation resist transformation. After (Robertson, Mehta et al. 2007). The crack growth rate data in Figure 6 focused on global measurements of crack growth, and the strain/crystallographic data shown in Figure 7 focused on the effects of local strains and stress-induced phase transformation. There are additional investigations that monitored the actual motion of the growing cracks with high-resolution techniques under several loading conditions. For example, fatigue cracks in polycrystalline Nitinol were investigated with a multiscale experimental framework for average grain sizes (GS) from 10 nm to 1500 nm (LePage, Ahadi et al. 2018). Macroscopic fatigue crack growth rates, measured by optical digital image correlation (DIC) are shown in Figure 8 from that study, whereby the tensile stress-strain curves in as a function of grain size (after various thermal treatments) indicate the substantial differences in mechanical properties (Figure 8a) (Ahadi and Sun 2013). Interestingly, the crack in the as-rolled Nitinol sheet (GS 10nm) grew in an oblique angle rather than along the expected x-direction since the force was applied perpendicular to the initial crack (mode I). The authors speculate that this crack path was due to two possible mechanisms: First, it may be related to a positive σ xx , that could induce crack instability. Furthermore, the unexpected crack growth direction could have been due to a strong crystallographic texture in the 10 nm GS that directed the crack along a path of lower resistance (LePage, Ahadi et al. 2018). The optical DIC also showed an interesting pattern in the crack path during cycling of the sample with 1500nm grain size as shown in Figure 8 (c). For this case, an initial crack was observed to initiate and grow and then bifurcated into a second crack after ~ 163,000 cycles. This second crack then continued to grow in the expected mode I path for the duration of the test whereas the first crack terminated propagation.