The effect of microstructural barriers on transient crack growth in shape memory alloys

There are several issues to be solved in the fracture mechanics of shape memory alloys, one of them being the resistance to crack growth and therefore to fracture. This paper discusses the crack growth in a single crystal CoNiAl shape memory alloy under cyclic loading and the effect of micro-structural barriers. To observe the crack growth in detail, tests are conducted on edge-notched specimens. The displacement field is obtained using digital image correlation (DIC), and the fracture parameters are calculated by fitting anisotropic crack tip displacement equations to DIC data. Similar crack growth behaviors are observed in both superelastic and shape memory specimens, with a comparatively higher crack growth rate in the superelastic case: first a crack initiates at the notch and grows, then new cracks are observed to form near the tip of the main crack, or on the notch when the growth slows down. Then, further cyclic loading leads to the growth of the main crack and the new crack simultaneously with the two cracks merging at the end. Test specimens are examined post-failure with optical microscopy to better understand this complicated behavior. Results showed the presence of a non-transforming secondary (γ) phase around the regions where the propagating cracks slowed down, deviated, and/or stopped, improving the resistance of the shape memory alloy specimen to fracture.

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