Effects of texture and grain size on fatigue crack growth in NiTi

This talk will discuss an investigation of the fatigue crack growth behavior of NiTi. New experimental approaches were utilized to connect macroscopic fatigue performance with microstructural effects, including grain size and crystallographic texture. Millimeter-scale experiments measured the accumulation of plasticity during cyclic uniaxial tension, as well as fatigue crack growth rates in compact tension samples. At the micron scale, crack tip displacements were measured with sub-20-nm precision with in-situ scanning electron microscope digital image correlation (SEM-DIC). For all of the grain sizes and crystallographic textures studied, smaller crack opening displacements (measured with SEM-DIC) correlated with slower crack growth rates (measured with optical DIC). Tracking the crack tips and crack growth rates with DIC was necessary because indirect techniques such as back-face strain gages or crack mouth opening gages rely on linear elastic fracture mechanics (LEFM), but the phase transformation in SMAs can violate LEFM. With respect to grain size, it was found that the largest grain size tested (1500 nm average) exhibited the slowest crack growth rates due to roughness-induced crack closure. With respect to texture, it was found that the resistance to plasticity was a stronger factor than the transformation strain in determining fatigue performance. Specifically, while the rolling direction (RD) of the sheet had about 1.4 times the transformation strain of the transverse direction (TD), the RD accumulated twice as much plasticity during tensile cycling and had twice the crack growth rates compared to the TD.