Determination of transformation stresses of shape memory alloy thin films: A method based on spherical indentation

The forward and reverse transformation processes of superelastic shape memory alloys (SMAs) under spherical indentation are analyzed. We found that there exist two characteristic points, the bifurcating point and the returning point, in an indentation curve. The corresponding bifurcation force and return force, respectively, rely on the forward transformation stress and the reverse transformation stress. A method to determine the transformation stresses of SMA from the measure of the bifurcation and return forces is proposed. Additionally, we suggest a slope approach to determine the values of the two forces with high accuracy.