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Method for Extracting True Stress from TEM in situ Compression Testing

Transmission electron microscopic (TEM) in situ mechanical testing approaches are increasingly being used to investigate plastic behaviors of materials at the nanoscale, especially in thin films, implanted or irradiated specimens, and other volume-limited materials. Recent results from depth-sensing TEM in situ compression pillar tests have shown the method is capable of providing meaningful quantitative measurement of yield strength and elastic modulus of specimens with low characteristic length (i.e. high number density of obstacles to dislocation motion) [1–3]. However, data analytics for these tests remain in their infancy. Tests output is typically comprised of the load and displacement at each time step. The researcher must then know the specimen dimensions in order to generate a stress-strain curve, from which yield stress can then be determined. In all reports in the archival literature, these are engineering stress-strain curves – i.e., the original specimen dimensions are used to calculate stress from load. However, since nearly all materials exhibit a nonzero Poisson’s ratio, most compression pillars will exhibit “barreling” during deformation. In such cases, the true stress-strain relationship may be of greater interest than the engineering stress-strain curve.