1094 Timothy J. Ruggles et al.
Figure 3. Norm of the erroneous two-dimensional strain detected via digital image correlation after a 10 μm translation in the hori- zontal direction. The strain contour is overlayed on a micrograph of the urethane polymer correlation pattern.
To evaluate the suitability of the stamped pattern for image
correlation, a simple displacement experiment was conducted using a stamped pattern on a piece of aluminum. The two- dimensional total strains were calculated locally using DIC. Imaging was performed at a 10.1mmworking distance with an accelerating voltage of 5keV, a beam current of 1.6nA and a pixel density of 5.12 pixels per micron. The subset size (an image correlation parameter that approximates the spatial resolution of the technique) was selected to be 55 pixels, or around 10.7μm. Subset size must be selected to be compatible with the size of the speckles in the pattern (Lecompte et al., 2006), and our subset size was determined using trial and error to achieve the desired strain resolutionwhile maximizing spatial resolution. Image drift calibration for the SEM was performed per the standard operating procedure of the image correlation software employed, VIC2DTM (Correlated Solutions, 2009). The area was imaged and then translated 10μmand imaged again. The resulting erroneous strains, pictured in Figure 3, had a mean Frobenius norm of 174 microstrain, approximately on the order of the strain resolution limit of HREBSD for high EBSD camera resolution [around 200 microstrain (Wilkinson et al., 2006)]. Note that increasing the subset size would reduce error in the measured strain at the expense of spatial resolution, but this subset size was selected to maximize the spatial reso- lution afforded by the 1μmspeckle size of the stamp, while still maintaining similar strain accuracy to HREBSD.
DISCUSSION
This work presents a new method of applying a speckle pattern for DIC in a SEM, which does not disrupt EBSDdata collection, urethane polymer microstamping. The residual layer of urethane left by this stamping process has sufficient mass for contrast at low accelerating voltages, but has a small effect on the quality of EBSD patterns that can be collected from the underlying surface. This allows for DIC and HREBSD, a technique particularly sensitive to the quality of EBSD patterns, to be carried out on the same surface at the same length scale in situ. The stamped pattern resulted in an ~28% increase in the measured noise of HREBSD. Note that this number could be improved by increasing the exposure time or beam current which were kept constant for the experiment. Currently, microstamps have been successfully
fabricated with a 1 μm speckle size, which results in a spatial resolution of around 10.7 μm and a noise level in the strain measurements of around 170 microstrain. The spatial reso- lution may be improved by refining the speckle size of the stamp, which has a resolution limit of around 30 nm. Fur- thermore, the error level of the HREBSD measurements of elastic distortion and DIC results of total strain are of com- parable magnitude, meaning that together they can be used to extract plastic strain. Future work will focus on applying the current 1 μm microstamp to the characterization and modeling of the deformation of oligocrystals as well as developing higher resolution stamps to work at the length scale of engineering materials.
ACKNOWLEDGMENTS
This work was supported by the National Aeronautics and Space Administration’s Aeronautics Research Mission Directorate through the Digital Twin effort within the Convergent Aeronautics Solutions project.
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