Development of Optical Technique For Measuring Kinematic Fields in Presence of Cracks, FIB-SEM-DIC

Abstract

Currently, kinematic field measurements for studying the mechanical behavior of materials and structures use common optical methods, such as mark tracking techniques, grid methods and correlation techniques (Sutton et al., 2009, Image correlation for shape, motion and deformation measurements, Springer, Berlin, https://doi.org/10.1007/978-0-387-78747-3). These techniques are used over a region of interest ranging from micro to millimeter scale. However, when studies need to be conducted on even smaller scales such as sub-micrometric scale, the use of more complex means of observation is required. In this case the work can be achieved using the scanning electron microscope SEM, or some specific marking techniques as the Dual-Beam FIB [ANR-11-LABX-0017-01].

For this application, the Digital Image Correlation DIC is chosen to investigate the material behavior. In the present approach, an artificial speckle having the depth of the engraving around (a few hundred nanometers) 200 nm was used. Statistical evaluations such as grayscale histograms, autocorrelations, defocusing effects, and rigid body displacement effects are used to evaluate the error measurement in a field of 100 μm width. Various tests were also performed to ensure the repeatability and reproducibility of the method. The order of the errors is much greater than those obtained in classical optical conditions, but is less than 0.05 pixel.

An application to study the mechanical behavior of a metallic composite is proposed. These composites Al/ωAl-Cu-Fe (Joseph 2016) have a local behavior depending on the local material structure, which can be brittle or ductile. An adaptation of DIC method (H-DIC) is proposed to study the mechanisms of deformation at these scales, taking into account local fractures (Valle et al. 2015) or local plastic strains. The particularity of this method extension lies in the good separation of the strain fields and the cracked part. Results of a single test are presented and discussed here, which focus on the comparison between a classical DIC analysis and its extension.