N. Vanderesse

Microscopic Strain and Crystal Rotation Measurement within Metallurgical Grains

This work describes an experimental procedure to measure the progressive strain localization and crystal lattice rotation within metallurgical grains. A digital image correlation software was implemented and associated with mechanical tests carried out inside a scanning electron microscope on specimens exhibiting nanometric grainy patterns. Cross-correlation analyzes between electron backscattering diffraction maps were also developed to quantify the corresponding local crystal rotation relative to the original structure. The microscale strain and rotation fields on the surface of a tensile-loaded specimen made of austenitic stainless steel 316L are presented as an illustration. Their direct spatial correlation between strain heterogeneities and the progressive activation of slip systems is put into evidence and discussed.

Inter- and intragranular delta phase quantitative characterization in Inconel 718 by means of image analysis.

This paper describes an image processing method for discriminating the inter‐ and intragranular delta phase precipitates in Inconel 718 (IN 718). The successive practical operations and the motivations of their choices are presented in detail. The method was applied to IN 718 specimens heat treated with different parameters to produce microstructures containing various amounts of both types of precipitates. They were characterized by electron microscopy in backscattered electron imaging. The main difficulty arose from the fact that the brightness distributions of inter‐ and intragranular precipitates partially overlap. Additional information on their morphology and their spatial distribution had to be exploited in order to differentiate them. The shape and the orientation of the precipitates were evaluated using the structure tensor, an operator that quantifies the directionality of the intensity distribution in an image. The distance between parallel precipitates was also used as an additional property to identify clusters of intragranular precipitates.

Measurement of deformation heterogeneities in additive manufactured lattice materials by Digital Image Correlation: Strain maps analysis and reliability assessment

The mechanical behaviour of porous lattice materials for potential orthopaedic applications was investigated at a fine scale by means of digital image correlation (DIC). Specimens with cubic, body cubic-centered reinforced (BCCZ), and diamond mesostructures were tested in quasi-static compression up to failure. Images were continuously recorded by an imaging setup and processed by a custom DIC program, OpenDIC. The resulting strain maps were analyzed in both spatial and temporal scales, displaying the onset and evolution of strain heterogeneities. The three geometries show different failure modes, i.e. collective buckling of an entire row for cubic, diagonal shearing band for BCCZ, and generalized crushing for diamond. The strain maps correlate well with these patterns. Most importantly, they show early strain localization below the macroscopic elastic limit. After failure, a phenomenon of strain release was witnessed and evaluated in the parts of the specimen that do not fracture. In the cubic geometry, the vertical struts sustain most of the deformation. Further analysis shows that the rows of vertical struts have similar, yet scattered, evolutions until failure. Interestingly, the row that leads to specimen failure is not necessarily the first one to deform. In addition to these experimental results, the uncertainties of the method were thoroughly assessed by means of calibration procedures.