Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
98 Chapter IV. STRAIN PARTITIONING
with the deformation, which may later be observed and measured. It is possible to measure the in-
plane strains at the surface of a specimen by taking images in the undeformed and in the deformed
configuration, and by finding the homologous points in both configurations. Homologous points are the
points in the images (undeformed and deformed) that belong to the same material point on the sam-
ple. The homologous points draw a displacement field, which can be differentiated to get the in-plane
strain fields. The outcome of the procedure depends strongly on the accuracy of the matching proce-
dure, on the density of the homologous points, and on the range of deformations. Several techniques
for defining homologous points are available. Here, it has been chosen to introduce only the two main
image analysis techniques.
IV.1.1 Grid techniques
Techniques commonly called “grid techniques” use markers arranged periodically on the surface of a
specimen. In the literature different grid patterns have been used: fine array of dots [98], crosses [99]
or lines (Figure IV.1).
a) b) c)
Figure IV.1. Illustration of different grid patterns; a) fiducial microgrids; b) fine array of dots; and c)
fine array of lines encircled.
Different marking techniques have been developed in order to create the desired contrast on the sur-
face of the specimen. Indeed, grids may be applied to the surface in a variety of ways: scribing [100,
101], printing, etching [14], embossing, embedding, reflection, depositing or lithography [98, 102-104].
Figure IV.2 shows respectively a grid processed by microlithography (Figure IV.2.a) and another one
by engraving via chemical etching (Figure IV.2.b). The selection of the marking technique is closely
related to the observation device. For instance, the painting techniques are not suited for with SEM
observations because the paints are usually not electrical conductors. In addition, it requires adapting
the marking technique to the conditions of mechanical loading applied to the specimen. In particular in
thermo-mechanical applications, it is necessary to choose markers that are high-temperature resistant.
This aspect will be discussed further (see section IV.2). The goal of the image acquisition procedure is
to create high-contrast, high-resolution pictures for subsequent determination of grid points coordi-
nates. These grid point coordinates are recorded numerically, after either manual determination, or
after digital capture of the grid feature locations. Initially, grid point coordinates were determined with
the help of a graduated ruler or micrometric tables. Now, thanks to numerical imaging, both pictures
(undeformed and deformed) are digitized in grey levels and analyzed by image analysis: threshold or
morphological operations (erosion, dilatation). Indeed, each marker showed a grey level different from
the material. As a consequence, it is easy to find accurately their position. Then, the local strains are
determined by calculations from the coordinates of the points in both undeformed and deformed confi-
gurations.