Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
100 Chapter IV. STRAIN PARTITIONING
IV.1.3 Microgrid vs. correlation
With both techniques, very wide fields can be examined. The limitation does not stem from the imag-
ing equipment, but from the area of the surface covered by the microgrid. Furthermore, measurement
of the deformation field is direct and unambiguous; qualitative assessment of the nature of the strain
can be made with even casual examination. Set-up of the microgrid technique is relatively simple,
requiring no specialized device, except for the manufacture of extremely fine submicrometer grids. The
analysis of the deformed grid pattern does not require extensive computing or highly specialized soft-
ware. In addition, because the grids are physically attached to the sample after deformation, the
record of deformation is permanent.
Unfortunately, small strains cannot be accurately determined, as the deviation of the grid lines from
their reference positions can be very small, compared with the width of the microgrid lines. Table IV.1,
from Schroeter and McDowell [122], shows relative advantages and disadvantages between both
strain-field measuring techniques discussed in this section.
Characteristics Digital Image Correlation Microgrid Technique
Physical basis
Amplitude
(image contrast figures)
Direct measurement
Data collection method
In-situ or post-mortem
(for small strains)
Post-mortem
Strain mapping
Direct numerical differentiation of
displacement data
Differentiation of grid lines or
crossing points locations
Imaging hardware/software Moderately demanding / Intensive
No specialized hardware / minimal
software
Displacement value Absolute Absolute
Surface quality requirement Diffusive, as-received Smooth
Surface decoration requirement
Random or ordered intrinsic or
artificial features
High-quality cross-line or speckle
grid
Surface decoration hardware Minimal or none Minimal
Deformation range Small or medium Medium to large strains only
Field of view
Wide field, limited only by the
specific image formation system
High, depending on the surface,
Wide field, limited only by the
area of the grid
Spatial resolution
and resolution of imaging equipment
Medium, depending on grid pitch
Data storage requirement Large Small
Table IV.1. Comparison between two strain mapping techniques: digital image correlation and microgrid
technique [122].
IV.2 Measurement of micro-scale deformation at high temperature
Ideally, during plastic deformation, all grains are strained homogeneously, following the polycrystalline
body, but in real materials, strain is not homogeneous. As seen in section IV.1, several techniques are
now available to measure the micro-scale strain distribution in polycrystals across a wide range of
scales of observation, including resolutions fine enough to reveal intergranular deformation gradients.
Marking samples with gold or platinum microgrids using electron lithography is now widely used to
characterize the low to moderate temperature mechanical behaviour of heterogeneous materials. The
temperature was so far restricted to a field ranging from 20 to 500°C because of the deterioration of
the microgrids at high temperature.