Integrated correction of optical distortions for global HR-EBSD techniques.

Optical distortions caused by camera lenses affect the accuracy of the elastic strains and lattice rotations measured by high-angular resolution techniques. This article introduces an integrated correction of optical distortions for global HR-EBSD/HR-TKD approaches. The digital image correlation analysis is directly applied to optically distorted patterns, avoiding the pattern pre-processing step conducted so far while preserving the numerical efficiency of the Gauss-Newton algorithm. The correction implementation is first described and its numerical cost is assessed considering a homography-based HR-EBSD approach. The correction principle is validated numerically for various levels of first-order radial distortion over a wide range of disorientation angles (0 to 14°) and elastic strain (0 to 5×10-2). The errors induced when neglecting such distortions as well as the influence of both the radial distortion coefficient and the pattern centre and optical centre locations are quantified. Even when both reference and target patterns are distorted, the correction appears necessary whatever the disorientation between those patterns. The required accuracy on the true distortion parameters for an effective correction is consequently determined.

Geometrically Nonlinear Field Fracture Mechanics and Crack Nucleation, Application to Strain Localization Fields in Al-Cu-Li Aerospace Alloys.

The displacement discontinuity arising between crack surfaces is assigned to smooth densities of crystal defects referred to as disconnections, through the incompatibility of the distortion tensor. In a dual way, the disconnections are defined as line defects terminating surfaces where the displacement encounters a discontinuity. A conservation statement for the crack opening displacement provides a framework for disconnection dynamics in the form of transport laws. A similar methodology applied to the discontinuity of the plastic displacement due to dislocations results in the concurrent involvement of dislocation densities in the analysis. Non-linearity of the geometrical setting is assumed for defining the elastic distortion incompatibility in the presence of both dislocations and disconnections, as well as for their transport. Crack nucleation in the presence of thermally-activated fluctuations of the atomic order is shown to derive from this nonlinearity in elastic brittle materials, without any algorithmic rule or ad hoc material parameter. Digital image correlation techniques applied to the analysis of tensile tests on ductile Al-Cu-Li samples further demonstrate the ability of the disconnection density concept to capture crack nucleation and relate strain localization bands to consistent disconnection fields and to the eventual occurrence of complex and combined crack modes in these alloys.

Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle.

Mantle flow involves large strains of polymineral aggregates. The strongly anisotropic plastic response of each individual grain in the aggregate results from the interactions between neighbouring grains and the continuity of material displacement across the grain boundaries. Orthorhombic olivine, which is the dominant mineral phase of the Earth's upper mantle, does not exhibit enough slip systems to accommodate a general deformation state by intracrystalline slip without inducing damage. Here we show that a more general description of the deformation process that includes the motion of rotational defects referred to as disclinations can solve the olivine deformation paradox. We use high-resolution electron backscattering diffraction (EBSD) maps of deformed olivine aggregates to resolve the disclinations. The disclinations are found to decorate grain boundaries in olivine samples deformed experimentally and in nature. We present a disclination-based model of a high-angle tilt boundary in olivine, which demonstrates that an applied shear induces grain-boundary migration through disclination motion. This new approach clarifies grain-boundary-mediated plasticity in polycrystalline aggregates. By providing the missing mechanism for describing plastic flow in olivine, this work will permit multiscale modelling of the rheology of the upper mantle, from the atomic scale to the scale of the flow.