Indent shape recovery

The repeated measurement of the size of indents, and the interpretation of indent geometry for the purposes of calculation, may be tedious, and operator bias is almost unavoidable. The edge of the impression is not always well defined, and misleading edge effects may be associated with anisotropic plasticity or plastic recovery. Faceted and elongated grains, or other microstructural features, together with the limitations of contrast and resolution in the optical microscope, complicate the interpretation, while the shape of the indent may differ in different materials so-called pin-cushion or barreled indents, associated with different constitutive relations and frictional shear on the faces of the indentor in contact with the plastic zone [3]. Mismeasurement of indent size is a major source of scatter in the experimental data and the relative errors in the results of different operators. 

An instant elastic recovery of the indentation depth upon load release. The loss of contact with the indenter allows for a change in the shape of the indentation. 

Uniaxial mechanical deformation produces a conspicuous anisotropic shape of the residual indentation (4). The anisotropy depends on the orientation of the diagonals of indentation relative to the axial direction. Two well-defined hardness values emerge. One value (maximum for a Vickers indenter) can be derived from the indentation diagonal parallel to the fiber axis, d. The second one (minimum) is deduced from the diagonal perpendicular to it, d . The former value responds to an instant elastic recovery of the fibrous network in the draw direction. The latter value defines the plastic component of the oriented material. It is useful to define the indentation anisotropy as AH = 1 - (d /d ). 

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