MECHANICS OF INDENTATION - Nanoindentation with Biological Applications - page 122

based on convenience; however, for time-dependent materials the
loading history is integrated into the data analysis and must be
considered in further detail.

2.1. Indenter probe geometry

Indentation testing considers the contact between a probe and a sample.
The probe geometry thus determines the deformation profiles obtained
during the test, and as such is an extremely important experimental
controllable. Flat punch tips are convenient in that the cross-sectional
area of the contacted region is constant, simplifying analysis. However,
due to the sharp corners, there can be stress singularities at the edges.
Particularly in extremely small-scale nanoindentation testing, it can also
be nearly impossible to align the tip's flat surface parallel to the sample
surface. Therefore two tip shapes dominate small-scale contact testing:
spherical and conical or pyramidal ( Fig. 5-1 ).

Figure 5-1. Schematic illustrations of cylindrical, spherical and conical or pyramidal
indenter geometries. The critical parameters are the indenter radius, R , for a cylindrical or
spherical indenter, the contact radius a < R for a spherical indenter, and the included
indenter half-angle for a conical indenter (or effective indenter half-angle for a

pyramidal indenter).

Spherical tips are advantageous in that there is a delayed onset of
plastic deformation, while conical or pyramidal tips are exactly the
opposite since there is usually an immediate onset of plasticity and thus
elastic-plastic deformation even at small displacements. In indentation
testing with conical or pyramidal tips there is also the advantage of
geometric similarity—there is no intrinsic length-scale associated
with the size of the tip, and indentation tests at large loads result in