MECHANICS OF INDENTATION - Nanoindentation with Biological Applications - page 129

Figure 5-4. Elastic-plastic indentation responses for a sphere on fused silica glass and
aluminum. The fused silica remains remains elastic while the aluminum demonstrates the
onset of plastic deformation and a transition from P ~ h 3/2 to P ~ h behavior at
displacements larger than 100 nm.
is the relative curvature between the indenter and the residual impression
on the sample surface. The challenge lies in identifying the elastic depth
h e as a function of the total depth, and separating out this relative
curvature term. In this regard a “partial unloading” protocol has been
developed for using multiple loading and (partial) unloading cycles—
a stepwise indentation protocol—to assist with identification of the
plastic depth and allow for elastic modulus measurements. The interested
reader is referred to Ref. 13 for further details; in many cases the
advantage of using a spherical indentation probe is the avoidance of the
onset of plastic deformation. For this reason spherical probes are often
used for elastic, viscoelastic and poroelastic analysis and less commonly
for elastic-plastic problems.
4.2. E and H deconvolution for sharp indentation
Elastic-plastic indentation testing typically uses a sharp “geometrically
similar” pyramidal (or, less commonly, conical) probe with a fixed
cone angle ( Fig. 5-1 ) . For conical pyramidal indentation with very sharp
probe tips (indenter radius R i = 50-100 nm), the large strains in the