COMPUTATIONAL MODELING OF INDENTATION - Nanoindentation with Biological Applications - page 174

in earlier examples ( e.g. Fig. 6-5b ) that despite the non-ideal constitutive
behavior of the specimen, when the reverse analyses are carried out
correctly, the extracted material elastoplastic properties represent an
“equivalent” power-law ideal material, whose indentation force-depth
curve can well reproduce that of experimental data. The same principle
applies to other non-power-law materials and time-dependent materials
as long as an effective model can be chosen to effectively describe the
constitutive behaviors of such material. The model always has certain
deviation with respect to the real material behavior, and it is pertinent
that the extracted material properties with respect to the model, could
reproduce the similar indentation behavior of the “black box” real
material. This is the criterion of a successful indentation technique.
There are several important issues related with correctly using the
experimental data. 91 First, since indentation is a surface measurement,
during sample preparation it is very important to avoid using mechanical
forces to harden or alter the surface; otherwise intrinsic specimen
properties are difficult to obtain. Second, the indentation depth should be
carefully chosen such that it should not be too small so as to interact with
surface roughness and other small-scale uncertainties, and at finite
indentation depth boundary effect may come into play, which needs to be
modeled using computational analysis. Third, the measurement of
contact stiffness or unloading properties are sometimes strongly affected
by instrument or indenter tip stiffness, which must be subtracted off. In
the establishment of indentation theory via computational modeling
(including those introduced above), a rigid indenter is assumed and thus
in order to apply such theory, the experimental data must be corrected to
remove any contribution from the instrument frame.
Many commercial indenters output the contact stiffness S
automatically (which includes the contribution of the indenter tip), using
the continuous stiffness measurement, CSM. By employing the widely
used diamond properties, 2 Young's modulus (

E ) of 1.141TPa and

Poisson's ratio ( ) of 0.07, the intrinsic contact stiffness S (which

corresponds to that produced by a rigid indenter) can be obtained
by deducting the indenter tip contribution from 0 S There are two
approaches of deri vi ng S in practice: (1) If the specimen modulus E is
already given,

ν

i

SES E =

*
0 /()

for both conical and spherical indentation,