NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 243

Thirdly, the mechanical properties of the enamel are intra-tooth site-
dependant. The inner enamel near the dentino-enamel junction (DEJ) has
on average a reduced hardness and elastic modulus by about 26%
compared to the outer enamel. 29 The size of enamel rod decreases with
distance toward DEJ, consequently the volume fraction of softer
interrods has a two fold increase, which undoubtedly contributes to a
decrease in composite hardness and stiffness. 36
Recently, the authors did a systemic nanoindentation investigation on
the elastic modulus of sound enamel of permanent teeth with different
tips and various loads. 33 The interesting finding to emerge from such an
investigation is: the elastic modulus of enamel was not a constant value
and had size effect dependent on the indentation contact areas.
Figure 8-3 shows a consistent decrease in elastic modulus for all
indenters in both rod orientations. This illustrates that elastic modulus,
in the case of enamel, is a function of contact area and the effective
volume of the contact stress field from which the elastic modulus is
determined, irrespective of what type of indenter tip is used. This can
be rationalized by the unique rod-interrod microstructure of the
biocomposite, in which the nanohardness and elastic modulus of interrod
region were respectively 73.6% and 52.7% lower than those of the rod. 6
Consideration of the rod structure of enamel, with increasing peak load
during nanoindentation, more and more prism-sheath units are involved
in the indentation stress field and contribute to the measured
nanoindentation elastic modulus. Due to the decreased modulus of the
protein rich sheath region, the more the sheath area becomes involved in
the deformation field, the lower the stiffness of the measured area. This
may also partially assist in explaining the wide range of elastic modulus
reported in other investigations and wider standard deviations from AFM
based nanoindentation tests.

3.2. Stress-strain relationship of enamel

In our previous study, we developed a nanoindentation-based method
for the measurement of indentation stress-strain relationship of the
material. 33 Based on Tabor's theory, In the case of a conical-spherical
tipped indenter, contact stress, , and strain, , can be estimated as: