NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 250

In conclusion, enamel showed much higher energy absorption
capacity than HAP, a ceramic material with similar composition. This
may be because of the minor organic component of enamel. For the
spherical indenters the energy loss was found to be a function of the
contact strain. Energy loss with a Berkovich indenter was greater than a
spherical indenter at equivalent contact strain.
Higher energy expenditure of the enamel was primarily related to the
minor organic components. Three different mechanisms were considered
to contribute to the measured energy absorption, namely fluid flow
within the sheath structure as proposed by Fox, 42 protein “sacrificial
bond” extension as proposed by Smith et al . 43 for nacre and Fantner
et al . 44 for bone, or nano-scale friction within sheaths associated with the
degustation of enamel rods as considered by Macho et al . 17 Further work
is required to more quantitatively assign the energy loss observations to
these or other mechanisms.
3.4. Creep behaviour of enamel
Viscous responses of natural materials play an important role in their
function to bear and distribute force, and especially for bone, and
has attracted considerable attention. 45-50 Jäger 46 summarized the
characteristics of typical viscous deformation of biological tissues as:
(1) the force necessary for deformation depends on the deformation rate;
(2) force-displacement hysteresis occurs during loading and unloading;
(3) application of a (constant) force leads to transient and possibly
permanent creep; (4) after removing the deformation force there is
recovery.
Restricted by measurement methods, the limited viscosity and
temperature range over which natural biomaterials operate, the viscous
response—especially creep behavior—is difficult to quantify. For
metallic and polymeric materials, traditional tensile or compressive tests
are used to investigate the creep response of a specimen. Two main
problems limit the usage of these methods for biological materials such
as enamel. Firstly, these tests need multiple identical specimens and are
time consuming to conduct. Secondly, the accuracy of these methods