NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 247

Figure 8-5. Stress-strain ( H- tan ) relationship of different materials. (Solid lines

represent plastic part and short dots lines represent elastic part of materials, which was
calculated from the Hertz expression, equation 5-5. )

energy adsorption, an approach that has received relatively scant
attention to date. From nanoindentation load-displacement curves, the
elastically recovered energy, U E , and inelastically absorbed energy, U I , of
one indentation test can be calculated; the total energy U T = U E + U I . The
ratio of the inelastic energy to the total energy, U %= U I / U T *100%, is
used to normalize the energy expenditure of each test. This value may
also be denoted the “plasticity index” to characterize the plasticity of the
material. This U I / U T ratio ranges from 0 for an ideally elastic material
(all energy is returned on unloading) to 1 for an ideally plastic material
(no energy is returned).
In a recent study, we investigated the energy absorption ability of
enamel by nanoindentation. 41 As shown in Fig. 8-6a , energy expenditure
ratio was plotted against the contact strain. In this way, data from
different tips, especially spherical tips, are consistent and form a
continuous curve. For enamel, energy expenditure ratio, U% , begins at
30% and has a monotonic increase with the contact strain, , with no

obvious elastic part. In contrast, from Fig. 8-6b , HAP showed a linear

ε