NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 195

generation of an indentation stress-strain curve and analysis of modulus
as a function of contact depth (for each incremental loading step).
However, due to the viscoelastic nature of bone, the rates of loading and
unloading are of critical importance to once again avoid a creep response
from being superimposed on every partial unloading event. Similar to the
ramp-and-hold method, such creep behavior results in an overestimation
of the measured indentation modulus at each incremental depth.
Constant Strain Rate with continuous stiffness measurement: The
continuous stiffness measurement technique utilizes a small sinusoidal
oscillation that is superimposed over the main loading function.
Continuous stiffness measurement, a standard feature on many
nanoindentation systems, enables the continuous determination of stiffness
from before making contact with a surface to the maximum contact depth
at each indent site. Measurements of continuous stiffness enable the
collection and analysis of modulus data as a function of contact depth at a
resolution that is much higher than in multiple partial unloading.
In the literature covering nanoindentation of bone, the effect of using
various oscillation amplitudes and frequencies is generally unexplored.
With continuous stiffness measurement, an increase in mechanical
properties seen with increased strain rate is similar to the increase
seen with increased loading rate. 55 Even for identical experimental
procedures, continuous stiffness measurement tests performed at a
constant strain rate take variable amounts of time during the loading
portion of the test and depend on the exact material response at each
specific indent site, and consequently may vary in loading rates. Bone
varies significantly in its inherent structural and material heterogeneity;
therefore, continuous stiffness measurement testing will naturally vary
from site to site. In addition, variable loading times introduce greater
complications when testing a viscoelastic material. The use of continuous
stiffness measurement for extracting nanomechanical property
information from bone is also complicated due to bone's natural resonant
frequencies and its viscoelasticity. 67
2.1.3. Preconditioning and repeated loading at single indent sites
Mechanical preconditioning, a technique that is commonly used in
biomechanical testing of viscoelastic soft tissues and elastomers, utilizes