Biomedical Engineering Reference
In-Depth Information
viscoelasticity and time-dependent plasticity. 66 In practice, the loading
rate and creep hold time can, and should, be determined experimentally
as the viscoelastic response of bone varies tremendously with many
factors. 43 For example, Berkovich indentations in equine cortical bone,
dehydrated in 100% ethanol and tested to a 5 mN maximum applied
load, show creep on unloading with no hold (0 s) at maximum applied
load. However a complete superposition of the unloading responses
occurs following unloading after both 120 and 240 s creep hold times -
thus indicating that a 120 s hold is sufficient for these samples and
testing conditions. 43
A limitation of many commercially available nanoindentation
systems is that they permit only creep, and not stress-relaxation, testing.
Because many systems are designed for testing stiff materials with no
time-dependence, they are instrumented to control for load and not
displacement. Advancements in instrumentation would assist in a
sophisticated analysis of viscoelastic, biological materials.
Partial Unloading : Unlike the ramp-and-hold approach, the
indentation modulus can be determined at a discrete number of contact
depths via a multiple partial unloading technique that is described in
detail elsewhere. 50-52 In brief, the indenter tip is incrementally loaded and
unloaded into a material over a preselected number of intervals. As the
initial unloading response of most materials is elastic, each loading event
(to a force, P 1 ) is followed by a partial unloading to a fraction of that
force ( P 2 ) and enables a calculation of modulus at a specific contact
depth. Successive loading continues, to a higher value of P 1 , and is again
followed by partially unloading to the same fraction of P 1 . The net effect
is that each successive loading event causes the applied load to increase
until the maximum desired applied load is attained. A new value for
modulus is collected for each pair of load-unload increments, thus
permitting modulus to be collected as a function of contact depth.
Typical values for partial unloading of bone using a small ( R
m
spherical indenter tip) might be a 5 mN maximum load with unloading to
75% of the applied load at each of 40 incremental loading events. 50
The partial unloading approach is advantageous in that the separation
of the loading and corresponding unloading points can be used to
indicate the yield point of the material. This method readily permits the
5
μ
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